Sound Based Airsoft Chronograph

airsoft chronograph

Seattle Airsoft has a great post on measuring BB velocity using a microphone. They use two pieces of paper separated by 10 feet with a microphone by each one. Audacity is their program of choice for recording. If you look at the waveform after firing you’ll see three distinct spikes: one from the firing mechanism and one from each paper strike. They compared the results from this method to those of an actual optical chronograph. The mic seemed to deviate far less.

[thanks aqua_scummm]

32 thoughts on “Sound Based Airsoft Chronograph

  1. Right on! Everything a good hack should be: simple, easy with a little know-how, and better than the commercial version. ;)

    I’m wondering what causes the discrepancy in the deviation of the measurements, though. He does say it’s a cheap model of optical chronograph, but I’m wondering if it has something to do with the measuring systems. Specifically, I’m wondering if the BB takes a tumbly sort of course through the air (well, it does), which might throw off the chrono.

  2. nice, very pimptastic.

    “simple, easy with a little know-how, and better than the commercial version.” – I love that

    Audio recordings are acutally great ways to messure things, I used mutiple microphones to messure the speed of an rc car I was building on time that worked amazinngly well. i would really recommend that any project you have that you want to messure the speed of something, you consider how audio could help.

  3. Jdoggy,

    You are right with the paper slowing the BB down. thinking about this more. Wouldn’t it work to measure the sound of the gun firing and then the sound of the bb hitting a solid object. Just make the gun a known distance from the object. That way the first paper doesn’t slow the bb down since it isn’t there. Also, saves on paper.

    –Kurtroedeger (off to get his air tank filled)

  4. I like this new idea, mic at the muzzle and a solid object. This way you could measure the speed of many different projectile weapons. Everything from bb guns, to paintball guns to a bow and arrow (with some working…) Awesome.

  5. In the RC plane world we use the same program to measure the speed of our planes. They are too fast and do not reflect enough radar to get an accurate reading with a radar gun. But if you perform several high speed passes over a microphone you can measure the speed based on the doppler shift of the engine noise and average the speed from a few passes to get a v-e-r-y accurate top speed for your model.

  6. andrew – you think so? I considered that (and obviously it depends on several factors like humidity, how brittle the paint is, distance from the paper, etc), and I’ll admit I’ve never shot at a taut piece of paper, but I’m pretty confident it would be able to rip through both. Still, going along with what someone said about the first piece slowing down the projectile, it seems to make the most sense to measure out the distance and use only one piece of paper.

    Too bad I saw this a year after I gave up paintball to pay for tuition, having a chrono at home would have rocked.

  7. You should be able to factor in the slowdown caused by the paper into the result and boost the acuracy. One way to do this would be to add a 3rd sheet of paper and then mesure the difference between the 1st and 2nd sheet vs. the 2nd and 3rd sheet. that decrease in speed could then be added to the first result to correct the diffrence.

  8. Detecting the sound of the gun firing and then hitting the target would also work.

    But the problem is, how do you make sure the distance between the muzzle and the target is a constant between all tests? Just holding the gun is never going to be accurate enough.

    You would have to hold the gun down in vice that was mounted solid so that it couldn’t move. Beyond the possibly of damaging the gun (let’s not forget, most AirSofts are made of plastic), that means there will be more setup and hardware required.

    While not the most accurate way to do this, it is certainly the easiest and quickest way.

  9. On the last page, in the second to last paragraph, they state that the paper took 3-5fps off, and that once they get their calculator operational, it will compensate for it. That seems rather fishy to me, but the important thing here is the fact that this has less deviance than the commercial chrono.

    IF you set it up to detect the sond of the gun firing, and then the sound of the BB hitting the paper, you could get much more accurate and consistent resuslts.

    As for keeping the gun the same distance for all shots: may be worth buying a bipod or something.

    Off-topic: Paintballs shouldnt break on only taut paper. Ive shot through a (old)shower curtain before (the field had improvised some of their bunkers. Was rather fun actually).

  10. Hey, I wrote the article and wanted to respond to a couple comments.

    The reason I did two pieces of paper instead of one is because I was going for maximum simplicity over maximum accuracy. Picking out the sound of a BB leaving the barrel vs the sound of the piston, motor, and random air movement was a little bit difficult, and I wanted the project to be accessable to as many people as possible, even if they weren’t used to looking at a sound wave. If I were making one for personal use I’d probably do it more like Andrew and use an optical system.

    The calculator was hacked together in a pretty short period of time by my friend who runs the site, but it does account for the loss of fps from hitting the paper. Again, simplicity over accuracy. My hope was that pretty much anyone could replicate my method, and more experienced people could modify it to fit their needs. I’m glad you guys got a kick out of it :)

  11. I guess in an airsoft context two pieces of paper would make sense, since the airsoft guns I’ve seen have all been fairly quiet. With paintball markers, on the other hand, there would be no mistaking the audio spike of the initial shot.

  12. My first thought was that the papaer slowing down the ball would make it really inaccurate, especially for cheap airsoft guns, and i was aggreeing with people that said use the spike that comes from the gun fireing, but since I do some programming on the side I came to realize that it was probably something with the sound from both paper strikes being kinda similar and is the same in nearly all cases, but each airsoft gun ussually has a slightly different sound to it fireing which would play hell with the programming.

    When I read the creators post it makes sence, and since paper ussually has a uniform consistency you could probably figure out the speed drop from the first paper strike.

    Also, it depeneds on what sort of paintball you are using, I think that if you got some cheap balls they would proably go through a sheet of paper pretty easily. Or you could freeze them before fireing them through the paper to insure they do not break.

  13. jonny_s, yes, in theory. Have you ever fired an airsoft gun? they’re loud. the motor is loud, the piston is loud, and the gearbox makes all kinds of fun noises. not to mention the pop of air that comes out the barrel. in our first few tests we spent a lot of time looking for the spike that was the bb hitting a piece of paper, and picking it out of the other random noise that happens right around then. Eventually we decided that it would be easiest just to turn the input volume way down, and put a mic by each piece of paper so that was essentially the only sound they’d pick up. with spring guns this wasn’t as big of a problem, and with gas guns the noise made by the slide would completely drown out the sound of the bb hitting the paper.

  14. I was thinking perhaps using tinfoil instead of paper, and making some kind of switch that detects the flick of foil.
    Just thinking about doing this with sub-sonic .22, but i dont quite like the idea of using sound. anyone have any more links?

  15. there is an instructable about that, a switch that sets of a flash, based on an airsoft bb going thru 2 layers of tinfoil, seperated by wax paper. when the tinfoil on each side meets as the bb passes thru, the circuit is completed. somewhere on the tinfoil wouldnt slow the .22 much

  16. Sound is actually the method used to capture the results of Olympic shooting events. The method, perfected by a Swiss firm, Sius Ascor, uses a sealed target frame, with either a light cardboard or thin self-healing rubber target face. When a projectile, any calibre from air pellet up to .50 cal, passes through the target face, three small microphones within the sealed target frame capture the sound, and triangulate the projectiles position on the target

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