Some acoustical properties of ships with respect to noise control Part 1

STUDIEENTRUM TN.O. VOOR SCHEEPSBOUW EN

NAVIGATIIE

NetherIands' Research Centre T.N.O. for Shipbuilding and Navigation

SHIPBUILDING DEPARTMENT _{MEKELWEG 2, DELFT}

SOME ACOUSTICAL PROPERTIES

OF SHIPS WITH RESPECT

TO. NOISE CONTROL

(Acoustische eigenschapen van schepen in verband met geluidbeperking)

PARTI

H.

.

..

by

ir J. H. JANSSEN

Technical Physics Department T.NO. & T.H - Delft

Issued by the Couíncil

This report is not to be published unless verbatim añd unabridged

CONTENTS OF PART I (Report no. 44 S)

page

Summary 5

Chapter 1: Introduction 5

5 1 Sorne general remarks - 5

§ 2 Aim

of the report ...6

§ 3 - Contents- of the report -6

Chapter 2: Measurement of sound 7

§ i Transducers

...-

. . . 7

§ 2 Calibration 9

§ 3 Electronic aids . . . ₁₀

§ 4 Plotting sound measurements IO

Chapter 3: Acoustical quality of ships 12

§ 1- Recent trends in noise criteria 12

- 2 Additional information on criteria . 14

5 3 SOme examples of cabin noises in six motorships ₁₉

Chapter 9: Some. conclusions 21

çhapter 10: Some calculated noise spectra compared

with measurements 24

References - - 25

CONTENTS OF PART II (Report no. 45 S)

Summary

Chapter 4:

Propeller nOise ...5

1 Structure-borne noise due to propellers ₅

s 2 Transmission through the hull 6

3 Transmission along the propeller shaft. ...7

chapter 5: Engine roOm noise . 9

s I Air-borne and structure-borne noise near diesel engines.. 9

§ 2 Air-borne and structurè-borne noise near reduction gears 11

s .3 The distribution of air-borne noise in the engine room ₁₂

§ 4 Structure-borne nOise in engine room boundaries 16

Chapter 6: Transmission experiments -1-9

1 Transmission in vertical directión ₁₉

§ 2 Transmission in horizontal direction 23

§ 3 Damping 23

Chapter 7: Sound pressure excited plate vibrations . 24

§ 1 The coincidence thodel 24

s 2 -Experimental facts 25

§ 3. A simple model of a finite plate - 26

s 4 Experiments on ä model casing - 29

Chapter 8: Radiation of sound by vibrating plates

30-§ 1 Pressure, intensity and power of radiated sound ₃₀

5 2 Plates excited by point- or by line-forces 31

§ 3 Constant -force or constant velocitr excitation;

experiments 31

§ 4 Plates excited by area-'forces ₃₂

5 5 Experiments in cabins ₃₃

§ 6 Air-borne sOund insulation ₃₅

SOME ACOUSTICAL PROPERTIES OF SHIPS

WITH RESPECT TO NOISE CONTROL

PART i by

Ir. J. H. JANSSEN

Summary

The aim of this report on the acoustical properties of ships is to present data for the assessment of the relative importance of noise sources, noise paths and noise reduction measures in ships as far as human comfort is concerned. lt is therefore a more practical parallel to preceding report "Acoustal principles in ship design".

The instruments used for the measurements by the Technisch Physische Dienst are described briefly. Also

some information on recent trends in noise criteria is given. Noise spectra, as measured in cabins of different ships, are compared, stressing the importance of the distance between noise source (e.g. engine room or propellers) and cabin.

-Results of measurements on structure-borne sound (vibrations) and on air-borne sound due to propellers or engines are compared with results obtained during excitation of parts of a ship's structure by means of a tapping

machine.

With the aid of theoretical and experimental models the various possible noise paths are identified and investigated as to their respective contributions to noise in a cabin.

The conclusions about the origins of a cabin noise may be of some value for an "engineering estimate", as is illustrated in the last chapter. A list of references is given..

For practical reasons the report, is published in two parts; part I (chapters 1, 2, 3, 9 and 10) containing

introduction, information on instruments, criteria and a summary and illustration of results; part ¡I (chapters

4, 5, 6, 7 and 8) containing the results of measurements and some theoretical considerations.

CHAPTER 1. INTRODUCTION

§ 1. Seme general remarks

In the series of reports published by the

"Studie-centrum T.N.O. voor Scheepsbouw en Navigatie" the following appeared: in 1953 report no. 12- M (Noise and abatement of 'noise in marine engine rooms, in Dutch); in 1957 26 M (Noise

measure-ments and noise reduction in ships) and in 1959

34 S (Acoustical principles in ship design) [1, 2, 3].

One may say that 12 M is an initial orientation

containing mainly a survey of the relevant

litera-ture, an4 that 26 M presents many data about

noise levels in engine rooms and some useful

sug-gestions about improvements. Moreover it

con-tains some recommendations for further

investiga-tions. Among other problems the lack of, numerical

data relating to the air-borne soUnd insulation and

to the transmission of structure-borne sound in

ships, and the n ise production of turbirie reduc-tion gears and of diesel engines were menreduc-tioned. Research to fill this gap already started before 26 M was published [4]. However, considerable difficulties were encountered, mainly due to

mal-functioning of piezoelectric accelerometers. It took

a long time finding causes and remedies on this point [S] the more so as the accelerometers in

question seemed to be above suspicion.

In the mean time report 34 S was published,

containing some information about noise criteria

and about acoustical principles. For example, theo-retical models of sound sources, of structure-borne

sound transmission and of air-borne sound insu-lation are treated briefly. It showed implicitly that

noise levels can be computed from- basic data.

Subsequently the interrupted investigations could

be continued as better accelerometers became

avail-able. Moreover newer literature appeared. There-fore' a supplementary report to 26 M as well as to

34 S seemed desirable. It should contain information about acoustical -measuring instruments, about' the

rating of typical 'ship noises, about sound trans-mission in ships and about the explicit application of acoustical calculatiOns to specific ship spaces.

Especially about the last subject some remarks may be made.

It is well-known, that vibration calculations are very complicated for constructions such as ships. Even for the relatively simple low-frequency re-gion the amount of theoretical and computational

labour is nearly prohibitive.

For the higher frequencies, where the, dimen-sions of the structural components become much

larger than the wave lengths of the acoustical

signals therein, this difficulty would be

unsur-mountable if

the exact shape of the vibrating

plates and beams had to be known as accurately as

for the low frequency vibrations. Acoustical

de-signing in naval architecture ,would appear

im-possible for the time being if one- would wait until

i % accuracy in acoustical calculations could be

'guaranteed.

A good combination of measurements in ships,

and-6

common sense may therefore offer a solution to

the problem of noise control in ships.

This report may show that some useful results

are obtained with respect to noise control,

not-withstanding the many remaining problems.

2. Aim of the report

According to the preceding section this report

should be a more practical parallel to 34 S. One

will find, therefore, as counterparts of "Descrip-tion of sound" and "Criteria" the chapters

"Meas-urement of sound" (methods and instruments

used) and "Acoustical quality of a ship" (how to rate the noise in specific spaces in a ship and how to compare with other ships), here. Moreover

in-stead of the general

"Sources of sound" now "Propeller noise" and "Engine room noise" are

included. "Transmission" has its parallels too, but

"excitation" and "radiation" are treated more in

detail than in 34 S. This turned out to be necessary

for a better indentification of sound transmission

paths (see also § 3).

The principles of noise control, as mentioned

also in 34 S, are: first of all one should make up one's mind as to what noise can be tolerated and

what not (criteria). For an adequate description

of noise at least octave band spectra (decibel and

hertz) are required. The sources of air-borne or of structure-borne noise, the transmission paths

and the' various possible reduction measures should be distinguished clearly.

Eventually, computed or measured levels are compared with the appropriate criteria; ensuing

noise reduction requirements 'are considered with

the aid of e.g. data presented in this report.

These data result from research on board large

sea-going cargo or passenger ships with trañsverse

frames. The results of this report might also be applicable to typical war ships, tugs or craft for

inland navigation; then, however, some

modifica-tions will be unavoidable.

This report is not intended as a collection of

clear-cut solutions to specific noise problems; its

only aim is to study the relative importance of

noise sources, noise paths and noise reduction meas-ures in ships as far as human comfort is concerned.

This may be stated otherwise. It is well-known

that a huge variety of more or less efficient noise

control measures can be applied on board ship. Certainly the effect of applying them all would

be good from the point of view of the individual

exposed to noise, he will not even notice any noise.

For the ship designer it is, however, not the

question how' to prevent noise but rather how to

prevent noise annoyance or damage economically.

He will ask therefore: if. one or more of the

pos-sible measures were not applied what would' be the effect then? This "effect" might be an

enor-mous saving in money and still an acceptable noise

level. Moreover, inexpensive measures sometimes

are very effective. Anyhow, the question has its

own merits. An answer in this respect, however, can only be an estimate; many details will remain

open to discussion. It is for this discussion that this

report is intended to present some facts; at the

same time the vacancies in our knowledge become apparent.

§ 3. Contents of the report

In the preceding paragraphs the Contents are

indicated. Some details may be mentioned here.

Chapter 2 ( "Measurement of sound" ) is not an introductory text on. acoustic measurements ; for that purpose excellent books are available [6, 7, 8]. It contains in fact some information on the

instru-ments used by the Technical Physics Department

for the investigations reported.

Also, chapter 3 ("Acoustical quality of ships") is far from complete. Of course, rating the noise in a specific ship space is essentially a matter of

taste. A useful general opinion, however, may

result from a widespread use of simple criteria for

such items as speech intelligibility, loudness, hearing damage or comfort.

'Some recent data about criteria are adopted from

international standardization and from literature.

A fair comparison of noises in cabins of

dif-.ferent ships, for example, will be possible by means

of a "computed loudness". Judging cabin nois:s, moreover, implies observing the fact

that the

noise level in a cabin depends considerably on the distance to the main noise sources on board: the

engine room and the propellers although local noise

sources might be more important, incidentally.

Some examples are given for illustration.

In chapter 4 some new experimental results about hull vibrations due to propellers are pub-lishéd, about vibrations and air-borne noise of

diesel engines and reduction gears in chapter 5 and

in chapters 4, 5 and 6 about structure-borne noise

transmission through the hull. The frequency range considered was from 30 up to 12,000 Hz.

Recent theoretical results from' literature are in-cluded in chapters 7 and 8 about the excitation of

plates by sound pressures and about sound radiation

by plates. In these mainly theoretical chapters

some mathematical models are investigated as to the possible mechanisms of the interaction of

vi-brating plates and a surrounding acoustic fluid.

Moreover, an expression is given for the

mechani-cal point impedance of a finite flat plate.

The probably correct identification of the very

important noise path. from the engine room to

near by cabins through the casing was facilitated

greatly with the aid of the models mentioned.

Already in 26 M [2] the probable importance

of this direct air-borne sound path was stressed.

ships seemingly indicated, however, a purely

strut-ture-borne mechanism; at present both paths ap-parently are important. One may derive from the

contents of this report that in many ships a

re-duction of some 10 dB in the air-borne noise

transmission path would reveal the remaining

im-portant structure-borne transmission. If more noise

reduction is required the latter mechanism must

be considered - too. Besides sound absorbing

ma-terials, mufflers and insulating partition walls,

vi-bratiOn damping layers and resilient mountings

will then be of use.

Another feature of the chapters 7 and 8 is the

assessment made possible for the relative efficiency

of damping layers for noise control. Controlled

experimental data obtained in ships are badly

needed in this respect, however.

In chapter 9 it is concluded implicitly that ships need not be revolutionary for a reasonable amount of acoustical comfort, whereas chapter 10 shows that cabin noise spectra might be calculated with-in fair limits of accuracy if some drawwith-ings of the

ship and some engine data are available.

It is hoped therefore that this report may be of some use in providing an "engineering estimate"

of the relative importance of various possible meas-ures regarding noise control in ships.

CHAPTER 2. MEASUREMENT OF SOUND

§ 1. Transducers

A sound field may occur in all material media.

The elastic and inertial properties of a medium

determine largely its acoustic behaviour. For a

quantitative description of a sound field (see also [3]) many quantities, might be used. Only two of

them are in common use, viz, pressure and particle

velocity. In some instances these alternating, or

fluctuating, quantities can be measured directly.

Mostly however yard-stick, barometer, Roman

bal-ance or even a microscope are of, little help for

acoustic measurements. As soon as the quantities mentioned, small though they are, may be

trans-formed into electrical ones, electronic equipment can be applied. This transformation is made pos-sible by means of so-called electro-mechanical or

electro- acoustical transducers.

From the many physical principles that night

be applied only the piezoelectrical and the

electro-dynamical are widely used. The former is based

upon a property of many crystalline materials to

produce. electric charges due to mechanical strain;

the latter upon the fact that an electric voltage is

induced in a conductor moving in a magnetic field.

Before treating some details, however, a sketch

of. the electrical equipment may be given (see

figure 2.01). A mechanical signal is, transformed into an electrical one by means of the transducer

(e.g. a microphone or a vibration pick-up). This

transducer cathode follower cable amplifier jrnagfl.tic tape recorder filter amellfier w,ltmeter loudspeaker oscilloscope

Fig. 2.01 Block diagram indicating typical arrangement of instruments used for measuring sound pressures or other acoustical quantities (e.g. acceleration).

Transducers are shown in figure 2.03. Calibration instru-ments are not represented; see, however, also figure 2.06. Temporary storage of 'signals is possible by means of a tape recorder. Various filters may be 'inserted for signal analysis (e.g. 1/, 1/y, 1/3 _{octave pass band or constant}

bandwidth filters). Numbers eventually are read out from

a voltmeter.

electrical signal is either stored temporarily in 'a

magnetic tape recorder or fed directly into.' an

am-plifier and frequency analyzer. Eventually, the "strength" of the signals is read on a voltmeter.

A simple battery-operated, portable instrument

combining some of these functions is shown in figure 2.02. In addition to this sound spectrum

analyzer, 'the operator carries a small tape recorder

which is also battery-operated. It is mainly used

for reverberation time measurements but some-times also for temporary storage of measuring

signals. The accuracy for the latter instance, how-ever, is not always' 'sufficient and, moreover, the

useful frequency and amplitude range is rather

limited. It is for these reasons that more elaborate instruments must be used. 'More will be found in the following sections and in the relevant

litera-ture (e.g. [8]).

Some transducers used for the noise

measure-ments reported are shown in figurç 2.03. From left

to right the first one is an electrodynamical

8

membrane which in turn causes a tiny conducting coil to move in the field of a permanent magnet. In this way electrical voltages proportional to the

exciting pressures are induced. The order of magni-tude of these pressures can be read from figure 2.12

for example.

Fig. 2.02 A portable battery-operated sound spectrum indicator (Peekel type GRB) is used here to measure the noise out-put of a rotary converter. It may be used also

for vibration measurements. Analysis in octave bands

(centre frequencies 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz) "in the field" is possible by means of built-in filters. The operator also carries a battery-operated magnetic tape recorder (Stellavox). It is used for

rever-beration time measurements, for discrete frequency analy-sis and for fluctuating sound signals.

Fig. 2.03 Transducers for acoustic measurements. From left to right: microphone (Philips), accelerometer (Rohde und Schwarz), force gauge, velocity pick-up and hydro-phone (TPD).

The smallest pressures to be measured in practice are slightly smaller than i O 8 of atmospheric

pres-sure ( 3 4 dB in figure 2 . i 2 ).An almost

intoler-ably loud noise of i 1 dB corresponds to slightly more than 0. 1 %o of atmospheric pressure; it is

however nearly the upper limit of the pressure

amplitude range to be measured. This range thus

comprehends quantities possibly differing by as

much as a factor io5.

The second transducer in figure 2.03 is a piezo-electric accelerometer. The cube is fixed to a

vi-brating surface by means of a very thin layer of

modelling clay or a special gluing "wax". In this way quick measuring is possible, in contrast to the

widespread use of fixing bolts. These bolts or threaded studs not only imply fixing holes and associated delay but also introduce a dangerous

situation: many vibration pick-ups turn out to be

rather sensitive to vibrations in directions other

than normal to the vibrating surface. The spurious

signals resulting from this effect may be

over-looked easily

if the pick-ups are bolted to the

vibrating object, whereas "the sticking method" allows for easy checks e.g. by turning the pick-up [5].

Inside the cube some very small bars of piezo-electric material are fixed in such a way that they may bend in response to accelerations performed

by the housing. The associated deformations (strains) in

the bending bars bring electrical

charges to electrodes connected to an amplifier.

Thus, the fluctuating voltage proportional to the

acceleration may be measured.

To illustrate the order of magnitude it may be

mentioned that displacement amplitudes as small

as 10_lo m sometimes must be measured, or in

other words displacements over a distance equal to

the diameter of a hydrogen molecule must be

known accurately!

The third transducer is a piezoelectric force

gauge; inside the cylindrical housing a pair of

piezoelectric plates is "squeezed" if the gauge is

placed between a force generator and a structure to be excited. The induced voltage is proportional

to the force.

The fourth transducer shown in figure 2.03 is an

electrodynamical velocity pick-up; a permanent magnet is mounted as a seismic mass, the poles of which enclose a conducting coil. This coil follows

the movements of a vibrating surface to which

the perspex housing is fixed. The induced voltage is proportional to the velocity of the surface. The

last transducer is a hydrophone; it is built in nearly

the same way as the force gauge but it is

water-proof. The force in this instance is a result of the incident sound pressure acting on the flat surface

of the small box at the lower end of the cylin-drical house. The hydrophone can be raised or lowered by means of the cable entering at the upper end. Inside the house a cathode follower

(a special amplifier enabling the use of long

ca-bles; c.f. 3) is mounted.

Of course, this hydrophone may also be used

as a microphone. The trañsducers shown thus far

are built and used for producing an electrical

volt-age proportional to a mechanical quantity (i.e.

force, pressure, velocity or acceleration); the as-sociated electronic equipment is inherently

com-plicated and delicate. In figure 2.04 two

trans-ducers are shown which also may be used as pick-ups but they are intended as mechanical exciters

i.e. for producing mechanical quantities propor-tional to an electrical input voltage. At the left hand side a normal loudspeaker cabinet may be

seen. The other transducer is a vibration exciter. Both instruments use the electrodynamical

prin-ciple in order

to produce air-borne sound or

mechanical force (structure-borne sound)

respec-tively. In cabins the loudspeaker may produce some 90 dB, whereas the maximum force

nor-mally produced by the exciter is

about 10 N

(i.e. approximately corresponding to one kilogram force).

The transducers mentioned up to now are

"rever-sible": an electrical input produces a proportional

mechanical output and a mechanical input produces a proportional electrical output.

This is not true for the transducers shown in

figure 2.05. The first instrument is

a tapping

machine; four spring-loaded hammers are raised

and released successively (this instrument is not

built according to international standardization for

impact noise rating of floors in buildings; its smaller dimensions however are sometimes advantageous).

Fig. 2.04 Electró-acoustical transducers used for gener-ating air-borne sound (loudspeaker) or structure-borne sound resp. (Philips exciter, right). They are reversible

and may be used as microphone or velocity pick-up as well.

The excitation force in normal use of the exciter is

approximately 10 N ( 2 lbs force).

Fig. 205 Mechanical "transducers" (irreversible) for low frequency excitation (< 40 Hz; in background) and

for broad band excitation (tapping machine). The working

principles are based on rotating unbalanced mass and

hammers resp.

They may excite a foundation or substructure by hitting it. In this way e.g. 10 impacts per second

are given. The structure-borne noise due to a

diesel engine is simulated more or less realistically.

In the low frequency range (f < 50 Hz) measure-ments are difficult if this type of exciter is used.

Then a rotating unbalance exciter gives better results. A small one used for the measurements underlying figures 4.04, 4.05 and 4.06 is shown

in the background of figure 2.05.

§ 2. Calibration

A great difficulty is presented by the long chain of electronic instruments used for sound measure-ments: plenty of opportunity for failing parts and malfunction, may even be unnoticed. Of course, they are calibrated in the laboratory in the usual

way [8]. The air-borne sound pressure microphone

shown in figure 2.03 turns out to be very stable

and reliable. Moreover a simple comparison of two independent sound level meters (e.g. one as shown in figure 2.02 and one consisting of other elements)

in the same sound field in most instances suffices

to establish the reliability of air-borne sound meas-urements.

Although the accelerometers of the type shown in figure 2.03 are very good as such and although

the calibration of the electrical sensitivity (by

means of an auxiliary 1 ohm series circuit) is reliable, still the measurement of structure-borne sound

meets with more unexpected inaccuracies than the

normal air-borne sound measurements. It was

there-fore thought necessary to calibrate in the field.

Figure 2.06 shows the main details. A hand-held

electrodynamical velocity pick-up (Philips) is used

as a calibrator. A known current input produces a known force, from which the acceleration of

lo

accelerometer to be calibrated is fixed on top of it.

The velocity pick-up and the meter used for

measuring the input-current are very stable. In this

way simple calibrations in the field were possible.

f

Fig. 2.06 Calibration of an accelerometer in the field.

The very stable electrodynamical velocity pick-up

(Philips) is used as a force exciter. From known current and sensitivity the acceleration of the house may be

computed. Thus, the piezo-electric accelerometer is

calibrated (f < 1000 Hz).

3. Electronic aids

In many acoustic measurements cables must

be used between the indicating instrument and the

pick-up. More often than not the recording or

analyzing equipment cannot be carried around in

all parts of the ship. Thus, long cables appear to introduce "cable attenuation" of the signal and

moreover spurious signals. Their detrimental effects

on accuracy may be minimized by using low

impedance cable transmission. This implies

cathode-followers for piezoelectric pick-ups. Speech

com-munication between the operators at both ends of the cable is another very useful feature to be

incorporated. It is therefore that some cables, boxes

and headsets are used besides the actual sound measuring instruments. In figure 2.07 a typical

vibration (acceleration) measurement is illustrated.

In the measurements of 1956 and before, the

analyzing and recording equipment often had to be arranged as shown in figure 2.08. Afterwards many improvements could be introduced resulting in a double channel recording set shown in figure

2.09 and in half-octave band excitation and

analyz-ing equipment shown in figure 2.10. All instru-ments are easily transportable. The magnetic tape recorder of figure 2.09 is provided with calibrated attenuators and with a variable sweep frequency

generator yielding a constant current electrical

calibrating signal. Its useful frequency range is

from 1.5 Hz to 30 kHz; for this report only

the audio-frequency range is used (this is also

covered by the equipment of figure 2.10).

§ 4. Plotting sound measurements

As mentioned before, sound power manifests

itself by way of fluctuations in particle velocity and

pressure. In fluids and gases these pressure fluc-tuations are measured by means of hydrophones or microphones. Statements about sound pressure

r

-.4

Fig. 2.07 Measuring the structure-borne sound (vibra-tions) in the foundation of a rotary converter. Operator

carries headset for communication with colleague at

tape recorder (other end of cable). The hand-held

cathode-follower is fed from the battery box (with voltage moni-tor, calibration input and spare room for accelerometer and cement). The cathode-follower permits up to 1000 m e.g. cable between measuring station and recorder.

Fig. 2.08 Instruments for 1956 measurements of

propa-gation and radiation of sound in ships. Seaworthiness could be improved upon!

Fig. 2.09 Magnetic tape recorder (TPD 1959) for

double channel recording and with variable sweep fre-quency calibrating generator, calibrated attenuators and extreme stability. Useful frequency range is from 1.5 Hz

to 30 kHz (direct recording).

-t

Fig. 2.10 _{"Source-(right) and receiver-boxes" for(1957}

and later) acoustic transmission experiments. From top

to bottom the "source box" contains a random noise

generator, 1/., octave bands filter set and a 20 watt

amplifier. Loudspeaker or vibration exciter may be fed from this equipment (c.f. fig.2.04). From bottom to top

the "receiver box" (left) contains three preamplifiers, a

octave bands filter set and a vacuum tube voltmeter with built-in monitoring oscilloscope and loudspeaker.

Between 30 Hz and 13000 Hz transducer signals may be analyzed.

measurements are easily done when use is made

of a decibel scale. At a first glance this scale seems

to complicate matters. Apart from being customary in acoustics it is very useful, however. One decibel is sufficiently accurate and moreover the

enormous range of values (e.g. a factor 1O, c.f.

chapter 2 §

1) is covered by decibel numbers

between 0 and

120 practically.

This may be

compared with a temperature scale; nobody wor-ries about the exact definition of a degree Celsius

when measuring a temperature.

Both the Celsius scale and the decibel scale are

efficient, for

heat or

sound respectively. The

quantity analogous to temperature is called "level".

A sound pressure level L» is defined according to

2.01 L» = lO log P2cjr/P2o

where log is the logarithm to the base 10, 2cjî is

the mean square of the sound pressure to be described

(in N2/rn4) and o is equal to 2 . iO N/rn2, the

international reference pressure [9].

The mean square value is determined in a

con-venient time interval e.g. i second; in view of

the accuracy aimed at this interval need not be

defined more precisely here.

It is very important, however, to mention the method which is used in this report and often in

the acoustical literature in order to describe the

frequency composition of a sound. As is well-known a sound may consist of several components. These components may be visualized as sinusoids.

For each of these sinusoids one must distinguish the frequency and the amplitude. The amplitude

determines the "strength" of the component, the

frequency determines the pitch.

Because signals of different frequencies may have different properties regarding physical noise control

as well as regarding annoyance it is important to

know the so-called spectrum of a noise.

This spectrum

- a

better name would be

"acustrum" - can be determined in various ways.

b...e

øS

Fig. 2.11 _{Automatic analyzing equipment for magnetic}

tape recordings. The frequency spectrum is presented in a graphical form. The tape is moved backwards and for-wards during analysis (no dubbing or cutting; no loop). The selected portions of the tape (arbitrary length) are

marked by coding pulses recorded on it. An 8

octave-bands analysis of 300 _{measurements (of}10 seconds

dura-tion each), for example, takes about 8 hours

12

Mostly electronic frequency filters are used. The "strengths" of the noise components whose

fre-quencies lie within a certain filter band are measured

only. By switching to adjacent frequency bands

successively a spectrum may be determined.

If the upper limiting frequency of a band is

two times thé lower limiting frequency it is called

an octave band. In this report most of the spectra are presented as octave band spectra. In diagrams

the respective levels are plotted at the centre frequencies

of the

band. The frequency scale (mostly abscissae). is divided according to the "ISO preferred frequencies" [10]. This scale is

logarith-mic, I mm being taken for two octaves. The

120 110 100 80 70 60 50 z o 40-0 2 30 20 10 0- -32 63 125 250 500 1000 2000 40CV 8000 Hz

centre freqency of + oàt. bando

27.5 II Il III I!I! a(440) II II 4186 Hz 01°/co of at,,,. preas. lpre H20

Fig: 2.12 Graph for plotting air-borne sound pressure measurements (the ratio of the f-requijcy scale to the decibel scale according to ISO recommendations). The key-board and the musical standard frequency (440 Hz or cycles per second) illustrate the abscissae. TEe order of magnitude of common sound pressure levels may be compared with the level corresponding to an effective

pressure equal to 10 m water "column" or to iO times

atmospheric pressure (see also relation 2.01) resp.

-decibel scale (ordinates, with some exceptions) of

course i! also logarithmic, 1 mm being taken for

1 dB. In figure 2.12 some air-borne sound pressure spectra are illustrated.

For the particle velocity measurements

(struc-ture-borne sound) the velocity level L is defined as 2.02 L = 10 log V2eIf/V20

where V2cff iS the mean square of the particle velocity

( or: "rapidity", for a cl-ear distinction from sound

signal velocityor speed of sound?) ; vo 4.7 10_s in/-s is a reference velocity ( chosen by the author)

in accordance with the relation for air-borne plane travelling waves p = cv (see also -[11]; instead of this value y0 5.0

. 108 rn/s would be more

elegant; the ensuing difference in level shoul4 be

only 0.5 dB).

This choice and its use may be explained as

follows. Apart from pressure, velocity is physical-ly more important than acceleration or

displace-ment because the power radiated by a sound source equals pressure times velocity times area (if proper-ly defined) and corresponding formulae are simple

and useful. Moreover, velocity as a descriptive

variable is a good compromise between displacement

and acceleration. Plane travelling waves are often

used as a model for sound fields. Because the

pres-sure p and the particle velocity y in a plane wave are related as mentioned above, the pressure level and the velocity level of a plane wave are

numeri-cally equal due to this choice.

In figure 5.06 three special straight lines are

drawn in addition to .some data about diesel engines, illustrating the velocity level mentioned above. t

a given frequency they indicate thevelocity level.

for a sinusoidal, vibration for which the effective

value of the velocity 'equals iO in/s (hocizontal line) or the displacement 10 10 m (lower oblique

line) or the acceleration 9.81 mIs2 (upper line).

A line parallel to the "g-line" but 20 dB lower would indicate the frequencies and corresponding

levels for a vibration acceleration (effective value) of 0.98 1 rn/s2. Approximately this value (within

the shaded region) clearly will be measured often

on diesel engine foundations.

In addition to the pressure level and velocity level

mentioned, a sound power L may be defined: 2.03

L = 10 log (P/l02 W)

where P is the air-borne acoustic output of a sound

-source in watts and 10_12 W is an international

reference power.

CHAPTER 3.

ACOUSTICAL QUALITY OF SHIPS

§ 1. Recent trends in noise criteria

judging the acoustical quality of a ship

details about noise criteria that may be used on board ships, because there is an ever increasing

need for rating noise with respect to conservation

of hearing, interference with speech and with

respect to annoyance.

One might put the question whether any other

criteria can be introduced for example with respect

to work efficiency, to bodily activity or to social

behaviour.

At present an answer could be summarized as

follows.

In general, criteria may be derived from daily

experience concerning the behaviour of man when exposed to noise. This behaviour or these reactions can be measured:

a man may be required to perform a task when

exposed to noise; then the efficiency on that

task may be a measure for the noise influence;

also, physiological measurements are possible

e.g. on blood pressure, heart rate, metabolism,

hearing damage etc.; or

annoyance caused by noise can be investigated

for example by asking a person exposed to

noise, to report on his own feelings, or by

investigating his social behaviour.

Research results, as summarized by Broadbent

[1], indicate that there is almost no measurable noise influence on work efficiency, in spite of

contrary opinions. This seems to be in accordance with what the man-on-the-ship says.

Broadbent mentions, however, a kind of internal

"blinking" effect due to noise

exposure. This blinking mechanism results in intermittent failures in performance, although the work will nog

neces-sarily be less efficient. An "internal blink"

is

supposed to last as long as I second but probably

does not cut off the incoming information

com-pletely as eye blinking does.

It may be very serious in tasks in which no

momentary relaxation is possible, in particular in watchkeeping tasks

in which short stimuli are

delivered at unpredictable times and in which quick reaction is required; to prevent this effect, exposure

to noise levels of more than 90 dB and high

fre-quency noise should be avoided.

Regarding the physiological measurements, there

are many literature references (e.g. [12] or [13] ).

Some of the rather alarming statements in this respect, however, are lacking comparison of the noise-induced effects with other effects due to

well-known factors influencing the human body. At present the evidence seems to be that no undue

and unhealthy response appears in prolonged noise, provided the noise spectrum does not exceed noise

rating curve number 75 _{(ISO, c.f. 2), although} it must be admitted that important noise effects on social behaviour have been found [13]. Cer-tainly, further research on this and other

uncon-scious response of man to noise is badly needed, the

more so as for example Jansen [I3 ] reports that

from 1005 steel industry workers interviewed the

intense noise-exposed group showed ' relatively

four times as much complaints regarding the in-terhuman relations in their job as the normal

noise-exposed group.

If the noise spectra on board do not exceed

noise rating curve number

75 -

and this will

nearly always be the case outside the engine room

-it turns out after all that only cr-iteria on noise

annoyance or on speech intelligibility offer means

of rating and judging the acoustical quality of

a ship. An important question

is now how to

describe a sound field and the corresponding

reac-tion that can be expected from "general people"

"objectively". In the past several systems have been

in use. A great confusion ensued with 'respect to noise rating limits, noise rating and even loudness

(c.f. the use of "phon" for subjectively determined loudness level and "Dinphon" for measured "weighted" noise levels).

Considerable progress has been made, however,

by the International Organization for

Standardi-zation [14]: a draft secretariat proposal for

"noise-rating numbers" with respect to conservation of

hearing, speech communication and annoyancewas

prepared. It has a twofold scope: firstly it shoulçl enable the user to forecast whether a future noise environment will be acceptable to people in that environment; secondly it should enable thê user

to decide whether an existing noise environment

is to be considered as acceptable or not. Obviously

one should realize that the final decision about the

acceptability rests upon the exposed individual.

Therefore, the rating system can oniy be statistically

correct. It should be based upon a statistical study of the reactions of individuals to noise exposure.

This problem is being studied; the available evi-dence confirms the principles of the rating system

involved.

For shipboard noises, however, sufficient data

have not been available as yet. Especially for

annoyance rating the list of recommended limits for specific noises, as presented in _§ 2, is to be

understood therefore as tentative. This list was

composed by the present author after simple

observations and cautious inquiries about various

noisy situations.

The system using a set of rating curves in a

decibel-frequency plane as introduced originally

by Bolt, Beranek and Newman [15], afterwards

combined with others, modified and now proposed

by the ISO, seems to be the best method interna-tionally agreed upon for rating noises. It is a prac-tical system anyhow.

Sometimes other quantities can be used for

char-acterizing a noise; the more important ones may be mentioned here. One may measure the strength

of a noise by means of a sound level meter;

14

loudness level of a noise may be determined by comparison with a 1000 Hz pure tone [8]. This

loudness level may, moreover, be computed

approx-imately by means of the Stevens method (c.f. § 2) [16] if the octave band spectrum is known.

All these, and other systems not mentioned, are

used to obtain one siñgle number describing or

characterizing a noise solely. It must be admitted

that it

is tempting indeed to do this. One must

not forget, however, that sound is not like heat:

temperature often suffices to describe a thermal

situation, although even here for human comfort humidity is as important a factor. Hearing at the

other hand is more analogous to vision: nobody will try and express a painting into one number charac-terizing it, althöugh the visible light spectrum

contains two octaves oniy instead of the ten acoustic octaves.

An indispensable minimum amount of data on

a specific noise for rating or control purposes seems

to bean octave band spectrum. This spectrum may

be compared with the rating curves mentioned.

This method of rating or judging is astonishingly

simple in view of the many types of noise affecting the various human activities.

§ 2. Additional information on criteria

In the report

"Acoustical principles in ship design" [3] the chapter on criteria started with

probably the most important acoustic problem

re-lated to ships: the audibility of warning signals.

The physical conditions for sound propagation in

the open air were idealized as an

ìnverse-4istance-proportionality of the sound pressure. In addition, audibility of e.g. a ship's whistle was considered

necessary at 4 miles' distance (incidentally, a

warn-ing signal level below 40 dB re 20 pN/m2 will

be insufficient practically;

this 40 dB is

to be

read instead of the misprinted 70 dB in [3] chapter 3 of course).

Admittedly, the sound propagation as well as the audibility conditions were simplified considerably. More detailed research on real conditions probably will result in a much greater recommended value for

the acoustic power output of a ship's whistle than

the minimum value of I W proposed in [3] (see

also [17], [18] and [19]).

The many shipping accidents wherç signal

audi-bility was of primary interest clearly demonstrate

the need for thorough investigation and regulation, anyhow. Especially on the bridge the air-borne noise level deserves utmost attention, therefore.

Also for other ship's spaces the acceptability of a noise must be known. The ISO standardization in this respect may thus be treated first. The scope

(of the Draft Proposal, August 1961) is to re-commend ratings for noises with respect to the risk of permanent impairment of hearing,

inter-ference with speech communication and with

res-pect to annoyance on the basis of which the

permissible limits for noises in various situations may be set by the competent authorites.

Use is made of noise rating numbers. These

num-bers may be illustrated by means of figure. 3.01. This figure replaces. figure 3.03 of [3]. Its use is essentially the same, the wording and the official procedure being slightly different.

32 93 125 250 500 1O) 2000 4000 8000 Hz

- centre frequency of + oct. bands

Fig. 3.01 Limits for specific noises. An octave band spectrum of an air-borne noise can be plotted in.this

graph. Only 24 curves are shown from the infinitely many

rating curves. The number N of the lowest curve just not exceeded (in three or eight octave bands, as indicated in the text) by the noise spectrum is the noise rating number of that noise. This number may be compared with the appropriate criteria (see text § 2).

A measured or computed noise spectrum is

handled as follows. First, an eight-octave-band

spectrum consisting of 8 values of the sound

pres-sure

level Lj for each filterband with centre

frequency f, according to the preferred ISO f re-quencies 63, 125, 250, 500, 1000, 2000, 4000, and

8000 Hz, is determined (the indicating meter is

set at fast response and read as the average of the maximum readings of the meter).

I

uui

12 11 9 8 7 6 E z-o 4

'3

f a b (Hz) (dB) (dB) 0.790 0.870 0.930 0.974 1.000 1.015 1.025 1.030 80 TABLE 31

for hearing conservation

jfor speech communication

70

1 2 5 10 20 ) 2

5)

.-allowable short term Contiruous exposure

Secondly, from these L1 values three or eight

numbers N1 are computed according to

3.01

N1 = (Lj a)/b

where a and b are constants as given in table 31.

For hearing conservation or speech

communica-tion purposes the three octave bands indicated in

table 3 I are considered important, whereas for annoyance rating the eight octave bands are of

interest (according to the ISO; may-be more than

eight bands must be used for ship noises).

Thirdly, the so-called Noise Rating Number N of the noise is then found as the highest of these,

three or eight, numbers N1.

Stated otherwise: determine the lowest noise

rating curve just not exceeded by the noise spec-trum after plotting it in figure 3.01 (at 500, 1000

and 2000 Hz only for hearing or speech!).

For hearing conservation N = 85 is proposed

as a limit, because habitual exposure to such a noise for 10 years or more may be expected to result in a negligible loss in hearing for speech

of an average individual.. This proposal replaces

the noise safety level limit (NSL) of figure 3.02

in _{[3]. For continuOus exposure less than five}

I min.

Fig. 3.02 Concerning the hearing damage risk. If normal human ears are exposed to broad band noise that is

continu-ously on for less than five hours per day the probably allowable continuous exposure (abscissae) based on the noise rating number N (ordinates) is given in this figure by means of the curve shown (ISO, Glorig [14] ).

90 80 o cff time in minute loe 50 - 20 -10 5 Noise rating number 40 50 60 70 80

Distaiice at which every day speech is.

considered to be intelligible if the level is

normal

raised

For more data about hearing conservation and about speech intelligiblity and telephone

commù-nication may be referred to the ISO Draft

Pro-posal [14].

Rating noise in respect to annoyance is a very

difficult and: complex problem. In this report some suggestions may be presented regarding noise rating

number limits for various ship's spaces following

the method, of the ISO proposal. These suggested

limits and corrections are given together with some

of the ISO. 63 35.5 125 22.0 250 12.0 500 4.8 1000 0.0 2000

3.5

4000 61 8000

8.0

5 10 15 20 25 30 35min.

-

on-time of exposure cycle

Fig. 3.03 Concerning the hearing damage risk. During in-termittent exposure to broad band noise the nóise rating limits for hearing conservation are determined by the exposure cycle, consisting of an "on-period" and an

"off-period" as indicated [14].

Example: an intermittent exposure consisting of 10

minutes in a noise for which the noise rating is 105

followed by 40 minutes quiet is just acceptable.

hours per day more noise probably may be admitted according to figure 3.02. For intermittent exposure

figure 3.03 may be used. It will be clear that short

term exposure to noise in an engine room, although the spectrum often exceeds curve 85 of figure 3.01, is not as dangerous as continuous exposure would be.

Frequently because of lack of apparatus or time a simple "screening method" may be useful. For

that case one may take as a criterion that no

further detailed study of

a noise is necessary

with respect to 'hearing conservation if that noise

is not so loud as to prevent easy conversation at

raised voice level between two persons at a distance

of 0.3 m. This follows also from tablè 311.

TABLE 311 120 z T1O C 100 o C 120 z 110 E C 100 90 m m 7 14 2.2 4.5 0.7 1.4 0.22 0.45. 0.07 0.14

After having determined the eight-octave band

spectrum and the corresponding noise rating

num-ber as mentioned above for annoyance purposes

one should correct this number

with the aid of

table 3111 which replaces table 3111 of [3], as far

as air-borne sound is concerned.

From this correction the final rating number for a specific noise as measured (or computed)

results; it may be compared with the suggested limits as presented jn table 31V (replacing thus the

list of figure 3.03 in [3] ).

In order to compare these suggested limits with those introduced by the See-Berufsgenossenschaft

Hamburg [53] as given in table 3V one must

know what a Din-phon is.

Summarizing one may say that a Din-phon is

not a phon (unit for loudness level as determined subjectively) but a unit for weighted sound pressure level.

This weighted level may he measured directly with the aid of a standardized Din-phon meter or

it may be derived from an eight octave band

spectrum. In the latter case the proper weighting

) What is me2nt here is the sometimes very irritating effect due to the periodic slcw variation of vibration amplitudes in a ship's hull,

when two or more main engines and/or propellers are operating at

slightly different rates; the peak level of the noise should be used for

deriving N.

Suggested limitsnot to be exceeded preferably by the rating numberN

(determined as the maximum of eight Nf values with the aid of equation 3.01 for the octave band spectrum, or by means of figure

3.01) as corrected according to table 3111for specific shipboard noises

hospital, quiet sleeping room 30

quiet conference room 35

luxury cabin (e.g: cabin of captain) 40

ressaurant, normal good cabin (e.g. officer's cabin),

bridge 45

"tourist class" cabin, crew, radio station (radio

signals excluded) 50

secretarial office (typing included) 55

public rooms on board short distance ferry 60

engine room at control panel 70

TABLE 3 V. See-Berufsgenossenschaft Hamburg noise

limits (tentative)

1. Engine rooms:

at control panel during normal watch during short repair or

checking intervals 2. Crew living space

3. Bridge, radio station

if keel laid before

I May 1961

level corrections must be added. There are three

sets of weighting corrections viz, for lower, for

middle and for high level noises.

For the present purpose all that would be needed

for- the comparison of the "Hamburg"-numbers

and those suggested here is the so-called B-weighting

(identifying for simplicity the German with the

American Standardization, [21] and [22]), which applies to the middle levels. In certain instances the A-weighting (for low levels originally) is also

interesting however. In table 3V1 two sets of

weighting corrections are given therefore; these must be added to the corresponding level of an

eight-octave band spectrum (C-weighting equals

O dB; above 85 dB).

if keel laid after

I May 1961 Influencing factor ' ..

Possible conditions Corrections to_{uncorrected N}

noise spectrum pure-tone components -

+

5

character wide band noise O

peak factor impulsive

+

5

non impulsive O

interference -

+

5 )

repetitive continuous exposure

character up to one per minute O

(about 0.5 min. 10-60 exp. per hour -

-

s

noise duration - 1-10 exp. per hour

- 10

assumed) 4-20 exp. per day

- 15

1-4 exp.perday

- 20

i 'exp. per day

- 25

adjustment no previous conditioning O

to exposure - considerable prey. cond.

-

5

extreme conditioning

- 10

time of day only during day time

-

5

at night

+

5

noise origin- radio in adjacent cabin

± 15

-talking in adjacent cabins ship's engines:

+ 10

cruising

- 5

manoeuvring

- 20

- fog signal

+

S

- walking on deck above

+

S

TABLE 3 VL Weighting corrections to be added'

Centre frequency of octave band A-weighting below s dB or between 30 and 60 Dinphon B-weighting between 55 and 85 dB or 60 and' 130 Dinphon 63

- 26 dB

9dB

125

- 16 dB

3dB

250

- 7 dB

1dB

500

2dB

i dB 1000

+ i dB

O dB 2000

+ 3 dB

+1dB

4000

± 2 dB

O dB 8000

- 2 dB

- 2' dB

16

TABLE 3111 TABLE 31V. Noise annoyance rating (tentative)

95 Dinphon 105 70

,

70 90 Dinphon 1-00 60 60

,

After adding these corrections the eight resulting

levels must be added according to the appropriate rules, viz, the level of two incoherent signals, of

x dB and y dB separately, measured simultaneously

is equal to 10 log (1001z + l001).

For most purposes sufficient accuracy is obtained

if one adds to the highest level 3 dB, 2 dB or i dB resp. for a difference in level lying between O and 1.5 dB, resp.

1.5 and 3.5 dB, resp. 35 dB and

8.5 dB.

Now, the loudest noise still not exceeding a

suggested noise rating limit of Fig. 3.01 and table

31V may be compared with the corresponding

limit of table 3v; of course the spectrum of

such a noise is exactly given by the noise rating curve in question. Let us call this type of noise

a 'Rating-curve-noise".

It turns out that la ("old" ships) 95 Dinphon corresponds very nearly to a 80

"rating-curve-noise", whereas the 90 Dinphon (new ships)

cor-responds to a noise spectrum just not exceeding

the noise rating curve no 75.

This is indeed a very good agreement.

The lb limits of table 3V are rather stringent;

for example, a 15 minutes exposure to 103 dB in

the 500 Hz band requires only some 20 minutes

"off-time" (c.f. figure 3.0.3). The total sound level

of a "rating-curve-noise" could thus be equal to

112 Dinphon, being 7 more than indicated in

table 3V for "old" ships.

In the same way a 'rating-curve-noise", the

spectrum of which does not exceed noise rating curve 50, would satisfy limit 2 of table 3V for

the crew's accommodation in old ships but not on board new ships (see also table 31V). One should

not forget

that sometimes discrete frequencies

(pure tones) may be audible or that propeller

shaft interference occurs; then a

"rating-curve-45-noise" although corresponding very nearly to

65 Dinphon would satisfy the suggested criteria

of table 3 IV. The 60 Dinphon limit seems to

be on the safe side, in some instances too safe. This is a good example of the advantages of the

ISO noise rating system: only if the noise actually

contains pure tones or interference effects, the

level must be lowered some 5 dB additionally whereas

the method of table 3V always requires the lower

levels although the spectrum may be of a broad

band continuous character. Of course, although

the ISO system is more complicated, it is more

economical!

For the bridge, it may be remarked that annoyance

is not the main objective but signal audibility;

table 3111 about the corrections for annoyance

purposes must not be applied to bridge noise

rating numbers. In general, the agreement between tables 31V and 3V is, very good apparently, except for the radio station.

As has been said in § 1 there are many methods for characterizing noise. in addition to the

eight-octave band spectrum and the ISO noise rating

numbers the Dinphon already was mentioned. It

is comparable to the American Standard Sound

Level Meter method.

Moreover, the "loudness level" of a noise is of some interest. It is a subjectively determined

quantity expressed in "phons", defined as follows.

A "sound jury"

is required to compare the

loudness of the noise in question with the loudness of a 1000 Hz tone (or a noise band between .900 Hz

and 1120 Hz) and to vary the loudness of the

latter until it

is found equal in loudness to the

unknown sound. The sound pressure level of the 1000 Hz tone be X dB then the loudness level of

the noise is said to be X phon. For every phon value there is one corresponding "sone" value Y given by

I

32 125 2S3 500 ¶000 2000 4000 8CO3 Hz

centre freqeney of band5

Fig. 3.04 Procedure for calculating loudness. Plot a

measured air-borne noise spectrum (1/i, 1/9 or 1/3 octave band width) in this graph. Find the corresponding

loud-ness in sones (S) for each band. Proceed and a.d according to the prOcedure described in the text (ISO, Stevens; note: a new ISO-draft proposal includes a similar graph in which

the curves 0.5 ---- 150 have been approximated by broken straight lines).

¿0 Io 30 90 eo -30 15 70 50

li,

Y 16 E z o N o C w t' t' t' w Q

18

Fig. 3.06 M.S. "Oranje". Length of ship and situation of engine room and casing are indicated In a cabin, sepa-rated frOm the casing by a corridpr, the airborne noise spectrum "a" of figure 3.07 was masured. This spectrum is characterized approximately by noise rating curve 50

120 110 100 go 60 70 60 50 z o o 30 e s o 40 20 10 o 63

(c.f. figure 3.01). This number- is written at the cor-responding deck close to the casing. Similar data for cabins in five other motorships are shown in figures 3.08,

10, 11 and 12 thus facilitating, a fair comparison.

so that by definition i sone corresponds to 40

phon, 2 sone to 50 phon and 4 sone to 6.0 phon. It

has been found that the loudness of two noises sounding simultaneously can be determined ap-proximately by adding the separate loudness in

sones but not by adding the loundness levels.

Another use of the sone scale might be the

in noise reduction work: if a noise has been reduced

from 103 sones to 42 sones it is correct to say

that the- loudness is about forty percent its former value (this example relates to a

"85-rating-curve-noise" which is reduced to a

"70-rating-curve-noise"). From data about the sound spectrum the loudness may be computed approximately instead

of determined as mentioned above.

One of the computing methods is called the

ISO-Stevens method [16]. Only this method may be mentioned here, although excellent other me-thods are available, because it has been used for

some time and even in the literature outside the

specialized acoustics field comjuted "Stèvens

loud-ness" values may be found. The procedure is

as follows

Measure the octave (resp. '/2 octave or 'A

octave) band spectrum of the noise.

Find the loudness corresponding to each band from figure 3.04.

The loudness of the band having the greatest

- number of sones is left unchanged (i.e.

multi-plied by a correction factor 1.0). The loudness values of the other bands are multiplied by a

correction factor 0.3 (resp. 0.2 or 0.15).

The total loudness of the noise equals the sum

of the loudness values of the various bands

multiplied by their respective correction factors.

In figure 3.05 some spectra together with the

computed loudness values are given.

Iñ addition to the vibration criteria, of figure

3.06 in [3], a source of secondary sound may be

treated: sometimes low frequency vibrations of

decks are so intense as to excite rattling and jingling noise of cups,. plates, knives etc. with which tables are laid. Although general rules in this respect can-not be given, one may expect this type of secondary

125 250

5i

1 2X) 4 8 Hz

centre fneqi,enc)' of + oct. bands

Fig. 3.05 Examples of spectra and corresponding com-puted loudness values: I 1.34, 111.09, III 3.79, IV 4.42,

V 23.6, VI 26.4, VII (ISO NR curve70) 37.2, VIII

27.5, IX 22.2, X NSL (see [3] ) 212 sones.

noise to start as soon as the infrasonic deck'vibrations exceed the region 3 of figure 3.06 in [3].

Laborato-ry experiments resulted in two different limits for inceptive rattling; viz, for infrasonic white noise excitatión: i m/s2, whereas for discrete frequency

excitation the limit was approximately 2 in/s2, both

values being the long time root-mean-square ac-celeration of the table-top on which some tea-cups

3. Some examples of cabin noises in six

motorships

No doubt, all criteria and rating methods

men-tioned cannot alter the fact that the best way

to know the acoustical comfort of a ship is to

be aboard at least for a week and meanwhile

listening to engines or air conditioning, or simply

relaxing, sleeping or speaking. However, this would

be of very little help in judging or rating the

va-rious specific noises found on board the ship in question. Rating implies of course comparison with other situatións. For noise control on shore

the ISO system as treated briefly in the preceding paragraph offers sufficient data although the f u-ture certainly will bring improvements. For ship

noise control comparison is difficult, partly because

data is scarce, partly because the sitüation is

com-plex.

Even if he could afford the time-consuming. study in order to compare different ships in the

way mentioned before, a shipbuilder would find that man has no good memory for noise he heard,

anyhow not as good as for things he saw.

9 8 7 6 20 o 32 63 125 250 Sm 10m

centr'e feency of oct

20 40m 60m Hz bands

Fig. 3.07 Comparison of noise spectra as measured in cabins af three different ships: "a" close to engine casing; "f" and "g" at about 40 m distance from the engine casing, three decks above cylinder heads of main engines (c.f. figures 3.06, 11 and 12). For comparison ISO noise rating curve 40 is shown; even the quietest cabin (on board "WillemRuys") of this series does not satisfy the "luxury cabin" requirement of table 31V completely.

From this point of view measured eight-octave band spectra and the ISO noise rating curves are

very efficient: a fair comparison of a ship with

other noisy living spaces is very well feasible thus. Sometimes, however, a more precise comparison

with other means of transport in general or with

other ships is desirable. In [ 2 ] many spectra mostly concerning engine room noise have been published.

Rating with respect to the conservation of hearing

or to speech communication is not difficult and

needs no special attention here. In passenger ships cabin noise spectra are more important. The main

noise sources are the propellers and the engine

room. One may expect that the distance of a

cabin to these sources determines the resultiiìg

spectrum considerably. Of course, in some instances local sources might cause the prevailing noise.

Illustrative examples in this respect are the

mea-surements on board two ships [23] and [24]; see also figures 3.08 and 3.11.

For a fair comparison of cabin noise spectra, especially on different ships, the distance to the

engine room and to the propellers must be given

therefore. Some examples are given in this respect in the figures 3.06 up to 3.12. The first ship shown,

figure 3.06, was built in 193.9. Attention was paid to noise control [25]. In 1954 measurements were

carried out [26]. The only cabin noise spectrum

available is represented in figure 3.07, curve a. The,

corresponding cabin location is shown (adjacent to the hull, a corridor between casing and cabin). During all noise measurements referred to in this

paragraph the ship travelled at normal

service-speed.

On board the second ship (built in 1958), figure 3.08, extensive measurements were performed in 1958. In the engine room and casing some sound

absorbing material has been applied. From the many

spectra available [24] two representative spectra are given in figure 3.09: "b" being the noise

spec-trum of a cabin similarly situated as the one

men-tioned before, "c" ditto but 20 m from there to

the bow.

Moreover eight numbers are shown in figure 3.08

to indicate the noise rating curve néarest to the

noise spectrum' prevailing in the corresponding

part of the ship. Thus the general noise distribution

in this ship can be seen clearly.

The third ones are two sister-ships [27], built in 1939. The spectra "d" (lounge first class) and

"e" (radio station) are shown in fig. -3.09. These

cross-channel ships running in day-service are

well-known for their intense low-frequency noise.

Two series of measurements

- before and after

a reconversion period - were carried out on board

the fourth ship, shown in figure 3.11 [23.]. She was

built in 1947; some noise abatement measures were

taken [28]. One typical cabin noise _{spectrum is}

shown in figure 3.07. Moreover many approximate noise rating numbers are given at the corresponding

" \ .. C S-'. .. .5,.. .5. -"J---.5 'J' 's,.,. ISO 40 .- .5--(5- .5 N,. s.

'J-20

16940

Fig. 3.08 M.S. "Randkntein". Length of ship and situa-tion of engine room añd casing are indicated. In a cabine,

separated from the casing by a corridor, the noise spectrum

of figure 3.09 was measured; it is characterized by noise rating curve 65 approximately. Moreover spectrum (N 55) was measured in a similar cabin 20 m more

forward. Other noise rating numbers are shown atseveral

locations, thus approximately characterizing the

corre-spon4ing noise spectra prevailing therç.

Fig. 3.10 M.S. "Koningin Emma" and "Prinses Beatrix",

cross-channel ships running in dayservice. Length of

ships and situation-of engine-room and casing are indicated. In the first, class lounge the noise spectrum "d" of figure 3.09 was measured; it is characterized by N 65 (c.f. figure

i.06). Moreover spectrum "e" (N 55 intense low

fre-quency noise exceedinghowever N 55) was measured in

the radio station.

Fig. 3.11 M.S. "Willem Ruys". Length of ship and situa-tion of engine róom ánd casing are indicated. In a cabin at 40 rn distance from the noise, spectrum "f" of figure 3.07 was measured; it is characterized by N 45 (c.f. figure 3.-06). Other N numbers are shown at several

locations, thus characterizing the corresponding noise

spectra prevailing there. It illustrates the importance of the distances to the two main noise sources: engine room

and propellers.

1530

Fig. 3.12 M.S. "Koningin Wilhelmina", cross-channel ship running in day-service. Length of ship and situation of engine room and casing are indicated. In the first class

lounge at 40 m distance from the casing the noise

spectrum "g" of figure 3.07 was measured; it is charac

terized by N 45 (c.f. figure 3.06).

63 125 250 1 2

40 8

Hz

centre frequency of f oet. bands

Fig. 3.09 Comparison of noise spectra as measured in cabins of three different ships: "b" and 'd" close to the engine casing; "c" and "e" at 15 m distance from the

casing approximately (c.f-. figures 3.08 and IO).

places in figure 3.11 to indicate the prevailing noise in various parts of the ship.

For the last ship, shown in figure 3.12, only one of two nearly identical spectra published in

litera-ture is presented in figure 3.07. This cross-channel ship, also running in day-service, was built in 1960;

considerable attention was paid to noise control

[29].

It is interesting to note that in a normal quiet motor-car (e.g. Opel Car-avan 1959 running on

brick-pavement, 50 km/h) a noise spectrum may

be measured higher by about S dB for the high frequencies, but even up to 20 dB for the low

frequency bands, than the spectrum "d" in figure 3.09 corresponding to a "noisy" ship. In running

railwaycoaches, moreover, generally spectra 10-15

dB higher than curve d can be found; yet these

coaches may be called queit!

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\

' d

N

b lOo 90 00 70 6 I1