JASON - Insonification for Area Denial

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Insonification for Area Denial

Study Leader:M. RudermanS. Treiman

Contributors Include:R.Garwin

January 1998

JSR-97-120

Approved for public release; distribution unlimited.

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Insonification for Area Denial6. AUTHOR(S) 13-988534-04

M. Ruderman, S. Treiman, R. Garwin7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATlON

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13. ABSTRACT (Maximum 2D0 words)

This report examines concepts for area denial by use of focused sound sources.

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Contents1 INTRODUCTION 12 RANGE 33 EFFECTS OF ULTRASTRONG SONIC INTENSITIES 54 INSONIFICATION LIMITS 95 COUNTERMEASURES 13

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1 INTRODUCTION

A need has been expressed for technology to deny large areas and fa-cilities within them to an invasive group in a way which does not kill orpermanently injure members of that group. Desired features for systemswhich might accomplish this include

1. area coverage which may extend to as much as 1 km2 ;2. persistent coverage (certainly many minutes, perhaps months);3. capability for use in unfriendly areas from a remote transportable plat-

form (e.g. a helicopter).

The production and maintenance of intense local air disturbances has beensuggested as a way of achieving this. Two methods have been proposed forinjecting a large amount of energy into air and rapidly directing that energyto another location.

(a) Exploiting an essentially incompressible type air motion, generallythe creation and propagation of vortices (e.g. "smoke rings").

(b) Using the compressibility of the air to store and transport thatenergy as sound or shock waves. There has been interest andsupport for using very intense insonification of an area to accom-plish the goal (see Figure 1) and we shall consider below only thatmethod. l

1A main contributor has been Scientific Applications and Research Associates, Inc.(SARA) sponsored by the Advanced Research Projects Agency (ARPA) and the U.S.Missile Command. We shall refer to SARA's Phase 1 Final Report "Selective Area/FacilityDenial Using High Power Acoustic Beam Technology" (10 March 1995) as SARA in ourdiscussion.

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t--:l 500 meterAltitude

Enemy Bunkerand WeaponsComplex

2.0 Kilometers

Zones of Non-LethalIncapacitating AcousticPressure Levels

Figure 1. Concept for Aircraft Based High Energy Acoustic Beam Incapacitation WeaponDenial of Access or Activity at a SelectedWMDFacility. From SARA, loc. at. (WMD = Weapons of Mass Destruction)


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2 RANGE

The range of high frequency sound is severely limited by the viscosityof air. Sound intensity zdrops with propagation range r according to

i(r) = ioexp( - ex r)j (2-1)in reasonable approximation

(2-2)Here

1J dynamic viscosity of air (18311 poises)po density of air ("" 10-39 cm-3)Cs - speed of sound ("" 3 . 104 cm 8 - 1)v the sound frequency.

Then for sound in air(2-3)

with v in Hz. Thus for a desired projection range of a kilometer or so theinsonification frequency should be limited to

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3 EFFECTS OF ULTRASTRONG SONIC IN-TENSITIES

Sound intensities are generally expressed in decibels (dB):Flux of sonic power in dB= 10 10glO ( 0 16 i -2) ,1 - watts cm

where i = (t1P)2 / PoCs and t1P is the pressure change amplitude. Then thesonic pressure in dB = 20 l o g l O ( 2 - l o - 1 ~ : t m o s ) .

Some effects of typical audio-frequency intensities (in dB) areTable 1.Effect or Source dBThreshold for hearing 0Normal conversation (at mouth) 65Threshold for audio-pain 120Next to a je t engine (Figure 2) 160Reports of lethality to small animals 170Next to a rocket engine exhaust (Figure 2) 180

The published literature about effects on humans of relatively brief exposures (several minutes) to extraordinarily intense sound waves (frequencies1I < 10 Hz) seems largely anecdotal.

According to SARA

"Strong incapacitation effects will set in between 140 dB and 150dB for nearly all frequencies ... Potential lethal effects will setin above the 160 dB to 170 dB regime for sustained or modestexposure."

Certainly in the 1I 0 limit the pressure change f::!.P for the insonifications of Table 1 are easily tolerated. For example 150 dB corresponds tot1P I'V 10-2 atmos., the pressure at the bottom of a bathtub filled to about

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dB200

180: ) ~

Gia; 160E

... rockets...,

I rIIIis jet enginesa;2-I:::: 140)0a..

"'0c0 120Cf)

100

300 700 1500 3000jet velocity (mls)

Figure 2. Acoustic power levels (from Chobotov & Powell: 1957 Ramo Woolridge Corp. Rep.EM-7-7). ocket; T turbojet (afterbuming); A turbojet (military power)

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4 inches of water. Variations of such pressures on a time scale of order afew tenths of a second or longer are clearly inconsequential. (The maximumnet force on a body exposed to such 6.P is comparable to the redistributionof external force difference on us when we lie down or roll over.) Clearlyimportant effects from insonification must begin only at higher frequencies.A summary given by SARA of reported effects at higher frequencies is shownin Table 2.

Table 2. Reported Effects on Intense Sound.Acoustic Frequency Intensity Reported Biological Effect7-17 Hz 110-130 dB Bodily discomfort. Intestinal pain'" 10 Hz 11(}-130 dB Severe nausea175 Hz 120-140 dB Severe nausea. Shutdown of air base196 Hz 120 -150 dB Internal organ damage103Hz 140 dB Incapacitation of personnel2.6 KHz 120 - 150 dB Permanent physical disability&.30 kHz 120 - 160 dB Cavitation, thermal burns, fatigue,lethality of animals

The SARA report gives the detailed frequency dependent response ofa microphone inserted inside a particular scientist's mouth to an externallyincident sound wave (Figure 3). It is indicated that this response reflectsthe frequency sensitivity of the scientist's internal respiratory structures tomonochromatic insonification. The sharp peaks' half widths and separationsare much less than the internal structure scale variations among people (certainly greater than 10%). Thus even if single frequency might give a sharplyresonant response for some individual, that same frequency would not doso for other members of a group. Only relatively broad frequency intervalswould be expected to have a robust significance in designing an insonificationstrategy. In this sense while one may accept the claim of SARA that "ourown analysis of human coupling physiology suggests that specific frequencieswithin ... 100 Hz to several kilohertz may have optimized effects," thoseprecise frequencies would not be predictable because they would vary greatlywithin a group.

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00

25 i TMPlThroat Open

20

c0 15uc:lU.Qju;c 10I-

5

100 200 300 400 500 600 700 800 900 1000Frequency (Hz)

Figure 3. Acoustic absorption spectrum for the human respiratory tract and sinus pas sages, indicating possible frequency bands forenhyancement of high energy acoustic weapon areaIWMD Facility Denial. From SARA, loco dt .


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4 INSONIFICATION LIMITS

We consider first the sound radiated by a spherical surface of radius Roscillating at an angular frequency w = 27rV and maximum radial speed

(4-1)(M is the Mach number.)

The maximum sonic power from this monopole acoustic source, generally the most efficient of all sonic radiators, is

(4-2)where A=47rR2 is the surface area, >. = c/w, and>' R. When R> >.

(4-3)For higher multi pole emission Equation (4-2) is reduced by an additionalfactor (R/>.)2n with n the multipolarity, but Equation (4-3) is unchanged.

The goal of a typical area denial geometry is sketched in Figure 4 from aSARA final report: a sound source about 2 km from a targeted area insonifiesa 1 km radius region to various intensities; insonification reaches 160 dB in thecentral region, and drops to 140 dB about half-way out. The total depositedsonic power would be of order 300 MW (rv 10-2 watts cm-2 x 3 x 1010 cm2)supplied from an acoustic source on a helicopter whose horn area may be,roughly, of order 1 m2 Is this a plausible goal? This would, of course, be ahuge sonic power output even for 100% efficient conversion of input energyinto sonic power. (Entire electric utility power plants are typically morepowerful.) How much acoustic power can be emitted from an acoustic sourceof effective area several square meters? Optimal power cannot come froma compact source for which >. > > R. so we assume >. < 102 cm, or v > 50Hz. The appropriate bound to emitted power is then Equation (4-3): the

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......a

High PowerAcoustic Array

Standoff Irradiation Configuration

WMO Facility

1 > 160 dBLethality for(t > 10 min.) 1 > 150 dBOnset of internaldamage (t > 5 min.) 1 > 140 dBOnset of strongincapacitation(t>5min.) 1>120dBincapacitation(t > 5 min.)

Central Placement Configuration

FIgure 4. Basic concept for ground based standoff and central illumination denial of a WMD facility by a high energy acoustic weaponsource. From SARA, loc. dt .


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radiated acoustic power from any effective diaphragm area should satisfy theinequality

(Area)Power < 3.101 M2 2 watts.meter

I f the Mach number M for the real or virtual diaphragm exceeds unity, shockwaves will be emitted giving a very broad sonic frequency range. For anassumed monochromatic emission, M < 1, so that radiated acoustic powerflux would not exceed

po c: f'V Patmos . Cs f' V 3 Kw/cm2over the diaphragm area, and even this would require an astonishingly largediaphragm motion. The diaphragm displacement2 would be = M;. f'V =50( lO:Hz) cm, a truly enormous oscillation amplitude for almost supersonicmotion. Fuel weight, conversion efficiencies for fuel burning to sonic power(10-2 seems a plausible goal), limits to source size, beaming inefficiencies etc.suggest very much more modest goals for area denial, perhaps even less than10-2 km2 at considerably less than 140 dB.

2In practice such motion, if achieved, might not efficiently transfer expended powerinto a monochromatic sound beam because the Reynolds number of the induced air flowvp A 1/2 c p A 1/2 ( A ) 1/2Re '" () '" S () ' " 105 _

ry ry emis so high that strong turbulence would be expected.

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5 COUNTERMEASURES

Suppose, however, that a very much smaller area than 1 km2 has beeninsonified to the intensity needed for denial (probably by sound waves withv 102 Hz because much lower frequencies are less efficiently produced anddirected). How might a determined area occupier defend against such a sonicbarrier? The answer appears to be - rather easily (quite apart from simplyshooting the sound source or a hovering helicopter which carries it).

We are all familiar with the very great reduction from outside audiointensities achieved by closing a window. Some commercial airliners mounta pair of jet engines adjacent to the passenger compartment. Despite whatmight be intolerable sound levels outside the fuselage during full power enginethrusts, the noise level for a passenger separated from the je t by a thinfuselage or a window is generally not even uncomfortable. This suggeststhat sound intensity in such a circumstance could be diminished by at leastseveral tens of dB. Completely surrounding individuals by effective sonicscreens can be accomplished in several ways - from employing simple heavyplastic envelopes to just climbing into a vehicle and rolling up the windows.To be somewhat more quantitative we will consider some idealizations ofsuch sound barriers.

Simplest of all is representing the separation of a targeted individualfrom the acoustic source by the interpolation of a thin infinite (i.e. no edges)wall. If the wall is free to move in response to an incident sound wave thefraction of the incident energy which passes through the wall is

(j2W2ITI2 = (1 + 4 2() 2 2 ) -1 ,cos PoCs (5-1)where(j - surface mass densi ty() angle between the incident beam direction and the surface normal.

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For u = 3g cm-2 and v = 103 Hz a wall would transmit a fraction

of the incident intensity. There would then be a > huge 50 dB reduction insound intensity behind such a wall, rendering any plausible incident insonification innocuous. 3

In the above idealization only wall inertia was included in the responseof the wall to the incident sound. There are changes when the separationwall supports structural resonances. We may represent a relevant one bymounting the wall on a spring so that it has the resonance frequency Wo forwall motion in the normal direction. Equation (5-1) becomes

[ U2(W2 - W2)2]-1ITI2 - 1 + 0- 4w2 cos 2 () p ~ c ~ (5-2)The frequency averaged transmission for a broad band of width D.w centeredon the angular frequency Wo is

ForITI2 "-J 1r cos ()Pocs 2D.wu

D.w wo/2 = 1r. 1038 - 1ITI2 ,...., 4 10-3

(5-3)

Plausible insonification is again reduced to an easily tolerable level butby a smaller factor than when Wo = o. We note that for frequencies w > >

3Results could be quite different behind a finite width or height thin shield. Soundcan then also be propagated to behind the shield by edge diffraction rather than onlyby transmission through it. This diffraction is generally maximized by a "sharp" edgein which the shield is terminated over a distance < < >.. The intensity behind the shieldwould then fall off with distance(s) from the edge to about a fraction )..j27rs of the incidentone. This fraction can be changed by more gradual shield edge fall off, curving the edge,etc. Unless s >. (50 em for v = 10 2 Hz) edge diffraction would be expected to be moreimportant than transmission through the wall.

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Wo Equation (5-2) of course reduces to Equation (5-1) but it gives a muchdiminished ITI2 in the small w limit where

(5-4)which approaches zero.

In the more complicated cases of greatest interest the interpolated infinite wall is replaced by a closed one around each targeted individual. Analysis of the resulting reduction are similar to the infinite wall case. There canbe many structural resonances as well as many in which changing pressuresfrom standing interior sound waves give "spring" responses. Again we wouldconclude that satisfactory protection could be achieved from being within aclosed shield (e.g. a vehicle with closed windows or specially designed mobileenclosures). Any potential worrisome structural resonance frequency couldrather easily be shifted and/or damped.)

At very low frequencies the important sound wave induced interior pressure fluctuations may be caused by airflow into and out of a "vehicle" throughthe many seams, cracks, and small openings in the enclosure, certainly thecase in an automobile. (In the v 0 limit outside pressure increments arecompletely matched by interior ones so the effective ITI2 1). The responseinside of a thin container of volume V and N holes, each of area A., to anexterior sound wave may be analyzed as a Helmholtz type resonator. The(off-resonance) ratio of low frequency sound intensity, (pressure change)2,inside such a resonator to that just outside the holes is

(5-5)

with "V t + b: t is the thickness of the container wall and b is essentiallythe minimum hole dimension. (For a round hole b=0.8 A.l/2.) For a vehiclewith rolled up windows, V f" V 5 m3, N A. f" V 10 cm2 and b + t f" V 1 cm the

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characteristic Helmholtz resonance frequency

(NA) 1/2

VHR =;; vi '" 7Hz (5-6)Such frequencies and smaller ones would not be expected to be effective

for area denial by insonification because a) sound radiators are so ineffectiveat such large wavelengths, b) no unusual human response resonances areexpected at such low frequencies because we know there are none in thev --+ 0 limit, c) whatever resonance might exist, the VHR would differ greatlyamong vehicles or within any single one from slight adjustments. At muchhigher frequencies the Helmholtz resonator model for holes, cracks, etc. givesan effective

(5-7)In summary, from the above analyses and, above all, our immediate ex-perience in moderating loud audio noise, realistically achievable large areainsonification does not appear promising as a way of denying an area to peo-ple who can enter and traverse it in vehicles or simpler mobile enclosures aslong as they take simple precautions.

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JASON - Insonification for Area Denial

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