Many communities are installing gunshot detectors on lamp poles to help triangulate the location of gunfire during holidays or to help in responding to "shots fired" 911 calls. Besides location, another piece of useful information is the caliber of the gun that was shot. Fortunately, studies have shown that each caliber has a distinct noise spectrum. Police can use this spectrum to identify the gun's caliber, rather than using loudness, which isn't reliable. The circuit in Figure 1 has been used to differentiate between different calibers of gunshots.

The circuit employs a seven-band graphic equalizer, the Mixed Signal Integration MSGEQ7, which has seven bandpass filters set at 63, 160, and 400 Hz and 1, 2.5, 6.25, and 16 kHz. The equalizer has a fixed 24 dB of gain on the input. The output is seven dc levels clocked out for interface with a microcontroller's analog-to-digital converter (ADC) input. A TLC081 amplifier supplies the additional gain needed for the electret's microphone. Figure 2 depicts the timing for the reset and strobe clocks relative to the output.

The two most popular calibers, 22LR rimfire and 9-mm Luger, were used as sound sources to demonstrate the circuit's operation. For the sound of the 22LR, the output of the MSGEQ7 showed spectrum peaks at 2.5 kHz and 6.25 kHz (Fig. 3a). For the larger-caliber 9-mm Luger, the spectrum peaks were at 400 Hz and 1 kHz (Fig. 3b).

Along with crimefighting applications, determining the caliber spectrum can help timers for field sports at busy shooting ranges. Shot timers for measuring the time it takes to engage targets use only the microphone sensitivity control to determine if the shot is close or far. Louder rounds fired by other shooters can cause false time readings. By adding the circuitry described in this Design Brief, a timer could set the equipment to measure only the desired small-caliber or large-caliber shots.