SECRET AGENT

rdEAT mE mE SI]BIIA O.UES.YOU WZ

.. -sonething caus-ed you and your client to finally auspect that he was bugged inthe first p1ace. some compelling overt event, or an accurulation of individualstraus -tha_t finally broke the caoeLrs back took p1ace. I,Ihether maJor businessproposal. u'ir strategies, technlcal. approaches, or iosting information -uas leaking,your client seened to lose nore than his share of businEss oppoitorrrti.s by veiysnal1 marglns. Did thes-e lossee stop? or did the nargins wraei araoaiicar ryz Diayour clientts truly profound ideas cease appearlng in Jonpetitorsr piopl"at"z

Did outside business writerst and financial analystsr lnsight into you,clientrs affalrs evaporate.after bugs in the board "oo,

,eie neutrali;ed? Did your:11:i!_gb:9lve stranse behavior begin to develop in certain k;t-;;;iry.us? 'Didvendors' sa_]esmen or your own saleg reps nake_ radical changes in iheir callingpatterns? Did some of yguq clientts executives begin to extritil ]rustr"tion oithootany obvious. cause? -Did some of then begin pushlng for reassignnents ororgaaizational changes?

IOU'II T{EVER RECEIVE IIEAT YOU REAIJ,Y SF.F.( _ 1OOZ .ASSI]MICE

..._ ..l.-ah" -I9: "nl {9!. cLient would far prefer ro have concrete asE}urance that everyDug was tound' eYery leak stoppered, and every neakaess shored. youtl1 never seianl-of this come to pass. The closest one in ftrty of you will "r". "or" i.s if theTSCM operator was able t-o track the bug Eo iEs perpetraEor. only then does yourattorney have auy chance for civil or crioinal proseiution.Finding bugs is at best a.9oz proposition. Tracking one is perhaps a 302 dea1.rhus, pure statistics indicate that approxiEarely I bug Io 4 wtlL'te aiscovereo anacorrectLy laid back at its owuerts feet. civil - j-udgpints in such

".""".r" maybe 1in 10. This oeans chat you have l chance in 40 oi o6t.ioio! ""ii"t""ii", in court.TI'NNI}{G A BUG TO ICIN ADVA}NAGE

,,,, t}_a-!?"gh .we have strayed a bit from evaluaring your ES expertrs perfornance,'rr oeverop thia one last tangen_tial thought first. Instead of removing the bug,I::r--"1_.:it E?{ walr ro consider.Leavi.n ii iu place and feedin! iilrausiUre Uutrarse intormatlon fron time-to-tine. This approach has the distlnct Ldvantages ofnot info-rni[g your opponent that he was uncovered Iby all of tris tugs going"dead]and at Lhe sane tine luring hin into coetly nistakeJ blsed upon false-data.

lXE Cl.ffir IOU'IJ, SEE rc A BESI AIISTR

. The_ only way to evaluate a TSCMT' effectiveness is to continue to observe thesubtle factors that caused you to ruo one in the first placl. ihi; i; why a frankand candld self-exanination wtl1 be the closest thing to a best uo"r.i yoirrir ."".receive. other than auccesa in court, your11 never hale a better basis.A SPECIAI. RESPOI{SIBITITT TO YOIn CLIIIIT

lack of conclusive proof is an unfortunate fact of TSCM life that could reflectupon both -the prlvate operative and the TSCM specialist. Therefoie, whenever youand.your client reach a pro-TSCM decision, you beLr a special responsi'bility tt Eak;certain he understands precisely what lies in the oifing. The consequences areobvious, possibly leading to. litigation if the client feel-s that he has been rakenadvantage of or worse yei, cheated-. To say nothing of damage to yo* iepotatroo.

PTJBTISHED BY EDM{ PRETSS 36

SECREf, AGENT

EI.EGTRONIC SIIRVETIJ.TICE DEVICTS AND STSTEMS

II{ OVBVIffI

lJe have established that, there are active and passive devices, also carledbugging and defensive or.anti-bugging d""i";" Active a"ui""r-"r. -,ild to provideaudio, visual, or location.r"1-Jiri.o"ll-'tt"t._are, of course, speclalizedconbinaLions and clever operarional "ppii".iioo" of rhese'catlc*i;;.'' '

Passive devices arethis section is conpleted,have taken close looks at

intended to detect "ld protect against acti.ve usages. whenwe will have considered all oithese categories and willthe perfornance of representative nodels oI each.

ECI{ ES RET,AIES IUE REUAINDM OF IUIS VOTTIME10

section xr will address visual surveillance and rcirl subsequently discuss theDarriage of optical devi..ces lrith renote aud.io d.evices. section xrr treats variousforns of "lie detectors" tor tnose ;.1;.;;..atives ,.i.h needs and lnterests inthis rather conrroversia_l area. In Section iiif, ,r" rri1l EelI you of sources andevaluations involvinc ES dev.ices. ri""iiy, -ii-'s".ai"rtv, ,l ili r"ioot at a fewall-source Iboth Voluie I and Votune iii-"-J*riio".rcRMAT AI{I) OBJECTTYES I}I DISCI'SSING ES DEVICES

- The subject of ES riterally extends fron DC to LighE_ and embraces tech,ologiestar bevond all but a specla.iisrrJ .orli"n.orion.' -tecrroi".i-Iiprr'"ticationnotwithst-anding, there needn?t

.be- anl topic so difficult but rhat it can bepractical ly explalned to the uninltiated. ri.i" i" oo. p.in.ip"i "u:."iir".

APPROAffi AI}IS AT BOIH NOVTCRS AITD EIPERTS

. The approach ftve implemented in SECRETr--nexperienced ES laymen as wel l as t,o talkThis is a particulari y ambit j.ous chal lenge,

AGENT undertakes to coach relativelyon a technical plane with ES experts.but I feel that rhis approach will besrsuit the fu11 spectrum of needs. Advanced ES operators r+.i1 1 find kernels of value

ll^_.T.j^Y'::I:1-li::l*i.o", .,a .,tir 1"*.;i;;;;-i.l'ir' .'#r", ^"".'Ji'j" H"l:+::they choose aloas the way. witi "i"ay-""a;;;;;r;,';ilrlr""'rEi'piil:::: ::TI?:I;":.1, ur:::*X ,?:r:,?1d -?dranced princlples of rechnolosy and operari.on of;::1"""t11":.^1"J.';",..y",:l::,:1.?r".r.ii--i",';;-.-1;:;T""i#'"Jff ;:l,L:;

::;j".,.J:1",:.:"J::,1- ":.j:F.i"did, r"i i"* ;i,;"liiii.iTJ"i;=:?""rT:i.rT. *if::ff '".y"*""."". j:1,1*^L1"i1r__,u=i*:od".;;;ir;;"J"-inl,?iin".t"r.",!liXi"" jXsuch as power requlrements' physical size, and rhe need for Jfi;'"Xl'rliJ"J.

You will gain powerful insight into auch EaEters as deterninlng the best

SECREf, AGEhIT

AN ADVANCB T{ORD OF THA}IKS AITD CREDIT

alnost all new audio devlces and performance profiles that we are about todiscues are the result of active courtesy extended to te during ,y r""""a"t, efforts.whiLe nuroerous sales outlets claim otheiwise, only a ser.ect t"I, "cto.iry design andI?n:facture_ rheir. orm products, or have then oEMid to Lheir

SE@Ef, AGEIIT

ur@oPEoNE - A 1"Ar{Srm@A microphone is a transducer. Transd.ucers are devices that convert one form ofenergy into another. a microphoners sole function iB to convert airborne pressurewavea lnto eLectrlcal currents.

_ Airborne pressure waves are caused by sound.s.

Hl:T-o.I acoustic energy is coupled into the atnosphere, pressure waves are created.lnese pre.sure waves are anplitude modulated and convey sounds of yariousfrequencies. suffice it to s;y, sound energy 1s not. traveiiog or", the wirea;electrici.ty 1s.

SOMID REQIIIRES A IRANSMISSION MEDII]M

To il lustrate the dependence of sound transmi.ssionany other suitable gas], one simply has to recall thescience experiment. In that demonstration, a doorbel 1jar and is connected vla very thin wires io a battery.clearly at thls point. The bell jar is placed on rop ofattached to a vacuum pump.

upon the presence of air [orclassic high school generalis placed inside of a bel1You can hear the bel 1 very

a f1at, metal plate which is

. _ shortly after the punp is turaed on, you rri1l notlce that. the beLl see's to begetting weaker, nuch as if.lt is novlng away from you. what is reai iy-trappening isthat as the air 1s renoved, sound creited -ty ttre ttapper striki;;-#e be1l is notconduct-ed as welL by rhe reduced arnosphere insiae tle'fJr -j"i--ri..ii.y, rhe soundis so faint -tha_r r9u can only hear the be11 by placing you.".u, ";;;; rhe knob onthe top of the be11 jar.

To show you that there is no trickery invol ved, the instructor then turnedthe vacuum pump and slightly opened a bleeder valve. As air s1ow1y bled backv*v'r4J v*!the partial vacuum, the be1 i became louder. Final 1y, it was restored tooriglnal loudness as the partlal vacuum inside the bel1 5Li *as eliminated.Another example of transmission mediun is

You can hear the traln conlng by listening to thethrough the air. Acoustic eiergy is transmittednediums. Because iron rails are more dense thanrail first. Most, people over thirty experimented

offintoits

Sound conduction and velocity are both affected by transnlssion Eed.iun density.I:i_.:flr_t-":- e_ven -thouBh

you. e"i is an inefficient underr.rater transducer, you cannear EI,e pops and snaps of a distant notorboatrs props and the whine of iis-enginewith your head under vater. yet, above watei vhere yorr ..."-ruoction mostefflciently in_air,,you can hear _nothlng. you can see the boat off in the dlstanceand that is_ a1.1. A1so, you would actually hear those sounas ,r.h soonlr under waterthar through the air.

HOI{ lHE I'trCROPEONE FI]NCrIONS AS A IRANSDUCER

Converslon of sound pressure waves into electricalways in microphones. These mechanisrns are ca 1 1edelectrostat,ic, and electromagnet,ic. There is a flfthmagnetostricLive .

THE C0t\tDUgIM MIcRopEOlrE _ a CARBON I,trKEfn conduction style microphones, loosely packed granules of carbon are part of

the railroad track demonsLration.rails long before you can hear it

llore efficiently in higher densltyair, you will hear the train in arith this during childhood.

energy occurs in one of fourconduct j.on, pi ezoel ectric ,quasi-method referred to as

PTIBLISHED BY EDEN PRESS 39

SECRET AGENT

atr electric circuit. As sound presaure waves strike the diaphragm sealing thecarbon-fiLle-d cavity, the granules are conpressed togetheruor. iightty. -Thisreduces the electrical reslatance of the micrbphone eldent, thereby .i lorring ^or"current to f1ow. when packing becones tighter, nore current flons. when the soundpressure wave has passed, t-he diaphragm resu,es its nor'al position and releases

Pressure on the granules. Circuit reslstance increases and c'urrent flow decreaees.Thus' circui.t current tracks sound pressure waves, or 1s said to be in phase withthe eound causing the naves.The conduction nicrophone is stiLl ln coonon ueage, although it. is on a rapiddecline. _Princlpal advantages are that it is rugged; io.*p.o"fr" to nan"tactuie,and is re1iab1e. Disadvantiges lnclude the need-ior .

":.riti" por". source an

SECRHT AGENT

noi-se amplifiers. Thus, they were favorite choices ofthey are being replaced by newer designs which we,l1popular and in widespread usage.the bugging community. Whiledlscuss next, they are st,il1

the loudspeakerThis coi 1 is

moves, it also

EI,EGI?O}IAGI{ErIC }TICROPEOT{ES _ DII{AUIC XIreSMany insEitutions wiLh internal public address syste's quietly utilize thisEike systeq. During my school years I sonetines notiied that the vatuun tube basedPA systen hunmed durlng perlois when the instructor uaa out or trre-.raJsrooilAnnounce,ents didn't come over the speaker, so we students ig.nored it. until Ehetlne we learned that the princlpal wai s-omehor listening in, a-nd p""i"tr""t for ourbad behavior was srift and sure. -Todayrs,pA systens arl solid stlte and aonli rr*,but the dynanri c mi-crophone effect is as-reliabfe nov as it nas then.

DWA}trC }trKE IHEORY OF OPERAION

The theory of a dynamic microphone lies in the construction ofitself. A Pernanent magnet surrounds a floating coil of wire.physlcally attached to the speaker diaphragn so that rhen the coilmoves the diaphragn.

. Th" basis of operatio,' of a dyuanic loudspeaker is trlat vhenever electricalsignals_are applied to the. coil windings, varying Eagnetic fierds are created by thecoil. . These varying fields are relatJd to the inpui signal. These fields interactwith the permanent nagnetic field in cycles of attiactioi aad repulsion which are intli"^"-iill 11 in!:r signal. Because rhe elecrromaeneric coil is floaring, it iBrree Eo xnove in accordance with the interactive EagneEic fields. Theipeakeidiaphragn is carried with it.

A SERIES OF' TRANSDUCERS

There are severar. transductions takin_g p1ace. F1rst, electrical energy isconverted to nagnetic energy in the coiL. Tihit nagnetic energy interacts with thefixed magnetic. fie1d, producing notion, or kinetiJ energ". T'ire kineiic energy iscoupled into the speaker diaphrign, thereby noving air .ia cr""tioj p."""rr" ,ur""io_the at,osphere -- or sound energy. I'I" thu" s-e e four fo..s ode'nergy at uork

158 IJII{ OF RECIPROCITY

- Iho" far, wetve discussed a dynamic speaker, a-nd havenrt nentioned microphones.To nake thi-s translation, we need io look at a the physical law of reclprocity. Agappl,.ied to this_ particular situation, we see thai'.".i"rt-ir""iriiin the coilcreates sound. rf the theory of reciprocity applies 1n Ehis case, ue s"hour d be ableto use sound waves to nove tire coiL in tt" iiiii-n"g'etic field and create a reratedvarying current in the col1. To highlight the essJnce of this tt o.ght,-r. postulatel*.1.,,.f-:::l"l:-il :h: coil causes-diapiraga '''otion, rhen mot,ion -of the aiapirragmsnouro cause current in the coi1.

RECIPROC]TY IIOI.DS TRUE

In practice, noThis device is oftenPOSSeSSeS a mlxture

physical laws are violated and the dynamic mike is a reality.us_ed in Place of carbon and plezoelectric models, although itof relative strengths and weaknesses. Dynanic mikes areare easily lnterfaced rrith audio circuits. With the advent ofsimple, re1iab1e, and

HTBLISHED BY EDEN PRESS 41

synthetic materials to replace paper diaphragms, their shock resistance is now anongthe best available. Disadvantages include size and ueight as compared to other mikedesigns. One class of size exceptions, known as line drivers, are extrenely soa1l,but are quite costly.

EI..ECTRONIC STIRVEILI.ANCE APPLICATIONS

Dynamic nikes are often used as bugglng devices in either of two basicconfigurations. The more obvious is to take advantage of an instal led PA systen.The other is with an energing class of highly effective line drivers which havebegun t,o appear on the narketplace. Wer1l" now look at. each one.

IMBMH BRIIrcB

To install an intercom-based bug, it is necessary that a dynamic Loudspeaker ispresent, in the target area. Using a speciaLized circuit, the speaker leads arephysically tapped and led to either a recorder or a headset. Conversatio[s lrithinrange of the supposedly benign PA speaker are thus intercepted.

The specialized iaterface circr,r'i t must acconplish three things. First, it Eustbe of an extreroely high input impedance in order to not load the speaker circultrsnoruaL function. Second, it nust contain an anplifier r,rhich will raise therelatively 1ow target slgnal-caused voltages to a usable level. Finally, thecircuit must have autonatic high input 1eve1 protection such that whenever the PAsysten is actual.ly used as intended, that nuch higher signal voltage 1eve1 doesnrtdo'nage the delicate tapping circult electronics and devices -- or your ears.

These speciallzed circuits are available, or can be deslgned and constructedquite readily. Most sourcea will advertlse then as diagnostic alds or Eaintenancedevices and usually call then dynrn{ c bridges. They are usually a combinatlon of anaudio transformer, two enitter follower audio amplifiers for high iaput lmpedance,and an output audio voltage anplifier. Bridges are built on sna11 boards and thentaped t,o a standard 9 volt battery. They are concealed in a dark corner near thesource leads that are tapped.

One sophistication in this circuit rrorth nentioning is that some units are alsoconstructed rl.ith a miniature radio comnand recei-ver installed. The receiver is anextremely sEal1 current drain [or load] on the battery. Its functj.on is to listenfor activate/deactlvate connands and then affect the desired bugging tap status.This applique materiaLLy extends battery 1ife, and thus the 1lfe of the bug.

IJilE DBIYER,

A newLy-perfected mediun inpedance dynarnic nike is avaiLable in an extrenelysnal1 package. It is about 1/32nd of a cubic inch in sj^ze lL/2 x L/4 x 1/41, has a5K ohn inpedance, and Batches with a 1ow noise preaoplifier. Ca11ed a line driver,this package has extended range capability and deEonstrates unusual sensitivity andfidelity. It is discussed nore fully in Section XIII. Suffice it ro note here,line drivers made of dynsm'i c mikes are far easier to use over long distances becauseof inherent stability. I,lhi 1e they Eay not be so sensitive as other types of mlkes,they require far Less precautionary design features and are therefore more popular.

ET.ECTRoSTATIC I'trCROPHONES-

FJ.ECTRETS

Electrets have become the wonder drug of the audio industry since they first

L PIIBTISHED BY EDEN PRESS 42

. The active ele,ent in an electrostatic mike is a thin filn of syntheticmaterial sinilar to teflon. A surface charge of several h*J;;d -"-olt" is bonded tothis fiLn. separated from a specially etchJd pc board base by an lnsulating spacer,the charged film serves as one.pl,aie iu a 'pre-cha"g.arr Jrpa"iio..- rrr" second.capacitor Plate is formed by a diiphragm vhich is sandiiched over the charged fl1nuith an lntervening insulating sp"ier.- FET chips and other circuit "orporr"nt"

...placed on the reverse side of-thi pc- board, "nd

-th" chargeJ

".-p-"-"-itoi-""rves as rninput to the qmFlifier circuit thus forned.'

There are several sound-porting nethods used to achieve noise-canceling whichconsist of vent holes througtr the pc board, but this i.s a nectranic.i a""igo functionEhat wiLl not be discussed 1n detail. uactr ml-nufacturer "".r" io t""" i proprietarydeslgn approach regardlng hole si.ze, -p1...r"nt,'"ra nunber.

when the- assenbly 1s prgperly sealed, the electret nike will have a rifetine ofseveral decades. Thev are relatively iopervious.to- s-hock and vibraiion] and so longas mechanical -or liquid contact with the charged fir.n is prevented, the nike rir.1vork quite we1I.Electrets act as voltage sources because

f eature. By selecti.on of r"li"tor values, lowcan be adjusted between 10 - 1000H2.Eigh frequency cutoff- and response flatness are adjusted by diaphragm tension,thereby controlLinc sound-induced iapacitance- oariations. Electrets can be nadefl'at to lOKHz if dJsired, altho,gh ;;;-;";;e nodels ro11 off ro r.heir 3dB valuessonewhere around 3-4KHz. This iJoften.,Llt""" of cost ," o..d.-- -----

. FEI_ confi.guration dcJernines device lnpedance. rn pracEice, i.f the second FETenitter is capacitor-coupled to the outpuE, ihe a.uplifier'."t" ""'. .on-siant voltagesource and Lherefore has,moderate inpedance -- 2.5K0has ". "o.

- wir"i ariu"o ty.standard.acousric signal__of I pascll Il Sevron per square meteri, -;;i#S;sensit,lvity is abour -40dBv. rn the second our?ur FET .oriiigur.-t-i;, the secorde'itter is directly coupled to the output aDd the derice ap-p-e'ars;;-;" . constantcurrent source. r', this configuration, inpedance is oa Ehe order of 25K0hms ornore, and sensitivlty to a standird audio i.nput is _l2OdBy.Average near-field voi-ce sould pressure 1evels are 3pasca1s, so a typi.calelectret will deliver 30ni11ivo1ts'.t i r:."ro"rDs ouEput. Additionar signalamplification is necessary, and rhls. i" ,ty-ioog-dii"" ,fJiiity ;;;;1.r" tend roarise. Electrets offer.distinct advantaies, hovever, including exceptionallyreliable perfornance in hosEile erriro*eni", e:tretrely good sensi-tivity, and top-notch frequency response over their design raage.Because the ele,ent- is. so ph-yslca11y saa11, it also offers a significantadvantage in noise cancellation. 'uit." *l' u" lcousticar ry excited fron both siclesof the ele,en. and if the. acoustic l"!r.,-i"{rr., d:.fferencL il-;;;; l-i-o.t,

"rui"otnoise sounds teod ro drive both sldes of -.he diaphragn at thl saie tine an

SECRET AGENT

the Eike, thereby striking one side directly. source sound nay echo fron a far walLand strlke the reverse si.de, but the patil length is severir. hundred - to a fewthousan-d tines longer than the acoust:.C path length difference of the olke. Echopower 1eve1s are weaker and are slgnificantly out of phase with stronger inputsignals, thereby not effecting projected sound leve1s.EI^ECIRoI{IC SIrRVErr.r.arIcE tppLIcAIIO[S

- capacitive nikes are proven to be superior in every application not requiringling -J'ine-driving distarces. The electret has 1ltera1iy revolutLonized Hoilywooi:19 !v ?y coupllng the, wirh radio__transoitirs, thereby 6rinr""tiai nany uoon-i:.teJand' cunberaome sound equiprnent. WhlLe one night obseive that nodeirn audio controlpanels look as if they belong in a deep-space rocket console, one look at the ordersor Eagnltude iEproveEenta -over a-udio t.echnol0gy fron even five years ago will showthe impact electrets have had. Note Johnny Caison t s tieclip n"*f tir".-'

Electrets were an initial answer to'r1ine drlver,r applicatlons; however, it wassoon found that iterative anplificatlon and conditioning nodules rere required inordr to not have targeE sound-induced squeals at t-he far end. Thi; Led tocunbersome_ support-ing components in order to utilize electrets. Thus, ny cormentsunder the Dyna'n{ c Mi.crophone sectlon on page 42. Electrets rri1l work wl1l as a 1lnedrive-r for perhaps 250 feet or so. Past ihat, one need.s to actively consirler thedynanic nlke instead.

TABLE 1O-1: SI]MI.{ARY OF PRIMARY MICROPHONE CHARACTERISTICS

Characteristic Conductlve Piezoelectric Electromagnetlc Elect,rostatj.cCornmon Name Carbon Crystal Dynamic ElectreL

FrequencyResponse

Fair500-3KHz

Very Good250-4KHz

Excellent200-5KHz

Excellent10-10KHz

Fideliry Good Excellent Very Good ExcellentNoise Cancel Poor Fair Good ExcellentHostile Usage Fair Poor Good ExcellentPhysical Size Large Large Medlum-Sna11* MiniatureCost Cheap Modest Modest-Highx Modest-High

Interface Lo-Z Hj.-Z Hi-Z Hi-Z

Worst Feature Interface Deli.cate Cost* CostBest Feature Cost, Interface Stabil:.ty 0vera11 Grade

-=-:=:Footnotes: rt*rt indlcaLes tradeoff considerations between Biaiature size and farhlgher costs. A non-oiniature dynaoic nike is nodest in cost,while a sub-Einiature line driver is very expenaive.

PIJBLISEED BY EDEN PRESS t+4

SECRET AGENT

lTB I{AGI{EIIX'MICTTYB IBTXSIXICER0n1y for sake of conpleteness do r mention this clasa of microphone. certainnaterials such as Bariun TiLanat.e, nocnerle iarts, and certain ceiamic ti.tanatemixtures exhibit a transducer

"tr.cJ k;;;; ;; Eagneros rricti on. tlhen varrousfrel.u.encf -vo-ltages are_applied to tfr.""- r.t-"ria 1s, they physical,ly grow as atunction of frequency and inplitude of ttre exciter. conversely, whenever they arestruck by pressure waves, they generate ."i"ieJ-r,o1t.g.".The dlfference,

devi.ces is a mattertitanate staves in aenergy int,o Lhe ocean.

ln laynenst terms, between piezoelectric and nagnet,ostrictiveof sca1e. Extremely large vo1 tages are applied to barium_rr -shipboard sonar atray, .oulling thoisands of watt,s of acoustj.c

used in thecommonly lndeep ocean

or carried, through anyLetts look at wha[

"o*e

;L'irg,I.o':t"'o ??.'."""t'rk "1'",""1"**::1"-:,.':^*u ie"tiov al l bu1 ;h;;; carerul rvpiezoelectric devlces.

Needless to comment further -- magnetostrictive devi.ces are noLconvent'ional e lectronic sur veil lance sense. They are appried quitemil itary "ACrNT" or ?'Acousti.c rnrel ligence'. .L d;;i;;-f ;", sonar andhydroacoustic devicgs. e ----- ev svr r v s.r t r

lEE PEYSICS OF SOMD IH AIR

S0UND is a

rhird-seoester engineeri.ng students in Eost universities receive their baptis,of fire in a veed-our 'rleat, Llght, "ou trurl;'- utrysics course. rrve both taken andtaught this course ,. !f-. ,ni-reisiil--r"lJr,' and will arrenpr a consrrucrivetreatnent of what the electro[ic "urueil lance expert needs to knor. There will beno exanination except your own individual p,roie"sion"1 p.."ti""", t or"u"..

SOTHD AI{D rIS REI.A]3D DEETNTTTOilSform of energy whlch may be transmitted,of elastlc compressions and decompressions.medlun by meansof these terms mgan.

l1i.!1"1: 0r,, for rhar. marrer, Irry Bds, liquid, J,iJ'"*oiii'iii ry-t?rrXj"ii: :l:^.iiducring marerial. For sound enersy, it may be air, water,

medlum, provided certain ru1 "" "." o"u"L;Jj;hi"ir' u]"ir"i ;rJ#;ELASTIC means that the.nediun nay be deforued, or moved s1ight1y, by sooepressure' I'/hen the Dressure is renoved,' tr," ."aiur t"ir i- "IJ"rl- i1'"'"o'iigirr.r "t"t..This is easy to observe in rubber, tui tt" "onl"pt beco.es less clear with air orbrlcks and stee1. But on a nicroscopic reuei, .ri ,.t".rr1" -e*a].liiiir" degree ofelastlc defornation/restoration

"rr"r-""i"ii"iils] and thus ,ir.i ""ra""J-sound energy.

TRANSMITTED simplypoint, and extracted attransnlssion occurs that

means that energy ruay beanother polnt. There

will be developed 1ater.

introduced into a nedium at oneare some rules regardi.ng how

higher and lower pressures. In airborneto fo11ow a complex sinusoidal pattern,focus upon a specific frequency in the

serve as an acoustlc1ater.

describe the range of elastic movement of a mediumrelaxations.COMPRES S ION/DECOMPRESS ION

under successive pressures and

PRESSIIRE wAvES are successive waves ofsound and noise considerations, they tendal though mosL academlc di.scussions tend to

PT]BLISHED BY EDEI{ PRESS 4s

SECREf, AGEI{T

general case, and then extend t,o the complex case.

FREQUENCT is define_d as -the r.elario--nshlp betweeq propagation velocfty andwavelength. F = v/w. (Norma11y, 'rl,andarr whith resenbie" in-inuerted y is usedinstead of rrl,lrr for ruavelength. This print font doee not have landa; wef11 use I,I).PROPAGATT0N vELOcrrY is the epeed'at which energy Eoves in a partlcular nediununder specified conditions. souod noves at approlinately 1100 feet per-second

[actuaL value is r087.42fps] ln air at srp. srp'is standaii r"Gi.t"i"'i-daj-;;Staadard Pressure [Sea Level, or 14.97psi]. V = I130fps at 719F.

WAVELENGTH ls the physical length occupied in the nediun of one cycle of theenergy.

-. . Applying,the F = V/W forn6r:,- w-e know that V = 1100 fps in air. If we want tofind the waveJ.ength of a tone of 1100 cycles per second irt, ," r.tt in the twoknowns: 1100 cps = 1100 fps/w, or by algebra, w = rroo tpsTttbo cps. Nore ar thispoint r carried not only the nunerical values ['t1100tt] but also the units [fps andcps] io the express-ion. T_hls is good practice, Just ln case eonebody slipe inEeters or lnches. ttcycLestr are defi-ned as ttdimens-ion l ess unitsrr and tirus wi canvisualize then ae rrlrr rhere appearing. The remainder of the units are treatedalgebraically, exactly as are the numerical values.F=v/lrl

1100 cps = 1100 fps/Wor W = 1100 fps/1100 cps

and W = 1100/1i00 = 1 lNunirical vaiue]and W = fps/cps = t/L = feer [Units]

T:=1::119==111i-i*alL::*i:For comparison, if we were talking about radi.o energy, whose propagationv.elocity is_ much higher, a Ir waveLength would

"o"responJ-to 9gg,s6b,og9.1 .p".This means that RF energy propagates at Agl,tS+- tines the speed of s'ound.

_- Su_ppose, instead, that the audio tone nas 400 cps. What is its wavelength?Using the algebralcal ly rearranged fornula, W = V/F, re have:[/ = 1100 fps/400 cps

orw=1100/400feetandW=2.75feet

Sinusoidal means sine-wave like.

we knew wavelength was 6.00 feet and wanted to find frequency of sound j.nwould use F = 1100 fps/6.00 ft,:

F = Ll00/6.00 = 183.333 [Numerical value]and F = tps/t = per second [Units]orF=183.33cps

Ifair, we

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SECRE|T AGENT

TTPIC,AL ACOIISTIC EIBGr VIIIIBSThe unit of neasure, trcyc-Ies per secondr, rras_rep1ace6 in 1965 by ,rHertzr,abbreviated fl,. Hertz was m6 eerna'n- prr;gilit vho first postulated the existenceof electronasneric Ior radio I""il",I;i;iil" rorg b"f";. l;;;;;ticurar energyrorn was definiEivelv discovered

"nir iaeiriiiied. -0il"";;-.;;o'oir J.Hiory refer roIlertzian waves in tiis context, as is a current practice in nany parts of Europe.For our purposesi L Hz = L "p".' io" ;";;-;;; up until no,, io'oiaei ro provideclarity in treatnent of unlts'duri;g .1g-"-br;i"'Lnipulatlons. rr11 non shift to IIz.

.: l8ft6ot:"?i'rr'T1"r.o1,,nT#.nfr'j1ftfi"#J',tf"lv".l;::ph1 "r","'""..r:',.i'H;:of hunan hearing is fron 500 to 3K ttr,-aoa Jo-Ji t.r.ptone systens operate on thlsbaai.s. Perhaps 902 of h",nan volce energy i"-ir-tfr"i-fi"iJ';r-ffi;;j;.rn the first caser rou'g, undamaged human earscorresponding to r4ravelengths of 731-4* T rs ill ao r-"""should note by norrr that hlgher f requencies have 1or+erexpressed as ,an^g r.ength iJ invers.ry proportioaal towavelengt,hs are 2'-2.4'i to 4.4r. rn prattice, ," usual lyfrequency and 1t is the nedian wavelength for'.pplication

can sense 15 18K Hz,than 3/4" [18 KHz]. youwayelengths this isf requency. Te l ephoni.c

say 1100 Hz is the nedian

One of the rur'es of thunb is practical application of the ter' rrnear fie1d,,.This is a rarher convoluted-marh#;i";f-f;:.iltioo or aistrrbuliJ,i o] por., specrradensity. In both audio and RF consid-e-iatio-n-"-,' -"n"., fieldrr is consider6d to be thatdistance corresponding to : ,r.r"rerl-tirsl--;;;;u". in audio r" rrur. -t.ia"d to adopt awavelensrh of 1 foot. . near f ield is 3 feer-.

^ tn ..touiiil.f-.;1;" telephonebandwidth,

'e "r" a"tr"rry. roo[ir,g "i-riya-ii-rrg from about rr ro about 6,-7'r.Near field considerations usually arise whenever microphones are beingevaluated. A r'r-pascal s-ource 1eve1 io rir"-n-".ir,iir?;ffi"fi, ;""'Jf 'rii.t trre sornaproJector was within 3' of rhe.microphone ;;J-;;" enitting 1 Nerrton per square ne'erof acoustic energy. you saw *:.s term :." t-fr1 Ji."tret discussions.

of rules of thumb.

a fu1 l-wave, cenLer-L2O Hz high volrage

You often hear a hr:m in oldertapped rectifier circuit, so thetB+] ripple. The audio wavelength

tube-t,ype radios. Most havehum is actuallI caused by aof L20 Hz is g1-2,,.

wi1 t hear a large power transformer substation hum. Sincephase systensr |ou are actual ly heari.ng a 1g0 Hz tone due tovoltage peaks. 180 Hz audio ""rui"ngth-is 6r-1-33,,.

0ccasional 1y, youmost of these are threethree successive 60 Hz

rrve Eenti'oaed that RF energy has a much hig-her propagation vel.city than doessound energy' RF energy. moves lt 983,569,089.3 teet per second, as does conducted!{,-Jji'?i:"::.:j*:ri "r:""";i'":i fl:F"r,?."r'"ir"..-"r.."r}x;"l*1.:ltri:r;}i".1'jt.:ior alooar 3'r05 milesr v ror ir-p-r8p"e"iiir- i" s6s,sos,oai ;;J;; second., or3,540,848,72L,000 niles per hour "l " o"n t-r-"-i "i with 3,960,000 nph for sound. r anbelaboring rhis Doinr nLw while rr"-;;;-I;-;;e audio section !o rhat 1r can belntroduced w-ithouf fanfare ,t

"n ,e ;;;; rie -Ri

chapt".s.

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SECRET AGEMT

NATIIRE OF SOIND PROPAGATTOI{

SPEERICAL SPREADI}IG

sound radiat.es outward. from a point source in what i.s known as a spherl.calepreading node unless it is somehow constrained. An exanple of consirainlng soundis the.old-tlne college cheerleaderrs negaphone, or Ru

instead moved inside of the imaginary 1source. And suppose that at 10cm trorn thetwice what it ls aE the reference point.would have:

SECRET AGENT

2 x 2 = 50.29 square meters

radius was doubled from 1 t,o 2 meters?

meter radius sphere, towards t,he soundsource, hre found the power to be exactlyIf we use the attenuation formula, we

sourceIf Itm

gain.

Attenuation = 10 10g(P."1 /p) = to x toe1/2) = 10 x (-0.30) = -3 dB [Loss!]GAI]IS rPE f,EGATTYE U}SSNS IilD YICE-YRSA

. , lhis concept 1s far more i,portant than a play on vords. A negative gain rs,by d.efinirion, a 1oss. -A_pro foot_b_a11 runninj u.Lr. rno ?i"i"ii"" i g"r" wirh -10yards rushing has actua-r,1y rost 10 yards. Th"eref ore, . o"g"tiu"-r.ols Eust be ag?ri.^_^Il1t-!l-

-!hr9u,Sh, especially if you have ever taken an accounrins course.rr so'eone lakes away (negative) a liability (10ss), then you have gained,- havenrtyou? Because we have chosen to use an .tt.nr.tioo torn oi

"!o.t-ion, negativeresults Eust be [egatlve atteEuations - or, by definition, gains.'-----'Looking back, doesn'L it seem reasonable that if I got closer Eo thethan the defined reference point, the signal r+ould be strJnger? Certainly.looking at, everything less than reference as a 1oss, then anlthing trore is a

A GAUOI{ OF PAI}{T A}ID A BAI,L(NN

. Back to spherical spreading 1oss. Assune Lhat you have exactly 1 ga11on ofpaint and tha_t you are going to iompletely use it to paint a baL100n that has a 1neter radius fot a 2 neter diaoeter]. ttre surface area tf that ba1100n ls:A=4xPixR2

Al = 4 * 22/7 x I x L = L2.57 square EetersNow, letts inflate that balloon until its radius is just doubled, or 2 neters:

A2= 4 * 22/7 xWhat happened to surface area when

AZ/ AL = 50.29/12-57 = 4.000r, you'd have t,o spread that ga11on of paint L/4 as thick inthe second ba11oon. order to stil1 cover

Let's now look aE a 3 neter diameter balloon:

A3 = 4 x 22/7 x 3 x 3 = 113.14 square tretersCompared to the original 1 meter balloon,

\/ \ = 113. L4/ 12.57 =0r, youtll now have to spread that gal1on of painEcover the third ba1 1oon.

I think by now that rhesound pressure at 1 neter fron

9.0c

1/9th as thick in order to sti11

point is beginning to come clear. rf r define theLhe source as reference, then rr11 hear l/4* of that

PIIBLTSHED BY EDMT PRESS 49

SECRBT AGBM

sound at 2 meters. 7/9th a.L 3 Eeters, l/16th at 4 neters, ... l/N2 at N meters andso forth. Ar l0 nerers,.rtre sounJ-p-r""""i"'flr"f vill be l/l}z = l/lOOth of the1eve1 at 1 meEer. Thls is spnerrcJi "p-r-""-ailg

and can be characterized in dB ofattenuaEion as follows:

. Ar. 2 neters, Pz= t/+ p."g, so e = l0 1oe(4) = 6.02 dBAt 3 neters, PZ = l/g pr"1; so I - 10 1og(9) = 9.54

Megaphones, and toprojectors. Figure L0-2

SECRET AGENT

@I{ICAI. S?READIIIG MODE

a certai.n extent, human throat/mouths are typical conlcalillustrates the geometry which we will discuss.

rn order to avoi.d resorting to solid geo,etry, ve will siuplify our model byconsidering the eound wave to b1 a plane i"r".a the circular basL of Lhe cone.This aLlows us ro conside' "p."'a1.ifi;;=-;a runction oi;;;;;;; of a growingcircle rather than having to talculaie surface ar-eas of

"onic "e.tioos of spheres.The net difference betweeln, ttrese tr.'o area= ll-r"r r within calculation error limitscreated by assuning V = 1100 fps in rhe ftrJ jiace.As distance'

'' tro' speaker to-oicrophone grorrs, area A of the circle alsogroua'

',e nust first assune- an .iflEerior- arti.,- a, value for the negaphone. Hunanvoice projection rvpicallv equares ro about" eo" tl.'iii" ffi"i:'=,T,;'J,t r" are ableto calculate rhe value of ihe-radius, n, oi-ti" circle by G-o-rtig ol"--'

Referring to figure l0-3, plane trigononetry gives the following:

We see Uhat the plane angle, b, 30o results in the follon.ing relationship:TANgent(3eo) = R/D = 0.577, ot R = 0.577 x D

- The plane area of the circle is pi x R2. Thus, the area of the ci.rcre as atunction of distance, D, froo "p".["r1o ri"i.i.r :.":

A = Pi- x [O.5l7D]2

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SECRET AGENT

Fron this relationshlp, completing all indj-cated nathr w have:

A = 1.05 x D2

I I{orE: in all nath ca1cu1_a_tions, r am using a calculator and alloring trailiagdeci.ml places to carry. If you perforn the sor'e nath by loaghaad, you nay fiod islight difference iu results, but lothJ ''g whlch rt11 invalidati our discussions.l

_ Conparing with the spherical node1, we find that conical spreading loss isabout 1/12th the value. That ls, using a 600 megaphone coocetrtratea sound-energy bya factor of 12. 0r, to express this gain i! dB,

G = 10 1og(P2 /Pr.) = 10 1og(12) = 10.8 dBAI'DIO GIItr II{D RII{GE

^RE RE.IIED, BITT-

_ Thls explains why soneone beyond hearing range may hear you calling if youshout through cupped hands. The fact a 600 negaphonJ givei you . 12:1 or a -10.79- dBgain does not Eean you can be heard 12 ti-nes further. Remember, we are dealing withlogarithnic and squared trigonoEetric values, not linear values.

To i 1 lustrate, 1et rs assumeand that all factors consldered,hearing at, that distance. Youadding the equivalent of 10.79 dB

that sotreone is standing exactly 100 yards fron youyour shouts are just barely below the threshold ofcup your hands to forn a 600 negaphone, therebyof power to your voice.

In the iniEial situaEion, A1^= 1.05 x 1002 = 10,500 sq yarcls. If we say thatsound pressure must eq{a l. 1x l0-1o watts/sq cm at,,,;hreshold- of hearing [ttri6 is agenerally accepted valuel, then we need 836I x 10-ro watts/sq yd to reach threshold.IDid you fo1low that conv,ef,sioa? There are 2.54 ca per inih, and 36 inches peryard, therefore ( 2.54 x 36)z = 8361 sq cn in 1 sC ydl. -

Total audio power at 100 yards, integrated qger the plane surface of the circLeat that range, is (10,500 sq yds) x (8361 x 10-16 *rarr"f sq yd) = 8.78 x l0-9 watts.

_Assuning no other path losses [not entirely correct, but it slnplifies what weare^doingl this neans-Ehat in the second case, we now have . totai of 12 x g.7g x10-v r.ratts = 1.05 x 10-/ watts present at 100 yards. I.Ie need to calculate what newdistance, D2r will reduce this ,rew power density back to threshold conditions. I{eknow Lhat aa ue increase D, we al,so i.ncrease A according to the fornula A = 1.05 D2.

- trllithout using the precise term, wetve also just noted t.hat power Density andplane area are directly related by a colstant factor which IrlI call k.I.Ie set k = 8361 x 10-16 watts/sq yrl, the threshord hearing 1eve1. we also knowthat total power divided by total area Eust equal power denslty. Therefore, k (inwatts per square yard) = pwatts/Asq yards.

In Lhe first case, A1 = 1.05 x 1002 = 10,500 sq y

SECRET AGEMI

So1 ving r we have A2 = (1.05 rc-7)/(8361 x 10-16) = 126,000 sq yd

?^^, If your voice could have been heardoU" -megaphone?

you calculate it out, andyards about 3-l/2 times __1ook again.

-of 75 yards, your megaphone distance should be aboutfactor of about 3-ln-l-actual ly,-3.40+t "rr"*ing upsquare root of L2 mlght be? rtrs Lhe same __ 3.4641 '

A}I T},TPORTAIIT ATIDTO RITIfi 'DIS@YEREDN

by any factor nominally results in its beingequal to the square root of the gain factor -- anmegaphone concentrates power by i2.

1"":"ll:!: s^1.1:"::^:-?iii. ":1r_.". for any orher megaphone, divide 72ao by rheil:'r'r'"%i ",.r1"",,"-t-

r.5-1"^r:ti,::":.-A jl-"-.;"!'i'",*:.?!i:T#;'i:,"oX'..Jj:; :l i?:gl"l:q:" *;S,:: I_" *J_1:i"_, ir.,1 .31s"ii"ii"ff #iili;Il? ffi.";"8:,.,1J.: ,icircle has 3600 in itufrLrc u'*' Jou- In r'r -- you could either divide -the ,eg.p-hon. angle by-2-and Ehendivide that into 3600 or simply divide into lwi-ce :eo - ot 71o ra

-haai- ,.,;+r. rF^r--your choice. twice 360 or 720 to begin with. Take

Using our 600 megaphone equation, wedistance becones 346-yards. About i_t/Zthis example starting at 100 yards.-

Use the L2Oo modelsufficiently that mi.crophone

0r at an initial distance260 yards --- do you see t,hateach time? I wonder what the

Concentrating audio powerprojected farLher by a multiplierimportant acoustics 1aw. Th; 600

A HODEL FOR ITORMAT ETII.{AN SPEECH

conic spr.eadi.ng by human speech is nore pronouncedopens his mouth wider and proSecls from his ,oiar cords.

A 1200 megaphone results in aboutof -1 600 negaphone. It11 1eav" ii rote 11 you thar rhe rangenr of 600;1f,"cause for diminished performance.

have 92 = 126,000/1.05 and the new heari.ngtimes, not 12 ti.mes the distance f oi

.!'-120 yards, how far would it camy with aif your answer is much different tt "r, 4L6

a_: the speaker or singerThe cavern forned by ifr"

When one ye 1 1 s, the same

1/3th of the concentrating abilityr or gain,you to verify t.iis nunber. As a hintr lriihalf-ang1e of a 120o negaphone, is the root

entire larynx and mouth approximates a 600 megaptrone.effect takes- p1ace, plus additional energy is imparted,carry much farther. thus allorring the voice to

[TTrt"1T;'-:,r,:iT:i_l:1.:11::, g: lor ,r"pricare -a 600 megaphone. Rarher, rhe:Ii,.:*1:?._"^ll_1".!,1.p,"l.il, resu1rs i"1r,"'"q"i,"i";; ";T'#';f,=;."1"u!T.";iiEfou should consi.dertypical values corresponding to a r2oo r"g"fhon" inst,ead.

for normal speech, anC your11placement calculatlons rt1 1 work

I{AVE I.IODE OF SPREADIIG

be in the bal lparkwel1.

There are two basicpropagates . One, the ,rr"ythe source and an observer.

schoo 1 s of thought regarding how 1 ightthat f. ighr rravelJ i, striight iirr"" berweenenergY is emitted in ttrays.tt-

is best illustrated by the proverbiar stone

sci.entif ictheory" says

0r, lightAnother is the ,twa ve theoryr? which

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