Cd quality sampling rate is 41,000 cycles per second.
Sound travels 1,100 fps (1,126 ft/s precisely, but it can vary depending on air density, etc.)
A sound card sampling at 41,000 times per second listening for the impact on a target 90.9 feet away (to make calculations easy).
Suppose a bullet were travelling 1,100 fps. The time between the report of the shot and the sound of the hit would be about 1/6 second. The time between report of the shot and the sound of the hit at 2,200 fps would be 1/8 second. At 3,300 fps about 1/9 second.
At a sampling rage of 41,000 times per second, a single cycle can differentiate
between 1100 fps and 1100.3 fps.
between 2200 fps and 2201.3 fps and
between 3,300 fps and 3,302.9 fps.
That is assuming the microphone and sound card will pick up the exact cycle, which I am guessing might be a bit of a stretch.
Still, the theory is sound.
Now, shorten the distance to 22.75 feet and the resolution changes to:
1100 to 1101.3
2200 to 2205.2
3300 to 3311.7
If the speed of sound varies, (as it does) your calibration will be off.
But if you are going to shoot at a target that clangs, you might consider using a two-microphone setup with one mic downrange and the other at the firing line.
That would eliminate the atmospheric speed of sound variation. Simple two-conductor wire going to the target location with the cheapest microphone you can get, located, say, 4' from the target (right channel of a stereo card laying on the ground with the target 4' tall) and a second mic (left channel) at the firing line 4' from the muzzle.
I think the only trick would be making sure there is no time difference between the sound of firing and the exit of the muzzle (like the blast from the barrel/cylinder gap and the blast from the muzzle).
Not as good as my idea of a radar gun capable of reading the reflection from the base of a departing bullet, but a lot cheaper.
p.s. The cheapest Chronographs are under $100. A portable PC, sound card, microphones and a program to read out the velocity rather than you having to read the traces from a sound card, input them into a spreadsheet and find the velocity is well worth $100 and the risk you might shoot your instrument. Besides, you still have to keep your muzzle a precise distance from the target (which must be able to make a decent amount of noise when hit as well as being the precise distance). Using a chronograph is a lot simpler.
Last edited by Lost Sheep; April 7, 2013 at 01:54 AM.