GL-TR-90-0225

Infrasonic Emissions From theOtis AFB Hush House

SAlan L. Kafkacc' Joseph 1. Blaney

SBoston CollegeWeston Observatory

I Department of Geology and Geophysics381 Conco d Ruad

SWeston, M,, 02193

DTIC10 September 1990 ELFCTE0

JAN 0 21991,

Scientific Report No. 6

approved for public release; distribution unlimited

GEOPHYSICS LABORATORYAIR FORCE SYSTEMS COMMANDUNITED STATES AIR FORCEHANSCOM AIR FORCE BASE, MASSACHUSETTS 01731-5000

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Infrasonic I1nissions from tJhe Otis AFB Hush House PR 600PR 7600TA 09

6. AUTHOR(S) WUAGAlan L. KafkaJoseph I. Blaney Contract r19628-66-C-005

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'--- Hush House infrasonics produced by F-100 engine runs at Otis AFBappear to be generated by a single point source located in the vicinity of theexhaust deflector. The resu!!s of this study suggest ihaw a nlcn-vertical jetplume might be an app'opriate source model for the Hush House infrasonics.When tested at military power, engines installed on a test stand produceslightly stronger infrasonic emissions than engines installed in an aircraft.The most significant factor in the overall level of infrasonic emissions.however, is whether the engines are run at military power or in afterburner.The azimuthal radiation pattern of infrasonics generated by the Otis HushHouse is, in a general sense, comparable to that reported for the Hush House atLuke AFB. The peak sound level observed behind the Hush House at audiblefrequencies is not generally observed for infrasonic pressure vibrations. Infact, at frequencies lower than about 10 Hz, pressure vibrations arc generally,.gicater along the side of the Hush House than behind the facility. "'

'14 SUbJECT TIRMS 1S NUMIJIIM Of PAGiES42

Jet engine noise, infrasonic emissions 4 PFIU Cool

acoustic vibrations, noise suppression (1 ) 14141CODE

17l. Si.CURITY C[AS$IIICATION ii SICURITY CLMASIfICATION 19 SECURIIITY (LA'SaFICATION 20 LIMITATION OF AUSI'ACITOf NAPORT OF THIS PAGE OF ABSTRACTUnclassified Unclassified Unclassfied Unclassified

TABLE OF CONTENTS

Page

1. Introduction .................................................................................................................. I

2 . F in d in g s ......................................................................................................................... 1

3. The Otis AF1l Hush House ............................................................................................ 2

4. The M easurement System and Recorded Data .................................................... 3

5. Ambient Pressure .................................................................................................. 4

6. Infrasonic Emission Level ................................................................................... 4

7. Source Location ................................... ...... .............................................. 5

8. Radiation Pattern of Infrasonic Emissions ....................................................... 7

9. Comparison of Otis Hush House with Hush House at Luke AFB ....................... 8

W0. Discussion and Conclusions .................................................................................. 9

11. References .................................................................................................................. 10

I. , ...... ....... .

Di t I; A-

v I'

i.J i"

1. INTRODUCTION

The T-10 jet engine noise suppressor (Hush House) at Otis AFB is part of an Air

Force wide effort to upgrade engine test operations with a standard test cell. Figure I

shows a plan view of a model T-10 Hlush House. The Hush House at Otis AFB was built

in 1985. Acceptance tests were concluded in October of that year with a J75 engine on

a test stand and installed in an F-106A aircraft. For acceptance, "A-weighted"

pressure levels (Beranek, 1971) were measured at 20 points over a pair of arcs

centered 250 feet from each end of the Hush House augmenter tube (Figure 2). Sound

levels recorded for those acceptance tests are shown in Figure 3. The sound level

around the Hush House is not uniform. The peak sound level radiated by the Hush

house generally occurred behind the facility (in the direction of the exhaust

deflector) when the engine was operated in afterburner installed on a test stand.

Acceptance and impact estimates for siting Hush Houses are based on

spherically propagated A-weighted pressure levels. These criteria, however, do not

fully account for vibroacoustic emissions from Hush Houses because the noise

suppression characteristics of a Hush House are achieved, at least in part, by the

transfer of energy from the audible (>20 Hz) to the infrasonic (<20 Hz) band. The

ability to forecast Hush House generated vibrations, therefore, requires estimating

pressure emissions at infrasonic frequencies, well below the audible (A-weighted)

band.

This study of the Otis AFB Hush House is a step towards constructing a data base

to dcfine the Hush House as at, infrasonic source. The resulting data base can be used

to test concepts for forecasting and controlling unwanted vibrations excited by

current and future engine "sound suppression" cells. The overall objective of this

type of study is to develop an infrasonic source model for Hush Hoiiszs and to test the

extent to which that model is independent of a particular location. In this report, we

present the results of a study of infrasonics emitted by the Otis AFB Hush House, and

we compare the results from the Otis Hush House with previous results obtained from

a study of Hush House emissions at Luke AFB.

2. FINDINGS

Hush House infrasonics produced by F-100 engine runs at Otis AFJ3 caa be

accurately modelled by a single point source, located in the vicinity of the exhaust

deflector. This type of single point source model was also hypothesized for the Hush

House at Luke AFB (Beaupre and Crowley, 1987). The results of this study indicate that

there is iso evidence of a second low-frequency source associated with the air intakes

at the front of the Hush House.

Although it was previously concluded by Beaupre and Crowley (1987) that the

Hush House infrasonics could be modelled as a vertical annular jet, the results of this

study suggest that a non-vertical jet plume might be an appropriate source model.

When tested at military power, engines installed on a test stand produce

slightly stronger infrasonic emissions than engines installed in an aircraft. The

most significant factor in the overall level of infrasonic emissions, however, is

whether the engines are run at military power or in afterburner.

The azimuthal radiation pattern of infrasonics generated by the Otis flush

House is. in a general sense, comparable to that reported by Beaupre and Crowl2y

(1987) for the Hush House at Luke AFB. The peak sound level, observed at audible

frequencies, behind the Hush House (in the direction of the exhaust deflector) is not

generally observed for pressure vibrations in the infrasonic frequency range. Infact, at frequencies lower than about 10 Hz, pressure vibrations are generally

greater along the side of the Hush House than behind the facility.

3. THE OTIS AFB HUSH HOUSE

The Hush House at Otis AFB is currently used for Air National Guard

verification tests with F-100 engines mounted either on a test stand or installed in F-

15 aircraft. Otis AFB is located on a glacial moraine, consisting largely of

unconsolidated sand and gravel. The ground structure underlying the Hush House

facility is revealed by nearby drilling logs, and is found to be uniform. The depth to

the water table is about 70 m, and the depth to bedrock is about 150 m.

A plan view of the facilities in ihe area surrounding the Otis Hush House is

shown in Figure 2. The area south and west of the Utis Hush House is flat and clear of

structures. To the east, a large aircraft hanger strongly reflects pressure loads

emitted from sources near the Hush House. Pressure levels cast of the Hush House

are likely to be significantly altered by reflections from this hanger. In contrast,

pressures to the weat of the Hush House are probably only weakly modified.

Based on the rest'its of the acceptance tests, the Otis AFB control tower and Fire

House lie in a strong acoustic lobe (i.e. behind the Hlush House). Occupants have

2

reported that strong, disruptive vibrations are excited in the Fire House (the closer of

the two facilities) when the test cell is in use.

4. THE MEASUREMENT SYSTEM AND RECORDED DATA

The Air Force Geophysical Data Acquisition System (GDAS) was used to measure

the flush House emissions. The system records and stores seismic and pressure

measurements taken from up to 16 sensors. These measurements are recorded at 100

scans/sec for later retrieval, display and analysis. The system is calibrated with its

sensors in place. A typical response for the GDAS pressure channels is shown in

Figure 4. The measurement band for this study was chosen to cover low mode

resonances of "large" class structures with a modest frequency overlap into the A-

weighkted acoustic regime.

In this study, pressure sensors were placed on the ground under a permeable,

low silhouette (molded "horse hair") wind shield that stagnates airflow in the

immediate vicinity of the transducer. The GDAS sensors were installed at the

following locations (Figure 2 and Table 1):

Location..1. A seven clement array was centered at a distance of 110 m and an

azimuth of 1150 from the center of the Hush House exhuast deflector. This array was

centered at appioximately the same azimuth and distance from the Hush House as one

of the measurent sites at Luke AFB in Arizona.

Locaton 2. Three recording sites (sites 1, 2, and 3) were installed in the

vicinity of the acceptance point located at an azimuth of 53'. Site 3 is located in the

same position as that acceptance point.

L. Three recording sites (sites 5, 6 and 7) were installed in the

vicinity of the acceptance point located at an azimuth of 1100. Site i is located at the

same position as that acceptance point.

For each engine run, 30 sec of pressure signal was recorded at each site. Each

30 sec trace was processed to produce an average power spectral density plot by

averaging spectra calculated from a series of 2.56 sec overlapping time windows.

These measurements extend the acoustic (A-weighted) measurements into the

infrasonic band, and they provide data to test the hypothesis that the single point

source model should be extended to include additional point sources.

3

5. AMBIENT PRESSURE

Hush House emissions, albeit intense, are only one of a number of disturbances

that affect the infrasonic environment at Otis AFB. Hence, measurements were

collected to establish ambient pressure levels close !o the flight line free of Hush

House generated disturbances. An example of the ambient pressure spectra is given

in Figure 5. The ambient pressure spectra exhibit the characteristic frequency "roll-

off" and spatial coherency associated with wind driven turbulence (Beranek, 1971).

The overall strength of these pressure fluctuations is controlled by topography,

surface "roughness". and wind speed.

GDAS hardware noise is a trivial contributor compared to the ambient pressure

levels. The signal-to-noise ratio was estimated from the ratio of pressure spectra at

recording site 2 (102 m from the Hush House) with the test cell active and dormant.

The results from that site show that, when the engines were running at military

power, the Hush House disturbance ranges from about 10 times greater than ambientat 5 Hz to about 50 times greater than ambient at 20 Hz. Signal-to-noise ratios were

significantly greater when the engines were running in afterburner. At the times

that these measurements were made, the Hush House could be considered to be the

dominant infrasonic "noise" source at Otis AFB, even for points quite close to the

flight line.

6. INFRASONIC EMISSION LEVEL

Observed spectra are shown in Figures 6 through 15 for pressures measured at

sites 3 and 7 with the Hush House operating F-100 engines at two differeat powersettings (and engines in and out of the aircraft). Of the 10 Hush House operations

investigated in this study, three runs involved engines running on a test stand and

seven involved engines installed in an F-15 aircraft (Table 2). Seven of the runs

were at military power, and three were with the engine settings in afterburner.

For the acoustic regime, A-weighted pressure levels are greater for an engine

installed on a test stand than for an engine installed in an aircraft (Figure 3). A

comparison of the spectra measured at a common point indicates that engine runs on

a test stand produce slightly sironger infrasonic emissions than an engine installed

in an aircraft when the engines are tested at military power (Figure 16). For the

three afterburner runs analyzed in this study, there was no systematic relationship

between the level of infrasonic emissions and the installation of the engines in an

4

aircraft versus on a test stand. In general, the most significant factor in the overall

pressure level at infrasonic frequencies is whether the engines are run at military

power or in afterburner.

Thz spectra obtained in this study generally exhibit two peaks, one at about 5

Hz, and another (broader) peak located between about 10 and 20 Hz. Although the

pressure level observed at frequencies higher than about 8 Hz is generally greater at

site 7 than at site 3. the pressure level for vibrations at frequencies lower than about

8 Hz are generally greater at site 3 than at site 7. The details of this difference in

radiation pattern at high versus low frequency are discussed below in Section 8 after

we first discuss the location of the source of the infrasonic vibrations in the next

section.

7. SOURCE LOCATION

After preliminary flush House studies at Luke AFB and Fort Smith, Arkansas, it

was hypothesized by Battis (1987) that the Hush I-louse infrasonic source could be

modelled as a tripole source. The dominant source in this model was assumed to be a

jel-like source located over the exhaust deflector at the end of the augmenter tube

with weaker "negative" jets in proximity with the air intakes along both sides of the

main structure. This hypothesis was developed on consideration of the spectral

structure of the infrasonics recorded during the two Hush House studies. This study

at Otis AFB provided an opportunity to test this hypothesis using array processing

techniques.

Acoustic measurements made during the Otis study were examined by J.C. Battis

(personal communicatior) using frequcncy-wavenum ber (FK) anilysis. The

procedure estimates the relative power in the signals at a given frequency that can

be fit to a plane wave front crossing the array from various azimuths and at various

apparent velocities A result of this plane wave assumption is that the sensors used

in the analysis must be tightly clustered relative to the source-receiver distance. For

this reason, only a subset of the available recording sites were used in this analysis

(sites 5, 6, 7 and 10 through 15). The configuration of the array used is shown in

Figure 17, where the y-axis (defined as 00 azimuth) is parallel to the axis of symmetry

of the Hush House.

During the Otis study, a series of small explosions were detonated

approximately 4.1 m behind the exhaust deflector and essentially at the ground

surface. As a preliminary check on our analytical techniques and bearing angles

5

from the array, FK analysis was performed on the signals from these shots. Figure 18

shows the signals recorded from the second shot of this sequence. Figure 19 is a plot

of the FK analysis results for these signals for a bandpass c,.atered at 25 Hz, the

approximate spectral peak of the detonations. First, note that the wavenumber used

in these plots is linear wavenumber (i.e. 1/wavelengih). The analysis shows a

maximum occurring at an azimuth of 287.60 with an apparent velocity of 0.338

km/sec. The remaining contours result from spatial and temporal aliasing. As

should be expected, the apparent velocity is reasonable for the speed of sound in air.

The results from all the shots agreed with this bearing angle and apparent velocity

within LO.5 0 and -2 m/sec, indicating that the shot azimuth from the array is well

determined through FK analysis.

Figure 18 shows a distinct echo arriving at approximately 0.6 sec after the

direct acoustics. Figure 20 shows the FK analysis for that echo. Again the apparent

velocity (0.342 km/sec) is close to that of the speed of sound in air, but the azimuth is

now found to be 100 behind the augmenter tube at 2770. From Figure 2, it can be seen

that this azimuth is appropriate for an echo from building 858, the large hanger to

the east of the Hush House. Although the configuration of the sites is not ideal for

azimuth determination at high frequencies due to spatial and temporal aliasing, the

separation of the direct and echo azimuths dernonstartes sufficient resolution for the

separation of the hypothesized Hush House sources.

Using bandpass filters of 3.0 Hz bandwidth and centered at 50 intervals from 50

to 25*, an FK analysis was conducted on all Hush House runs. Figure 21 shows typical

data, in this case collected during Hush House operations with the engine in the

aircraft and operating in afterburner. The pressure spikes seen in Figure 21 (at

about 5 and 7 sec) have not been previously observed, and their cause is uncertain.

It is not known if they are a result of a Hush Hlouse property or an engine condition.

During the observing period, some of the engines had mechanical problems that

might be responsible for these impulses.

Figure 22 shows the mean spectrum derived from the array outputs in nominal

units of spectral density. As an example of the FK analysis, Figure 23 shows the

results of FK analysis for this run at a frequtency of 10 Hz. In this case maximum

power is found for an arrival with an apparent "7c-l'cicy of 0.344 km/sec coming from

290.5'. The lesser local maxima seen in the plot are the result of temporal and spatial

aliasing of the data. All analyses produced essentially the same results as this

particular run and the variations ý,oserved over the runs are within the expected

errors of the procedure. It is of particular notw that azimuth to the infrasonic source

6

appears to be insensitive to the power setting of the engine. Table 3 lists the azimuth

and apparent velocity determined from each run in the five frequency bands.

The apparent source of the Hush House infrasonics at all frequencies lies

within a narrow arc from the array between approximately 2860 and 2920.

Assuming that the source lies along the center line of the Hush House, this would

place it somewhat behind the deflector plate, suggesting a non-vcrtical jet plume

source. There is no indication from this analysis of a second, hypothesized low-

frequency source thought to have bcen associated with the air intakes at the front of

the flush House. That hypothesis must now he abandoned, and it appears that a

monopole source, located in the vicinity of the exhaust deflector, must be assumed.

It was previously concluded by Beaupre and Crowley (1987) that the Hush

House infrasonics could not be modelled as a vertical annular jet. Observation of the

exhaust plume and the results on source location as given above, however, suggest

that a non-vertical jet plume over the augmenter tube might be a good replacement

hypothesis.

8. RADIATION PATTERN OF INFRASONIC EMISSIONS

A-weighted pressure levels at site 7 are substantially larger than levels

encountered at sit- 3 during the acceptance tests for a common engine installation

and power setting. The results of the acceptance tests show that, in the audible

frequency band, the level of pressure vibrations around the Hush House is not

uniform. The peak sound level radiated by the Hush House generally occurred

behind the 'acility (in the direction of the exhaust deflector). In this section, we

discuss the zzimuthal radiation pattern of the flush House at infrasonic frequencies.

Figure 24 shows average spectral values corrected for amplitude to a constant

distance of 76 ni (assuming an attenuation of l/r) for the infrasonic data obtained

during this study. Results are shown for center frequencies of 5, 10 and 20 Hz in

Figures 24(a), (b) and (c) rcspectively. For each of the points shown in Figure 24, the

spectral values were averaged over 5 frequencies in the vicinity of the center

frequencies, and the resulting average values were again averaged over all runs at

military power and in afterburner. At frequencies of 10 and 20 Hz, the pressure

levels are greater at azimuths near that of site; 7 than at azimuths near that of site 3.

At 5 Hz, however, the pressure levels are lower in the direction of site 7 than in the

direction of site 3.

7

This observationi olf lower levels of iiifra'aouid in the dlirect ion of site 7 cannot

be explained by invoking a different source location f~or the 5 Ilz. spectral peak. The

results of' the FK analysis indicate that the source is located in essentially thle samec

place for all frcqucricies investigated. Thus, the di ffecrneL in radiation pat tern for

low versus high frCquIctwLy in US? be cawwd' by i~omne other phienonieloon.

9. COMPARISON 01: OTIS It (SfI I Ilot iSFL WITH I HIl [15011~.SE AT LUKE AF3

Figure 25 shows a comuparison of- the r~idiatioti patienti observed at Otis with

that reported by Beaupire aind Crowvlcy (] ()87) for (the lflush Hlouse at Luke AFB. The

data shown for Otis are averaige spectial values corrected for amplitude to a cornstant

distance: of' 76 rn. Results are shown for center frequencies tif 6.250. 10.937 and 29.687

liz, which are the frequencies that were1 rcported in the 13eAupre anid Crowley st udy.

For the Otis recording sites, the spectral values were averaged over 5 frequencies ill

the vicinity of the center frequectieis, anid the resulting average values were agairl

averaged over all runs with engine scttings at military power- Lind in afterburner-.

The data were then normalized by dividing by the values observed at site 15 f-or each

specific frequenfcy anid sie rhii providecl a direct conmparison between the Otis and

Luke studies, sin.ce Beaupre arid C'rowley normalized their data to observed Spectral

values at a site that was Lit approx iniate ly the san-e. aztntimlt anid distance as site 15.

Since there were only 3 runs in afterburnecr. only thre averages of the scvcn runs at

military power- are shown. The results for the afterburner rutns are similar to those

shown for military power-, although the after-burnecr data exhibit somewhat more

scatter.

The azimuthal radiation patterti of i ufrasonics tjbscrved for the Otis Ifush

[louse is, in a general sense, comparable to that rep~orted by tBeiuprc and Crowley

(1987) for the f-lush Ifouse at Luke AFB. Because ,he azimuths covered by the

expcrirmcntal setups of the two Ii fferenti studtcs otnly overlap across a fairly narrow

are, it is difficult to compare the details of ilhe differences itt radiation patterns. Also,

the experimental design at Luke AFB included a string of sensors located along it line

almost parallel to the axis of symmetry of thc flush [louse. Trhus, the experimental

setup at Luke was more appropriate for investigating the source radiation pattern

than thc experimental setup at Otis. Nonetheless, several points of comparison

between Lhe two sites amc quite clear from the data shown in Figure 25.

8

Thc pronounced peak in the A-weighted sound level behind the Hush House

(i.e. at azimuths grcater than 1000) is also observed at both sites for frequencies

between 20 and 30 Hz (i.e. at the lower end of the audible frequency band). At

frequencies lower than 20 liz. this peak becomes less pronounced, and at frequencies

lower than about 10 Hz, pressure vibrations arc generally greater along the side of

the hush [louse than behind the facility [Figurcs 24(a) and 25(a)].

10. DISCUSSION AND CONCLUSIONS

The results of this study indicate that Hush House infrasonics produced by F-

100( engine runs at Otis AFB can be accurately modelled by a single point source,

located in the vicinity Of the exhaust deflector. This type of single point source

model was also hypothcsi,,ed for the [lush House at Luke AFB (Beaupre and Crowley,

1987). There is no cvidcnce of a second low-frequency source associated with th air

intakes at the front of tie Hush [louse. Although it was previously concluded by

fieaupre and Crowley (1987) that Hush House infrasonics could be modelled as a

vertical annular jet. based on this study a non-vertical jet plume appears to be a more

appropriate source model.

For audible frequencies, A-weighted pressure levels are greater for an engine

installed on a test stand than for an engine installed in an aircraft. In the case of

this ,tud& of the Otis flush House, there is some evidence that "bare" engine runs (at

militar, power) produce slightly stronger infrasonic emissions than an engine

instaiiud in an aircraft. The most signilicant factor in the overall pressure level at

infrasonic frequencies, however, is whether the engines are run at military power

or in afterburner.

"T., azimuthal radiation pattern of infrasonics observed for the Otis Hush

Housý. is. in a general sense, comparable to that reported by Beaupre and Crowley

(1987) fur the Hush House at Luk-e A•P. The peak audible pressure observed at sites

behind ti Hush Hlousc is not generally observed for pressure vibrations in the

infrasonic frequency range. In fact. at frequencies lower 'han about 10 liz, peak

pressures are generally greater along the side of the Hush House than behind the

facility.

9

11. REFERENCES

Battis, J.C., Hush House Induced Vibratians at the Arkansas Air National Guard

Facility, Fort Smith, Arkansas, AFGL-TR-87-0320, 13 November 1987. ADA195389.

Beaupre, J. T., and Crowley, F.A., Hush House Infrasonic and Seismic Emissions

Produced by F-1Q0 Engine Tests at Luke AFB, Arizena, Weston Observatory Scientific

Report, 30 April 1987.

Beranek, L.L., Noise Vibration and Control, McGraw-Hill Book Company, New York,

NY, 1971.

10

Table 1.

Sensor Locations for Otis AFB Hush House Study

Site ft Rangc (mecters) Azimuth (degrees)

1 93.3 60.72 102.1 58.33 89.5 53.25 72.5 98.76 67.8 108.47 76.3 110.19 100.9 114.7

10( 105.8 110.411 114.9 110.812 119.2 114.713 114.9 118.714 105.8 119.015 110. 1 114.7

--------- -- -- -- -- - -- -- -- - -- -- -- -- - -- -- -- - -- -- -- -- - ---1 1- -

Table 2.

Hush House Operations Investigated at Otis AFB

Run # Engine Installation Engine Setting

I In Aircraft Military Power3 In Aircraft Afterburner4 In Aircraft Military Power5 In Aircraft Military Power6 In Aircraft Afterburner7 Test Stand Military Power8 Test Stand Military Power9 Test Stand Afterburner

11 In Aircraft Military Power12 In Aircraft Military Power

12

Table 3.

Results of FK Analysis

5 Hiz 10 Hz 15 Hz 20 Hz 25 Hz RUN MEAN-. . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. - - - - - -- - - - - - - - - - - - - - - -- - - -

RUN AZ C AZ C AZ C AZ C AZ C AZ C(deg) (m/scc)

I-ML 288.5 332 290.0 349 287.8 344 289.2 350 289.1 344 288.9 3423-AB 291.3 359 292.2 336 288.4 346 288.9 348 289.0 344 290.0 3474-ML 293.4 353 289.5 339 289.3 343 289.7 341 288.1 343 290.0 3445-ML 297.6 350 290.7 350 286.5 348 290.5 348 287.8 343 290.6 3486-AB 292.3 354 290.5 344 287.9 341 285.7 337 288.7 337 289.0 3437-ML 290.4 355 288.0 339 291.8 357 290.0 351 289.7 350 290.0 3568-ML 289.7 348 287.3 353 289.2 347 290.3 347 288.2 347 288.9 3469-AB 280.9 337 291.3 349 288.5 350 289.2 352 289.3 348 287.8 348

MEAN 290.5 347 28M.9 347 288.7 347 289.2 347 288.7 346 289.4 347S. . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..3.

13

AI •RCRAFT ENTRANCE 1

16 M-- ,

! I7

A C O

N T R

O L

EUIPMENT O MAIN TEST AREAROOM

AIR

- I.

INLET AREA<tjIN

AIR COOLED MUFFLERCAUGMENTOR TUBE)

DEFLECTOR

Figure 1: Plan view of a model T-10 "Hush Hlous'" jct cngine rioise supicssor.

14

128 Ti

N 0"° I-I Lctin 3 -;-

LNoa 154

1 4e or t iDud

135

1134234 Locaotiton 3

1 172 8 Location. I Locaion

Figure 2: Facilities in the area surrounding the Hush H~ouse at Otis AFB, and sensorlocations for Otis AF'B Hush House study. The y-axis (defined as 00 azimuth) isparallel to the axis of1 symmetry of the Hush House. Open squares indicate locations of

- ~acceptance points, and s•mall open squares indicate recording sites for this study thiat: coincide with acceptance points. CIc .ed squares indicate locations of other recordingi sites for this study.

11

9j0

85.

o80

75 toA11

<70 F I Oj6A(MIL

0 90 I00. 150 200 250 300, 35

A7IULTH- (de-rees'

Figure 3: Sound pressure lcvcls; recorded for the flush Hlouse acceptance tests'at Otis AFB.

16

SENSOR 6 OTIS AFG 5TUDYS- MI¶MGNITLIUE + - PHASE

ieee. _ 206.

160. 85.n )'

•--35 o LuUj°- 10.

Lu -155. w

o -275.

0.1 _______________________________-395.

6.1 1. 10. 100.

FREQUENCY (HZ)

Figure 4: Pressure response of the GDAS system.

17

OTIS BACKGROLUD• SPECTRA LOCATION 2 CHANNEL 2POWER z 5.2132E-e9 PSI*PSI/HZ

2 .E-033

0 .E-11

*18

oc 1 "E-I1~

6.1 !. • u. Iee.FRE JUEtiC'r 'Hz)

Figure 5: Example of ambient prcssure spectra recorded in the vicinity o•fthe Otis Hush H-ouse.

18

CaCOWI ;,?T C -,,IT• I IRA~

I L)WtOIL J AIIkfRMI M.P.

l * -. 02741[-68 P-, SO

S 43 -

~I.E I . .. . . .wWv ,*.1 I. Ia. 1 .

FILE)Ru 41(8 SIn Aircraft)~fXS

811 TCI- O tL 8IR~.., CC T PIP

0I. 1- I.0 10

Run #i (Jn Aircraft)

Figure 6: Infrasonic spectra for pressures measured at sites 3 and 7 during Run #1

with the Hush House operating an F-100 engine at military power (with the engines

in the aircraft).

1.9

GTCOa. TOTAL GKtCTA

FILE) 044*3.STT MISEW OF SnEUTS> 1Z[A1313 ChWME1. 3 AIRCRAT A.8

KNEWi~ - 0•.267EF-mr PSI S,•

i.E-I

I .E--4

POW -I&.-I PSI . .

L .1 I, Is. too.

Run #3 (In Aircraft)

Figure 7: Infrasonic spccira for pressures measured at sites 3 and 7 during Rut) #3with the Hush House operating an F-100 engine in afterburner (with the engine', in•the aircraft).

20

I

GTCfDJ * TOTAL PI CTOAFILE) ..0,44 S STI t O 9"EQ .gfTS 12I" 4 C.iW4.L 3 FRCAA T iI.P.

[PO R . %.3156 It_4 PSI SQ

I .E-1

6.1-I

I .ea-rIc t

I• 4 CJ~t~L. 7 aIII 2 H.P 1

I..'. IR

Run #4 (In Aircraft)

Figure 8: Infrasonic spectra for prcssures measured at sites .•and 7 during Run #4with the Hiush ltousc operating an F-IO0 engine at military power (with the enginesin the aircraft).

21_

rHi_-

GTC... TOTAL ,P( CTRFILE) O4*I .STT r11( OF" SEQWMTS, 1I

"LK 5 QIWNU 3 A Raw , N.P.[PON"I . e.54"X-q PSI so~

! .C-lO .

I ,E--tI I

0.1I. 11 . Ito .

rCi•cC, TOT-A& .C-TcRiFIL E I OWC .STT fjW OF S EGMEHIS> 12

"I I CHANNEL 7 AIRCRAT M.P.PUOC * 5'aie'-.1 PSI tSQ

I ,-t-1

I. -I

4,1 Ia.O IlO

Run #5 (in Aircraft)

Figure 9: Infrasonic spcctra for pressures measured at s•itcs 3 and 7 during Run #5with the flush House operating an F-100 engine at military power (with the enginesin the aircraft).

22

GTCOW, TOTA•. GKCWTA

* 1.4742[-.,? PSI SQ

I .- 47

E-10

S.1[ -qI~

4.1 I. to. 16 .

rI U> ORC".SITY • S"3M•T$ 12RL1 4 CHNE ? Alllec T A.S.

6.1 aI 10. 1Wll.

Run #6 (In Aircraft)

Figure 10: Infrasonic spectra for pressures measured at sites 3 and 7 during Run #6with the flush House operating an F-100 cngine in afterburner (with the engines inthe aircraft).

23

lGTf*C, TOTAL SECTR

FILE 44> 7,SrY hI OF" 9COVIsTS) Q2

P 3.976ft-M PSI SO

I .E- 1

1. . 49. 490.

GYC-QHC TOTAL SPECTRAFILE) OM4H7.STT ?%AMR OF S. •.Nr6"> 12

RUNl 7 CMN4#1E4 7 EM0329 M.P.POWR - 9.3)44*W- PSI SO

1 1-09

4.1 .

Run #7 (Test Stand)

Figure 11: Infrasonic spectra for pres.urcs measured at sites 3 and 7 during Run 47

with the Hush House operating an F-100 engine at military power (with the engines

installed on a test stand).

24

F ILE> OW4CW.ST? MBFR OF SEcDENTS) 123A C B C1EL 3 IJNlM M~. p

P06ER 3.6044E-EM PSI SO~

A-49

I E5-1i

6.1 1. Ia. Too

FILE) 0$44C45.STT t9Ij4W OF SEG(?4Tý1 12"RJ 0 CJ-BttCL ? EGIlK hp.POWER - Q.M I -40 PSI SQM

E-f-

E-09ý

I.S. 1 too.fRE~.tcy (4z)

Run #8 (Test Stand)

Figure 12: Infrasonic spectra for pressures mecasured at sites 3 and 7 during Run figwith the Hush House operating an F-100 engine at military powcr (with the enginesinstalled oil a test stand).

25

GICOWC& TOTAL 6PECTRAFILE) O44C9.Mrl INDEN O SENTS. I2MIN 9 CHAWBL 3 M114 A.8.

POE 9.1"M-4S P~SI SQ

I E-4

4. .1 toISO,

GTCONCs TOTAL EPCTFFILE) O.4C9.STT MGM OF CQ'EMTS> 1•AM 9 CKVNI. 7 &M4Irt 4.2.

P • E.44-W' WlSl SQ

.E-47

I .t-,E•

C SI. 1. l[s .

Run #9 (Test Stand)

Figure 13: Infrasonic spectra for pressures measured at sites 3 and 7 during Run #9with the Hush House operating an P-100 engine in afterburner (with the enginesinstalled on a test stand).

26

RUN . ,.v1ME1 3 *CKrkT nM.PPOE3 ?.1eW-m PS1 vo

I .E41g

,kv

I. I . IA. hvAS.

GTcCOK ? TOfAL SPECNAFI LE) U4CII.•T1, PRWU Of•[ O4HIS' 1?

LUMI II CWF^fk 7 AINCWT M.P.

POE - 36 ftPIs

It.E -0i9

4.1

0.1 I, LA•. ISB.

Run #11 (In Aircrafl)

Figure 14: Infrasonic spectra for pressures measured at sites 3 and 7 during Run#11 with the Hush House operating an F-100 engine at military power (with theengines in the aircraft).

27

I•GTC 1TOTOl. SPWCTRA

I ILE) G4FI2.ST mix.eE or SEGMUMTS, 2ZRI 22 W-ICI. 3 A2l? M.P

I .E-If

POER 0.17 1. O

QGCOI TOTUAL. SKCTR

FIL I f'* i9- T , ,LM OF .. ..3 1" t~12 CKAMNEL 7ICW M".1

POE .83, -so PSI SO

I.E-a9

I.E-Ig

t,E-11 .

Run #12 (In Aircraft)

Figure IS: Infrasonic spectra for pressures measured at sites 3 and 7 during Run#12 with the Hush House operating an F-100 engine at military power (with theengines in the aircraft).

28

I 000r-8Site 3

* R I (MP)- AC* R3(AB)-AC

* R4(MP)-AC* R5(MP) -AC* R6(A8)-AC

0 R7(MP)-TSO R8(MP)-TS

I R9(AU)-TS* RI I(MP)-AC

I .O00E-91 *'RI2(MP)-AC

aI10

Frequency (Hz)

Site 7

i O00E-RMP)-AC * R3(AG)-AC

* R4(MP)-AC

* R5(MP)-AC

* R6(AB)-AC

0 R7(MP)-TS

o RU(MP)-TSW R$(AB)-TS

0 RI (MP)-AC* R12(MP)- AC

bI O00E-94

10

frequency (Hz)

Figure 16: Comparison of 5, 10, and 20 Hlz spectral values measured at (a) site 3 and(b) site 7 for engines operating at military power and in afterburner (in aircraft andinstalled on a test stand). Vertical axis is power spectral density in PSI 2 /Hz, andhorizontal axis is frequency in Hz. Open symbols indicate engine runs at militarypower (MP), and closed symbols indicate engine runs in afterburner (AB). Diamondsindicate engines tested in aircraft (AC). and squares indicate engines installed on atest stand (TS).

29

OTIS

Y0

0

0

0Q

S -X

0

20 M

Figure 17: Configuration of array used for FK analysis. The y-axis (defined as 0'azimuth) is parallel to the axis of symmetry of the Hush House.

30

OTIS SHOTO2O-P AMP 9.13E-01 PASr- ' " -, I " i i 1 I i l "

051 1 144- 1 1114 1141 II

06 -

0 7 4 *-4 1 ''--- 4I-- - - - - - - -4 - - - -4 - -

091

10

12

13

14

"-F"-4'4-I I I | t I I ±-4 ' -±- ,I--- ,•+ - - I' I I----4-I-4--4•---4-±-

0 0C 0 498 0 997 I dq5 1 .993 2 492 2.9901988 176 10 14 11 5 TIME (S)

Figure 18: Signals recorded from a small explosion detonated approximately 4.1 rnhchind the exhaust deflector and essentially at the ground surface.

31

SHOT02 OTIS 25HZ

VEL 0.338 AZ 287.6

FREQ 25-00

5 0 .0 /1--'r 0,--rr

50.0 I' c

-0.0

-100.0 -50.0 0.0 50.0 100.0

K(X)

Figure 19: FK analysis results for the signals shown in Figure 18 for a bandpasscentercd at 25 H~z. the approximate spectral peak of thc detonations.

32

SHOT 2 - ECHO

VEL 0.342 AZ 277.2

FREQ 25.00100.0

50.0

47

-50.0

-1 00. 0-100.0 -50.0 0.0 50.0 100.0

K(X)

Figure 20: FK analysis for a distinct ccho obscrvcd in thc signals shown in Figure18, The echo arrives approximatcly 0.6 scc after the dircct acoustics.

33

OTIS HUSHO60-P AMP 4.92E-01 PA

05

06 .....

07

10 - - 1

44i4i lii il llt111111

12

14

15

0.0 2.0 4.0 6.0 8.0 10.0 12 01988 169 18 40 13 0 TIME (S)

Figure 21: TAypical data collected during Hush House operations with the engine inaircraft and in afterburner.

34

i

HUSI-806 STACKED

-K -

Q.

•10

1- a} p . . __-J•_J_.... •--__ _ _ J I II .L_

Figure 22: Mean spectrum derived from the data shown in Figuyrc 21.

35

/\II .?

I0~ 10

HUSH06 OTIS 10HZ

VEL 0.344 AZ 290.5

FREQ 1000

/

-30.00

./ /

-60.00@

-60.00 -30 00 0.00 30.00 60 00K(X)

Figure 23: The rCsults of FK analysis for thc data shown in Figure 21 (at afrequency of 10 Hz),

36

a L.00W- 8 .

9.00t-9S HZ

0.0001-9

7 000f-9

6 000£-9

0 AV(MP)5 000E-9, o AV(AB)

4.000[-9,

3 0001:-9

2 000•-9

I OOOE:- 950 60 70 80 90 1W0 110 120 130

Azimuth (dog)

1.400-E-8

b 10Hz1 200E-8

80001-9r AV(MP)

0"-9t 0 AV(AB)

4 O001--9

2 000E-9.

50 ,0 70 80 90 100 110 120 13o

Azimuth (deog)

C i.800(-a 20Hz

I.60O1-G

i.400(-O

I 200E-6

1.Oo01-e * AV(HP)o AV(AO)

8 0001-9Q

400C1-9

2 00O-9 "

S60 70 80 90 100 110 ,20 IS0

Azimuth (dog)

Figure 24: Radiation pattern of the Oqis Hush House at frequencies near (a) 5 1|z, (h)10 tlz, and (c) 20 Hz. Vertical axis is power spectral density in PSi 2/lIz, and horizontalaxis is azimuth in degrees. Closed circles indicate engine runs at military power, andopen circles indicate engine runs in aftcrburncr.

37

09

COLLwu

C-C6

0 0 7

-s -00

II2C

Ej

Q~ -: '

CD U

cu~

- 38