The View Up Here

Random scribblings about kites, photography, machining, and anything else

Archive for November, 2015

Rebuilding the Contact Mic

Posted by Tom Benedict on 07/11/2015

The problem with prototypes is they’re never robust enough to exist in the field. Such was the case with my sadly lamented contact mic. Shortly after making the line scream recording it developed a bunch of noise and variable gain. Turns out my none-too-friendly treatment of the piezoelectric crystal had finally taken its toll. One of the wires had popped off, and the crystal itself was cracked.

So much for my $3.50 piezo buzzer from Radio Shack!

I didn’t want to go through the drill of gutting another Radio Shack piezo buzzer, so I bought ten piezo elements from SparkFun. The Alex Rice preamp was still fine, so I salvaged it and put a new piezo element on the end of the cable, but with an actual housing. It now sports a 1″ diameter aluminum shell and a 0.010″ stainless contact plate that the piezo is epoxied to. The shell and contact plate also act as an RF shield, so I had to isolate the piezo from the stainless. Turns out thin strips of Scotch Tape do a wonderful job of keeping things from contacting. And with the rigid housing I can clamp this thing down nice and hard without risking damage to the piezo.

Contact Mic Reboot\

While I was at it I made two other mics. One is attached to an alligator clip, and the other is attached to a magnet salvaged from a dead hard drive. The clip should make it easier to record line scream, and the magnet will cling like nuts to anything ferrous. (Sorry, no photos of these yet.) These won’t have hard housings on them since they won’t require clamps to use, but I potted the wires so they should be plenty robust in the field.

To make all this more modular, I’m planning to mount three Alex Rice preamps in a nice case and provide jacks for each one so I can mix and match contact mics / hydrophones on three of the four XLR inputs on my Tascam DR-70D recorder. (Why not four preamps? I only built three! But I’m planning to lay the box out for four in case I wind up needing quadraphonic contact audio.) The DR-70D has 1/4″-20 mounting hardware top and bottom, so I can put a tripod socket in the preamp box and mount it rigidly to the recorder.

This weekend is packed, but I hope to be back in the line scream business soon.

The aerial microphone array? That’s a whole ‘nuther story.

Tom

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