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.

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Projectile speed sensor

[Mike] built a sensor rig to measure projectile speed. The setup uses a tunnel with two sensors in it. Each consists of a laser diode on one side focused on a photodiode in the other. The two are monitored by an op amp and measured by an ATmega128 microcontroller. When the beams are broken the elapsed time between the two events is measured in order to calculate speed. There is a setting to adjust the calibration for a range of speeds, which came in quite handy as [Mike] initially tested the device with rubber bands before moving on to a pellet gun and then a rifle.

It seems like he’s tempting fate by shooting a target just a few inches below his exposed circuitry but his marksmanship prevailed. We’ve seen bullet speed detectors in the past, used just for the delight of seeing how fast the projectile is moving, and also to capture an impact at just the right instant.

Beauty in Destruction

This is not a hack. In fact it’s a promotional montage for a collection of scientific equipment that few of us could likely afford. But like yesterday’s giant marionettes over Berlin, sometimes even a costly and delicately-orchestrated achievement transcends its own not-a-hack-ness, fulfilling our brains’ lust for wonderment all the same.

Kurzzeit of Germany produces ballistics measurement equipment. The video depicts various combinations of projectiles and targets at up to one million frames per second, revealing unexpected beauty in hitherto unseen phenomena, and is the best damn ten minutes you will waste on the internet all day!