Using Audacity Recording Software to Find Bullet Velocity at Various Distances

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By using audio recording software such as Audacity, it should be possible to calculate the speed of bullets fired at metal targets by measuring the time it takes the sound of impact to reach the microphone after the bullet has been fired. This would be useful as both a basic chronograph, and for ballistics studies of various bullet shapes.

Since we had a weekend free, we decided to give the idea a test run. This was our first attempt, and it was intended strictly as a proof of concept. Like all good first experiments, we found two things we did wrong for each thing we did right.

The idea was simple. Since the speed of sound through air is dependent only on temperature, we should be able to record the sound of a bullet impacting on a metal target at a known distance, and subtract the time it took the sound of impact to travel back, to get the speed of a bullet over the distance. All we needed were some big chucks of metal, a good private shooting range, a tape measure, and a recording platform...

Interesting idea ... I think they need a Chronograph to check their figures.

I sense a project for Billl

 

[Zak Smith]

I fail to see the usefulness of this for their intended purpose of determining velocity because they are measuring the average verlocity over a fairly long distance. This doesn't give them the terminal velocity, nor the muzzle velocity. Deceleration is not at a linear (or even a simple) rate.

On the other hand, if they used local transducer at the muzzle and target (to reduce air propagation inaccuracies), over a long distance, and measured the time, they'd be in a good position to back-calculate B.C.

 

[Michael Courtney]

I fail to see the usefulness of this for their intended purpose of determining velocity because they are measuring the average verlocity over a fairly long distance. This doesn't give them the terminal velocity, nor the muzzle velocity. Deceleration is not at a linear (or even a simple) rate.

On the other hand, if they used local transducer at the muzzle and target (to reduce air propagation inaccuracies), over a long distance, and measured the time, they'd be in a good position to back-calculate B.C.

You're right that things are simpler with two microphones. However, even with a single microphone, you can use the manufacturer's published BC and a ballistic calculator to determine the muzzle velocity with an accuracy of better than 1%.

I worked with one of my Physics students to develop a similar technique that works with any soundcard and software that allows viewing of the waveform. A single microphone can be used to determine the muzzle velocity with accuracy comparable to a chronograph. The key issues for accuracy are a careful measurement of the distance to the target, knowing the temperature to accurately determine the speed of sound, and being able to discriminate the sound of the bullet hitting the target from other sounds.

Michael Courtney

 

[Rockstar]

Never start an experiment based on a false premise, "since the speed of sound through air is only dependent on temperature..."

 

[Henry Bowman]

Uh... A digital chrony is pretty cheap, and reads out in fps. This proposed solution seems to be inherantly less accurate, more cumberson, will require calculations to be made each time, and the equipment will probably cost as much (or more, since you probably are using a laptop) as a top of the line chrony. Did I miss something here?

But it is an interesting scientific discussion. And if I was stuck on an island with just my guns, reloading equipment, my laptop, a couple of microphones, and access to download the necessary software... I'd just send an e-mail with a voice message saying, "Come rescue me when I run out of powder."

 

[Michael Courtney]

Never start an experiment based on a false premise, "since the speed of sound through air is only dependent on temperature..."

At a fixed location, the speed of sound will also vary a tiny bit with humidity and barometric pressure, but these corrections will be much smaller than 1%. For most hobbiests, knowing their bullet velocoties to 1% accuracy is more than adaquate.

Michael Courtney

 

[Michael Courtney]

Uh... A digital chrony is pretty cheap, and reads out in fps. This proposed solution seems to be inherantly less accurate, more cumberson, will require calculations to be made each time, and the equipment will probably cost as much (or more, since you probably are using a laptop) as a top of the line chrony. Did I miss something here?

The manual calculations can be automated, and the principles involved put us one step away from using any notebook computed with a soundcard as a chronograph with no additional hardware investment.

To quote from the report my student and I wrote on the subject:

Measuring the velocity of a bullet with a chronograph is a relatively simple process and the price of these instruments has become more affordable in the last several years. However, shooters who use chronographs regularly and under a variety of conditions have probably noticed some drawbacks. Replacing skyscreens and electronics can become a recurring operational expense, especially if one’s aim is poor or if one regularly chronographs shotgun loads or saboted muzzleloader projectiles. Wads and sabots can quickly separate and damage the equipment. Instruments placed in front of black powder firearms are subject to corrosion if not carefully cleaned. An important rule of firearms safety is never allow the muzzle to point at anything you are not willing to destroy. Many chronographs end up vivid demonstrations of this important safety rule.

Chronographs can also be inconsistent in some lighting conditions such as late afternoon, early morning, or the covered shooting area of a range. Cables and sky screens can be an irritation to set-up. Fiddling around with a chronograph in front of the firing line is an unwelcome activity at many range facilities.

This article describes a simple method for using a PC soundcard to accurately measure bullet velocity. Soundcards are inexpensive and readily available, and the majority of computers today already include a soundcard. The biggest advantage of the method is that no expensive equipment needs to be placed in front of the gun.

To summarize, measuring bullet velocity with a PC soundcard eliminates the need to put pricey electronics in front of the muzzle of a gun. It also allows anyone with a PC+soundcard to measure bullet velocities without investing in a chronograph. A third advantage which has not yet been mentioned is that it is possible to infer the BC of a bullet from velocity measurements across different distances. In other words, if one knows the average velocity for 50 yards, the average velocity for 100 yards, and the average velocity for 200 yards, one can determine the ballistic coefficient. Determining BC with a chronograph requires shooting through the chronogragh at different ranges, which can greatly imperil the electronics. (One can also use a chronograph and the PC soundcard technique together to determine the ballistic coefficient, or one can use two microphones with a soundcard, one at the muzzle and the other at the target.)

Perhaps the most interesting potential application of this technique is forensic. One can determine the velocity of a bullet in a shooting event if the shooter, target, and microphone are at known locations and an audio recording of the event is available. Alternatively, if you know the locations of the target and microphone and the velocity of the bullet, you can determine the location of the shooter.

Michael Courtney

 

[james481]

Being someone who has access to much computer equipment (I've probably shot more of it than most people will ever own), and no chronograph, this is kinda interesting. Hasn't the military already started to deploy a system to track the location of gun shots using an array of microphones? Does this operate on similar principles?

 

[Michael Courtney]

Being someone who has access to much computer equipment (I've probably shot more of it than most people will ever own), and no chronograph, this is kinda interesting. Hasn't the military already started to deploy a system to track the location of gun shots using an array of microphones? Does this operate on similar principles?

There are added complications, but the principles are similar. Some urban areas have systems deployed also.

Michael Courtney