11 Replies Latest reply on Jul 8, 2017 9:10 PM by Bob Howes

# An Experiment

I have a twelve year old son (an afterthought--his brothers and sisters are in their early 30s) who's into science.

Today he came to me with an idea for an experiment.  He's built a sort of sling shot device that involved a bottle top and part of a balloon and has been shooting those hard silver balls you can use to decorate cakes.  He gets enough power that he can penetrate a pop can.

Anyway, he wanted to know how fast he was shooting in feet per second and worked out that if used a couple of mics recording him firing at a target over a known distance, we could calculate the velocity.

So, a couple of mics and a six foot range and over 4 shots we got a variance between 172 and 222 feet per second...I'd call it an average of 200fps.  I wouldn't want to be hit!

I certainly proved useful to be able to zoom in to ten thousandths of a second on Audition though.

Frankly, I'm more proud that he worked out the method and did the maths than I am with the shooter thingie!

• ###### 1. Re: An Experiment

Way to go (as we say over here).  Sounds like a lad with proper values, and I suspect he will go far.  I am wondering what the microphones are picking up though?  Are you using the time between sound of the release, and the sound of the soda can being hit?  Or how does this work?

If you could drill a tiny hole through one of the projectiles and make it whistle, you could maybe work out the speed with Doppler?  That was how the Brit MOD measured artillery shell speed at Foulness — my last boss before I retired from Ford Research used to work there doing exactly that, and it was fascinating hearing about it.  Luckily he didn't have to kill me after telling us how it was done.

At one time, they looked at building a third London airport off Foulness, but one of the problems was clearing the thousands of shells they'd fired out onto the MOD owned mudflats (Maplin Sands)

• ###### 2. Re: An Experiment

Two SDCs...in this case AKG C451s as that's what I had handy.  On was a

few inch from the target (a piece of wood leaned against a blanket to

avoid too much reverb) and the other at where he was shooting (resting

on the edge of a chair a measured distance from the target.  Then I just

hit record and told him to "Go".  We did four takes to get a good average.

The measurement was just zooming tightly in on the wave forms, far

enough to get 4 decimal places.   Then a little math got us the time for

how long  it took from shooting until hitting the target then more math

to get the feet per second number.

You've got me interested in trying a doppler test though, if we can find

a way to make the "ammo" whistle.

• ###### 3. Re: An Experiment

A bit off topic, but true story, my wife once used those little silver balls to decorate a cake and they were terrible. Upon examining the bottle, the label said they were "not for human consumption" It turns out they were craft silver balls that accidentally found their way to the kitchen.

• ###### 4. Re: An Experiment

Trevor.Dennis  wrote

If you could drill a tiny hole through one of the projectiles and make it whistle, you could maybe work out the speed with Doppler?

Wouldn't that affect the velocity of the object?

• ###### 5. Re: An Experiment

I love the ingenuity of this experiment.

Dave

• ###### 6. Re: An Experiment

yes it adds drag... slows it down

• ###### 7. Re: An Experiment

Trevor.Dennis   wrote

If you could drill a tiny hole through one of the projectiles and make it whistle, you could maybe work out the speed with Doppler?

Wouldn't that affect the velocity of the object?

Yeah.  After sleeping on it, I came to the same conclusion as Ussnorway.

Well, that and I don't have anything to drill a small, precise hole without just crumbling the whole thing!

• ###### 8. Re: An Experiment

This guy does it with a single microphone. I'm wondering how the delay of the sound reaching the microphone will affect the experiment.  The mic looks to be equidistant from gun and target — does that cancel out speed of sound error?

• ###### 9. Re: An Experiment

The mic is 1 meter from each sound generator. The speed of sound is about 350 m/sec. So the sound of the shot registers 1/350 sec later on the mic, and the sound hitting the pan registers 1/350 sec later. Does that mean the total time measured is 2/350 sec longer than the actual time between the shot and the pan? Or is the total time measured 2/350 sec shorter than the actual time?

• ###### 10. Re: An Experiment

Yes, a microphone at exactly the halfway point would cancel out the effect of sound travel - adding the same delay to both sounds. The actual time between those sounds remains the same.

Dave

• ###### 11. Re: An Experiment

I'd gone the same calculations--sound travels approximately 1.1 feet in a millisecond so if the mic is exactly central.the two delays should cancel each other out.  However, I had two mics on stands sitting there so it was just as easy to us two.