# Supersonic Speed Measurement With A Sound Card

You might think that if you have a need to measure the speed of a projectile that is too fast for your high-speed camera, you would have to invest in some significantly expensive equipment.

That was the problem facing [Nick Moore], and the solution he arrived at is extremely elegant in its simplicity. He’s arranged a pair of foil tapes in the path of the projectile, as it passes through them they break, and he measures the time between those breaks. The clever bit though lies not in the tapes, but in how he measures the timing. Instead of relying on a lab stuffed with equipment, he’s using his computer sound card. The outputs send a tone through each tape to the inputs, and using Audacity he can capture both tones and measure the time between the end of each one on left and right channels.

In the video below the break he demonstrates measuring the speed of a supersonic particle at 496.5 metres per second, which for such relatively simple equipment is rather an achievement. He could certainly improve his resolution by increasing the sampling frequency, but we are guessing that the choice of 48 kHz owes much to the quality of his sound card. Still, to achieve this with such a relatively basic piece of equipment is a neat achievement.

This project isn’t the only attempt to measure high-speed projectiles we’ve shown you, there was this high-speed Nerf dart speedometer. We’ve taken this field further into the realm of science though, measuring the speed of sound.

Thanks [Matthew Simmons] for the tip.

## 17 thoughts on “Supersonic Speed Measurement With A Sound Card”

1. would’ve made it a lot easier for some of the mythbusters tv episodes… :) Brilliant!

2. Internet says:

Can also amusingly use something similar to this to trigger high speed cameras.

3. Thanks, I would have used a scope but a)mine has no storage and was made in the 60’s and b)this is something that damned near anyone can do.

1. doppler says:

Made a chronograph using a sound card. Unless you are measuring something faster than a bullet. You can get a chronograph for under a \$100. Under \$200 if you want a fancy one.

1. Megol says:

1. Internet says:

Not quite \$0 in parts but your time still has value.

2. TheRegnirps. says:

Hey, it works. OTOH, a \$5 Arduino clone using the counter-time would also work off the broken conductors, with a lot more resolution.

2. macona says:

Did this in high school around 1992 with one of the macs at at school but used a single mic, a wall, and something loud to make a noise (I think hitting a piece of metal with a hammer) The mic was by you and then you measured the time between source sound and the echo on the computer with SoundEdit 16.

1. macona says:

Yes, I realize not the same thing, but one of many things you can do with a sound card. A friend took a mac and had it play a spectrum of sound while the computer listeners for things resonating with the output sound. Then he could “play” things in the room, like a rack of pans.

3. imqqmi says:

Very clever, love the simple and effective solution! I’d use a microcontroller with a Charge Time Measurement Unit (CTMU) which can be setup with one pin as a start trigger and a second as a stop trigger (first and second foil strip). Depending on the mcu it can measure down to around 7 pico seconds. It’s fast enough to measure reflection time of a square wave signal in a 1m+ coax length cable.

4. Paul says:

The coolest “sound card” hack/app I saw was on the NeXT: it transmitted a CW tone, and listened for the echo, computing (on its DSP) FFTs in real time and looking for the Doppler shift of echoes. It could easily “see” people walking around the room. Maybe not so special now, but it was pretty awesome to do this in real time in 1989.

As for the chronograph: you could get much better precision than one sample period by fitting the sinewave to the signals. But it would be simpler to just forget using a tone source at all: use instead a DC bias, and look for the edge when the tape gets broken. Fitting an edge to the signal makes it trivial to get close to microsecond timing precision. At that point it would be important to characterize the relative delays between the two channels.

Or even better: forget the tone generator and forget using breakable tapes you destroy and have to rebuild for each shot. Use instead two microphones and measure the time of arrival of the shock wave at each. (rather, cross-correlate the samples sound and measure the delay).

1. I thought about using a DC bias but audio-cards usually have DC blockign caps. There would still probably be a spike at the moment of breaking but this way I didn’t have to use any external power just some gator clips.

I like the microphone idea but it would only work for super sonic projectiles (not every shot is a ringer). I may try that int he future however to show the difference between sound moving at the speed of sound and a real sonic boom. Thanks!

5. Werner says:

“You might think that if you have a need to measure the speed of a projectile that is too fast for your high-speed camera, you would have to invest in some significantly expensive equipment.”

.. or use a 2\$ microcontroller.

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