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Audio Engineering SocietyConference PaperPresented at the Conference on Forensic Audio2017 June 15–17 Arlington, VA, USAThis paper was peer-reviewed as a complete manuscript for presentation at this conference. This paper is available in the AES ELibrary (http://www.aes.org/e-lib) all rights reserved. Reproduction of this paper, or any portion thereof, is not permitted withoutdirect permission from the Journal of the Audio Engineering Society.Gunshot acoustics: pistol vs. revolverRobert C. Maher and Tushar K. RouthElectrical & Computer Engineering, Montana State University, Bozeman, MT USA 59717-3780Correspondence should be addressed to R.C. Maher (rob.maher@montana.edu)ABSTRACTAudio forensic investigations may require interpretation of recordings containing gunshot sounds. These sounds arenotable because of their impulsive nature: very high sound pressure and very short duration compared to other soundsrelevant to forensic analysis. In this paper we examine the acoustical characteristics of muzzle blast sounds from twohandguns: a Glock 19 pistol and a Ruger SP101 revolver. The muzzle blast sound of each handgun was recorded atseveral azimuth angles between 0 and 180 degrees with respect to the barrel using a quasi-anechoic methodology.Compared to the pistol, the revolver exhibits a more complicated acoustical pattern due to sound emanation from twosources: the cylinder-barrel gap and the muzzle.1 IntroductionGunshots are among the acoustical signals that may beof interest in audio forensic investigations. Gunfire maybe recorded by an emergency call center as part of atelephone conversation, a dashboard or vest cameraused by a law enforcement officer, a surveillancesystem at a business or private residence, or a bystanderor newsgathering team with a cell phone or camcorderat the scene of an incident. Audio forensic questions forgunshot recordings include determination and/orcorroboration of the sequence of events before, during,and after the shots, the shot timing, firearm position,type of firearm, and frequently the question of “whoshot first?” when multiple shots have been recorded[1, 2].Gunshot sounds present a challenge for recording andforensic interpretation. The very high peak soundlevels, especially near the firearm, often overload therecording microphone and preamplifier, resulting inclipped and distorted waveforms. Even in the situationin which the recording system is sufficiently far awayfrom the firearm so that the recorder is not overloaded,the recording will generally contain a mixture of thedirect sound of the gun and reflections andreverberation from the acoustical surroundings. What’smore, many contemporary surveillance recordingsystems use digital data compression algorithms thatmay be optimized specifically for speech intelligibilityand general audio purposes, but not for the very briefand high amplitude sound characteristics of nearbygunfire.For these reasons, the common forensic audio questionsabout the type of firearm and “who shot first?” can bevery difficult to answer because of the limitations of therecording. Thus, experiments under controlledconditions are needed.In order to study the acoustical characteristics of thegunshot sound itself without interference from the

Maher and RouthGunshot acoustics: pistol vs. revolverreflections and reverberation of the environment andfrom the distortion and other limitations of therecording system, for research purposes we havedeveloped a quasi-anechoic recording methodology.The procedure involves elevating the firearm and therecording microphones in an open outdoor area so thatthe firearm’s muzzle blast can be recorded in its entiretybefore the first reflection (from the ground) arrives atthe microphone [3, 4].Distinguishing among different firearms based upon thegunshot sound is only possible if there are distinctiveacoustic features the distinguish one shot from another.Our recent research using carefully-controlled gunshotrecordings is starting to reveal the subtleties of thesedetails.handguns are semi-automatic, which means the gun hasa mechanism that automatically ejects a spent shellcasing and positions a new round in the firing chamberafter each pull of the trigger, without a manual cockingmechanism. Each pull of the trigger for a semiautomatic firearm shoots one bullet. 1There are two common types of repeating handguns thatdiffer in the manner in which the ammunition ispositioned into the firing chamber: pistols andrevolvers. We use the term pistol (Fig. 1) to refer to ahandgun that has a single firing chamber positioned atthe back of the handgun’s barrel, and revolver (Fig. 2)to refer to a handgun with a cylinder containing multiplefiring chambers that are rotated into position behind thebarrel for each shot.2 Gunshot acousticsA conventional firearm uses an ammunition cartridgeconsisting of a projectile mounted in a shell casingcontaining the primer and propellant material. Thecartridge is positioned at the back end of the gun barreland the gun’s trigger mechanism causes a firing pin tostrike the cartridge, igniting the primer material, andrapidly combusting the propellant. The ignitedpropellant creates heat and expanding gases thataccelerate the projectile through the gun barrel and outof the firearm’s muzzle. The high pressure gas andcombustion debris emerges out of the barrel behind theprojectile, causing the acoustical ‘bang’ referred to asthe muzzle blast. The muzzle blast sound, lasting only afew milliseconds, propagates outward in all directionswith highest sound levels typically observed in thedirection that the firearm is pointing [5, 6].Figure 1: An example pistol (Glock 19).3 Handguns: pistols and revolversHandguns are firearms designed to be operated with onehand: the gun has a hand grip, trigger, barrel, andmounted ammunition, and is fired by grasping thehandle, pointing the gun, and pulling the trigger. Manymodern handguns are repeating guns, meaning that theyhave multiple rounds of ammunition stored in anintegral compartment or magazine that can be firedsuccessively without manual reloading. Some repeatingFigure 2: An example revolver (Ruger SP101).A fully automatic firearm shoots a rapid succession ofbullets as long as the trigger is held down (i.e., a“machine gun”).1AES Conference on Audio Forensics, Arlington, VA, USA 2017 June 15–17Page 2 of 7

Maher and RouthGunshot acoustics: pistol vs. revolverAmong the differences between pistols and revolvers isthe manner in which the firing chamber connects to thegun barrel. A pistol typically has a closed chamber sothat the expanding combustion gases are confinedwithin the gun and the barrel until emerging from themuzzle. A revolver, with its multiple chambers rotatedinto position for firing, has a small gap between thefiring chamber and the gun barrel at the point where therotating cylinder and the back of the gun barrel meet(Fig. 3). Although designed to be small, the gap is oftensufficiently large that some of the high pressurecombustion gases leak out from the cylinder-barrel gapand cause a separate sound report before the muzzleblast [5]. The time interval between the report from thegap and the report from the muzzle is the 200microseconds required for the bullet to travel from thefiring chamber to the muzzle.Twelve microphones are located on a mounting rig of3 meters radius surrounding the shooting position(Fig. 4).The microphones are G.R.A.S. type 46DPmicrophone sets, consisting of type 40DP 1/8"diaphragm 200 volt externally polarized condensercapsules, type 26TC ¼" preamplifiers, and type 12AAand 12AG power modules providing the 200 voltpolarization and 120 volt preamplifier power.The microphones provide 2dB frequency response to140 kHz, with dynamic range specification between 46dB lower limit and 178 dB upper limit (132 dB dynamicrange). A multichannel 16-bit digital audio recorder(500 kHz sampling rate per channel, NationalInstruments NI PXIe-1071 chassis equipped with a NIPXIe-8840 Core processor and NI PXIe-6358 dataacquisition card) simultaneously records eachmicrophone. The firearm shooting position is at thecenter of the microphone arc.Aluminum FrameMicrophones(12 GRAS 46DP)ShootingPosition(3 meters)Figure 3: Gap between cylinder and barrel for arevolver.Plan View4 Gunshot recordingsTo investigate the acoustical differences betweengunshots from a typical contemporary pistol and arevolver, we obtained carefully-controlled recordings ofa Glock 19 pistol and a Ruger SP101 revolver using thequasi-anechoic methodology described in our priorstudies [3, 4].Figure 4: Microphone mounting frame surroundingthe shooting position. The frame holds 12 GRAS46DB 1/8" diaphragm microphones 3 meters from theshooting position and 3 meters above the ground.AES Conference on Audio Forensics, Arlington, VA, USA 2017 June 15–17Page 3 of 7

Maher and RouthGunshot acoustics: pistol vs. revolver8000Amplitude [pascal]6000400020000-2000-400001234567Time [milliseconds]Figure 6: Glock 19 pistol waveform, on-axis(0 azimuth), 3 meters.8000Figure 5: Handgun data collection using themultichannel recording system at the shooting range.As shown in Figure 5, the shooting platform and themicrophone rig are both elevated 3 meters off theground of the shooting range. This provides a sufficienttime delay between the arrival of the direct sound at themicrophones and the arrival of the first reflected soundfrom the ground, ensuring that the entire muzzle blast isrecorded anechoically [3].Ten test shots were recorded for both the Glock 19pistol and the Ruger SP101 revolver. The ten shots wereexamined for consistency and repeatability [7].5 Glock 19 pistolFigure 6 shows an example of the quasi-anechoic signalrecorded on-axis for the Glock 19 pistol. The waveformshows a sudden onset as the muzzle blast sound arrivesat the microphone, including an impulsive peakattributable to the abrupt pressure disturbance emittedby the muzzle. The acoustic pressure declines andinverts rapidly with a secondary positive spike beforereturning to the ambient background pressure withinabout 2 milliseconds of the onset.Amplitude [pascal]6000400020000-2000-400001234567Time [milliseconds]Figure 7: Glock 19 pistol waveform, 98 azimuth, 3meters.The Glock 19 waveforms as a function of azimuth areshown in Figure 8. The muzzle blast signatures differwith azimuth, primarily reflecting the directionality ofthe sound source: the level in front of the firearm (lowazimuth angles) is higher than the level as the azimuthincreases.6 Ruger SP101 revolverThe Ruger revolver produces an on-axis soundwaveform that is similar to the Glock 19 pistol, butexhibits a higher peak sound pressure and slightlylonger duration. The on-axis waveform is shown inFigure 9.The quasi-anechoic pressure waveform recorded to theside of the firearm is shown in Figure 7 (98 degreesazimuth). The waveform is lower in amplitude than theon-axis report, but has similar general characteristicsand duration.AES Conference on Audio Forensics, Arlington, VA, USA 2017 June 15–17Page 4 of 7