Three experimental studies of long range sound propagation carried out or sponsored in the past by NASA are briefly reviewed to provide a partial prospective for some of the analytical studies presented in this symposium. The three studies reviewed cover (1) a unique test of two large rocket engines conducted in such a way as to provide an indication of possible atmospheric scattering loss from a large low-frequency directive sound source, (2) a year-long measurement of low frequency sound propagation which clearly demonstrated the dominant influence of the vertical gradient in the vector sound velocity towards the receiver in defining excess sound attenuation due to refraction, and (3), a series of excess ground attenuation measurements over grass and asphalt surfaces replicated several times under very similar inversion weather conditions

Sutherland, Louis C.

English

NASA Technical Reports Server

N91-16683
LONG-RANGE SOUND PROPAGATION -
A REVIEW OF SOME EXPERIMENTAL DATA
Louis C. Sutherland
Consultant in Acoustics
27803 Longhill Dr.
Rancho Palos Verdes, CA 90274
SUMMARY
Three experimental studies of long range sound propagation carried out or sponsored in the past by
NASA are briefly reviewed to provide a partial prospective for some of the analytical studies presented in
this symposium. The three studies reviewed cover (1) a unique test of two large rocket engines conducted
in such a way as to provide an indication of possible atmospheric scattering loss from a large low-
frequency directive sound source, (2) a year-long measurement of low frequency sound propagation
which clearly demonstrated the dominant influence of the vertical gradient in the vector sound velocity
towards the receiver in defining excess sound attenuation due to refraction, and (3), a series of excess
ground attenuation measurements over grass and asphalt surfaces replicated several times under very
similar inversion weather conditions.
INTRODUCTION
Experimental data on long range sound propagation sound from three unique programs carried out
over the last 25 years that were conducted or sponsored by NASA can provide a useful background lor
some of the analytical models treated in this symposium. These measurement programs are very briefly
reviewed here to insure that the existence of these data may be more widely known to researchers in the
field of long range sound propagation. The sources of the data are identified for the reader who may wish
to pursue the information in more detail.
EXPERIMENTAL DATA ON PROPAGATION OF
LOW FREQUENCY ROCKET NOISE AT LONG RANGES.
On March 24, 1964 at approximately 1340 CST, the NASA George C. Marshall Space Flight
Center, in Huntsville, Alabama conducted a static test firing of a Saturn S-I first stage rocket booster on a
test stand for which the deflected exhaust blast was directed due north. This rocket consists of a cluster of
eight engines with a total thrust of about 1.5 million lbs. Seven minutes later, a static test of a Saturn F-I
rocket engine (a single chamber rocket engine with the same total thrust), was conducted on the same basic
test stand but with the deflected exhaust blast directed due south. Major results of acoustic measurements
conducted out to a distance of 15 Km along a line of microphone stations on a 45 ° azimuth line from the
test stand towards the city of Huntsville, as shown in Figure 1, were reported by Tedrick. 1 However,
most of the detailed results presented here are contained in an internal NASA Memo. 2 Also shown in
Figure 1 are the vertical sound velocity profiles measured in this direction at the time of each test f'wing and
the resulting calculated sound ray paths in this same direction. The sound velocity profiles differ slightly
in the first 2 Km but the resulting ray paths differ significantly. Based on a comparison of the ray paths
for the two firings, one would expect to see a greater refraction loss for the second test due to the greater
upward refraction of the sound ray for this test. As will be shown, precisely the opposite condition
prevailed.
Not shown here are the same type of sound profiles and ray paths for a 226 ° azimuth direction -
essentially 180 ° from those shown in Figure 1. The results were very similar - minor differences in sound
profiles and a ray path for the second test showing more upward refraction in this direction than for the
first test - again suggesting a greater refraction loss for the second test.
https://ntrs.nasa.gov/search.jsp?R=19910007370 2020-03-19T19:20:25+00:00Z
Althoughthetwo rocketboostershaveavery differentgeometry,theresultanttotal soundpower
levels and spectraarevery similar1 and,asshownin Figure 2, the directivities for the overall sound
pressurelevelatadistanceof 1000ft from theenginesarevery similarwhenthedifferentdirectionof the
exhaustblast for the two testsis recognized. In the directionof themicrophonepositions, theoverall
soundlevels of the two rocketenginesdiffer by about12dB at a 1000ft radius. Figure 3 showsthe
valuesof excessattenuationin octavebands,includinganyair absorption,for theS-I test,asafunctionof
octavebandcenterfrequencywith distanceasaparameter.It wasconvenient,for thisplot, to use1.6Km
asareferencedistancefor evaluatingexcessattenuation.Thedatashow,roughly, theexpectedtrendof
increasingexcessattenuationwith distanceandfrequency.Figure4 showsthe samedatafor theS-I test
re-plottedasafunctionof distancewherethevaluesof excessattenuationhavebeenaveragedoverpairsof
adjacentoctavebandsto simplify thedatapresentation.Figure5 showsthesameinformationfor theF-I
test.
However,it is not thepurposeof thisreviewto examinetheabsolutevaluesfor theexcessattenuationfor
eachtestbut ratherexaminethedifference in excess attenuation between the two tests. This is shown in
Figure 6 in terms of the excess attenuation for the S-I test (i.e., maximum lobe of noise along the
measurement direction towards Huntsville) minus the excess attenuation along the same line, for the F-I
test (i.e., maximum lobe of noise in opposite direction).
The excess attenuation along this same path decreased between the two tests, conducted only 7 minutes
apart. This decrease is most significant for a distance of 9 Km and is more dependent upon frequency at
this distance than at any other point. This decrease in excess attenuation could be attributed to a change in
sound refraction between the two tests. However, as suggested by the sound velocity profiles and
calculated ray paths in Figure 1, this effect would have been expected to be just the opposite from what
was observed - i.e., an increase in excess attenuation due to the expected increase in refraction loss for the
second test. An alternative hypothesis is that the decrease in excess attenuation could be attributed to the
effect of scattering by atmospheric turbulence. This scattering would tend to increase the apparent excess
attenuation in the measurement direction for the first test (i.e., remove energy from the main sound lobe in
this direction) and decrease the excess attenuation for the second test by adding back-scattered energy to
the weaker lobe in this direction.
This hypothesis, admittedly not proven, is consistent with the observations and with theoretical
predictions.3, ;_ Further research is needed to more fully evaluate and experimentally validate sound
attenuation by atmospheric turbulence. Practical applications include definition of correct excess
attenuation models for the directive sound fields of jet aircraft and long range warning sirens.
LONG-TERM MEASUREMENT OF EXCESS ATFENUATION
WITH REFRACTION
The second sound attenuation program was conducted at the NASA Mississippi Test Range over a
one year period by Tedrick and Polly. 5 The program utilized the pure tone siren/horn sound source
system shown in Figure 7 mounted on a 60 ft. tower to propagate pure tone signals at 40, 80, 120 and 160
Hz at distances up to 3 Km over a flat terrain heavily covered with a deciduous rain forest. Over 29,000
excess attenuation measurements were made over the one year test period. The results were correlated
with the vertical gradient of vector sound velocity from the source to the receiver as measured over the first
300 meters above the ground. Typical results for two distances are shown in Figure 8 in terms of the
excess attenuation at 160 Hz as a function of this sound velocity gradient. As for all of the frequencies and
distances measured, the data collapsed in the form illustrated. At any given frequency and distance, the
mean excess attenuation was essentially constant when the sound velocity gradient was equal to, or greater
than zero and decreased approximately linearly as the gradient decreased below zero.
The mean excess attenuation, Ao for sound velocity gradients equal or greater than zero varied
linearly with distance and systematically with frequency as shown on Figure 9 which is taken from Ref. 5.
Although the excess attenuation includes air absorption, the latter is a relatively small part of the observed
2
excessattenuationwhich is believedto bepredominantlygroundattenuation.Notethattheh-iterceptvalue
of Aofor zerodistanceis roughlyproportionalto frequencybut therateof increasewith distanceincreases
only slightlywith frequency.
For negativesoundvelocity gradients,Tedrickand Polly showedthat the slopeof the plot of
excessattenuationversussoundvelocity gradientincreasedlinearly with distanceand approximately
linearlywith frequency(seeFigure10).
While the abovepresentsavery simplified definition of the datatrends,it hassubstantialface
validity on the basisof the very large numberof measurementsinvolved and shouldprovide useful
benchmarksfor comparisonwith the latesttheoreticalmodelsfor groundattenuationin the presenceof
refraction.
Anotherresultfrom this longtermtestprogramwasthedeterminationof thestatisticaldistribution
in themagnitudeof focusingamplification(i.e., excessattenuationwhich is positive) correspondingto
soundattenuationlessthaninversesquarespreadingloss. While very likely a site-specificstatistic,the
distribution datashownin Figure 11,developedfrom tabulardata in Ref. 5, showsthat this focusing
anomalyincreaseswith distancefor valuesof theanomalylessthanabout15dB. Note that in this case,
thedatacoveramuchlongerpropagationrangeandindicatethat,on rareoccasions,anomalousincreases
in levelabovethatpredictedby sphericalspreadinglossof up to 30dB wereobserved.
GROUNDATrENUATION MEASUREMENTSFORINVERSIONCONDITIONS
OVERGRASSAND ASPHALT SURFACES.
The final testprogrammentionedherewassponsoredby NASA andis fully describedin Ref. 6.
Copiesof the full report may be available through NASA, Langley. The program involved the
measurementof groundattenuationoverasphaltandgrasssurfaceson, or next to, anaircraft runwayat
NASA'sWallopsIslandfacility. Thetestswereconductedwith anelevatedloudspeakersourcelocatedat
2.5, 5, and 10 meters aboveeach of the surfaces. For most of the tests, the weather conditions
correspondedto amild inversionconditionthatwasreplicatedseveraltimesfor eachmeasurementsource
elevation/groundsurfacecondition. Thebasictestgeometryandmicrophonearrayemployedis illustrated
in Figure 12. Note thatat onedistance(225meters),microphoneswerelocatedessentiallyat theground
surface,andat 1.2and10meters. At 450 meters,microphoneswerelocatedat 1.2and10meters. (Note
thatfor thetestsovergrass,asmallstripof asphaltexistedalongthe"grass"pathbetweenthe450and675
m positions.)
Along with the excessattenuationmeasurements,the meanweatherconditionswereevaluated
extensivelywith meteorologicalinstrumentationon7 and10metertowersanda captiveweatherballoon
repeatedlyraisedto andloweredfrom aheightof 100m. For thesakeof brevity,only asmallfractionof
theavailableexcessattenuationdataareshownherein Figure 13. Thefigure shows,for two distances,
the two surfacesandthreesourceheights,thearithmeticallyaveragedexcessattenuationfor onethird
octavebandsof noisefrom 50 to atleast3200Hz for thefour to six replicationsof nominallyverysimilar
inversionconditions. Eachexcessattenuationmeasurementwasbasedon anenergyaverageof sound
levelsovera 15secondperiod. Thestandarddeviationof theexcessattenuationvaluesoverthefour to six
replicationsfor eachmeasurementconditionandfrequencywasnormallymuchlessthan1.5dB.
Theresultsshowthecharacteristicincreasein excessattenuationdueto groundabsorptionat frequencies
in therangeof 125to 630Hz dependingon thesurfaceandmeasurementdistance.Theexcessattenuation
dataareaugmentedby somevery limited measurementsof surfaceimpedanceemployingthe simple
techniquedevelopedby Piercy andEmbleton.7 Thus,thesedataprovideanother,and,in someaspects
morecomplete, set of measurementsof groundattenuationin the presenceof documentedrefraction
conditionsthanhadbeenavailablepreviously. Theyoffer auseful setof measurementsfor comparison
with correspondingtheoreticalmodels.
CONCLUSIONS
Resultsfrom threedifferent NASA conductedor NASA sponsoredtestsof long rangesound
propagationhavebeenvery briefly reviewed.Theobjectivehasbeento identify theseuniquesourcesof
data,two of whichareover25yearsold, for thebenefitof modelersof longrangesoundpropagationwho
maynotbeawareof theirexistence.Theyofferpotentiallyusefuldatasetsfor comparisonwith theoretical
modelsfor theevaluation,respectivelyof: scatteringattenuationby atmosphericturbulence,long range
groundpropagationunderawiderangeof definedrefractionconditions,andgroundattenuationover two
surfacesfor nearly identical mild inversion conditions. As further advancesaremadein theoretical
models,newandmoresophisticatedmeasurementswill berequiredto validatethetheory.
REFERENCES
°
.
.
Tedrick, R.N. "Acoustical measurements of static tests of clustered and single-nozzle rocket
engines," J. Acoust. Soc. Amer. 36:2027-2032, 1964.
Polly, R. "Acoustic levels resulting from F-I Test TFW-016 and Saturn Test SA-19,"
Memorandum R-Test-ICA-73-64, Acoustics Section, Components Instrumentation Branch, Test
Laboratory NASA GCMSFC, Huntsville, Alabama, April 16, 1964.
Sutherland, L. C. "Scattering attenuation of sound in the lower atmosphere," J. Acoust. Soc.
Amer. 49:129(A), 1971.
. Brown, E.H. and Clifford, S.F. "On the attenuation of sound by turbulence," J. Acoust. Soc.
Amer. 60:788-794, 1976.
. Tedrick, R.N. and Polly, R. "Measured acoustic propagation parameters in the Mississippi Test
Operations area," NASA TM X-1132, August 1965.
. Sutherland, L.C. and Brown, R. "Static tests of excess ground attenuation at Wallops Flight
Center," NASA CR-3435, June 1981.
. Piercy, J.E. and Embleton, T.F.W. "Excess attenuation or impedance of common ground surfaces
characterized by flow resistance," J. Acoust. Soc. Amer. 65(S1), $63(A), 1979.
4
Figure 1A. SoundVelocity Profile Figure lB. Microphone Positions
ORIGINAL PAGE IS
OF POOR QUALITY
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Figure 2.
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Directivity of S-I and F-I Rockets
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Figure 3. Excess Attenuation (S-I) Versus Frequency
REFERENCE DISTANCE = 1.8 KM, NO CORRECTION FOR AIR ABSORPTION
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12


Long-range sound propagation: A review of some experimental data

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