Initial calibration and physiological response data for the travelling-wave sonic-boom simulator

INITIAL CALIBRATION AND PHYSIOLOGICAL

RESPONSE DATA FOR THE TRAVELLING-WAVE

SONIe-BOOM SIMULATOR

by

Richard Carothers

•

INITIAL CALIBRATION AND PHYSIOLOGICAL RESPONSE DATA FOR THE TRAVELLING-WAVE

SONIC-BOOM SIMULATOR

by

Richard Carothers

Submitted, July 1972

Acknowledgement

I would like to thank Dr. G. N. Patterson for providing the facilities which made this research possible.

For his guidance and continued interest in my work, I would like to express my sincere appreciation to Dr. I. I. Glass.

To Dr. J. R. Brown and Dr. H. S. Ribner I would like to offer my thanks for their help in the fields of human physiology and acoustics.

I gratefully acknowledge the time and consideration that

Mr.

J.

J. Gottlieb has shown and would like to thank him for his encouragement and many helpful ideas.

For their invaluable assistance in carrying out many of the experiments I would like to express my appreciation to

Mr.

A.Falkiewicz,

Mr. M.

Millan and

Mr.

J. Horwood.

I would also like to thank Miss

M.

Instone for her assistance with the typing of the original manuscript.

To the twenty subjects who volunteered their time, special thanks are due for their efforts which made the physiological response experiments possible.

, . . . - - - -- -- -- - - -- - - _._---_._ -

-/

Abstract

To allow the study of sonic-boom phenomena and to establish how it

relates to the Canadian situation, two lsonic-boom simulation facilities have been designed and built at the University of Toronto Institute for Aerospace Studies. This report deals with the initial calibration of one of these facilities: the travelling-wave horn-type sonic-boom simulator, and presents the results of tests showning the effects of sonic booms on human heart rate and hearing.

The horn is an 80 ft long pyramidal structure. The useful test section extends from a

3

ft square cross-section 25 ft from the horn apex to a 10 ft square cross-section at the open base. Within this test section there is room for large structural models or human and animal subjects.

Both shock-tube drivers and a mass-flow valve have been used to generate sonic booms and for both of these methods the main sonic-boom para-meters of peak overpressure, duration and rise time have been measured. The mass-flow valve is capable of producing high peak overpressures (> 25 psf) and durations ranging from 70 to 500 msec, while the shock-tube drivers can produce short duration sonic booms with rise times less than 0.1 msec.

It was necessary to install inside the horn a fiberglass acoustical filtering section in order to attenuate the jet noise which was superimposed on the mass-flow-valve generated sonic booms.

Additional measurements of the particle velocities which were in-duced by the simulated sonic booms wer~ carried out, and the boundary-layer growth along the walls of the test section was also investigated.

1.

2.

4.

"

TABLE OF CONTENTS

Notation (explained in the text) List of Abbreviations

INTRODUCTION

EXPERIMENTAL PROCEDURE 2.1 Shock Tube Drivers 2.2 The Mass-Flow Valve

2.3

Superimposed Jet Noise

2.4

Velocity Profiles and Bounàary Layer Measurements

2.5

Physiological Response Studies

RESULTS

AND

DISCUSSIONS

3.1

Shock-Tube Drivers

3.2

The Mass-Flow Valve

3.3

Superimposed Jet Noise

3.4

Particle Velocity Profiles

3.5

Boundary Layer Measurements

3.6

Physiological Response to Simulated Sonic-Booms

3.7

Hearing Tests

3.8

Subjective Loudness

3.9

Heart Rate CONCLUDING REMARKS PAGE 1 1 1 1 2 3 3

4

5 7 9 9 11 11 11

13

14

17

REFERENCES

18

APPENDIX A: The Operation and Description of the Travelling Wave Sonic-Boom Simulator

APPENDIX B: Physiology of the Human Ear APPENDIX C: Physiology of the Human Heart

APPENDIX D: A Method ~or Predicting the Subjective Loudness of the Superimposed Jet Noise

FIGURES

DRC ECG IL PI'S TTS SPD

UTIAS

LIST OF ABBREVIATIONS Damage Risk Critèria

Electrocardiogram Intensity Level

Permanent Threshold Shift Temporary Threshold Shift

Sound Pressure Level - taken as a rms value unless otherwise indicated as a peak value

University of Toronto, Institute for Aerospace Studies

,....---~---

--'" ,

1. INTRODUCTION

Supersonic transport aircraft have been developed; their possiIDle wide-spread use necessitates the study of the effects of the associated sonic~boom

phenomena. The travelling-wave, horn-type, sonic-boom simulator provides a means of exploring in a laboratory environment the effects of sonic booms including psychological, physio1ogical (both human and animal) and structural responses.

The purpose of this report has been to complete the initial calibra-tion of the sonic-boom simulator so that suitably accurate reproduccalibra-tions of actual sonic booms cou1d be produced and th en to carry out two physiological response tests indicating the effects of sonic booms on human hearing and heart rate.

2. EXPERIMENTAL PROCEDURE

It is possible to operate the travelling-wave, sonic-boom, simulation facility at UTIAS using two different generating mechanisms: shock-tube drivers or the mass-flow valve. (See Appendix A for the operation and description of the sonic-boom simulation facility).

2.1 Shock-Tube Drivers

Work began ~n the simulator calibration using three constant cross~

sectional area (1 in. ) shock-tubes of 0.5, 1.0 and 2.0 ft lengths (See Fig. 1). :Qiaphragms of "red zip" cellophane using one or, more ',thicknesses

(depending on the driver pressure ratio) were pressurized near to their rupture point and then burst with a needle. Pressure profiles were monitored at the 70 ft station in the horn using a Brüel and Kjaer (B

&

K) 4145 microphone, a B

&

K 2631 carrier system and a Tektronix 555 oscilloscope, for diaphragm-pressure ratios up to 5. A further survey of the diaphragm-pressure profiles alo~ the axis of the horn was made for the 1.G and 2.0 shock-tubes using a diaphragm pressure ratio of 2. The B

&

K 4145 microphone was best suited for recording these pressure signatures. However, at stations less than 30 ft from the horn apex there was a danger of exceeding the upper dynamic limit of the 4145 micro-phone, therefore a B

&

K 4135 microphone and a 2803 pre-amplifier were substi-tuted for tests at the 30, 20, 13.5, 4.7 and 4.0 ft stations. Later a Kist1er gauge, model 70lA, and a model 504 charge amplifier were used to measure the shock strength 0.5 ft and 2.0 ft from the diaphragm for the 1.0 ft shock-tube driver,which was agàin at .a- diaphragm pressure ratio of 2.

As an exploratory step in trying to improve the wave shape generated by the shock-tubes a maleable putty was moulded inside the 0.5 ft shock-tube driver such that the 7.2 degrees taper angle of the pyramidal horn was continued inside the shock-tube (see Fig. 1). The success of this procedure led to the construction of the 0.667 ft tapered shock-tube required to continue the taper of the pyramidal horn to its apex.

Tests previously carried out on the three original shock-tubes were repeated identical1y for this new tapered shock-tube.

2.2 The Mass-F1ow Valve

To achieve sonic-boom durations typical of actua1 aircraft sonic-booms, the mass~flow va1ve was designed and built at UTIAS, based on experience gained

from previous work. (Ref. 1). The operation and description of this mechanism is sumroarized brief1y in Appendix A.

Analysis (Ref.1) had predicted that the overpressure of the simu1ated sonic-boom generated by the mass=flow va1ve wou1d be a function of three basic parameters: pressure in the reservoir tanks, duration of the simu1ated sonic-boom? and the axia1 position at which the sonic-boom signa1 was observed inside the horn.

An experiroenta1 investigation of these mass-f1ow-generated sonic booms was undertaken as foi1ows.

First, the valve motion was examined to determine whether or not the 1inear (constant speed) va1ve motion assuroed in the theoretica1 predictions was indeed being achieved in practice. Second, the simu1ated sonic boom overpressure was examined as a function of each of the previously mentioned parameters. With a B

&

K 4147 microphone located at the,50 ft station and the sonic-boom duration fixed a1ternatively at 100 and 200 msec, overpressures were recorded for reser-voir tank pressure ratios ranging from 1.2 to 18.2. Then? with the reservoir tank pressure ratio equal to 4 and with the microphone at the 50 ft station, the sonic-boom durations were varied from 70 to 520 msec and for each case the overpressure was noted. Fina1ly, with a reservoir tank pressure of 2 atmos-pheres and a duration of 80 msec, recordings of overpressures were ~de at axial positions of 15, 2G, 30, 40, 50, 60, 70 ft from the horn apex.

Rise times were measured of both the long (360 msec) and short (80 msec) duration sonic booms.

2.3 Superimposed Jet Noise

During measurements of these simulated sonic booms it was found that each sonic-boom signal had superimposed upon it a broadband jet noise. It becaroe quickly apparent that the interference presented by this jet noise?

particular1y for the longer duration (20~-500 msec) sonic-booms warranted serious study in order to reduce or eliminate its effects.

A Krohn Rite model 3202 electronic filter was used? in a low pass mode, to determine the lower cut~off frequency of the jet noise. At the same time it was noted that the elimination of the higher frequency components in-creased the rise time of the 80 msec and 300 msec duration simulated sonic-boom signals.

An acoustic filtering section extending from 16 to 24 ft? as measured axially from the horn apex, was constructed. It consisted of lining the horn wa1ls with 1 in. thick fiberglass boards (7 lbs/ft3) and the installation of horizontal and vertical splitter p1ates of the same material (Fig. 2). This acoustic filter was used to achieve an attenuation of the higher frequency components of the superimposed jet noise before the simulated sonic-boom signal reached the test section of the harn. Later a further filtering, with a

reduced cut-off frequency was obtained by constructing the transverse fiberglass panel which was installed perpendicular to the axis of the horn. (See Fig. 2). Again the increase in rise times of 80 msec and 300 msec duration sonic booms was~ recorded for each particular acoustic filtering methode

2.4 Velocity Profiles and Boundary Layer Measurements

A Thermosystems Hot Film Anemometer, model 1031-2 with a 1034~4 linearizer unit, was used to measure partiele veloeities indueed by the

simu-lated sonie-boom signal generated ~y the mass-flow valve. The partiele velo-eity profiles of simulated sonie booms, with 80 and 300 msee durations were

observed at the 25 and 50 ft stations with the probe first mounted in the eentre

of the horn. During subsequent runs at the 25 ft station the probe was moved progressively eloser to one of the vertieal horn walls, eoming finally within

1/8 in. of the surfaee, in an éffort to deteet a boundary layer growth resulting

from the simulated sonie boom.

Sinee it was neeessary to compare the partiele velocity profiles of

several simulated sonie booms, the motion of the ~ss-flow valve was monitored

using a Tektronix 5103N storage oseilloseope to indieate the variability in

successive sonie-boom signals. As an additional check, pressure profiles were also reeorded for eaeh probe position and, for both pressure and velocity measure -ments, five signals were superimposed for each reading.

Physiologieal Response Studies

Upon completion of the initial ealibration work on the travellig

g-wave, sonie-boom simulator, tests showing some physiologieal responses (i.e., percent ehange in heart rate and temporary threshold shifts in hearing) to

simulated sonie booms begane

For the first set of tests, 20 subjeets were exposed to 50 sonic ~00ms

at the rate of 25 per minute. These booms were of the "noisy" type shown on

Fig.

3

with an overpressure of 2 psf, a duration of 80 msee and a rise time of

3 msee. Electroeardiograms were performed, with eaeh subject in a sitting posi

-tion, during the first sonic-boom exposure. Records were made of the initial

(immediately before boom exposure) and inereased (immediately af ter boom exposure)

heart rates as well as the time taken fo~ the increased heart rate to return

to its pre-stimulus level. Audiograms were completed before and about 2 minutes af ter the 50 sonie-boom exposures.

All subjeets were volunteers chosen from office workers, machine shop

staff and university students and professors; both male and female subjects were

ineluded in the group and ages ranged from early twenties to mid-sixties. Two of the subjects showed signs of a hearing loss in one ear and another subject

showed marked indieations of presbycusis (deterioration of hearing at high fre-queneies due to age). All other subjeets showed normal hearing in the initial audiograms and no one reported any heart disorders.

Further development ofthe sonie-boom simulator had shown that the su~­

jeetive loudness of the simulated sonie boom depended very mueh on the amplitude of the superimposed broad-band jet noise visible in Fig.

3

.

.

With the acoustie

filtering seetion in place, there was a eonsiderable reduetion (approximately

12 dB) in the jet noise amplitude and, sub§equently, the su~jective loudness of the simulated sonie boom. Oscilloseope readings had shown the unfiltered jet noise to he at a SPL of 108-112 dB over a frequeney range from 300 to greater than 19,000 Hz for a 2 psf, 80 msee duration sonie boom. This "redueed noise" sonie boom is shown on Fig.

3

and the relative amplitudes of the jet noise for a

4

and

8

psf sonie boom of the same duration is indieated on Fig.

4.

A more detailed study~ using the "reduced noise" sonic booms of 2~

4

and

8

psf overpressures, 80 msec duration and

3

msec rise times, was t~en made of hearing and heart rate responses. Audiograms were again performed before and about 2 minutes aft er exposure to 50 sonic booms. The heart rate measurements however, were somewhat modified. Each subject was in a reclining position and was allowed to rest for a short time. Before any sonic-boom exposure, each subject had to answer a simple question~ perform a simple task and solve a simple mathematical problem; e.gs. "In which country were you born?,"lift your left leg for the count of' ten"; "what is 56 divided by 8?". Heart rate was

monitored for these events as well as during the first five sonic-boom expo~ures. After each event (question, task, problem or sonic-boom) time was allowed for the subject's heart rate to return to its initial rate (approximately 30-40 sec) before the next event began. The question~ task and problem sequence served two purposes: first it gave a basis of comparison of a change in heart rate due to a normal stimulus; second, it helped subjects to relax in the somewhat unfamiliar surroundings of the test section of the sonic-boom horn.

Because the second set of sonic-boom measurements required more time to perform, it was necessary to limit the experimental group to 8 subjects, all of' whom had been part of the original 20 and seemed to represent a fair cross-section of the larger group as may be seen from the results displayed in Fig.

5

.

In the analysis of ,most of the electrocardiogram work, the heart rate was averaged over the

5

beats just before and just af ter the stimulating event. In a few cases it was clear that noticeable change in heart rate had lasted for _only 2 or

3

beats. For such cases, an estimate was made of the average heart

rate over these 2 or

3

beats.

3.

RESULTS AND DISCUSSIONS

Before beginning a discussion of the results for the UTIAS Travelli ng-Wave Sonic-Boom Simulator it is worthwhile to state briefly the main features of

actual, aircraf't-generated sonic booms.

A typical Concorde (SST) sonic boom (Fig.6-) shows the characteristic N-wave shape, consisting of a sharp rise above ambient pressure (due to the bow or first shock wave)~ followed by a gradual linear decrease in pressure below ambient, followed in turn by a sharp ri se in pressure returning to ambient

conditions(due to the tail wave or second shock wave). The terms peak overpressure and duration are defined in Fig.

6.

Rise time, for the case of an ideal N-wave~ may be simply def'ined as the time required to reach peak overpressure (Fig.6). However, in actual sonic-boom signals the distortions in their shape away f'rom the ideal N-wave, particularly visible on close examination of the shock wave

portions of the signal (see Figs. 7~8 and

9),

indicate that an alternate definition of' rise time is desirable. In applications to human response studies, a defi

-nition of rise time as being the time required to complete the first sharp

pressure rise, this being of ten to only one half or less of the peak overpressure,

is more useful. It is seen f'rom the records that an unexpanded time scale yields

a rise time of about 5 to 15 msec~ whereas from the expanded scale it is actually

001 to 1 msec.

The UTIAS Travelling-Wave Sonic-Boom Simulator

3~1 Shock-Tube Drivers

During the initial operation of the UTIAS Travelling-Wave Sonic-Boom

Simulator three shock-tube drivers, of constant cross-sectional area, with

lengths of ~.5, 1.0 and 2.0 ft (see Fig.i), were used to generate short duration

« 10 msec) simulated sonic booms

.

•

The wave forms produced at the 70 ft station of the horn were not

N-waves typical of sonic-boom pressure profiles. They were distorted as shown on Fig.

lp.

These wave forms may be considered in three main sections. First, there

is the initial shock wave

(SJ

followed by a gradual decay in overpressure

ampli-tude. These features are results of the shock wave formation within the tapering

horn (See Fig. land Appendix A for the operation and description of the UTIAS

Travelling-Wave Sonic-Boom Simulator). Second, a rarefaction wave

(R),

producing

a more rapid dec.ay in overpressure, follows the initial shock wave. It is the

rarefaction wave, originally produced by bursting the shock-tube diaphragm,

which reflects from the end of the shock-tube driver and propagates into the

horn. The third portion of the wave form is a compression wave, resulting

from reflection processes occurring when the rarefaction wave propagates from ·the constant cross-sectional area driver into the pyr~midal hGrn. This

com-pression wave can coalesce into a second shock wave (8) (Fig. 10 for the 1.0

and 2.0 ft drivers) or it can remain as a compression wave as for the 0~5 ft driver. An analysis by Gottlieb (Ref. 2) has shown good agreement with experi~

mental data, in predicting these wave shapes, within the limits of the acoustic

(linear) theory used (Fig. 11). This linearized theoretical approach indicates the formation of rarefaction shock waves (for tc/L

=

2.0, Fig. 11) which in reality do not occur as a result of nonlinear wave action. Portions of the "rarefaction shock wave" above amlilient pressure have a sound speed greater than

the amsient value and therefore tend to overtake the front of the wave, whereas

the portions of the "rarefaction shock wave" below ambient pressure, for similar

reasons, tend to lag towards the tail of the wave form. Therefore the "rare -faction shock" becomes in actuali ty the rarefaction wave (see 1.

7

<

tc/L

<

3.2, 1.0 ft driver, 70 ft station, Fig. 11).

A further limitation of the linear theory is the neglect of "wave

stretching". as it propagates down the horn (Ref. 2). From the diagrams of Fig. 11 we see that even 0.5 ft from the diaphragm the wave form appears elongated

compared with that predicted theoretically. This stretchigg increases with

distance from the diaphragm, as may be seen from the profiles recorded at the 2.0 and 70 ft stations.

The same nonlinear processes responsible for the spreading out of the

theoretically predicted rarefaction shock wave, are at work to cause the coales-cence of the compre?sion wavê into a second sheck wave by the time the 2.0 ft

station is reached and grows in relative amplitude to the 70 st. station. For the case shown on Fig. 11 the wave form at the 70 ft station is

approximately 30 per cent longer than predicted from theory. Consequently, neglecting the nonlinear factors in the theoretical approach results in some

discrepancies in comparisons to experiment. However, these differences are not

large enough to outweïgh the simplicity offered by a linearized theoretical

approach, which gives reasonasle guide lines for predicting results.

As earlyas 1956 (Ref.3) linear analysis had predicted that an N-wave pressure profile would be generated by a shock-tube driver having the same pyra

-midal taper angle as the horn. The entire shock-tube thereby forms a segment of a sphere with a divergence angle of 7.2 degrees, and can therefore be treated as a weak spherical explosion. Putty was used to create such a taper inside the 0.5 ft shock-tube and although the pyramid was truncated 0.167 ft short of its apex in this case (Fig. 1), reasonably good N-wave pressure profiles were generated

(Fig. 12).

Upon construction of the 0.667 ft tapered shock-tube (the pyramidal taper continued to the apex) again reasonably good N-wave pressure profiles were achieved (see Fig. 13). At diaphragm pressure ratios much above 2.0 there was some rounding of the second half of the N-wave profile. This was inherent in the formation and propagation process and was an effect which became more pro-nounced as higher diaphragm pressure ratios (see case for 5.1) were reached.

A numerical solution for a weak spherical explosion was obtained using a computer program developed by Brode (Ref. 4). Good agreement was obtained with experiment in predicting the wave shape 3.3 ft from the diaphragm (Fig. 14). The noticeable spreading of the shock front in the theoretical profiles shown in Fig. 14 arises from the use of an artificial viscosity that spreads shock fronts to avoid discontinuities in the computer program.

Comparisons of peak overpressures were made between the experimental data and two simplified theories. The linearized theory of Warren (Ref. 5) which is applicable to the 0.667 ft tapered shock-tube for diaphragm pressure ratios close to unity, predicted an initial peak overpressure of the shock wave equal to one half of the driver overpressure (or driver gauge pressure). This peak over

-pressure was then assumed to decay acoustically (inversely with distance) inside the pyramidal horn.

A second approach used the planar shock-tube equation (Ref. 6) to de

-termine the shock strength at the diaphragm station. Again acoustic theory was used to predict the decaying overpressure with axial distance from the diaphragm.

A comparison of the experimental data shovn on Fig. 15, with the analysis, shows that considerably larger overpressures are predicted. This discrepancy is due to the omission of the nonlinear wave action incurred as the shock waves form and propagate inside the pyramidal horn. As mentioned previously the approach used in Ref. 5 is strictly applicable only when the diaphragm pressure ratio is close to unity. Application of this theory to the cases used here, (diaphragm pressure ratios ranging from 1.5 to 5.1) resulted in predictions of initial peak overpressures considerably in excess of what would be expected from the planar shock-tube equation, (by 14 to 76 percent). The latter has been well verified by experiment for conventional shock-tube applications. The theory of Ref. 5 then may be used to predict only approximately overpressures inside the horn for dia-phragm pressure ratios less than two. The planar shock-t~be equation, at least for thef constant cross-sectional area shock-tubes does seem to produce reasonable agreement with experiment in predicting shock wave peak overpressures close to the diaphragm (see Fig. 16 for the stations close to the diaphragm). However, acoustic theory, when applied to the shock waves within the pyramidal horn, neglects two major nonlinear effects. No account is taken of the wave form "stretching" as noted previously. This phenomenon results in a more rapid decay in peak overpressure than that predicted by simple acoustic theory and would apply to the signals generated by both the constant cross-sectional area and

- - - _ . _ - - - -- .

tapered shock tubes.

An

additional nonlinear effect becomes impD~tant for the

latter case. The rarefaction wave, visible in Fig. 10 for the constant

cross-sectional area shock tubes, overtakes the shock front, in the tapered

shock-tube case, immediately upon rupture of the diaphragm, because of the spherical

nature of the flow (Ref. 2). With the rarefaction wave attached in this way to

the shock front, there is a further decay in peak overpressure as the wave form propagates through the diverging horn. It is this effect which produces the

differences in peak overpressures between the constant cross-sectional and tapered drivers shown on Fig. 15.

Results of the survey of peak overpressure as a funct.ion ofaxial distance from the diaphragm station show that the effect of nonlinear wave action is most important for stations close to the diaphragm, where the shock

strength is greatest. For the 1.0 ft planar shock-tube the peak overpressure

is already 20 percent below that predicted by acoustic theory at the 20 ft station

while thère is only a further reduction of 5 percent at the 70 ft station (Fig.16).

For the case of the tapered shock-tube driver the numerical results, discussed earlier in connection with wave shape, were used to obtain a theoretical prediction of overpressure 3.3 ft from the diaphragm. From this point acoustic theory was appl~ed to specify the continuing decay in peak overpressure to the 70 ft station. The results displayed on Figs. 14 and 15 agree well with the experimental data indicating that the dominant nonlinear action is occurring

within the first 3.3 ft from the di~phragm.

A particular advantage of the shock-tube drivers in simulating sonic

booms is their ability to achieve very short rise times

«

100 ~sec),as may be

seen from Fig.17. The importance of this fact will become clear in the

subse-quent discussion of human response to sonic booms.

3.2 The Mass-Flow Valve

The mass-flow valve, designed to simulate long duration (70 to 500

msec) sonic booms, operates on the principles put forward originally in Ref. 1

and later extended by Ref. 2. To generate an N-wave pressure profile, the

mass-flow valve must release into the apex of the horn a mass flow of air having a parabolic time history. This result is obtained, within certain limitations,

for a linear motion of the valve plug (Fig. 18), first opening and then closing

the throat into ±he horn apex.

There are two limitations which affect the mass flow of air into the

horn. First a detailed examination of the geometry of the throat created as

the valve plug opens or .closes was carried out by Gottlieb (Ref. 7). This showed that the actual mass flow into the horn apex was slightly distorted from the

desired parabolic profile and as aresult an 11 percent decrease in peak over-pressure of the simulated sonic boom could be expected. A second limitation was that the velocity of the mass-flow valve during the opening phase was less than the closing velocity (Fig. 19). The difference was very slight for sonic-boom durations less than 100 msec, however it became more significant when sonic-boom durations of 300 to 50Q msec were desired.

Since the theory presented by Ref. 1 indicated that the peak overpressure of the simulated sonic boom would vary inversely with its duration, it was necessary to determine this duration by the following method as indicated on Fig. 19.

The valve plug began to

move

within the teflon seal at point A. (see Appendix A for a description of the mass-flow valve). Af ter travelling 1/4 in.

(point B), the ~hroat into the horn apex began to open and the initial shock wave of the sonic boom ~as generated. Af ter moving a total distance of 2=1/4 in. the valve plug reached its full open position at point C and then reversed direction and began to close. Two closing motions are depicted on Fig. 19. Since the theory of Ref. 2 had assumed that the mass-flow valve opened and closed with the

same speed, a line C D'E' was drawn to show this theoretically assumed closing motion. This would indicate that the valve plug would enter the teflon seal at point D', thereby generating the tail shock wave of the sonic boom, and would finally come to rest inside the teflon seal at E'.

The theoretical duration of the sonic boom is then the time between the initial and tail shock waves indicated by the line BD'. In practice this time was obtained by doubling the time required for the valve plug to travel from point B to point C. This meant that in the theoretical computations to predict the peak overpressure only the opening portion of the valve mot ion was considered since it was only this portion of the mass-flow valve motion which affected the initial peak overpressure of the simulated sonic boom.

Steps presently under way are being taken to provide identical opening and closing velocities.

The simulate~ sonic boom peak overpressure is dependent on three basic parameters. There is a direct variation with the reservoir tank pressure

ratio (Fig.20) and an inverse dependence on both duration and axial distance from the horn apex (an acoustic decay). The effects of each of these parameters on peak overpressures were examined separately, varying one while holding the other two constant.

The finite rise time of the simvlated sonic boom (3 to 4 msec) was

originally not taken into account in the theory of Ref. 2. This omission produced higher predicted peak overpressures, an error which could be as large as 10 per cent for shorter duration signals (aO msec) when the rise time is 4 per cent of the duration. The error became less significant for longer duration sonic booms

(300 to 500 msec) where the rise time was only 1 per cent of the duration.

For each case displayed on Figs. 21,22,23 and 24, the finite rise time was considered in calculating the theoretical curves by reducing the original theoretically predicted peak overpressures by the appropriate percentages de-pending on the sonic boom duration. As the reader can observe, reasonably good agreement between experiment and the theory of Ref. 2 has been achieved. In general, discrepancies are less than the experimental measurement error.

Originally it was believed (Ref. 1) that N-wave pressure profiles could be generated only if the flow through the orifice into the apex of the horn was choked. Analysis by Ref. 2 indicated that N-waves could also be generated by a flow which was subsonic at the orifice. The measured waveforms on Fig. 25 show that N-wave profiles are generated by both types of flows.

Good agreement is also obtained, using the subsonic flow theory, in predicting the peak overpressures of the simUlated sonic booms (See Fig. 21).

3.3 Superimposed Jet Noise

A broad band jet noise, inherent in the mass-flow valve method of genera

-ting sonie booms, was superimposed on the N-wave signal. An investigation using

eleetronie filtering showed the frequeney eomponents of the jet noise to lie mainly

between 300 and 10,000 Hz. Fourier analysis (Figs. 26 and 27) indieated that the

frequeney eomponents eomprising a sonie boom with a

3

to

4

msee risê time (as was

typieal of the simulated sonie booms generated), lay mainly below 300 Hz. This

result led to the eonstruetion of an aeoustie low-pass filter or jet noise absorber

eonsisting of a fiberglass lining and horizontal and vertieal splitter plates shown

sehematieally in Fig. 2. This filter attenuated the higher frequeney jet noise eomponents without interfering signifieantly with the sonk Qoom signal (Fig. 28).

For longer duration sonie booms (durations greater than 150 msee) the

jet noise was still substantial (Fig. 29). Eleetronie filtering had indieated

that, for the longer duration sonie booms, frequeney eomponents below 300 Hz were

signifieantly present in the jet noise. Sinee the frequeney of jet noise is

inversely proportional to the effeetive jet diameter, the lower frequeney jet noise

eomponents would be generated mainly when the valve plug was nearly or fully opened.

Sinee the valve plug was moved more slowly for the longer duration (300 msee) sonie

boom there was more time to establish the lower frequeney jet noise eomponents.

Comparison of Figs. 29 and 30 show that the overall intensity of the jet no~e

superimposed on the 300 msee duration sonie boom is mueh greater than that

super-imposed on the 80 msee duration sonie boom. Therefore, although the frequeney

eomponents of the jet noise below 300 Hz may be negligible for the 80 msee

dura-tion sonie boom, the overall inereased noise level present for the 300 msee

dura-tion ease would aeeount for an inereased presenee of jet noise eomponents below

300 Hz where these eomponents were then no long er negligible. To eliminate these

it ~as neeessary to install a transverse panel whieh spanned the entire

eross-seetion of the horn (Fig. 2)~ By increasing the thiekness of this panel it was

possible to inerease the attenuation of the lower frequeney jet noise eomponents.

The results of two cases (for panels with a mass per sq ft of surfaee area of 1.5

and

4

lbs/ft2 ) are displayed on Figs. 29 and 30. (It should be noted that the

distortion away from an N-wave for the 300 msee duration sonie bdom shown in

Fig. 29 is a eonsequenee of the refleetions from the end of the horn. A refleetion

eliminator is presently under development).·

-Although the transverse panel does provide a good method for redueing

the jet noise amplitude, the elimination of some of the frequeney eomponents below

300 Hz produees an inerease in rise time as noted for both the 80 and 300 msee

duration sonie booms on Figs. 29 and 30.

3

.4

Partiele Velocity Profiles

Ineluded in the analysis of Ref. 2 is an expression for predieting the

partiele veloeity whieh results from the sonie-boom pfessure disturbanee. Beeause

of the nature of the sonie-boom generation procedure the partiele veloeity at the

horn apex is parabolie. For stations in the very far field, simple aeoustie theory

prediets that the partiele veloeity will be in phase with the pressure disturbance,

ie., an N-wave. At intermediate stations, whieh in praetiee ineludes the entire test

seetion of the horn, the partiele veloeity is a combination of a partial parabolie

and partial N-wave profile. Good agreement has been aehieved between the1theory

of Ref. 2 and experiment in predieting these profiles as may be seen fr om Figso 31

and 32. The slight diserepaney oeeurring for the 300 msee duration sonie boom is

a eonsequenee of the asymmetrieal mass-flow valve motion mentioned earlier. The

time for the mass-flow va1ve to open fully (170 msec) is greater, and the time required to cldSe fully is less (130 msec), than those values assumed in theory

(150 msec for both opening and closing).

Since the maximum particle velocity predicted by the theory depends on the time at which the va1ve plug reaches its full opening position, the increased

opening time effectively delays the occurrence of the actual maximum particle velocity. The asymmetrica1 valve motion also produces particle velocities which are slight1y lower during the opening phase and slightly higher during the closing phase, than those values predicted by the theory of Ref. 2 as may be seen from

Fig. 31. This effect is not as noticeable for the Bo msec duration sonic boom since the valve motion becomes more symmetrical for shorter durations.

Since the magnitudes of the particle velocities are small it is possible to neglect the effect of the dynamic pressure, resulting from these velocities,

in corrparison with the actual sonic-boom overpressure. For example at the 50 ft station, a 300 msec duration simulated sonic boom with a peak overpressure of 2

psf has a particle velocity less than 2 ft/sec. This produces a dynamic pressure less than G.Ol psf or less than 1/2 percent of the peak overpressure and is there-fore negligible.

Comparison of Figs. 31 and 32 shows that the longer duration (300 msec) soni~ boom had a velocity profile which was more parabolic in nature than the

short er duration (Bo msec) sonic boom. The latter was approaching an N-wave profile at the 50 ft station (see Fig. 32). These results were also predicted by the

theory of Ref. 2 and were a consequence of the mass flow of air required to generate

each particular sonic boom. Because a greater amount of air was released into the

horn apex during the generation of a longer duration (300 msec) sonic boom, the distance which the waveform must propagate in order that the particle velocity came into phase with the pressure (ie., became and N-wave) was increased. Therefore at a particular measuring station, the particle velocity profile of a longer duration

sonic boom was more parabolic in nature (less like an N-wave) than a shorter dura -tion sonic boom. The parabolic-shaped particle velocity profile then reaches its maximum during the mid portion of the sonic boom signal. As a result we find at

the 25 ft station the maximum particle velocity of the 300 msec sonic boom was greater than that of the Bo msec sonic boom {compare

4

ft/sec, Fig. 3l with

3.B

ft/ sec, Fig. 32) even though the latter was almost twice the peak overpressure of the

former (compare B psf with 5 psf). If we examine the initial particle velocities which correspond in time to the peak overpressures we find that for the two cases on Figs. 31 and 32 there is a proportional increase in the initial particle velocity with an increased peak overpressure, ie., the peak overpressures were in the ratio of 5 psf (3Qo msec duration) to B psf (B~ msec duration) where

5/B

=

0.6, and the

initial partic1e velocities were in the ratio of 1.B ft/sec (300 msec duration) to 3 ft/sec (Bo msec duration) where 1.8/3

=

0.60

Similar results also hold at the 50 ft station.

Thus we may conclude that the initial particle velocities, which are in phase with the peak overpressures, are related by simple proportions to those peak

overpressures. However partic1e velocities other than the initial values are not necessarily in phase with pressure and therefore a more complex approach was derived

by Ref. 2. This has been shown to agree well with experiment in predicting the

actual particle velocity profiles.

3.5

Boundary Layer Measurements

In measurements of partic le velocity some "scatter" in experimenta 1

re-sults was observed (Fig.

33).

When an attempt was made to measure the boundary layer close to the horn walls no change in particle velocity greater than this general "scatter" couHl. be observed even 1/8 in. from the wall, Le., the boundary layer

was thin enough that even 1/8 in. from the wall the particle velocity was still

virtually the same as the free stream velocity. An approximate theoretical approach was then undertaken to see whether theory would also predict a negligib le boundary.

layer growth.

For all particle velocities capable of being generated within the test section of the horn, the flow Reynolds number was at least one order of magnitude less than the critical Reynolds number required for transition from a laminar to a turbulent boundary layer (Ref.

6).

Thus any boundary layer growth along the horn walls would be laminar. It was then possible to adapt the "weak shock" case of Mirels (Ref. 8) to the flow behind the initial shock wave of the sonic boem. It was necessary to assume a constant "average velocity" shown on Figs.

31

and

32 behind the shock wave, a limitation which precluded the application of the ensuing analysis to the latter portion of the velocity profile where the particle velocity actually reversed direction and became negative.

The adapted version of Mirels' analysis was ~sed to compute the boundary layer thickness, as a function of time, behi~d the initial shock wave. From this estimate, the particle velocity 1/8 in. from the wall was calculated by assuming an error function velocity profile through the boundary layer. These results are

shown as the dotted lines on Fig.

33

along with the experimental velocity profile measured at the centre of the horn. Although the analysis did estimate the boun-dary layer to be slightly thicker than 1/8 in., particularly for the longer dura-tion (300 msec) sonic booms, the resulting change in particle velo city 1/8 in • . from the wall was less than the changes occurring due to the experimenta1 scatter,

i.e., the dotted lines which indicate the theoretical estimate bf particle velo-cities 1/8 in. from the wall shown on Fig.

33,

lie within the scatter in experi-mental results.

Both the theoretical estimate and experimental measurements indicate that any boundary layer forming along the walls would be negligible in comparison with the dimensions of the test section of the horn ·and therefore would not pose a problem to future sonic-boom response studies.

3.6

Physiological Response to Simulated Sonic Booms

Two physiological responses to simulated sonic booms are dealt with in this report: the effect on human hearing and heart rate.

3.7

Hearing Tests

In the results of the simulated sonic boom tests it is necessary to con-sider the combined effect of the sonic-boom signal and the superimposed jet noise mentioned previously when evaluating the subjective loudness of the entire

simu-lated signal.

As mentioned earlier, Fourier a~alysis had shown the frequency compo-nents of the simulated sonic boom to lie below 300 Hz while electronic filtering

had indicated that the frequency spectrum for the superimposed jet noise on an 80 msec duration sonic boom (this particular duration was used for all the

physiological testing) ran~ed from 300 Hz to above 10,000 Hz. Thus in

consider-ing the effects on human hearconsider-ing of the simulated sonic booms, a temporary thres

-hold shift (TTS) in hearing at a frequency below 300 Hz is attributed to the

sonic boom itself while a TTS above 300 Hz is assumed to be a consequence of the

jet noise. (For readers unfamiliar with the physiology of human hearing or the

related terminology, a brief summary of this topic is presented in Appendix BJ.

The peak sound pressure levels (SPL's) for the simulated sonic booms

and the superimposed jet noise are tabulated on Fig. 4 for all of the cases used

in the physiological studies. Comparison of these SPL's with the limits set by

Ref. 9 for impulsive noises (Fig. 34) indicate that an 80 msec duration sonic boom and the associated jet noise are both at SPL's below the 75 percentile limit.

This means that 75 percent of the normal hearing people exposed to these acoustic

impulses would not suffer significant TTS's even for sonic-boom peak overpressures

up to 8 psf. Acceptible TTS's specified by Ref. 9 are:

TTS

<

10 dB for frequencies

<

1000 Hz TTS

<

15 dB at a frequency of 2000 Hz TTS

<

20 dB at a frequency

>

3000 Hz

These TTS's must disappear within 24 hours from the time of the noise

exposure. If a shift in hearing persists for longer than 24 hours it is no longer

terms "temporary" and would indicate a more serious danger to hearing.

The limits displayed on Fig. 34 have been obtained using impulsive

noises of instantaneous rise times

«

l~ec) and repetition rates of

6

to 30

impulses per minute. The peak SPL's (or peak overpressures) and durations were

varied and from this it was learned that higher peak SPL's co~ld be tolerated

for shorter duration impulses. The choice of a tolerable peak SPL was somewhat

arbitrary and subsequently an alternative 95 percentile limit has also been pre

-sented on Fig.34.

Because the rise times used by Ref. 9 were much shorter than those

typical of the simulated sonic boom (compare 1 ~sec with

3

to 4 msec) the subjec=

tive loudness and therefore the potential damage to hearing would be less severe

for the simulated sonic boom. Reasons for this will become more clear in the

following section on subjective loudness.

It is not surprising th en to find that for all cases, exposure to the

50 simulated sonic booms, at the rate of 25 per minute, did not produce sig4ifi

-cant TTS's.

The data obtained from the 20 subjects originally exposed to the 2 psf

"noisy" sonic boom (Fig. 35) showed no TTS's above 10 dB. Further, the figure

indicates that TTS's of 10 dB were more common at frequencies above 300 Hz due to

the jet noise, than below 300 Hz due to the simulated sonic boom. This result

had pointed out the need to attenuate the jet noise in order that the simulated

sonic boom could be better used for physiological response studies.

The hearing response data obtained from the "reduced noise" tests did

show fewer cases of TTS's at frequencies above 300 Hz. Figure

3

6

shows onlya

few cases of TTS I S for the 2 psf "reduced noise" sonic booms. Although increasing

the sonic-boom overpressure to

4

and

8

psf did increase the nurober of cases of TTSt s (note particularly frequencies of 125 and 250 Hz on Fig. 36) none of these exceeded the TTS limits quoted previously from ReL

9.

It is clear then, that exposure to these simulated sonic ~ooms, with up to

8

psf peak overpressure and repetition rates of 25 per minute, wo~ld not produce a haza.nd_. to norrnal hearing. This represents a much more severe exposure than would ever be likely to occur in actual supersonic overflights and therefore such flights would likewise have no detrimental effect on hearing. It remains to show that the simulated sonic booms do represent as serious an acoustic hazzard as actual air

-craft-generated sonic booms.

3.8

Subjective Loudness

Brief treatments of the subjective loudness units, phons and sones, are given in Appendix Band a method for predicting the subjective loudness of the jet noise is found in Appendix D.

Robinson and Johnson (Ref. 10) have demonstrated good agreement between their method of predicting the subjective loudness of a sonic boom with the results of experimental huroan response studies where actualssOO±c booms were used. Figure 37 indicates the loudness level in phons, as a function o:f rise time and delay time (the time between the incidence of the sonic boom and the arrival of its re~ flection from the ground surface; throughout this report delay time is considered to be 0 msec) for a 2 psf, 350 msec duration sonic boom. The stated overpressure includes the effects of peak overpressure doubling due to ground reflection.

Considering zero delay time, this being the case where the subjective loudness is greatest, Fig. 37 shows that the loudness level in phons increases from

99

phons for a rise time of 16 msec to 117 phons for a rise time of 0.1 msec. This increase in subjective loudness as rise time is shortened has also been

predicted from experiment and theory by Zepler and Harel (Ref. 11) as rnay be seen from their results displayed on Fig.

38.

The Robinson and Johnson method of predicting loudness level in phons of a sonic boom was based on a model where peak overpressure, rise time, delay time and duration are specified. By applying the techniques of Fourier analysis they were able to arrive at the loudness level'curves of Fig. 37. The model neg-lects any superimposed fine structure which rnay occur along the transverse section of the sonic boom. Robinson and Johnson maintained that any such noise had no significant effect on subjective loudness. Electronic filtering of a typical Concorde sonic boom (Fig.

39)

indicated no noise or fine structure visible over the background noise incurred during the tape recordings of the signals (100 dB) and is therefore in agreement with the findings of Robinson and Johnson.

Further work by Robinson and Johnson had indicated almost no variation in subjective loudness with sonic booms of different durations, the difference in loudness level being only 0.1 phons for sonic booms ranging from 100 to 500 msec duration.

Already mentioned was the fact that actual sonic booms rnay have rise times as short as 001 mseco From Robinson and Johnson's results we can conclude

that for a given overpressure (and zero delay time) this would constitute the subjectively loudest sonic boom likely to occur from a Concorde aircraft. Using

this short rise time the loudness levels in phons were calculated for actual sonic booms of 2,

4

and

8

psf peak overpressures following the method of Ref. 10. The.

loudness levels in phons were then converted to loudness in sones using the nomogram and graph of Fig. 40 (Refs. 12 and 13). In a similar manner the loudness of the 80 msec duration, 3 msec rise time sonic boom was computed in sones for peak over-pressures of also 2,

4

and

8

psf. The lack of dependence of subjective loudness on duration allowed the curves of Fig. 38 to be used in computing the loudness levels in phons for the

8Q

msec duration simulated sonic booms.

The method outlined in Appendix D was used to obtain the loudness in sones ofthe superimposed jet noise. This value was added.to the loudness of the simu-lated sonic boom in order to obtain the total loudness of the simusimu-lated signal (sonic boom and jet nOise). From these results (Fig. 41) it is possible to compare the loudness of the simulated signal with an actual sonic boom of the same peak overpressure. Such a comparison shows that the 2 psf "noisy" simulated sonic boom is subjectively much louder than the actual sonic boom of the same overpressure. (Compare 320 sones wi th 220 sones). Also, since Fig. 41 shows that the superimposed jet noise is actually louder than the 2 psf "noisy" simulated sonic boom, the fact that there were more cases of TTS's at frequencies above 300 Hz due to the jet noise than at frequencies below 300 Hz due to the sonic boom is understandable.

Examining the results for the "reduced noise" sonic booms on Fig. 41, it can be seen that although the

3

msec rise time simulated sonic boom by itself is less loud than an actual 0.1 msec rise time sonic boom, the additional loudness added by the jet noise increases the loudness of the entire simulated signal so that it is equal to or slightly greater than the loudness of the actual sonic

boom. This holds true for each of the peak overpressures used (2,

4

and

8

psf) and it is.therefore possible to conclude that the entire simulated sonic boom signat would be at least as severe an acoustic haza:rd~~ as any actual sonic boom likely to occur from supersonic overflights. This is to say that even though the greater rise time of the simulated sonic boom tends to reduce its subjective loudness, the superimposed reduced jet noise is at a level so as to add a compensating increase in loudness.

As mentioned, the actual sonic boom treated in Fig. 41 with a 0.1 msec rise time is the most severe case likely to occur for a particular peak overpressure. Also peak overpressures as large as

8

psf would not be normally expected from

aircraft-generated sonic booms. Thus it is possible to conclude that the sonic booms used in this study would pose as great or greater an acoustic hazard to human hearing as any actual sonic booms likely to occur. The insignificant effect of the simulated sonic booms on human hearing then indicates that a similar

insignificant effect would result from actual sonic booms.

3.9

Heart Rate

A discussion of the physiological reasons for changes in heart rate due to environmental stimuli is presented in appendix C.

Since an increase in heart rate due to an acoustic stimulus is a classi~

cal startie reaction, it should be pointed out that all of the subjects expected to hear a sonic boom although they did not know the precise time that it would occur. Thus the startle effect of these sonic booms may be somewhat less than for completely unexpected sonic booms.

The results of the electrocardiograms (EeG) performed during the s0nic boom exposures are displayed in bar chart form on Figs. 35 and 42 to 46. The average human heart rate is approximately 72 beats per minute.

The largest percent change in heart rate (35 percent) and the longest durat.ion of this changed rate (40 sec) occurred for the case of the 2 psf "noisy" sonic boom (Fig. 35). In general however the heart rate changes due to this simu -lat~d sonic boom were less than 15 percent and disappeared af ter 15 seconds.

The effect of the superimposed jet noise in increasing the subjective loud.n_ess of this "noisy" sonic boom is assumed to be responsible for the larger heart rate changes. Because of the poor simulation of subjective loudness, when compared to the actual sonic boom (mentioned in the previous section and shown on Fig. 41) the "noisy" simulatéd:C, sonic-boom results are treated only briefly. Results from the "reduced noise" simulated sonic booms are considered to be more indicative of what should be expected from actual sonic booms, and are therefore discussed in greater depth.

Two typical electrocardiograms are displayed on Fig. 48 for one SUID~

ject where he was exposed to a 4 psf sonic boom and asked to solve a simple mathematical problem. For the sonic boom exposure we find that the heart rate rose from 60 beats per minute to 70 beats per minute, an increase of 16 percent. When the same subject was asked to solve a simple mathematical problem his heart rate rose from 64 beats per minute to 71 beats per minute, an increase of about 11 percent.

The time for the heart rate te return to its pre-stimulus level was 14 seconds af ter the sonic-boom exposure and 7 sec aft er the problem was pos ed.

In the following section the changes in heart rate, similar to those presented on Fig. 48, for all of the subjects in response to the sonic boom exposures or the question-task-problem sequences are dealt with.

For the first exposure to a "reduced noise" sonic boom the percent change in heart rate was usually less than 10 percent and lasted under 5 seconds, although for a few cases 20 percent increases lasting up to 25 seconds did occur

(see Figs. 42 to 46).Surprisingly an increase in sonie-boom peak overpressure did not show a marked effect on the size of the increase in heart rate. For example, Fig. 42 shows that for the first "reduced noise" sonic-boom exposure, the subjects did not show a significantly greater percent change in heart rate due to the 8 psf sonic boom than for the 2 psf sonic boom. Figures 43 to 46, for the second to fifth sonic-boom exposures, indicate a similar result where the percent increase in heart rate does not vary significantly with overpressure. This effect may be attributed to a conditioning of the subjects to the simulated sonic booms since each subject was exposed first to the 2 psf, then the 4 psf and finally the 8 psf overpressures. Familiarity with the soniC-IDoom signal could compensate for the increased overpressure therehy producing the similar percent increases in heart rate for the three overpressures used.

The hypothesis that subjects do become familiar with the simulated sonic booms and therefore undergo less of a heart rate change is further sub-stantiated by a comparison of Figs. 42 to 46. Examining these figures for each of the three peak overpressures separately, it becomes clear that the percent increases in heart rate become progressively less during the five sonic~boom

exposures; i.e., with each exposure to a sonic boom the subjeet's familiarity

with the stimulus increases and then the next sonie boom produces less of a startling effect, indicated by a smaller percent change in heart rate.

Comparison of the results for heart rate response to the simulated sonie boom with the responses to the simple question, task and problem sequ-enees (eompare Figs.

42

to

46

with Fig.

47)

showed in faet that a simple ques-tion, task or problem could produce changes in heart rate of 20, 25 or 30 per-cent respectively. These changes could persist for up to 10, 30 or 20 sec respeetively.

It ean be concluded then that even though sonie booms at peak over

-pressures of 2 psf or more are likely to produce some change in heart rate, such changes are not significantly greater than the heart rate changes due to a simple question, task or problem whieh would represent ordinary daily stimuli.

Similar experiments, monitoring the change in heart rate due to a sonie-boom exposure, were carried out by Thaekery, Touehstone and Jones (Ref. 14) using a sonie-boom simulator designed by the Stanford Research Institute. Rise times of their simulated sonic-boom signals ran~ed from

7

msec for a 1 psf sonie boom at 21 msec for a

4

psf sonic boom. These longer rise times would decrease the subjeetive loudness of the simulated sonie booms as indicated by Refso 10 and 11 when compared to actual sonie booms of the same peak

over-pressures with rise times of 0.1 msec. Using the criteria of Ref. 10, Thaekery's (et al) 1 psf sonie boom would have a subjective loudness of 70 sones comparing with 140 sones for alpsf, 0.1 msec rise time sonic boom. Similarly the longer rise time,

4

psf sonic boom would have a loudness less than

90

sones while an aetual 0.1 msee rise time sonie boom of the same peak overpressure would have a loudness of 310 sones. Because of the redueed loudness of these simulated sonic booms the potential startle effect is redueed.

Thackery (et al) found that heart rate decreased af ter a sonic-boom exposure. Physiologically speaking, this result was unexpeeted since it would indieate a stimulation of the parasympathetie nervous system to slow the heart without a prior sympathetic stimulation whieh would have inereased the heart rate. This represents the reverse of the normal heart response to a startle impulse (see Appendix C).

The unexpeeted response may be attributed to the faet that Thaekery's et al subjeets were performing a ~raeking experiment throughout the thirty minute period during whieh they were exposed to foUT simulated sonie booms. Work by Kagan and Rosman (Ref. 15) showed that during an attention attitude, i.e., when a subject was paying more attention to a partieular assignment, his heart rate slowed. It seems feasible then, that the sonie boom used by Thaekery was not loud enough to produce a startle effect, but was enough of an aeoustie stimulus to break the monotony of the traeking task and inerease momentarily the attention

bei~g paid to the assignment. The increased attention was th en responsible for

slowing the heart rate. This hypothesis was further substantiated by the faet that Thaekery et al did observe a temporary improved traeking performance af ter the sonie-boom exposure.

Work by Fleshler (Ref. 16) with rats showed that, for these animals, it was neeessary to eonsider both the rise time and the peak overpressure of an aeoustie impulse when trying to produce a startle reaetion. If the rise time of the impulse was inereased it beeame necessary to increase the peak overpressure in

order to continue achieving a startle reaction.

Although Fleshler's data could not be applied directly to human beings, it does seem likely that there would be a similar threshold in human beings that must be reached in order to achieve a startle reaction. From the previous section on subjective loudness, Fleshler's findings concerning the need to conside~ both rise time and peak overpressure is equivalent to saying that a certain subjective loudness must be reached to produce a startle reaction. We may then conclude that Thackery's et al simulated sonic booms were not loud enough to produce a startle reaction whereas the simulated sonic booms of this report were at a loudness level to produce startle. Further, it is apparent that actual sonic booms can be loud enough to cause startle reactions indicated by a slight momentarily increased heart rate in many people.

Although the findings of this report have not shown any significant physiological danger to normal hearing or heart rate, the subjective evaluations of the simulated sonic booms by the subjects (Fig. 49) indicate that particularly the

4

and

8

psf peak overpressure sonic booms are objectionable. It appears then that psychological rather than physiological responses to sonic booms are more cri tic al and it is this field of research which should be explored in future studies.

4.

CONCLUDING REMARKS

The initial calibration of the UTIAS horn-type sonic-boom simulator has been ~ompleted. Both the shock-tube drivers and the mass-flow valve have been investigated as methods of generating simulated sonic booms. For each case the important sonic-boom parameters of peak overpressure, duration and rise time were measured. With this sonic-boom-simulation facility peak overpressures of over 25 psf and durations ranging from 70 to 500 msec have been achieved using t,he mass-flow valve. With the shock-tube drivers it was possible to generate short duration sonic booms with rise times less than 0.1 msec. The fiberglass acoustic filter has been successful in signficantly reducing the superimposed jet noise which was associated with the mass-flow-valve generated sonic boams. Measurements of p,rticle velocity (induced by the simulated sonic boom) within the test section of the facility, have shown that the resulting dynamic pressure is negligible when compared to the peak overpressure of the sonic boom. Further measurements showed an insignficant boundary layer growth along the walls of the test section.

Two physiological response tests have been carried out using mass-flow-valve generated sonic booms. The results indicated that sonic booms of up to

8

psf peak overpressure would not have a detriment al effect on human hearing or heart rate. However, the subjective evaluation of the sonic booms indicated that peak overpressures of

4

psf or more would be unacceptible to most people.