دانلود رایگان مقاله انگلیسی تحلیل مشخصه های صوتی شلیک گلوله به همراه ترجمه فارسی
|عنوان فارسی مقاله||تحلیل مشخصه های صوتی شلیک گلوله|
|عنوان انگلیسی مقاله||Acoustical Characterization of Gunshots|
|رشته های مرتبط||فیزیک، فیزیک کاربردی|
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|نشریه||آی تریپل ای – IEEE|
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IV . بررسی دیگر موارد کاربردی
بخشی از مقاله انگلیسی:
Criminal, terrorist, and military actions involving firearms are of increasing concern to police, soldiers, and the public. A variety of commercial and experimental acoustical detection and classification systems designed for gunshot sounds are available. These systems can be intended to detect acoustical “gunshot signatures,” to classify or identify specific firearm types, and to detect and localize snipers. The degree to which a system can achieve satisfactory performance is typically limited by the assumptions required to estimate the firearm and/or projectile behavior based on the available acoustic evidence. Assessing and evaluating acoustic gunshot detection systems requires a thorough understanding of the characteristics of gunshot sounds and the significance of sound wave reflection, absorption, and diffraction from the ground, buildings, and other nearby objects.
A. Muzzle Blast A conventional firearm uses a confined explosive charge to propel the bullet out of the gun barrel. The sound of the explosion is emitted from the gun in all directions, but the majority of the acoustic energy is expelled in the direction the gun barrel is pointing [1-5]. The explosive shock wave and sound energy emanating from the barrel is referred to as the muzzle blast, and typically lasts for less than 3 milliseconds. The muzzle blast acoustic wave propagates through the air at the speed of sound (e.g., 343 m/s at 20°C), and interacts with the surrounding ground surface, obstacles, temperature and wind gradients in the air, spherical spreading, and atmospheric absorption. If a recording microphone is located close to the firearm, the direct sound of the muzzle blast is the primary acoustical signal. On the other hand, if the microphone is located at a greater distance from the firearm the direct sound path may be obscured and the received signal will exhibit propagation effects, multi-path reflections, and reverberation. Some handguns and rifles can be equipped with an acoustic suppressor. Suppressors are designed to reduce the audible report (and often the visible explosive flash) of the muzzle blast to reduce the likelihood of detection and/or to prevent hearing damage. Thus, gunshot acoustical detection systems that rely on the muzzle blast must accept the possibility of suppressor use by clandestine individuals.
B. Mechanical Action For some firearms the sound of the mechanical action may be detectable. This includes the sound of the trigger and hammer mechanism, the ejection of spent cartridges, and the positioning of new ammunition by the gun’s automatic or manual loading system. The mechanical action is, of course, generally much quieter than the muzzle blast and projectile shock wave, so this acoustical signal is only relevant if the microphone is located close enough to the firearm to pick up these subtle, telltale sounds. For example, personal surveillance recordings or recorded phone conversations that take place in proximity to the shooter may contain this information.
IV. OTHER PRACTICAL CONSIDERATIONS
A subsonic rifle or handgun without a suppressor will produce a muzzle blast acoustic signal, but the subsonic projectile will not create a shock wave or any other appreciable acoustic signal as it propagates through the air. A recording of a handgun and subsonic bullet (HK USP compact, 40 Smith and Wesson, Federal Hydroshock) is shown in Fig. 6. Note that no shock wave signature is present. The muzzle blast of this particular handgun is less intense and shorter in duration than the muzzle blast of the rifle shown in Figs. 2-4. It would be desirable for criminal forensic analysis to be able to identify a specific firearm from an audio surveillance recording, such as a 911 call or a tape of a land mobile radio conversation in which a gunshot was captured, but conventional audio recordings have not been shown to be reliable for identifying particular firearms . However, recordings obtained in a controlled manner such that the orientation of the firearm and the distance between the gun and the microphone are held constant do show consistency from one shot to another . At distances far from the bullet’s trajectory, the shock wave will have expanded sufficiently by spatial spreading that it may no longer be detectable compared to ambient noise. Also, as noted previously, the situation is much more complicated if the acoustical surroundings include obstacles and reflecting surfaces so that the received acoustical signal contains multipath interference, diffraction effects, and other propagationrelated flaws. The very short duration of the muzzle blast and the acoustic shock waves act like acoustic impulses, so gunshot recordings obtained in complicated surroundings will consist of the convolution of the gun’s report and the acoustic impulse response of the local reverberant environment, resulting in substantial temporal smearing. In fact, reverberant recordings will typically contain more information about the acoustical surroundings than about the gun or the projectile. Deconvolution of the gunshot from the reverberant background can be attempted, but no completely reliable means to accomplish this task for gunshots has been published.