Trans IMarE, Vol 107. Part 3, pp 195-207
Noise prediction and correlation with
full scale measurements in ships
C G Holland,
B S C , C E n g , M I M a r Eand S F Wong,
B E n g , C E n g , M I M a r ELloyd's Register
The demand for a comfortable environment on cruise vessels and the stringent habitability standards recommended by autfiorities to protect shipboard personnel against hearing damage prompt the necessity of an accurate noise prediction at the ship design stage. Whilst it is recognised that noise prediction is a continuous research and development activity in many institutes and universities throughout the world, the intention of this paper is to discuss noise prediction techniques in a general manner and to describe the approach to noise prediction developed and used by Lloyd's Register (LR). There is no guarantee of absolute accuracy in any noise prediction technique. This paper first briefly outlines the problems and factors affecting the degree of accuracy. The development of LR's own noise prediction programme is then addressed. In order to demonstrate the correlation between predicted and measured results, two practical cases, which have been completed recentiyon a modem containership and a luxury yacht, are discussed. The final section of the paper is devoted to a discussion of how future developments may improve the accuracy of noise prediction.
INTRODUCTION
S h i p b o a r d acoustics h a v e been t r a n s f o r m e d in recent years d u e to f u n d a m e n t a l changes i n s h i p d e s i g n . In the 1950s a n d 1960s s h i p p i n g w a s d o m i n a t e d b y general cargo vessels f e a t u r i n g a n a c c o m m o d a t i o n b l o c k situated a m i d s h i p s a n d this d e s i g n resulted i n l o w levels of p r o p e l l e r generated noise i n the a c c o m m o d a t i o n . The general a d o p t i o n of steam p r o p u l s i o n plant, rather than diesel engines, p r o v i d e d a r e l a t i v e l y quiet w o r k i n g e n v i r o n m e n t a n d also w a s b e n e f i -cial i n terms of a c c o m m o d a t i o n noise. In a d d i t i o n , s h i p s w e r e not g e n e r a l l y e q u i p p e d w i t h heating, v e n t i l a t i o n a n d a i r c o n d i t i o n i n g ( H V A C ) systems. M o d e r n c a r g o vessels n o r m a l l y feature diesel engine p r o p u l s i o n , a n aft a c c o m m o -d a t i o n b l o c k a n -d c o m p r e h e n s i v e H V A C systems. A l t h o u g h these d e s i g n changes d o not necessarily m e a n that the l i v i n g quarters i n m o d e r n ships are more n o i s y , it is clear that a m b i e n t noise levels are m o r e l i k e l y to be affected by the p r o p e l l e r , diesel engine a n d H V A C s y s t e m noise sources. T h e d e m a n d s i n s o m e s h i p types f o r h i g h installed p o w e r
a n d f o r h u l l s f e a t u r i n g m i n i m u m scantlings h a v e also re-sulted i n more onerous d e s i g n c o n d i t i o n s f o r the noise c o n t t o l engineer.
T h e w i d e s p r e a d a w a r e n e s s of noiserelated health p r o b -lems a n d the need to protect personnel against h e a r i n g d a m a g e have p r o m p t e d the i n t r o d u c t i o n of stringent habita b i l i t y sthabitandhabitards. T habita b l e 1 presents the noise levels r e c o m -m e n d e d by the I n t e -m a t i o n a l M a r i t i -m e O r g a n i s a t i o n ( I M O ) a n d the U K D e p a r t m e n t of T r a n s p o r t , n o w the M a r i n e Safety A g e n c y . ' * N o i s e levels s p e c i f i e d f o r recent passenger s h i p s are i n c l u d e d f o r c o m p a r i s o n . A l t h o u g h the a p p l i c a t i o n of the s t a n d a r d s to passenger a c c o m m o d a t i o n is not a legisla-tive r e q u i r e m e n t , they are o f t e n used as g u i d e l i n e s d u r i n g negotiations b e t w e e n s h i p o w n e r s a n d s h i p b u i l d e r s .
In recent years the cruise i n d u s t t y has e n j o y e d increased p o p u l a r i t y a n d this is reflected by the level of n e w b u i l d i n g orders. Passengers w h o j o i n c r u i s e ships f o r v a c a t i o n p u r -poses are, to a large extent, c o n d i t i o n e d to expect noise levels s i m i l a r to those o f hotels ashore o r of their o w n homes. N o n e of the s t a n d a r d s m e n t i o n e d a b o v e d e f i n e l i m i t s f o r passenger a c c o m m o d a t i o n , but c o n f i n e their attention to the w o r k
-Authors' biographies
Christopher Holland served an engineering apprenticeship with the C E G B , graduating with an Honours degree in Mechanical Engineering from the University of Bath in 1975. He joined Texaco as a seagoing Engineer Officer, serving on a variety of tankers. During this time he obtained his DTp Class 1 Certificate of Competency (Motor). In 1985 he joined the Technical Investigation Department of Lloyd's Register and was promoted to Senior Surveyor in 1989. In 1991 he transferred to the Machinery Design and Dynamics Department and in 1994 was appointed Head of the Health, Vibration and Noise section of the Technical Investigation, Propulsion and Environmental Engineering Department.
Mr Wong graduated as a marine engineer from H o n g Kong Polytechnic in 1981. He then worked for Orient Overseas Container Lines Ltd on ocean going vessels, up to the rank of Second Engineer Officer. From 1984-1986 he stiidied at the University of Newcastie-Upon-Tyne and obtained an Honours degree in Mechanical Engineering. In 1989 he gained the U K ' s D O T Class 1 Certificate of Competency (Motor) and joined Lloyd's Register as a Surveyor in 1989, working in the engineering investigation field. He was promoted to Senior Surveyor in 1994 and is currentiy the Deputy Head of the Health, Vibration and Noise section of the Technical Investigation, Propulsion and Environmental Engineering Department.
Table I Recommended noise levels for specified areas
Location Noise levels Code of practice for Noise levels
onboard ships noise levels in slups specified for recent IMOA468 (XID UK Dept of Transport passengerships
dB(A) dB(A) dB(A)
Pas.senger cabins
-
-
40-55Public area
-
-
55-60Shopping areas
-
-
60-70Crew cabins 60 60 55-60
Crew mess room 65 65 60-65
Ho.spital 60 60 55-60
Conference room
-
65 50-60Offices 65 65 55-60
Recreation rooms 65 65 60-65
Open recreation area 75 75 65-70
Galleys 75 75 65-75
Lifeboat stations
-
-
65-70Wheelhouse 65 65 60-65
Radio room 60 60 55-60
Engine control room 75 75 70-75
Workshop 85 85 80-85
ing a n d c r e w resting a n d recreation spaces o f the s h i p . In hotels noise levels i n the range 30-40 d B ( A ) c a n be expected for the better s t a n d a r d rooms. In conttast, f r o m L R ' s e x p e r i -ence of c o n d u c t i n g field measurements, m o d e m passenger cabins have noise levels i n the range 50-55 d B ( A ) i n general, r e d u c i n g to 4 0 - 5 0 d B ( A ) in the case of s o m e l u x u r y cabins. T h e changes i n s h i p c o n s t t u c t i o n , the emergence of noise regulations a n d the i n c r e a s i n g d e m a n d s of passengers, have r e i n f o r c e d the i m p o r t a n c e of noise c o n t r o l c o n s i d e r a t i o n s as early as p o s s i b l e i n the d e s i g n stage i n o r d e r to a v o i d a n y costly r e m e d i a l actions that m a y be r e q u i r e d after consttuc-tion. T h e f o r m u l a t i o n o f a n effective noise c o n t t o l sttategy w i l l i n v o l v e noise p r e d i c t i o n c a l c u l a t i o n s a n d this p a p e r describes this process.
MAIN ELEMENTS OF NOISE PREDICTION
T h e process o f noise p r e d i c t i o n i n v o l v e s c o n s i d e r a t i o n o f three related elements: the noise source, the t r a n s m i s s i o n p a t h a n d the receiver. W h i l e each element appears to be i n d e p e n d e n t , they c a n , jointly o r i n d i v i d u a l l y , affect the p r e d i c t i o n results s i g n i f i c a n t l y . A s s u c h , the u n d e r s t a n d i n g of each element, a n d p a r t i c u l a r l y the i n p u t data r e q u i r e d , is essential b e f o r e p r o c e e d i n g w i t h the c a l c u l a t i o n .
Noise sources
T h e m a j o r noise sources i n a s h i p are: the m a i n p r o p u l s i o n m a c h i n e r y , the a u x i l i a r y engines, the p r o p e l l e r a n d trans-verse p r o p u l s i o n unit, a n d the H V A C s y s t e m . T h e relative s e n s i t i v i t y o f one source, w i t h respect to the n o i s e l e v e l at the receiver, w i l l d e p e n d o n the s h i p type, m a c h i n e r y arrange-ment a n d the receiver location. A c a b i n at a location far f r o m the m a i n engine a n d p r o p e l l e r w i l l more l i k e l y be affected b y a n y H V A C noise. T h e m a j o r i t y of m a i n a n d a u x i l i a r y m a -c h i n e r y is d r i v e n either b y d i e s e l engines or steam turbines.
G e n e r a l l y s p e a k i n g , steam turbines generate less noise than diesel engines w i t h s i m i l a r o u t p u t p o w e r , a n d are t h u s less l i k e l y to p r o d u c e a n n o y i n g b a c k g r o u n d noise i n a c c o m m o -d a t i o n areas.
M a c h i n e r y generates n o i s e i n t o the s u r r o u n d i n g a i r a n d also i n d u c e s v i b r a t i o n i n t o a n y structure to w h i c h it is connected. T h e l e v e l s of n o i s e a n d v i b r a t i o n that are gener-ated w i l l be g o v e m e d by the acoustic p o w e r a n d m e c h a n i c a l force of the m a c h i n e itself. F o r p r o p e l l e r s the m a i n d i f f i c u l t y is the e v a l u a t i o n of c a v i t a t i o n generated noise, w h e r e a s the c a l c u l a t i o n o f H V A C noise is r e l a t i v e l y s t r a i g h t f o r w a r d .
Machinery
T h e acoustic o u t p u t p o w e r o f a p a r t i c u l a r m a c h i n e is ex-pressed b y the s o u n d p o w e r l e v e l , w h i c h describes the rate at w h i c h energy is r a d i a t e d (energy p e r u n i t time) f r o m the s o u n d source, b u t is i n d e p e n d e n t of the n a t u r e of the space s u r r o u n d i n g the s o u r c e - t h e s o u n d field. If the s o u n d p o w e r level is k n o w n , the s o u n d pressure level c o r r e s p o n d i n g to the characteristics o f a p a r t i c u l a r s o u n d field m a y be d e -d u c e -d . E m p i r i c a l f o r m u l a e have been u s e -d f o r m a n y years to estimate the s o u n d p o w e r level o f certain types of m a c h i n -ery, d e p e n d i n g o n the p h y s i c a l size, o u t p u t m e c h a n i c a l p o w e r , o p e r a t i n g speed etc, b u t the results are, i n general, unsatisfactory. In fact, the t r a d i t i o n a l a p p r o a c h is to measure the s o u n d pressure level d i r e c t i y a n d d e r i v e the s o u n d p o w e r level. H o w e v e r , c a r e f u l i n t e r p r e t a t i o n i s r e q u i r e d as the m e a s u r e d s o u n d pressure l e v e l w i l l be affected b y :
1. the distance at w h i c h the m e a s u r e m e n t w a s carried out; 2. the d i f f e r e n c e o f the acoustic properties b e t w e e n the
spaces w h e r e the m a c h i n e w a s tested a n d its f i n a l h o u s -ing e n v i r o n m e n t ;
3. the presence of other s i g n i f i c a n t noise sources. I d e a l l y , the m a c h i n e u n d e r test s h o u l d be p l a c e d i n a n anechoic chamber, b u t i n practice this is not a l w a y s possible.
Trans IMarE, Vol 107. Part 3, pp 195-207 n o o D i A i r D o r n e . - o i s i ?
4
ENGINE! S t r u c t u r e b o r n e N c i s e»
Fig 1 Noise transmission path
T h e a v a i l a b i l i t y of s o u n d intensity meters i n the late 1980s e n a b l e d the direct m e a s u r e m e n t of s o u n d intensity. T h e d e t e r m i n a t i o n of s o u n d p o w e r t h r o u g h s o u n d intensity m e a s u r e m e n t is more direct a n d accurate than the d e t e r m i -n a t i o -n t h r o u g h s o u -n d pressure measureme-nt. T h i s is d u e to the fact that the intensity meter takes a c c o u n t of the d i r e c t i o n of the p o w e r flow as w e l l as the m a g n i t u d e . T h u s measurements c a n be taken i n any s o u n d field w i t h o u t b e i n g i n f l u enced by b a c k g r o u n d noises f r o m other m a c h i n e r y . S t a n d -ard m e t h o d s of t a k i n g s o u n d intensity m e a s u r e m e n t s are still b e i n g f o r m u l a t e d a n d these w i l l be r e q u i r e d to ensure a consistent a p p r o a c h .
T u r n i n g to the subject of m e c h a n i c a l force d e t e r m i n a t i o n ; i d e a l l y the m e c h a n i c a l force generated by the source s h o u l d be m e a s u r e d a n d the v i b r a t i o n levels that c o u l d be i n d u c e d into the structure c a l c u l a t e d , based o n the m o b i l i t y of the seating ( m o b i l i t y is a f r e q u e n c y response f u n c t i o n to de-scribe the v i b r a t i o n response of a s t r u c t u r e to an i n p u t force).
In practice, h o w e v e r , it is not possible to measure the force d i r e c t l y a n d , instead, v i b r a t i o n levels at the foot of the m a c h i n e are n o r m a l l y m e a s u r e d at the test bed a n d the results used as the i n p u t data in noise p r e d i c t i o n . U n l e s s the m o b i l i t y of the test b e d seating is the s a m e as that installed on the s h i p , test bed results s h o u l d o n l y be tteated as e x p e r i m e n t a l data. T h i s m e t h o d also does not m a k e any a l l o w a n c e f o r the flexible n a t u r e of the s h i p structure, c o m -pared to the s o l i d floor at the test site.
Propellers
T h e m a i n d i f f i c u l t i e s i n d e t e r m i n i n g p r o p e l l e r noise are associated w i t h uncertainties i n the d e s c r i p t i o n of c a v i t a f i o n g r o w t h a n d collapse o n the p r o p e l l e r blades, the c a l c u l a t i o n of w a k e field d i s t t i b u t i o n a r o u n d the aft e n d of the s h i p , a n d the shell plate response to w a t e r b o r n e s o u n d pressure. A l -t h o u g h -there exis-t b o -t h s e m i - e m p i r i c a l f o r m u l a e d e r i v e d
Table II Insertion loss of common construction materials
Material Octave band centre frequency (Hz)
Mean value 63 125 250 500 1000 2000 4000 8000 Steel plate 3.2 mm thick 32 16 22 28 34 40 44 32 41 6 mm thick 37 22 28 34 40 45 37 42 51 12 mm thick 42 28 34 40 45 37 41 51 60 Mineral wool (120 kg/m') 25 mm thick 19 4 9 14 19 24 29 34 39 50 mm tfiick 24 9 14 19 24 29 34 39 44 75 mm thick 27 12 17 22 27 32 37 42 47 100 mm thick 29 14 19 24 29 34 39 44 49 Mineral wool (200 kg/m') 25 mm tfuck 22 7 12 17 22 27 32 37 42 Cement 20 mm thick 37 22 27 32 37 42 47 52 57 40 mm thick 42 27 32 37 42 47 52 57 62
Calcium silicate board
6 mm thick 23 8 15 19 23 24 24 34 35 9 mm thick 27 11 18 22 26 27 27 37 38 13 mm thick 29 14 21 25 29 33 40 41 41 19 mm thick 32 17 24 28 32 33 33 43 44 Glass wool (17 kg/m') 50 mm thick 12 0 2 7 12 17 22 27 32
Swedac on steel deck 59 32 43 50 52 67 67 76 90
TNF on steel deck 49 22 38 46 55 53 59 60 60
tiom m o d e l experiments a n d p r e d iction m o d e l s of the e q u i v a -lent acoustical source sttength (based o n the speed of revo-l u t i o n , t r a n s m i t t e d p o w e r a n d the p r o p e revo-l revo-l e r geometry), the results d o not c o m p a r e f a v o u r a b l y w i t h p r a c t i c a l measurements. T h e l i f t i n g surface m e t h o d , n o r m a l l y used in p r o p e l -ler a n a l y s i s , is p o t e n t i a l l y capable of e s t i m a t i n g the r a d i a t e d pressure f i e l d of a c a v i t a t i n g p r o p e l l e r i n the l o w f r e q u e n c y range. T h e m e t h o d is p r e d o m i n a n t l y based o n a time d o m a i n a n a l y s i s of c a v i t a t i o n g r o w t h a n d collapse, f o r p r o p e l -lers o p e r a t i n g i n a k n o w n w a k e f i e l d . It is adequate f o r e s t i m a t i n g pressure sources at b l a d e p a s s i n g f r e q u e n c y , but the q u a h t y of c o r r e l a t i o n b e t w e e n m e a s u r e d a n d p r e d i c t e d pressures at h i g h e r m u l t i p l e s of b l a d e p a s s i n g f r e q u e n c y reduces as the h a r m o n i c n u m b e r increases.
T r a n s v e r s e p r o p u l s i o n units are recognised as a m a j o r noise source w h e n they are u s e d f o r d o c k i n g m a n o e u v r e s . A u n i t of this type is n o r m a l l y integrated into the h u l l structure rather t h a n h a v i n g a fluid m e d i u m b e t w e e n it a n d the h u l l surface, as i n the case of the p r o p e l l e r . A noise p r e d i c t i o n m e t h o d d e v e l o p e d b y the Institute of A p p l i e d P h y s i c s , D e l f t , is based o n a large n u m b e r of m e a s u r e m e n t s o n b o a r d d i f f e r -ent types of s h i p s a n d is described i n Ref 3.
Transmission path
N o i s e ttar\smission c a n either be w a t e r b o r n e , a i r b o m e or s t t u c t u r e b o m e . E x t e m a l , w a t e r b o r n e noise t r a n s m i s s i o n is m a i n l y of c o n c e m to specialised vessels s u c h as seismic
s u r v e y s h i p s , o c é a n o g r a p h i e r e s e a r c h s h i p s , f i s h e r y s h i p s a n d n a v a l vessels. F o r m e r c h a n t s h i p s , the internal noise e n v i r o n m e n t is of greater c o n c e m a n d is affected p r i m a r i l y by a i r b o r n e a n d s t r u c t u r e b o r n e noise. F i g u r e 1 s h o w s t y p i c a l a i r b o r n e a n d s t t u c t u r e b o m e noise transmis-s i o n pathtransmis-s.
Airborne noise
A i r b o r n e noise is transmitted b y e x c i t i n g the s u r r o u n d i n g air particles. A s the noise propagates, part of the energy w i l l be lost t h r o u g h the b a r r i e r s it crosses a n d the distance it travels. G e n e r a l l y , the noise level w i l l d r o p b y 6 d B f o r every distance d o u b l e d a n d the a m o u n t of noise a t t é n u a ted t h r o u g h barriers w i l l d e p e n d o n the total i n s e r t i o n loss (IL) t h r o u g h the barriers (insertion loss is a m e a s u r e of the decrease i n transmitted p o w e r i n decibels). In the s h i p e n v i r o n m e n t , the barriers i n c l u d e d e c k s , b u l k h e a d s a n d c a b i n p a r t i t i o n s . Table II lists the I L of s o m e c o m m o n c o n s t r u c t i o n materials. It s h o u l d be n o t i c e d that the I L is h i g h e r at h i g h frequencies a n d w i t h t h i c k e r m a t e r i a l . T h i s is because the w a v e l e n g t h s of h i g h f r e q u e n c y c o m p o n e n t s are shorter a n d are thus more easily i n t e r r u p t e d . In fact, noise at remote areas is n o r m a l l y d o m i n a t e d b y l o w f r e q u e n c y c o m p o n e n t s w h i c h , i n prac-tice, are v e r y d i f f i c u l t to e l i m i n a t e .
A s stated, a i r b o r n e n o i s e is attenuated b y b u l k h e a d par-titions a n d distance f r o m the source. It is, therefore, f o u n d to be the m a i n t r a n s m i s s i o n m e c h a n i s m i n spaces w h e r e e x c i -tation sources are located but its i n f l u e n c e o n remote areas is
Trans IMarE, Vol 107. Part 3, pp 195-207
Table Radiation factor of common construction materials
Construction Octave band centre frequency (Hz)
63 125 250 500 1000 2000 4000 8000
Steel plate (20 mm thick) 25 22 15 10 6 -2 0 0
TNF floor 16 17 13 10 1 -4 1) 0
Swedac floor 19 18 17 16 13 5 -5 0
Cement on steel 19 18 15 2 -2 0 n 0
Cabin wall/lining 19 16 15 10 5 0 0 0
Table IV Absorption coefficients of common construction materials
Description Octave band centre frequency (Hz)
Mean coefficient 63 125 250 500 1000 2000 4000 5000
Room partition 0.04 0.10 0.09 0.05 0.02 0.01 0.01 0.01 0.01 Room ceiling 0.64 0.45 0.50 0.60 0.65 0.75 0.80 0.75 0.65 Floor with vinyl tile 0.05 0.02 0.02 0.04 0.05 0.05 0.10 0.05 0.05 Floor with carpet 0.20 0.05 0.10 0.15 0.25 0.30 0.30 0.30 0.20 Tiled deck 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.02 0.02 Steel plate 0.02 0.01 0.02 0.03 0.03 0.03 0.02 0.02 0.02
n e g l i g i b l e . A t y p i c a l e x a m p l e is the d o m i n a t i o n of a i r b o m e noise generated i n an engine r o o m .
T h i s c o u l d be a n y i n d o o r or o u t d o o r space o f interest i n a s h i p .
Structureborne noise
S t r u c t u r e b o r n e noise is the p r o p a g a t i o n of v i b r a t o r y energy t h r o u g h the sttucture. I n contrast to a i r b o r n e noise, a n i n s i g n i f i c a n t a m o u n t of e n e r g y w i l l be lost t h r o u g h ' u n -treated' steel sttuctures i n the t r a n s m i s s i o n process, d u e to l o w inherent d a m p i n g . In fact, energy c o u l d w e l l be g a i n e d in the t r a n s m i s s i o n process d u e to c o u p l i n g a n d d u e to resonances of sub-systems as the structureborne noise passes t h r o u g h . Because the r e d u c t i o n of s t t u c t u r e b o m e noise is m a i n l y a c h i e v e d t h r o u g h the process o f d i v e r s i f i c a t i o n o f energy at s t r u c t u r a l d i s c o n t i n u i t i e s rather than distance f r o m the source, i t is the m a i n cause of n o i s e levels i n spaces remote f r o m e x c i t a t i o n sources.
V i b r a t o r y energy travels t h r o u g h sttuctures i n several types of w a v e f o r m s , m a i n l y b e n d i n g , l o n g i t u d i n a l , ttans-verse a n d e v e n t o r s i o n a l . O f all the w a v e f o r m s c o n s i d e r e d , b e n d i n g w a v e s are by f a r the m o s t i m p o r t a n t type f o r s t r u c t u r e b o r n e noise ttansmission as they are f o u n d to be w e l l c o u p l e d to the r a d i a t i o n o f a i r b o m e noise. H o w e v e r , this does n o t necessarily m e a n that b e n d i n g w a v e s c a r r y m o r e v i b r a t o r y energy than other types o f w a v e f o r m s . In fact, the w a v e types are interrelated because b e n d i n g w a v e s at s t r u c t u r a l joints c o u l d be ttansformed into other w a v e types a n d vice versa. A l t h o u g h it is k n o w n that w a v e f o r m ttansformation takes place at joints, the m e t h o d by w h i c h the energy is disttibuted in the process is still uncertain. T h i s
uncertainty is c o m p o u n d e d w h e n c o n s i d e r i n g a c o m p l i c a t e d ship structure i n v o l v i n g a large n u m b e r of discontinuities.
Receiver
T h e receiver is the area o r enclosure w h e r e the final noise level d u e to the noise sources c o n s i d e r e d is to be d e t e r m i n e d .
Enclosed space
T h e noise level i n an enclosed space is a f f e c t e d b y the total acoustic p o w e r e n t e r i n g the space a n d the acoustic p r o p e r -ties of the space itself. T h e total acoustic p o w e r e n t e r i n g the space is a c o m b i n a t i o n o f b o t h a i r b o r n e noise a n d r a d i a t e d a i r b o r n e noise d u e to s t t u c t u r e b o m e noise ttansmission. W h e n s t r u c t u r e b o m e noise reaches the receiver, p a r t o f the e n e r g y w i l l excite the s u r r o u n d i n g a i r to generate a i r b o r n e noise. T h i s m e c h a n i s m is c a l l e d r a d i a t i o n . A s structureborne noise w i l l travel a l o n g a l l surfaces o f the enclosure t h r o u g h s o l i d connections, the r a d i a t i o n factor of each s u r f a c e w i l l c o n t r i b u t e to the total p o w e r r a d i a t e d . T h e r a d i a t i o n factor is p r i m a r i l y a f u n c t i o n of the material of the sttucture a n d also d e p e n d s o n the m o u n t i n g c o n d i t i o n . Its v a l u e s are f r e q u e n c y d e p e n d e n t a n d , p r e f e r a b l y , s h o u l d be d e t e r m i n e d b y meas-u r e m e n t o n site. T a b l e III lists the r a d i a t i o n factors f o r s o m e c o n s t m c t i o n materials u s e d i n ships. For a 20 m m thick steel plate, the most effective r a d i a t i o n w i l l occur at a 2000 H z centte frequency, increasing the noise level by 2 d B. A t a 63 H z centte frequency, the radiated noise level w i l l be reduced b y 25 d B .
T h e acoustic properties o f a n enclosure are g e n e r a l l y expressed as the ' r o o m constant'. T h e r o o m constant is d e f i n e d b y the a m o u n t of e x p o s e d s u r f a c e area i n the enclo-sure a n d the acoustic a b s o r p t i o n properties of the surfaces. The larger the r o o m a n d the m o r e a b s o r p t i v e area the sur-faces have, the larger w i l l be the v a l u e of the r o o m constant a n d the s m a l l e r w i l l be the noise level i n the enclosure. T a b l e I V presents the a b s o r p t i o n coefficients of c o m m o n materials used i n a c c o m m o d a t i o n areas. A g a i n , the v a l u e s o f these coefficients are frequency dependent.
Outdoor area
A n o u t d o o r area is n o r m a l l y treated as a ' f r e e f i e l d ' c o n d i -tion s u c h that a n y noise ttansmission w i l l not be reflected
AIRBORNE NOISE SOURCE A OCTAVE BAND 31.5 Hz - 8000 Hz A n E N U A T O N TMHOOQH STRUCTURE ATTENUATION T>4ROUOHROOM ROOM A c o o s n c PROPERTY ROOM SPL DUE TO SOURCE A (AIRBORNE) OTHER NOISE SOURCE-' STRUCTUREBORNE NOISE SOURCE A OCTAVE BAND 31 5 Hz - 8000 Hz ATTENUATKIN THROUGH MOUNTING ATTENUATIOM THROUGH STRUCTURE ROOM PARTITION ROOM ACOUSTIC ROOM SPL DUE TO SOURCE A (STRUCTUREBORNE) OTHER NOISE SOURCE'' HVAC SYSTEM ATTENUATKIN DUE TO OUCTUENGTW ATTENUATK3N THROUGH ATTENUATK3N THROUGH SILENCER ROOM ACOUSTIC r>Hn;'i n r v ROOM SPL DUE TO HVAC NOISE
ROOM SPL DUE ROOM SPL DUE
TO ALL NOISE TO ALL NOISE
SOURCES SOURCES
(AIRBORNE NOISE (STRUCTUREBORNE
TRANSMISSION) NOISE TRANSMISSION)
ROOM SPL DUE TO AIRBORNE AND STRUCTUREBORNE NOISES (ALL SOURCES) RESULTANT NOISE LEVEL dB (lln), (1B(A)
Fig 2 Noise prediction programme flow chart
f r o m s o l i d surfaces a n d w i l l n o t encounter any barriers. F o r e x a m p l e , the c a l c u l a t i o n of the noise level o n a n a v i g a t i o n b r i d g e w i n g d u e to f u n n e l exhaust gas considers o n l y the d i r e c t i o n of the s o u r c e a n d the distance b e t w e e n the source and receiver.
NOISE PREDICTION METHODS
O v e r the years a n u m b e r of noise p r e d i c t i o n techniques have been d e v e l o p e d , based o n v a r i o u s hypotheses a n d theories. Three m a i n approaches can be i d e n t i f i e d . These are: the finite element m e t h o d ( F E M ) , statistical energy a n a l y s i s ( S E A ) a n d the s e m i - e m p i r i c a l m e t h o d . T h e f i r s t t w o a p p r o a c h e s a i m to a d d r e s s t h e p r o p a g a t i o n o f structureborne n o i s e f r o m theoretical stand p o i n t s . T h e last a p p r o a c h uses e x i s t i n g w e l l - d e v e l o p e d e m p i r i c a l for-m u l a e to calculate a i r b o r n e a n d s t r u c t u r e b o r n e noise attenu-ation.
Finite element method
The a p p l i c a t i o n of the F E M i n e n g i n e e r i n g a p p e a r e d as early as the mid-1950s. T h e basis of this m e t h o d is to s u b d i v i d e a c o m p l e x sttucture into a finite n u m b e r of discrete parts o r elements. S t t u c t u r a l r e l a t i o n s h i p s can then be d e r i v e d f o r
each f i n i t e e l e m e n t w h i c h l i n k f o r c e a n d d i s p l a c e m e n t c o m -ponents at the n o d a l p o i n t s . A l t h o u g h the great sttength of the F E M is its v e r s a t i l i t y , as there is v i r t u a l l y n o l i m i t to the type of structure that can be a n a l y s e d , the c o m p e t e n c e r e q u i r e d to select suitable elements i n b u i l d i n g u p the m o d e l can o n l y be g a i n e d t h r o u g h research w o r k a n d / o r p r a c t i c a l experience. T e c h n i c a l l y , p r o v i d i n g the e l e m e n t types are s u i t a b l y selected, a n increase i n the n u m b e r of elements that are u s e d to represent a s t t u c t u r e w i l l g i v e better results. H o w e v e r , the use of m o r e elements i m p l i e s l o n g e r c o m p u t -i n g t-ime a n d , as s u c h , a b a l a n c e has to be d r a w n between these t w o c o n f l i c t i n g criteria. E x p e r i e n c e gathered i n the past suggests that F E M is p a r t i c u l a r l y suitable f o r p r o b l e m s c o n c e r n i n g l o w f r e q u e n c y v i b r a t i o n s .
Statistical energy analysis
T h i s m e t h o d i n v o l v e s the e v a l u a t i o n of v i b r a t i o n energy d i s s i p a t e d b e t w e e n c o n n e c t e d resonant structures by a sta-tistical m e t h o d , a n d is based o n the a s s u m p t i o n that the flow of acoustical energy b e t w e e n t w o s u b s y s t e m s is p r o p o r -tional to the d i f f e r e n c e i n energy levels b e t w e e n these sub-systems. T h e m a j o r c o n d i t i o n s u n d e r w h i c h S E A can be a p p l i e d are that the c o u p l e d s y s t e m s are resonant a n d that the m o d a l d e n s i t y (the n u m b e r of resonant v i b r a t i o n m o d e shapes w i t h i n a f r e q u e n c y b a n d w i d t h ) of each s y s t e m is s u f f i c i e n t i y h i g h . T h e h i g h e r the n u m b e r of m o d e shapes that
Trans IMarE, Vol 107, Part 3, pp 195-207
Table V Structureborne noise attenuation of various abatement measures
Description Octave band centre frequency (Hz)
Mean attenuation 63 125 250 500 1000 2000 4000 8000 Resilient mount 18.5 12 16 20 20 20 20 20 20 Floating floor 17.4 2 6 12 17 18 24 30 30 Cement floor 6.4 -2 2 3 6 9 10 11 12 TNF floor 24.1 0 3 12 14 22 37 50 55 Swedac floor 26.1 17 24 23 23 26 26 34 36
appear i n each system, the better is the accuracy. The method generally p r o v i d e s f a i r l y g o o d results at h i g h frequency bands but not i n the l o w frequency bands, a n d it is n o r m a l l y used o n stmctures w h i c h are less c o m p l e x than ships.
Semi-empirical method
P e r h a p s this is s t i l l the most c o m m o n l y a d o p t e d m e t h o d f o r noise p r e d i c t i o n . T h e m e t h o d uses e m p i r i c a l f o r m u l a e for the c a l c u l a t i o n but the accuracy of the results w i l l u l t i m a t e l y be d e t e r m i n e d b y the q u a l i t y of the i n p u t data. The i n p u t data c o u l d be o b t a i n e d from v a r i o u s sources. W i t h respect to the noise source data, m a n u f a c t u r e r s are u s u a l l y able to s u p p l y noise data associated w i t h the m a c h i n e r y s u p p l i e d , i n terms of b o t h a i r b o r n e a n d s t t u c t u r e b o m e noise levels. E v e n w i t h o u t this i n f o r m a t i o n , h o w e v e r , data c a n be inter-p o l a t e d f r o m noise m e a s u r e m e n t s o n s i m i l a r m a c h i n e r y , o r e v e n calculated a p p r o x i m a t e l y from e m p i r i c a l f o r m u l a e . R e g a r d i n g the data f o r the receiver, the acoustic properties of the sttucture are n o r m a l l y w e l l d e t e r m i n e d either b y l a b o r a t o r y tests o r site measurements. T h e characteristics of the t r a n s m i s s i o n p a t h present the m o s t p r o b l e m s i n terms of the c a l c u l a t i o n of structureborne noise attenuation. O v e r the years, L R has c a r r i e d out extensive m e a s u r e m e n t s o n v a r i -ous k i n d s of s h i p s to establish the characteristics of noise attenuation t h r o u g h d i f f e r e n t types of c o n s t r u c t i o n . These experimental results are stored i n a data bank, together w i t h noise source a n d receiver data. Because of the large a m o u n t of i n f o r m a t i o n available, the s e m i - e m p i r i c a l method is used as the basis for the L R ' s in-house noise p r e d i c t i o n p r o g r a m m e .
LR NOISE PREDICTION PROGRAMME
F i g u r e 2 s h o w s the flow chart of the p r o g r a m m e . It is b u i l t u p f r o m three m o d u l e s to c o v e r a i r b o r n e , structureborne a n d H V A C noise c a l c u l a t i o n s separately. T h e merits of u s i n g a m o d u l a r structure are that the effects of each m o d u l e o n the noise level at the receiver c a n be e v a l u a t e d easily a n d the d o m i n a n t sources c a u s i n g a n y excessive noise can be i d e n -tified.
Airborne noise calculation module
T h e c a l c u l a t i o n is based o n the a s s u m p t i o n that the a i r b o r n e o v e r a l l s o u n d pressure level i n the r e c e i v i n g space is e q u a l to the c o m b i n e d effect of the s o u n d p o w e r levels e m a n a t i n g
f r o m a l l s i g n i f i c a n t noise sources. The p a r t i t i o n i n s e r t i o n loss in w a y of the noise t r a n s m i s s i o n p a t h a n d the acoustic p r o p e r t i e s of the r e c e i v i n g space are c o n s i d e r e d i n the p r o c -ess. A f t e r the s o u n d pressure level at the receiver has been c a l c u l a t e d , d u e to each noise source i n every octave b a n d (centte frequency of 63 H z to 8000 H z ) , the resultant noise level can then be obtained b y logarithmic a d d i t i o n to obtain the overall a i r b o m e noise level w i t h i n the space or compartment.
Structureborne noise calculation module
T h e m a j o r a s s u m p t i o n u n d e r l y i n g this c a l c u l a t i o n is that v i b r a t o r y energy ttansmits into the structure from noise sources i n the f o r m of acoustic f r e q u e n c y v i b r a t i o n , w h i c h is attenuated t h r o u g h d i s c o n t i n u i t i e s a n d w i t h distance f r o m the source. F o r c o n v e n i e n c e of c a l c u l a t i o n , the attenuations a r e c o n s i d e r e d to be concentrated atstructural discontinuities a l o n g the p a t h b e t w e e n the source a n d the r e c e i v i n g e n d . T h e d i s c o n t i n u i t i e s a c c o u n t e d f o r i n the c a l c u l a t i o n s are: d e c k plate s t i f f e n i n g , j u n c t i o n s of deep frames a n d b u l k h e a d s , a n d j u n c t i o n s of shell p l a t i n g w i t h decks.
T h e degree of s t t u c t u r e b o m e noise attenuation t h r o u g h each j u n c t i o n is not the same f o r a l l ships d u e to d i f f e r e n c e s in the geometry of c o n s t r u c t i o n . S t u d i e s of data collected t h r o u g h measurements, h o w e v e r , p r o v i d e n o m i n a l values, a n d as a r o u g h g u i d e , s t r u c t u r e b o m e intensity reduces o n average b y about half a d e c i b e l per f r a m e . S t r u c t u r e b o r n e noise also falls w i t h v e r t i c a l distance f r o m the source; f o r e x a m p l e , the noise r e d u c t i o n o n the f i r s t t w o d e c k s is ap-p r o x i m a t e l y 5 decibels ap-per deck a n d o n subsequent d e c k s is a p p r o x i m a t e l y 2 decibels per deck.
N o i s e abatement measures are w i d e l y a p p l i e d i n a s h i p to c o n t r o l the noise a n d , as s u c h , the c a l c u l a t i o n also considers the noise r e d u c t i o n s a c h i e v e d . T y p i c a l measures i n c l u d e resilient m o u n t s f o r m a c h i n e r y , floating floors f o r the ac-c o m m o d a t i o n , silenac-cers f o r v e n t i l a t i o n outlets a n d d a m p i n g materials a p p l i e d to steel structures. T h e attenuation data for s o m e general abatement measures are g i v e n i n T a b l e V . It s h o u l d be n o t e d that the effectiveness is f r e q u e n c y de-p e n d e n t a n d , as s u c h , the selection of measures de-p r o de-p o s e d requires c a r e f u l c o n s i d e r a t i o n .
T h e s o u n d pressure level r a d i a t e d f r o m the v i b r a t i n g surface of each b o u n d a r y i n the r e c e i v i n g space is calculated in a s i m i l a r m a n n e r to the a i r b o r n e noise c a l c u l a t i o n , t a k i n g into account the acoustic properties of the receiver. The c a l c u l a t i o n is repeated f o r a l l noise sources i n a l l selected octave bands. The resultant s o u n d pressure level c a n then be used to evaluate the o v e r a l l a i r b o r n e noise levels i n the space d u e to the s t t u c t u r e b o m e noise.
HVAC noise calculation module
T h e c a l c u l a t i o n of H V A C noise d e p e n d s o n a k n o w l e d g e of the s o u n d p o w e r o u t p u t of the fans i n the air c o n d i t i o n i n g s y s t e m . T h e noise e m i t t e d f r o m a i r outlets i n the r e c e i v i n g space w i l l d e p e n d o n the a m o u n t of noise attenuation d u e to the length of d u c t i n g , types of d u c t branches, a i r flow rates a n d the characteristics of any silencers f i t t e d . T h e resultant s o u n d pressure level i n the space is then c a l c u l a t e d , t a k i n g into account the acoustic properties of the space itself.
O n c e the s o u n d pressure levels i n the r e c e i v i n g space due to the airborne, s t r u c t u r e b o m e a n d H V A C noise sources have been c a l c u l a t e d , the o v e r a l l noise level can be o b t a i n e d by l o g a r i t h m i c s u m m a t i o n . T h e results are expressed i n l i n e a r a n d A - w e i g h t e d v a l u e s f o r e v e r y octave centte fre-q u e n c y b a n d . T h e o v e r a l l A - w e i g h t e d noise level at the receiver is also g i v e n .
CASE STUDIES
Case 1: 4450 TEU containership
A noise p r e d i c t i o n w a s carried out d u r i n g the s h i p ' s early d e s i g n stage. T h e p r i n c i p a l p a r t i c u l a r s of the s h i p are listed i n T a b l e V I .
T h e first step i n any noise p r e d i c t i o n process is to i d e n t i f y the sources. A n e x a m i n a t i o n of the general l a y o u t revealed that the a c c o m m o d a t i o n block w a s located b e t w e e n f r a m e s 70 to 90 a n d , therefore, p r o p e l l e r i n d u c e d noise w a s not expected to h a v e a n y s i g n i f i c a n t effect o n the b a c k g r o u n d noise levels, even t h o u g h it w a s i n c l u d e d i n the c a l c u l a t i o n . A l s o , the a c c o m m o d a t i o n b l o c k w a s not expected to be affected by noise f r o m the b o w thruster, w h i c h w a s located remotely at frame 151 a n d is o n l y operated intermittently d u r i n g m a n o e u v r i n g . Further examination of the submitted d r a w i n g s c o n c l u d e d that a c c o m m o d a t i o n area noise w o u l d be caused by the m a i n engine, a u x i l i a r y machinery, ventila-tion a n d exhaust fans, and the air c o n d i t i ç n i n g units. The fans a n d the air c o n d i t i o n i n g units w e r e located w i t h i n the accom-m o d a t i o n area. The starboard side of the a c c o accom-m accom-m o d a t i o n w a s expected to be exposed to higher noise levels in c o m p a r i s o n w i t h the port side, d u e to the p o s i t i o n i n g of the machinery.
There w e r e n o special acoustic treatments a p p l i e d to the s h i p a n d c a b i n c o n s t t u c t i o n w a s t y p i c a l l y based o n the f o l l o w i n g elements:
1. steel d e c k c o v e r e d by a t h i n layer of concrete a n d p o l y v i n y l tiles;
2. c a b i n l i n i n g r i g i d l y connected to the decks a n d the b u l k h e a d s ;
3. c a b i n l i n i n g surface m a d e of steel sheet c o v e r e d b y a P V C film.
It w a s expected that a v o i d space i n t r o d u c e d i m m e d i -ately b e l o w the superstructure w o u l d reduce a i r b o r n e noise t r a n s m i s s i o n but h a v e little effect o n structureborne noise p r o p a g a t i o n .
A s u m m a r y of the p r e d i c t e d a n d m e a s u r e d results is g i v e n i n T a b l e V I I . T h e accuracy of the c a l c u l a t e d o v e r a l l
Table VI Particulars of a 4450 T E U containership Main engine Auxiliary machinery Propeller mcr: 41 310 kW at 93 rev/min ncr: 36 170 kW at 89 rev/min Mounting: .solid
4 off: 1880 kW each at 720 rev/min Mounting: elastic
1 X 5-bladed Diameter: 8400 mm Mean pitch: 9156 mm
Table VII Comparison of predicted and measured noise levels (4450 T E U containership)
Deck Space Noise level, dB(A)
Predicted Measured
2nd Workshop 86 82
Engine control room 72 76
Upper Suez crew 64 63
Deck control room 66 64 Shipper's office 66 61
A Hospital 59 56
Crew recreation room 62 57
Galley 70 68
Crew mess room 57 61
B Gynasium 66 66
Chief cook 58 56
Crew (K) 59 54
Officer mess room 62 58 C Open swimming pool 74 77
Bosun 59 55
Chief steward office 62 58 Conference room 63 •56 Officer lounge 62 •56 D Cadet (A) 66 •51 4th engineer 62 •52 Engineer office 59 55 2nd engineer dayroom 58 53 E Chief engineer dayroom 64 55 Chief engineer 54 53
F Radio room 53 49
Captain bedroom 61 •55 Radio officer 53 51 Navigation Bridge 52 •60
More than 5 dB(A) deviahon
noise levels w a s expected to be w i t h i n 5 d B ( A ) . In general, the p r e d i c t e d noise levels correlated w e l l w i t h the measure-ment results, w i t h the e x c e p t i o n of cabins o n d e c k s C a n d D . T h e a c c o m m o d a t i o n a r r a n g e m e n t s of decks C a n d D i n F i g 3 s h o w that the engine r o o m v e n t i l a t i o n fans a n d the sanitary exhaust f a n are located o n these decks. F r e q u e n c y a n a l y s i s of the p r e d i c t e d results i n d i c a t e d that the m a j o r i n f l u e n c e o n the o v e r a l l noise l e v e l s of spaces o n these t w o d e c k s w a s the s t r u c t u r e b o r n e noise e m i t t e d by these fans. T h e p r o b l e m w a s related to the fact that i n f o r m a t i o n o n these fans w a s not s u p p l i e d by the m a k e r s , a n d so the i n p u t data used i n the calculation w e r e exttacted f r o m a data bank based on s i m i l a r types of e q u i p m e n t . A s o n l y the cabins i n the v i c i n i t y of the f a n s d i s p l a y e d d i s a p p o i n t i n g results, it is suspected that the data exttacted from the data bank d i d not t r u l y represent the actual acoustic p o w e r of the machinery.
Trans IMarE. Vol 107, Part 3, pp 195-207 O F F I C E R LCUi-:GE cO.-.T C O I : F E ? . E I 3 C E P . C G M C H I E F STEWAP.r F.OCH O I L E R O F F . (B) OF] L I F T O F F I C : ENGINE ROOM V E N T I L A T 10:: Fi^^J ROOM E N G I N E C A S I N G BOSUN ENGINE ROOM vENTIL.i.TIO: FA:^^ .ROCH —i L I F E BOAT 2 / E N G O F F I C E 2 / E N G BED ROCH ENG ' R O F F I C E 4/ENG R O C H F . O C H 3 / E N G E E D R O C H CADET C A D E T L I F T L A U Î ' J D R Y S-Z-IHTARY F.y-:i ROOM Fb'NNEL JlJiCIOR E N G ' R D E C K D
Fig 3 The cabin arrangement on decks C and D
Table Vlil Particulars of a 70m yacht Main engine 2 off: 1432 kW each at 1800 rev/min
Mounting: elastic
Auxiliary machinery 3 off: 145 kW each at 1500 rev/min Mounting: elastic
Propeller 2 X 4-bladed highly skewed cpp Diameter: 1700 mm
Mean pitch: 20 094 mm
Case 2: 70m yacht
P a r t i c u l a r s of the yacht are g i v e n i n T a b l e VIII. T h e noise p r e d i c t i o n w a s p e r f o r m e d u n d e r t w o c o n d i t i o n s : a l o n g s i d e a n d u n d e r w a y .
T h e a l o n g s i d e noise c a l c u l a t i o n w a s c a r r i e d out a s s u m i n g the o p e r a t i o n of t w o generator sets, one e n g i n e r o o m v e n t i -l a t i o n f a n a n d the a i r c o n d i t i o n i n g s y s t e m . T h e u n d e r w a y noise p r e d i c t i o n w a s based o n the r u n n i n g of b o t h m a i n e n g i n e s a n d p r o p e l l e r s , p l u s the noise d u e to the e n g i n e
r o o m v e n t i l a t i o n f a n a n d air c o n d i t i o n i n g s y s t e m . In contrast to C a s e 1, p r o p e l l e r noise w a s c o n s i d e r e d as a s i g n i f i c a n t noise c o n t r i b u t o r because of the l o c a t i o n of the c a b i n s a n d recreation areas. T h e diesel generators were, h o w e v e r , e x c l u d e d f r o m the u n d e r w a y noise calculation as the noise levels w e r e f o u n d to contribute at least 9 d B p e r octave less noise a n d 18 d B per octave less v i b r a t i o n than the m a i n engines.
E x t e n s i v e noise a n d v i b r a t i o n attenuation m e a s u r e s w e r e a p p l i e d to the yacht, w i t h m o s t a c c o m m o d a t i o n areas b e i n g fitted w i t h floating floor arrangements. T h e a p p l i c a t i o n of s p e c i a l acoustic treatment o n s m a l l vessels is quite c o m m o n practice i n o r d e r to a c h i e v e m a x i m u m noise attenuation i n the light of the c o m p a r a t i v e l y s h o r t t r a n s m i s s i o n p a t h be-t w e e n noise sources a n d receivers.
T h e c o m p a r i s o n b e t w e e n the p r e d i c t e d a n d m e a s u r e d results is g i v e n i n T a b l e I X . U n d e r the a l o n g s i d e c o n d i t i o n the c o r r e l a t i o n is v e r y i m p r e s s i v e , s u g g e s t i n g that the q u a l ity of the i n p u t data w i t h respect to the noise sources c o n s i d -ered i n this c a l c u l a t i o n w a s g o o d . H o w e v e r , the p r e d i c t e d
Table IX Comparison of predicted and measured noise levels (70m yacht)
Noise level, dB(A)
Deck Space Alongside Underway
Deck Space
Predicted Measured Predicted Measured
Lower Engine control room 56 57 76 •55-60
Engine room 96 93-98 101 98-103
Crew cabin (IV L2) 46 50 63 •50
Guest cabin, P fore 45 43 53 •46
Guest cabin, P aft 45 42 52 50
Main Crew mess 45 46 48 49
Dining room 45 44 49 47
Main saloon 45 43 55 •48
Upper Owner's suite 45 43 47 44
Top VIP suite 45 46 46 45
Outside sitting area 57 59 72 67
* More than 5 dB(A) deviation
results f o r the u n d e r w a y c o n d i t i o n f o r some areas are gen-erally h i g h e r than the measured data, m a i n l y at the l o w e r deck. F r e q u e n c y a n a l y s i s reveals that the noise i n the E n g i n e C o n t t o l R o o m w a s generated m a i n l y b y p r o p e l l e r a i r b o r n e noise, w h i l e the noise i n c r e w a n d guest cabins w a s affected b y structureborne noise f r o m b o t h m a i n engines a n d p r o p e l -lers. It is b e l i e v e d that the i n p u t data f o r the n o i s e sources, especially p r o p e l l e r s , c o u l d w e l l be overestimated.
T h e above t w o case studies d e m o n s t r a t e the i n f l u e n c e that the i n p u t data has o n the accuracy of the final results. H o w e v e r , d e v i a t i o n between the p r e d i c t e d a n d m e a s u r e d results is also g o v e m e d b y h o w w e l l the f i n a l c o n s t r u c t i o n agrees w i t h the d e s i g n d r a w i n g s .
FUTURE DEVELOPMENTS
P r o p e l l e r noise c a l c u l a t i o n s u s i n g the c o m b i n e d l i f t i n g surface a n d b u b b l e - c l o u d c o l l a p s e m e t h o d are a v a i l a b l e . T h e v o l u m e of c a v i t a t i o n c o m p u t e d i n the w a k e peak is related to the d i s t r i b u t i o n of free b u b b l e s generated d u r i n g the collapse phase of b l a d e sheet c a v i t a t i o n . C o m p a r i s o n s w i t h s h i p a n d m o d e l scale data s h o w e n c o u r a g i n g results, a l -t h o u g h -the c o m p l e x i -t y of -the a p p r o a c h is no-t i d e a l f o r p r e l i m i n a r y d e s i g n studies.
R e g a r d i n g sttucturebome noise ttansmission, a l t h o u g h the s e m i - e m p i r i c a l method can give g o o d results if the ana-lysed sttucture is s i m i l a r to p r e v i o u s designs stored in a data bank, adjustment of parameters s u c h as frame s p a c i n g , steel thickness etc, carmot be s t u d i e d for d i f f e r e n t designs. In re-spor\se to the r a p i d change i n s h i p d e s i g n concepts, the feasi-b i l i t y of u s i n g a n S E A technique to address the structurefeasi-bome noise ttansmission is currently u n d e r investigation.
T h e target of any f u t u r e d e v e l o p m e n t is to i m p r o v e the accuracy of noise p r e d i c t i o n results. T e c h n i c a l l y , three areas in the noise p r o d u c t i o n process can p r o v i d e scope f o r i m p r o v e m e n t : d e t e r m i n i n g the s o u n d p o w e r l e v e l s of excitation sources accurately; i m p r o v i n g the accuracy of e s t i m a t i n g p r o p e l l e r noise; a n d u n d e r s t a n d i n g better the b e h a v i o u r of structureborne noise p r o p a g a t i o n i n s h i p struc-tures.
U n t i l recently, s o u n d p o w e r o u t p u t f r o m m a c h i n e r y c o u l d o n l y be d e r i v e d f r o m s o u n d pressure measurement a n d the results are i n f l u e n c e d by the v a r i o u s factors d i s -cussed earlier. W i t h the i n f r o d u c t i o n of the s o u n d intensity meter, s o u n d p o w e r can n o w be d e t e r m i n e d more c o n f i d e n t l y f r o m the results of intensity measurements. T e c h n i q u e s f o r u s i n g the e q u i p m e n t a n d the measurement p r o -cedures to be f o l l o w e d are still at the d e v e l o p m e n t stage. L R is w o r k i n g w i t h m a j o r engine b u i l d e r s to f o r m u l a t e s t a n d -ard m e a s u r e m e n t p r o c e d u r e s i n m a r i n e a p p l i c a t i o n s .
ACKNOWLEDGEMENTS
T h e a u t h o r s w o u l d l i k e to express their thanks to the C o m -mittee of L l o y d ' s Register f o r p e r m i s s i o n to p u b l i s h this paper. T h a n k s are also e x t e n d e d to the m a n y colleagues i n the T e c h n i c a l Investigation, P r o p u l s i o n a n d E n v i r o n m e n t a l E n g i n e e r i n g D e p a r t m e n t w h o have c o n t t i b u t e d to the c o n -tent of the paper.
REFERENCES
1. Noise Levels On Board Ships, Intemational Maritime Organisation (IMO) Resolution A.468 (XU) (1982).
2. Code of Practice for Noise Levels in Ships, U K Department of Transport, Second Edition (1990).
3. J S Carlton, Marine Propellers & Propulsion, Butterworth-Heinemann Ltd (1994).
Discussion
Trans IMarE, Vol 107, Part 3. pp 195-207
A J Sinclair (Brookes, Bell & Co) P r e d i c t e d a n d measured noise levels f o r a c o n t a i n e r s h i p are presented i n T a b l e V U i n the paper. It is not stated w h e t h e r the m e a s u r e d v a l u e s w e r e o b t a i n e d d u r i n g ballast ttials o r at f u l l s p e e d , f u l l l o a d service c o n d i t i o n s . H o w w o u l d the a u t h o r s expect these figures to change b e t w e e n the ballast, part l o a d a n d f u l l l o a d c o n d i t i o n s ?
C G H o l l a n d and S F W o n g (Lloyd's Register) T h e c o n t a i n e r s h i p noise levels presented i n the p a p e r are f o r the f u l l speed, f u l l l o a d service c o n d i t i o n .
A i r b o m e a n d s t t u c t u r e b o m e noise t r a n s m i s s i o n f r o m m a c h i n e r y is g o v e m e d b y m a n y factors, i n c l u d i n g the m a -chine l o a d a n d s p e e d , the m o u n t i n g arrangement, a n d the acoustic properties of the enclosure. C h a n g e s i n the vessel's d r a u g h t w i l l affect the a m o u n t of d a m p i n g a p p l i e d by the m a s s of w a t e r a c t i n g o n the h u l l b u t the effect is n o r m a l l y a m i n o r one. O f greater interest is the effect o n the ttansmis-s i o n of p r o p e l l e r noittansmis-se. P r o p e l l e r c a v i t a t i o n ittansmis-s g e n e r a l l y m o r e extensive i n the ballast c o n d i t i o n than i n the l o a d e d c o n d i t i o n . H o w e v e r , this does not necessarily m e a n that m o r e e n e r g y w i l l be ttansmitted into the s h i p ' s sttucture because, i n c o m p a r i s o n w i t h the f u l l y l o a d e d c o n d i t i o n , the ballast c o n d i t i o n m a y result i n a less effective c o n t i n u o u s w e t t e d surface area to absorb the energy. In s o m e instances a n aerated layer is p r o d u c e d close to the free s u r f a c e , w h i c h m a y also c o n t t i b u t e to a r e d u c t i o n i n the e f f i c i e n c y w i t h w h i c h p r o p e l l e r noise is transmitted to the vessel. D u r i n g the e a r l y stages of p r e p a r i n g a noise p r e d i c t i o n the effect o f d r a u g h t v a r i a t i o n is assessed a n d the w o r s t c o n d i t i o n is used as a basis f o r the calculations.
B G M Rice (European Marine Contractors Limited) T h e a u t h o r s are to be c o n g r a t u l a t e d o n their clear presentation of a v e r y p r a c t i c a l p r o b l e m . T a b l e II i n the p a p e r s h o w s an u n u s u a l l y l o w v a l u e of s o u n d attenuation f o r 19 m m thick c a l c i u m silicate at 2000 H z , c o n s i d e r i n g the adjacent values. F r o m T a b l e II one m i g h t c o n c l u d e that c a l c i u m silicate b o a r d is the best s o l u t i o n f o r c a b i n b u l k h e a d s w i t h o u t u s i n g excessively thick m i n e r a l w o o l panels. It is p e r h a p s w o r t h p o i n t i n g out that reputable m a n u f a c t u r e r s c l a i m 40-45 dB( A ) r e d u c t i o n s f o r S O m m thick systems usingSOOkg/m-* m i n e r a l w o o l , w h i c h has a s i m i l a r w e i g h t per square metre as 19 m m c a l c i u m silicate. I a m a d v i s e d that these s o u n d reductions are m e a s u r e d at 500 H z .
D o the a u t h o r s have a n y experience of noise p r e d i c t i o n f o r s e m i - s u b m e r s i b l e s a n d are t h e r e a n y p a r t i c u l a r p r o b l e m s associated w i t h s u c h vessels i n m a k i n g noise p r e d i c t i o n s ?
D o the a u t h o r s have a n y experience of m a k i n g noise p r e d i c t i o n s f o r vessels w i t h n u m e r o u s large (35 M W ) a z i -m u t h thrusters, p a r t i c u l a r l y p r e d i c t i o n s i n the adjacent thruster c o m p a r t m e n t s ? O n e w o u l d p r e s u m e that at h i g h p o w e r the m a i n noise source is the p r o p e l l e r . H o w e v e r , the paper suggests that p r o p e l l e r i n d u c e d noise is c u r r e n t l y one of the less w e l l m o d e l l e d aspects.
T h e c o n t a i n e r s h i p a n d y a c h t e x a m p l e s g i v e n in the paper s h o w a c o r r e l a t i o n between p r e d i c t e d a n d m e a s u r e d noise levels that indicates the m e t h o d has a practical a p p l i c a t i o n .
Is the sort of accuracy a c h i e v e d i n these t w o cases representative of the accuracy o b t a i n e d f o r other studies u n d e r taken b y L l o y d ' s Register, o r is there u s u a l l y a w i d e r v a r i a -tion?
C o u l d the a u t h o r s p r o v i d e a n i n d i c a t i o n of the cost a n d t i m e r e q u i r e d to u n d e r t a k e a noise p r e d i c t i o n f o r a large vessel, s u c h as the c o n t a i n e r s h i p d e s c r i b e d i n the paper?
C G Holland and S F W o n g (Lloyd's Register) T h e s o u n d i n s e r t i o n loss data of c o m m o n c o n s t m c t i o n materials s h o w n i n T a b l e II s h o u l d be interpreted c a r e f u l l y a n d s h o u l d be u s e d f o r g u i d a n c e rather t h a n as absolute v a l u e s . T h e objec-five of the table is to h i g h l i g h t the fact that the degree of s o u n d attenuation t h r o u g h v a r i o u s materials d e p e n d s o n the f r e q u e n c y of the s o u n d . T h e mean s o u n d r e d u c t i o n v a l u e , to a large extent, is of l i m i t e d use to the a c o u s f i c engineer, especially w h e n d e a l i n g w i t h the s o l u t i o n of noise p r o b l e m s . W h e n selecting the a p p r o p r i a t e s o u n d i n s u l a t i o n m a t e r i a l , m a n u f a c t u r e r s s h o u l d a l w a y s be c o n s u l t e d for a d v i c e . A p a r t f r o m the d e s i r e d s o u n d r e d u c t i o n , the selec-t i o n of selec-the r i g h selec-t maselec-terial w i l l also be g o v e r n e d by facselec-tors like the cost, area of a p p l i c a t i o n , e n v i r o n m e n t a l c o n d i t i o n s , fire r a t i n g , etc.
T h i s p a p e r has concenttated o n the noise e n v i r o n m e n t o n b o a r d ships. W h e n c o n s i d e r i n g o f f s h o r e units there are certain areas that require m o r e detailed attention d u e to their greater relative i m p o r t a n c e . F o r e x a m p l e , i n o r d e r to achieve acceptable noise levels i n a c c o m m o d a t i o n areas the s i t i n g of process p l a n t , the r o u t e i n g of h y d r a u l i c p i p e w o r k a n d the attenuation measures d e s i g n e d f o r the H V A C s y s t e m are a l l i m p o r t a n t c o n s i d e r a t i o n s . In the p a r t i c u l a r case of s e m i -s u b m e r -s i b l e -s w i t h a z i m u t h i n g t h m -s t e r -s f o r p r o p u l -s i o n or d y n a m i c p o s i t i o n i n g there are the a d d e d p r o b l e m s of p r o -p e l l e r a n d g e a r i n g generated noise. T h i s can be e v i d e n t not o n l y i n the adjacent c o m p a r t m e n t s b u t c a n also be ttansmit-ted u p the p o n t o o n legs as structureborne noise, l e a d i n g to n o i s e breakout i n the m a i n deck areas. The w a t e r b o r n e noise can also interfere w i t h acoustic p o s i t i o n sensor systems.
W i t h s e m i - s u b m e r s i b l e s the greater d e p t h s at w h i c h thrusters are located helps to s u p p r e s s c a v i t a t i o n , h o w e v e r the units operate i n the r e g i o n of b o l l a r d p u l l c o n d i t i o n s , w h i c h are m o r e c o n d u c i v e to c a v i t a t i o n . In a d d i t i o n , f o r m u l t i p l e installations, the interference effects between thrust-ers are larger than o n c o n v e n t i o n a l s h i p s a n d hence the p r o p e l l e r s m a y operate i n m o r e t u r b u l e n t flow, o r may be starved of s o m e flow by the adjacent thrusters.
P r o p e l l e r noise p r e d i c t i o n m e t h o d s d o exist f o r the types of p r o p u l s o r a n d l o a d i n g v a r i a t i o n s experienced o n d y n a m i -c a l l y p o s i t i o n e d vessels. H o w e v e r , su-ch m e t h o d s rely o n a s m a l l base of f u l l scale data.
T h e level of accuracy a c h i e v e d w h e n p r e d i c t i n g noise levels varies not o n l y for d i f f e r e n t s h i p types b u t also be-t w e e n cabins i n be-the same s h i p , as d e m o n s be-t r a be-t e d by be-the be-t w o cases in the paper. H o w e v e r , an o v e r a l l accuracy of ± 5 dB( A ) is a v e r y practical a n d achievable figure.
L F Moore (Noise and V i b r a t i o n C o n s u l t a n t ) In T a b l e V , o n p201 of the paper, 1 see that f o r resilient m o u n t s the
