After reading Brian Litz' Applied Ballistics book, I was intrigued by his method for measuring G7 BCs by using downrange mics at 200 yd intervals, and a sound waveform analyzer. Laptops are common, mics are cheap, and there seems to be many freeware sound waveform packages out there (eg Sonic Visualizer). It seems like all one would have likely have to buy is two mics (and a way to mix their output if your card can't record in stereo. All the bits are battery-powered, so no worries on that note.
I did a brief error analysis. The two sources of error are the mic separation and resolving waveform leading edges.
Assume mics at 10' and 40' (10 yds spacing), sound card set to sample at the highest rate you normally can record digital sound files at - 96 khz. We'll use a notional muzzle velocity of 3,000 fps. Assume you can measure the mic separation to within 1" (0.28%). Hard mounting them could improve on that uncertainty.
The time of flight is .010 seconds or 10 msec. time measurement accuracy is .021 msec (0.21%, Nyquist sampling)
combined RSS error is .35% or 10 fps, which seems quite useable.
Obviously you're measuring velocity averaged 10 yds away from the muzzle, where the bullet has already lost about 10 fps. You could always add that back in as from JBM that seems to be representative.
I haven't tried this yet, but it's tempting to whip something up and compare side by side. It would be cheap and light-insensitive. Just not quite as easy to use- you'll get a readout in msec and need to convert to MV by dividing the mic spacing by that.
just edited to show freeware which will allow measuring time between shots in a sound waveform
This is a screenshot from SoundCard Oscilloscope. It's freeware by C Zeitnitz. easy to install an use
I did a brief error analysis. The two sources of error are the mic separation and resolving waveform leading edges.
Assume mics at 10' and 40' (10 yds spacing), sound card set to sample at the highest rate you normally can record digital sound files at - 96 khz. We'll use a notional muzzle velocity of 3,000 fps. Assume you can measure the mic separation to within 1" (0.28%). Hard mounting them could improve on that uncertainty.
The time of flight is .010 seconds or 10 msec. time measurement accuracy is .021 msec (0.21%, Nyquist sampling)
combined RSS error is .35% or 10 fps, which seems quite useable.
Obviously you're measuring velocity averaged 10 yds away from the muzzle, where the bullet has already lost about 10 fps. You could always add that back in as from JBM that seems to be representative.
I haven't tried this yet, but it's tempting to whip something up and compare side by side. It would be cheap and light-insensitive. Just not quite as easy to use- you'll get a readout in msec and need to convert to MV by dividing the mic spacing by that.
just edited to show freeware which will allow measuring time between shots in a sound waveform
This is a screenshot from SoundCard Oscilloscope. It's freeware by C Zeitnitz. easy to install an use
