ABSTRACTIn this paper, we describe a concealed weapons detection concept, based

on nonlinear ultrasonic beam mixing in air, which offers advantages overtraditional techniques. Two ultrasonic frequency signals at high soundpressures undergo nonlinear beam mixing to generate audio range differ-ence frequency signals which can probe through thick clothing. While thelow frequency beams penetrate, the beam collimation and resolution is de-termined by the ultrasound. We present our research from proof of conceptto laboratory prototype tests to address two major challenges faced by thelaw enforcement community. The first is detection of hidden objects at ashort range for use in the corrections field. Here, images contrastingweapons against a tissue simulating background, both of which are con-cealed under fabric, are shown with image resolution not possible usingconventional acoustics. For the detection of weapons at distances up to4.5 m (177 in.), custom parametric array transducers and dish receiverswere employed to excite and analyze the acoustic signatures of concealedguns, knives, scissors and cell phones for use by the street level law en-forcement community. Resonance excitation of objects, chirp signal excita-tion, database generation, spectrogram analysis and signal processingusing advanced correlation algorithms at various stages in the develop-ment process are described. Some issues involved in realistic implementa-tion are also discussed. Keywords: concealed weapons detection, nonlinear acoustics, signatures,resonance, scattering.

INTRODUCTIONIn the aftermath of the 11 September 2001 attacks, the nonde-

structive testing (NDT) community has been focusing on enhanc-ing NDT technology and approaches for applications in homelandsecurity and terrorism prevention (Baltzer, 2002). Most of the tech-nologies being deployed for security applications are similar tothose already being used by the NDT community. For example,current commercially available systems for concealed weapons de-tection are based on metal detection, X-rays, camera-based millime-ter wave technology and terahertz imaging systems. Each ap-proach has advantages and serious limitations (Electronic PrivacyInformation Center, 2005). The development of a handheld, cost ef-fective, reliable, street-deployable system for the detection of con-cealed weapons from a standoff distance of 5 m (197 in.) is of greatvalue to security officials and law enforcement personnel. A hand-held concealed weapons detection device can be extremely usefulwhen deployed in public places like malls, parks, schools and

subways, where potential hostage situations can be avoided. An-other overriding concern in the criminal justice field is the safety ofcorrectional officers. While there are several commercially availablealarm systems for responding to an assault in prisons, the growingnumber of attacks on prison staff by inmates indicates room for im-provements. For example, the Bureau of Justice reports 14 165 at-tacks on prison staff by inmates, with 14 deaths and a 32% increasein attacks in five years (Hart, 2003). Prison managers face addition-al challenges, including identifying and screening visitors and thecontrol of inmate movement (Nacci and Mockensturm, 2001;Paulter, 2001; Spawar Systems Center, 2003). There is a growingneed for new technologies that will aid in the safe detection ofhandcrafted concealed weapons from items such as toothbrushes,alarm clocks, pens and razors.

Nonacoustic devices like the examples given above have seriouslimitations in detecting plastic weapons, are not cost effective andhave complex hardware implementation. While some imagingtechniques provide high resolution pictures of a person carrying ahidden weapon in real time, they raise serious privacy concernsdue to anatomically precise images (Graham-Rowe, 2004) and radi-ation exposure (National Council on Radiation Protection and Mea-surements, 2003). Other limitations include speed of testing, real-time implementation, detection at a distance, outdoor noise andportability issues. Conventional acoustic techniques have the dis-advantage of attenuation at higher frequencies and a large beamwidth at lower frequencies, which limits the ability to detect smallweapons (like guns, knives, plastic or ceramic blades and box cut-ters) from long standoff distances (Kinsler and Frey, 1962). Thework presented here is a team effort to develop a nonlinearacoustic weapons detection tool for standoff detection in streetsystems and a wand system for the corrections institute. We dis-cuss the nonlinear acoustic concept, theory and its relevance toconcealed weapons detection. We describe the proof of concepttests for penetration capability of nonlinear acoustic waves andshort range, high resolution imaging of concealed objects. Longerrange acoustic propagation, hardware design, collection ofacoustic resonant signatures from weapons and the classificationof targets will also be discussed. Finally, we will discuss some ofthe limitations and difficulties encountered and describe futurework plans and conclusions.

CONCEPTNonlinear acoustic generation via parametric acoustic arrays

(Westervelt, 1963) has been studied for approximately 40 years andhas been applied widely in several underwater sonar applicationssuch as communications and subbottom profiling (Hamilton andBlackstock, 1998), medical ultrasound (Fatemi et al., 2002) andacoustic microscopy for materials characterization (Lima et al.,2005). When two high frequency waves f1 and f2 combine at highsound pressures, the resulting sum signal driving the transmitter

Materials Evaluation/December 2005 1195

Nonlinear Acoustic Concealed Weapons Detection

by Anjani Achanta,* Mark McKenna,† Samuel Guy,† Eugene Malyarenko,† Ted Lynch,† JosephHeyman,‡ Kevin Rudd** and Mark Hinders**

Submitted October 2005

* Luna Innovations, 130 Research Drive, Hampton, VA 23666; (757) 224-5723; fax (757) 224-2019; e-mail <achantaa@lunainnovations.com>.

† Luna Innovations, 130 Research Drive, Hampton, VA 23666; (757) 224-5723; fax (757) 224-2019.

‡ Luna Innovations, 130 Research Drive, Hampton, VA 23666; (757) 224-5723; fax (757) 224-2019; e-mail <heymanj@lunainnovations.com>.

**Department of Applied Science, College of William and Mary,Williamsburg, VA 23187.

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will undergo self demodulation as it propagates. This results in thegeneration of additional frequency components at the integermultiples of the sum and difference of the original frequencies(nf1 ± mf2).

The parametric array in air was first experimentally demonstrat-ed by Bennett and Blackstock in 1975. Audio applications of theparametric array were described by Blackstock in 1997. Recently,commercial parametric arrays have come on the consumer marketfor audio applications including targeted advertising and localizedmuseum soundtracks (Croft and Norris, 2001; Pompei, 1999;Sennhieser, 2004). All use somewhat different forms of ultrasonictransducer arrays and transmitter electronics, but operate under thesame parametric array principles described above.

Since the absorption of acoustic energy in air is frequency de-pendent, as the acoustic wave propagates the higher frequenciesare quickly attenuated and the difference frequency (f1 – f2)becomesthe dominant frequency present in the acoustic wave. However, be-cause the resulting acoustic beam’s directionality is determined bythe transducer size relative to the original high frequencies f1 and f2,a very narrow sound beam at the low difference frequency (f1 – f2)can be achieved.

Using this concept, a nonlinear ultrasonic system has been de-veloped to provide an enhanced capability for detecting hiddenweapons over traditional ultrasonic techniques. The approach iscalled nonlinear acoustic concealed weapons detection. In the setupshown in Figure 1, when the modulated signal is radiated from afocusing source, the difference frequency sound generated due toacoustic nonlinear propagation is concentrated at the focus. Theamplification from focusing the beam compensates for the low con-version efficiency from the source frequencies to difference frequen-cy. Thus, a small size low frequency probe beam can be realized bysetting the two frequencies close to each other.

THEORYIn order to model the generation of the difference frequency and

to study the interaction of the acoustic difference frequency and thetarget, it is necessary to understand the nonlinear generation of theacoustic signal and the scattering. The theory and modeling will bedescribed below.

The source of the classification signals is derived from thebasic linear and nonlinear interaction equations that treat theacoustic wave as a significant modifier of the local velocity ofsound. The velocity of sound depends on the local density, which

itself is modulated by the propagating high power sound wave(Blitz, 1963). This can be understood in the equations that follow.

(1)

whereρ0 = the static densityδ = the incremental increase in density produced by the

acoustic wave.

(2)

wherec = the adiabatic sound velocity (with ρ0c the characteristic

impedence of the air)A = the amplitude of the oscillatory displacement that a small

element within the air sees when in the presence of anacoustic wave.

For two interacting beams of frequency f1 and f2:

(3)

We find the acoustic pressure in the presence of acoustic waves:

(4)

(5)

X and Y are corresponding angular arguments:

(6)

(7)

Table 1 shows the various terms of the source equation, the

P c A X A Y c

A X A A

= + +

+

ρ ρ δ0 1 2 0 0

12 2

1 22

sin sin

sin siin sin sinX Y A Y + 22 2

X f t kx Y f t kx= −( ) = −( ) 1 2 and

P c A X A Y c A X A= + + +ρ ρ δ0 1 2 0 0 1 2sin sin sin sin Y 2

P cAi= ρ δ02

0

A A f t kx A f t kx A= −( ) + −( ) =1 1 2 2 0sin sin and δ δ

acoustic pressure: P cA= +( )ρ δ0 1

air density: ρ ρ δ= +( )0 1

1196 Materials Evaluation/December 2005

Figure 1 — Experimental setup used to generate a parametric acoustic beam mixing two frequencies along the propagation path.

Table 1 Nonlinear acoustic concealed source equation terms

Dependence Term 1 Term 2 Term 3 Cross Term 4 Term 5Amplitude ρ0cA1 ρ0cA2 ρ0cδ0A2

1 2ρ0cδ0A1A2 ρ0cδ0A22

Frequency f1 f2 2f1 f1 – f2; f1 + f2 2f2

components f1; f2

Average overone cycle 0 0 [ρ0cδ0A2

1]/2 0 [ρ0cδ0A22]/2

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frequency of those terms and the average force integrated over onecycle. Terms 3 and 5 of Equation 7 produce a nonzero force when av-eraged over one cycle. This force is the radiation pressure, or the ul-trasonic wind as it is sometimes called. This force generates ultrasonicstreaming. Term 4 is the critical nonlinear term for nonlinear acousticconcealed weapons detection in the generation of the acoustic prob-ing wave, derived from the two ultrasonic waves. It comes from thef1 – f2 nonlinear mixing that occurs in the air.

All of these terms at all of the frequencies interact with the targetand result in the total signal presented to the nonlinear acousticconcealed weapons detection system for analysis. Some of theterms are absorbed over very short distances, such as the f1 + f2 termthat is highly attenuated at its higher frequency. In concealedweapons detection, the targets have complex geometries and aresurrounded by multiple clothing layers and tissue. The impedancedifference between air and most objects is very large, so that the re-flection and interaction of the beam with an object is a complexmixing of many specular and diffuse wavefronts. In addition, theultrasonic beam exhibits beam spread caused by the finite size ofthe source with respect to the wavelength of the disturbance as wellas the geometry of the reflecting object. Hence, analysis of the re-ceived data becomes extremely important. We developed a signalprocessing tool for this application to extract features that havebeen classified as indicative of hidden objects or geometries. Onemajor advantage of nonlinear acoustic concealed weapons detec-tion is that the detection process is not based on imaging; instead itis the acoustic response and resonance features of the target thatclassifies it as a weapon. Once an indication has been detected, theidea is to instruct a sophisticated sweep to increase the classificationcertainty.

One of our early tasks during this project was modeling thepropagation of the mixed ultrasonic and audio beams and later im-proving these simulations to study the interactions of the beamswith concealed objects. The Khokhlov-Zabolotskaya-Kuznetsovequation (Lee and Hamilton, 1995) is the most widely used waveequation to model nonlinear acoustic beams. It accurately modelsthe combined effects of nonlinearity, absorption and diffraction. Lee(1993) developed a finite difference method based on theKhokhlov-Zabolotskaya-Kuznetsov equation to model pulsedacoustic emissions from axial symmetric sources. Using a similarunderlying concept, a model was developed to visualize thewavefront of a dual-frequency ultrasonic signal as it undergoesnonlinear audio generation in air.

As an example, we present the results of two simulations of thesame parametric configuration with different degrees of nonlinear-ity in Figure 2. The first simulation uses the appropriate coefficientof nonlinearity for air (β = 1.2) and the second does not include anynonlinear effects; at each distance shown on the left, the plot is splithorizontally, with the nonlinear simulation on top and the linearsimulation on the bottom. A610 mm (2 ft) diameter transducer witha geometrical focus of 8 m (26.2 ft) is excited with a short pulse thatcontains two frequencies: 45 and 55 kHz. The initial sound pressureis 120 db. Each waveform is recorded at 2 m (6.7 ft) intervals start-ing at the face of the transducer and extending to 10 m (32.8 ft). At0 m (0 in.), both ultrasound frequencies are present. As the wavepropagates away from the transducer, the ultrasound is quickly ab-sorbed due to the viscosity of the air.

As can be seen from the plot, the nonlinear and linear (upperand lower) plots are almost identical until 6 m (19.7 ft) where the ul-trasound frequencies are attenuated and the difference frequencybecomes the dominant frequency in the nonlinear simulation. Thisunambiguously shows that the creation of the difference frequencyis a result of the nonlinearity of air.

EXPERIMENTS

Near Range Concealed Weapons DetectionUsing the test setup shown in Figure 1, an ultrasonic air-coupled

transducer was mechanically scanned along the surface of a targetat a liftoff of 140 mm (5.5 in.). This is an intermediate test between acontact test and a fully remote, long distance test for detection ofconcealed weapons. From the practical standpoint, one potential

application for this technique is airport security screening with anultrasonic wand.

A 140 kHz spherically focused transducer was used as a signalsource. The focal spot size of this transducer was 5 mm (0.2 in.) at adistance of 140 mm (5.5 in.) from the face. The transducer was drivenwith a mixed frequency signal at frequencies f1 and f2, slightly differ-ent from its resonant frequency. Adirectional “shotgun” microphonewas used as a receiver. The data in Figure 3 are for one spot on atarget with the source frequencies set at 137 and 144 kHz (differ-ence frequency is 7 kHz). The frequency domain signals areshown to highlight the narrow band nature of the acoustic differ-ence frequency.

Figure 4 shows the scan system used to generate the imagedata of a sample under test. The sample is an unconcealed pair ofscissors on top of a human torso model made of tissue simulantgel. The transducer/microphone assembly was translated in ascanning mode to build a spatial image of the interaction zone atthe target. The scan dimensions in the tests below are 300 by250 mm (11.8 by 9.8 in.) and scan step size is 1 mm (0.04 in.).Traces using continuous wave excitations were collected at eachlocation at a sampling frequency of 100 kHz. For each scan, thecomplete set of time domain signals was saved for postprocess-ing. Postprocessing of the continuous wave data included gener-ating a Fourier transform of each trace and computing the energyof the peak corresponding to the difference frequency. For pulsedsignals, images were formed by computing appropriate time-gated reflected energy (C-scans).

Materials Evaluation/December 2005 1197

Figure 2 — Parametric pulse propagation: sound pressure field of anonlinear versus linear wave as it propagates away from the transducer.The X axis represents time and the Y axis represents radial direction.

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Scanning the system over the scissors in Figure 4 produced animage with a resolution far beyond what is possible from a 10 kHzwave (wavelength of 33 mm [1.3 in.]). The resolution clearly islinked to the shorter wavelength of the two high frequency beams,generated by the narrowband ultrasonic transducer. This transduc-er transmits efficiently around its resonant frequency of 140 kHz.The sum frequency generated through nonlinear mixing (Table 1) isattenuated by the time it reaches the receiver. Remaining sum fre-quencies are further attenuated by a low pass filter set with a cutofffrequency below the driving frequencies of 135 and 145 kHz. In theabove experiment, preliminary tests were run to ensure that the dif-ference frequency, detected by the microphone, was not generatedby nonlinearities in the microphone or the ultrasonic transducer.

Figure 4 also shows a scan of the plastic gun covered by cloth.This test was also done at 10 kHz, and the presence of a foreign ob-ject under cloth is detected. The shape of the concealed gun is notdefined as sharply as the shape of the scissors, mainly due to the in-troduction of a new variable — scattering from the surface of thefabric. A new series of improvements to the scanning techniquewas necessary to reach the next level of resolution.

The imaging results shown above provide a preliminary proofof concept of the superior focusing and resolution of parametric ar-rays. Following this proof of concept, a more rigorous set of scanswere conducted on the real weapons shown in Figure 5. These

weapons were provided by the National Institute of Justice. For allsubsequent scanning tests using the nonlinear acoustic approach,the experiments were set up such that the presence or absence of aweapon could not be detected visually, a test that most closely sim-ulated an actual real life target.

To enhance resolution, these scans were later conducted in thepulsed mode using two 20 ms high frequency beams of 140 and147.5 kHz. The combined beam generates a 7.5 kHz difference fre-quency that can penetrate clothing and produce echoes from thehidden object sufficiently strong to be received by the microphone.While the transducers, scanning hardware and electronics werekept the same, the image of the scissors, generated in the continu-ous wave mode, was analyzed in the fast Fourier transform domainand the images of the improvised weapons were processed in thetime domain.

In these tests, we also optimized the orientations of both thetransmitting transducer and the receiver microphone to enhance re-ception of the acoustic echo from the hidden object. We lowered themicrophone to point right at the transducer’s focal spot and direct-ed it as close to the normal as was possible with the geometricalconstraints, mainly by the diameter of the transducer. Figure 6shows the setup of the concealed weapons scan. Two layers of thinfabric were covering the test articles such that there was no visualevidence of the presence of an object underneath. The arrangementof the objects on the tissue mimicking gel pad is also shown in thefigure.

Figure 6c shows the time domain C-scan image of the three hid-den objects. The image was acquired when the double layered clothwas stretched at approximately 12.7 (0.5 in.) above the weapons.The image resolves all three hidden objects. We have found thatstretched fabric produces uniform offset while loose fabric distortsthe reflected signal in a nonuniform way.

The above experiments show the feasibility of short range detec-tion of concealed weapons by scanning a focused, parametricallyexcited, low frequency acoustic beam. A low frequency acoustic

1198 Materials Evaluation/December 2005

Figure 3 — Sample data recorded at the target with a shotgunmicrophone used as a receiver: (a) the fast Fourier transform plot of thedifference frequency; (b) the fast Fourier transform plot of the sourcefrequencies. The nonlinear interaction produces the very narrowbandwidth difference frequency signal.

(b)

(a)

Figure 4 — Test setup for scanning the nonlinear acoustic concealed weapons detection system: (a) basic setup; (b) unconcealed scissors; (c) resultantimage of scissors as scanned lying on top of tissue-simulating gel; (d) plastic gun used in scanning; (e) resultant image of gun as scanned lyingbetween fabric and the tissue-simulating gel. Note the resolution is consistent with the ultrasonic, not acoustic, frequency.

(a)

(b)

(c)

(d)

(e)

Figure 5 — Improvised weapons made by inmates (courtesy of theNational Institute of Justice).

06_1195_1218_TPs_24pgs 11/10/05 9:01 AM Page 1198

beam can be used for high resolution imaging, if it is produced bymixing two high frequency beams. The resolution is not compro-mised when simulated and real weapons are placed on a humantissue simulator and hidden under several layers of fabric. As a re-sult of this demonstration, we have also developed guidelines forfurther improving the technique and moving it to the next stage —a handheld wand for security screening.

Long Range Concealed Weapons DetectionFor a longer range street system, the National Institute of Justice

would like to deploy systems which can detect concealed weaponsat distances of 4.5 m (14.8 ft) or greater. They would also like sys-tems with a compact size, which are easily deployable by a singlelaw enforcement officer, and which provide detection in 1 s or less.For detection at distances of 4.5 m (14.8 ft), we employed a resonantspectroscopic approach to acquire the resonant signatures of theweapons, rather than raster scanning the person to make an image.This approach has clear speed advantages. In fact, the current teststake approximately 1 s. This spectroscopic technique is similar toaccepted techniques in the NDT community such as resonant ultra-sound spectroscopy (Maynard, 1996), which has been applied totest a variety of materials from engineered samples such as com-posites and ball bearings (Spooer et al., 1997), superconductors(Maynard et al., 1992) and geological samples (Leisure and Willis,1997). In all these cases, the resonant signature of a sample is usedto determine the elastic constants of the sample or to measure engi-neering properties for quality control. A variety of transducers havebeen used to excite the sample resonances, from piezoelectric, laser-coupled ultrasonic and electromagnetic acoustic transducers (Ogiet al., 2002). Kadachak et al. (2000) have used an air-coupled reso-nant signature measurement to detect the presence of liquids insealed tanks.

In the experimental setup, the transmitters were commerciallyavailable systems designed for audio spotlight applications. Thesetransducers allowed generation of sufficient sound pressure levels,of approximately 100 dB, to generate enough acoustic energy topenetrate the clothing at distances up to 4.5 m (14.8 ft). Rather thanusing a single piezoelectric transducer, such as those used for para-metric arrays in underwater applications, we used a multiple ele-ment transmitter, 0.6 m (23.6 in.) in diameter, with 120 individualcapacitive transducer elements, each 50 mm (2 in.) in diameter anddriven simultaneously in phase by a low frequency modulated,high voltage signal. This has several advantages, primarily for sizeand weight. Use of the capacitive transducer elements also reducesgeneration of a difference frequency in the transducer itself, whichcan occur due to saturation of piezoelectric ceramics transducerssuch as lead zirconate titanate ceramic transducers. A low frequen-cy modulated signal was used to excite the transducer to generate

the acoustic signal, using either a summing circuit and an audio am-plifier or a proprietary amplifier for the transducers. Two transmitterarrays were used: the first was a flat unfocused array and the secondhad a curved front surface to focus the beam at 4.5 m (14.8 ft). Withthe focused array, there was approximately a 10 dB increase in soundpressure levels as compared to the unfocused array.

Before designing a focused transmitter, we used an array todemonstrate that sufficient energy could be generated using the air-coupled signals to cause a metallic sample to acoustically resonate(Heyman et al., 2005). For this test, a 50 mm (2 in.) outer diametersteel tube sample, 150 mm (5.9 in.) in length was used as the testsample. First, the test sample was placed at the focal spot of the re-ceiver, held by one end and tapped to measure the natural resonantfrequency. This is shown as the darker curve in Figure 7. The reso-nant quality factor of the sample was approximately 70. Then, thesample was placed on an absorbing background and excited by theair-coupled transducer array. In this case, the ultrasonic frequencieswere centered on 65 kHz and the difference frequency was variedfrom 1 to 2.8 kHz. The resonance of the tube is clearly shown, witha much smaller quality factor for the resonance and a slight shift ofthe resonance frequency from 2.05 kHz to approximately 1.9 kHzdue to the mounting.

Materials Evaluation/December 2005 1199

Figure 6 — Concealed weapons detection system setup and results for imaging improvised weapons: (a) the weapons placed on the tissue-simulatinggel; (b) sample covered by two layers of fabric; (c) nonlinear acoustic imaging result, clearly showing the concealed weapons. For these tests, the fabricwas placed in such a way that there was no visual indication of the weapons’ position beneath the fabric.

(c)

Figure 7 — Two resonant spectrum measurements for a 50 mm (2 in.)outer diameter steel tube sample, 150 mm (5.9 in.) in length. Thedarker trace is for the tube held from one end and excited with a tap.Here the quality factor is approximately 70, because the tube wasundamped. The lighter trace shows the same resonance mode, with thetube supported horizontally on an absorbing cloth background. Thequality factor for the resonance is approximately 8.

(b)(a)

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For actual concealed weapons detection tests, the transmitterwas designed using nonlinear beam simulations. We worked withan outside laboratory to design a focused transducer. The focusspot size was limited by the size of the transducer (handheld re-quirement) and the 4.5 m (14.8 ft) distance produced a spot size of250 mm (9.8 in.). Since the weapons that were to be tested were 100to 200 mm (3.9 to 7.9 in.) in size, we fabricated a lightweight ellipti-cal receiver dish that is approximately 1 m (39.4 in.) in size and de-signed such that the microphone and the interrogation zone thatwere 4.5 m (14.8 ft) apart were at each of the foci of the ellipse form-ing the dish. The elliptical receiver dish was able to isolate a100 mm (3.9 in.) spot size.

The carrier frequencies of the element in the transmitter unitshown in Figure 8 are 65 kHz and the capacitive transducer is dri-ven through a controller provided by the manufacturer. The maxi-mum sound pressure of the ultrasound at the focus was found to be134 dB and the maximum sound pressure level of the audio was111 dB at 6.4 kHz, both measured at the focus using an audio mi-crophone. Different objects have different acoustic responses de-pending upon their physical structure and boundary conditions.Since each of the weapons would be unknown to the user, a chirpsignal 28 ms in length ranging from 5 to 15 kHz was used as the in-cident signal from the speaker. The 28 ms chirp was determined tobe optimal based on the room size, wall reflections and crosstalk be-tween source and receiver. The chirp frequency range was deter-mined based on examination of the frequency response of a varietyof weapon objects in the database. The amplitude of the linear chirpwas varied to linearize the amplitude response of the transducerarray and electronics.

Figure 9 shows the results generated from various weapons hid-den in a person’s pocket under his shirt. These results are repre-sented in the form of spectrograms (joint time frequency) of the re-ceived chirp signal. Each recorded signal was averaged five timesand the specular part of the signal was removed by postprocessing.

The signal was also time gated to remove reflections from sur-rounding walls. In Figure 9a, there is no target in the focal zone ofthe transducer. There is little or no information in the spectro-gram. Also, time gating of the signal to eliminate room noiseworked effectively.

In the spectrogram shown in Figure 9b, a portion the originalsignal is reflected back and the rest is absorbed by the clothingand human tissue background. When the person holds a weaponin the interrogation zone, additional features show up in thespectrogram (Figure 9c) which are characteristic of the weapontype, position and orientation. To further support this, the spec-trogram plot in Figure 9d was from a signal reflected off a personholding a gun. Figures 9c and 9d are different based on the na-ture of the weapons.

A number of different chirps were experimented with depend-ing on the type of weapon being tested. Some of the variations thatwere tested during the analysis were weapon type, orientation andboundary conditions. Signatures of objects were prerecorded byvarying these parameters and constituted the database for the cor-relation algorithms developed in the next stage of the work.

Using this database, unknown concealed objects were detectedand classified correctly. Table 2 provides a summary of correlationalgorithm results for two unknown weapons that were tested froma database of signatures that contained acoustic features of objectsincluding scissors, a box cutter, a cell phone, a plastic gun and apocket knife. The values in the table are the correlation coefficientsobtained after matching the incoming signals with the signatures inthe database. These objects are approximately half the size of thebeam at the focus. When recording the database, care was taken tomeasure the acoustic signatures of these weapons with a direct inci-dence of the input chirp signals on the weapon and the return sig-nal recorded by the microphone. For each weapon, several signa-tures were generated and averaged. When testing the unknowncases, two weapons from the database of six weapons were selectedfor running correlation algorithms. The unknown weapons wereconcealed behind a thick coat 4.5 m (14.8 ft) away from the trans-mitter and receiver.

The actual weapons that were concealed behind the thick coatwere: Unknown 1, box cutter; Unknown 2, scissors; and Unknown3, a combination of box cutter and scissors. These results indicatethat objects that belong to a class of weapons can be distinguishedfrom nonweapons with some certainty. The low values indicatethat the database needs to be improved. These correlation coeffi-cient values are input to the software that provides a security offi-cial with a decision making tool for easy interpretation and visual-ization of results.

For realistic implementation of the nonlinear acoustic concealedweapons detection system, current software provides an automaticdetection result that displays the warning message when a weaponis present. A well trained database can detect and classify a weapon(Figure 10). For successful tests in uncontrolled conditions, classifi-cation of objects to identify a threat or no threat would be realistic.The results in Table 2 have shown that objects that have similarphysical features, like the blade of a scissors or a box cutter, are hardto distinguish from each other and instead can be commonly classi-fied a threat.

Propagation through thick clothing has been attempted and itis observed that fabric that is more absorbing eliminates the hugereflection from the input ultrasound carrier frequency and,hence, provides a cleaner return signal containing the weapon’sfeatures. However, this is a not the case for a tight weave fabric,where processing of the return signal to remove specular reflec-tion becomes more important. This has been addressed in the fre-quency domain.

As expected, as the standoff distance increases, it becomes hard-er to focus the beam to a small spot with a 610 mm (2 ft) transducer.Hence, we are currently working on building a dynamically fo-cused system and controlling the beam spot size electronicallythrough a phased array approach. Another advantage of such asystem would be the capability to assess moving targets in a realtest situation. We are also interested in detecting concealed suicidevests and explosive material in our future work.

1200 Materials Evaluation/December 2005

Figure 8 — Long range concealed weapons detection: (a) test setup;(b) spectrogram display containing the fast Fourier transform domain,time domain and joint time frequency domain plots.

(b)

(a)

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Materials Evaluation/December 2005 1201

Figure 9 — Spectrograms of received signals picked up by a dish microphone 4.5 m (14.8 ft) away: (a) no target; (b) person with no weapon;(c) person with a concealed box cutter; (d) person with a large concealed gun. The position of the person was set to coincide with the focus spot of thetransducer.

(c) (d)

(a) (b)

Figure 10 — Nonlinear acoustic concealed technique to detect and classify different weapons: (a) the weapons used; (b) classification software;(c) results from a gun; (d) results from a box cutter. The fast Fourier transform signal for a gun is significantly different than that for the box cutter.

(c) (d)

(a) (b)

Table 2 Signature identification results

Comparison Box Cutter Cell Phone Pocket Knife Plastic Gun Scissors Metal TubeCombinationUnknown 1 0.5252 0.2872 0.1493 0.4213 0.3383 0.2269Unknown 2 0.03366 0.0155 0.0832 0.0373 0.2057 0.0228Unknown 3 0.3413 0.0192 0.0798 0.1356 0.3392 0.0468

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CONCLUSIONThe use of nonlinear acoustic beams for concealed weapons

detection has been demonstrated for direct application by theDepartment of Corrections and the development of a street sys-tem for the Department of Justice. Objects such as guns, knifes,cutters and scissors that can be used as potential weapons havebeen detected successfully from a short distance as well as a wellcontrolled distance of 5 m (16.4 ft). Having proved the concept,the application of this technique in multiple situations is nowhardware driven. A phased array based dynamically focusingsystem with improved signal processing will be the final field de-ployable system that we envision. The database of weapons’ sig-natures needs to be further expanded. The speed of testing, abili-ty to detect moving targets and ability to see through thickclothing will be determined by the transmitter and receiver de-sign, which will be determined in the continuation phase of thisprogram. A cost effective detection system like the nonlinearacoustic concealed weapons detection system would be of greatvalue to law enforcement.

ACKNOWLEDGMENTSLuna Innovations thanks the National Institute of Justice, De-

fense Advanced Research Projects Agency for their support ofthis work and the management oversight provided by the AirForce Research Lab Information Directorate as technical repre-sentatives. In addition, this work was supported by the Home-land Security Advanced Research Projects Agency. The contentof the information provided herein does not necessarily reflectthe position or policy of the government and no official endorse-ment should be inferred. We also greatly appreciate the assis-tance of M. Bhardwaj, of the Ultran Group, for loaning the shortrange air-coupled transducer.

REFERENCESBaltzer, Keren R., “Is There a Role for Nondestructive Testing in Preventing

Terrorism and Increasing Homeland Security?,” Materials Evaluation, Vol.60, 2002, pp. 513-517.

Bennett, M.B. and D.T. Blackstock, “Parametric Array in Air,” Journal of theAcoustical Society of America, Vol. 57, 1975, pp. 562-568.

Blackstock, D.T., “Audio Applications of the Parametric Array,” Journal of theAcoustical Society of America, Vol. 102, 1997.

Blitz, J., Fundamentals of Ultrasonics, New York, Butterworth, 1963.Croft, J.J. and J.O. Norris, Theory, History, and Advancement of Parametric

Loudspeaker: A Technology Overview, San Diego, American TechnologyCorporation, 2001, <www.atcsd.com/pdf/HSSWHTPAPERRevE.pdf>.

Electronic Privacy Information Center, “Spotlight on Surveillance: Trans-portation Agency’s Plan to X-ray Travelers Should Be Stripped of Fund-ing,” <www.epic.org/privacy/surveillance/spotlight/0605/>, June 2005.

Fatemi, M., L.E. Wold, A. Alizad and J.F. Greenleaf, “Vibro-acoustic TissueMammography,” IEEE Transactions on Medical Imaging, Vol. 21, 2002, pp. 1-8.

Graham-Rowe, Duncan, “Handheld Terahertz Wand to Unmask Terror-ists,” New Scientist, July 2004, <www.newscientist.com/article.ns?id=dn6118>.

Hamilton, M.F. and D.T. Blackstock, Nonlinear Acoustics, Boston, AcademicPress, 1998.

Hart, Sarah V., “Making Prisons Safer through Technology,” CorrectionsToday, Vol. 65, 2003, pp. 26-28.

Heyman, J., A. Achanta, M. Hinders, K. Rudd and P. Costianes, “NonlinearAcoustic Concealed Weapons Setection (CWD),” Proceedings of SPIE: De-fense and Security Symposium, Vol. 5807: Automatic Target Recognition XV,2005, pp. 162-169.

Kaduchak, G., D. Sinha, D. Lizon and M.J. Kelechner, “A Non-contact Tech-nique for Evaluation of Elastic Structures at Large Stand-off Distances:Applications to Classification of Fluids in Steel Vessels,” Ultrasonics, Vol.37, 2000, pp. 531-536.

Kinsler, L. and A. Frey, Fundamentals of Acoustics, New York, John Wiley andSons, 1962.

Lee, Y.-S. and M.F. Hamilton, “Time-domain Modeling of Pulsed Finite-amplitude Sound Beams,” Journal of the Acoustical Society of America,Vol. 97, 1995, pp. 906-917.

Lee, Y., “Numerical Solution of the KZK Equation for Pulsed Finite Ampli-tude Sound Beams in Thermoviscous Fluids,” Ph.D. dissertation, Austin,Texas, the University of Texas at Austin, 1993.

Leisure, R.G. and F.A. Willis, “Resonant Ultrasound Spectroscopy,” Journalof Physics: Condensed Matter, Vol. 9, 1997, pp. 6001-6029.

Lima, R.S., S.E. Kruger, G. Lamouche and B.R. Marple, “Elastic ModulusMeasurements via Laser-ultrasonic and Knoop Indentation Techniques inThermally Sprayed Coatings,” Journal of Thermal Spray Technology, Vol. 14,2005, pp. 52-60.

Maynard, J.D., “Resonant Ultrasound Spectroscopy,” Physics Today, Vol. 49,No. 1, 1996, pp. 26-31.

Maynard, J.D., M.J. McKenna, A. Migliori and W.M. Visscher, “UltrasonicMeasurements of Elastic Constants in Single Crystals of La2CuO4,” Ultra-sonics of High TC and Other Unconventional Superconductors, PhysicalAcoustics, Vol. XX, M. Levy, ed., Boston, Academic Press, 1992, pp.381-408.

Nacci, Peter L. and Lee Mockensturm, “Detecting Concealed Weapons:Technology Research at the National Institute of Justice,” CorrectionsToday, Vol. 63, No. 4, 2001, pp. 70-74.

National Council on Radiation Protection and Measurements, PresidentialReport on Radiation Protection Advice: Screening of Humans for Security Pur-poses Using Ionizing Radiation Scanning Systems, Bethesda, Maryland, Na-tional Council on Radiation Protection and Measurements, 2003.

Ogi, H., Y. Minami and M. Hirao, “Acoustic Study of Dislocation Re-arrangement at Later Stages of Fatigue: Noncontact Prediction ofRemaining Life,” Journal of Applied Physics, Vol. 91, 2002, pp. 1849-1854.

Paulter, N.G., Guide to the Technologies of Concealed Weapon and ContrabandImaging and Detection, NIJ Guide 602-00, Washington, DC, National Insti-tute of Justice Law Enforcement and Corrections Standards and TestingProgram, February 2001.

Pompei, F.J., “The Use of Airborne Ultrasonics for Generating AudibleSound Beams,” Journal of the Audio Engineering Society, Vol. 47, 1999, pp.726-731.

Sennheiser Electronic GmbH, “Target Your Sound, Audio Beam ProductSheet,” Wedemark, Germany, <www.sennheiserusa.com/newsite/pdfs/AudioBeam.pdf>, 2004.

Spawar Systems Center, Correctional Officer Duress System — Selection Guide,document number 202947, San Diego, California, 2003.

Spoor, P.S., J.D. Maynard, M.J. Pan, D.J. Green, J.R. Hellmann and T. Tanaka,“Elastic Constants and Crystal Anisotropy of Titanium Diboride,” AppliedPhysics Letters, Vol. 70, 1997, pp. 1959-1961.

Westervelt, P.J., “Parametric Acoustic Array,” Journal of the Acoustical Societyof America, Vol. 35, 1963, pp. 535-537.

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