Monday, March 26, 2007

How to trigger a strobe for photographing bullets

We trigger the strobe by sound, as Bryan suggested. In particular, for a supersonic bullet, we use the sound of the shockwave that accompanies any supersonic object. This shockwave is also known as a "sonic boom".

Part (d) of this figure is a shadowgraph showing (in sillouette) a supersonic bullet, the V-shaped shockwave of the round, and the report from the exploding gunpowder (the circles emanating from the muzzle).

If we place a microphone just below the line of flight of the bullet, the microphone will "hear" the shockwave, generating an electrical signal that can be used to trigger the strobe. The bullet will only be an inch or so down range from the mike when the strobe fires.

Notice that if we don't like the position of the bullet in the photo -- say, the bullet is 2 inches further down range than we like -- we simply reposition the mike. In this example, pulling the mike 2 inches up range will ensure that the bullet is in the right spot for the next photo.

If round is subsonic (like the paintball), then it always lags behind the sound of the weapon firing (the report). In this case, the mike must be well down range from the target, and small variations in the projectile's speed from one shot to the next are more of a problem.

In the comments, El Tiberon asks what he won. If El Tiberon, Fargus, and J. will send me an email address (send it to my account, where my user name is "jimbales") I'll send you a hi-res bullet photo. Bryan, I've got your email address already. (It may be the end of the week before I can send it.)



Blogger Bryan said...

Thanks, Jim. My Dad would have been pleased to learn that I was paying attention when he was building his projects, from Heathkits to the guidance systems for ICBMs.

7:47 AM  

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