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.