Friday, October 26, 2007

Aiwa Rochelle Salt Crystal Microphone

The crystal microphone is an orphan. I cannot think of any air coupled crystal mics in production today, though there are many piezoelectric transducers, mainly contact or water coupled, or ultrasonic, out there for sale.

The crystal mic as you see it here was a typical Japanese style lapel mic, sometimes called a "spy" mic, because it could be concealed, and was supplied with numerous inexpensive 3" reel to reel tape recorders. The assembly is simple and consists of a two piece steel can with the element within. A conical perforated disc protects an aluminum foil diaphragm with a center pin attached internally to a piece of Rochelle salt, which is a piezoelectric substance. Rochelle salt generates a lot of voltage when deformed, and this type of mic has quite a good output. Its tinny sound is more a product of the packaging and the diaphragm than the inherencies of Rochelle's matter. In fact there are many Rochelle salt crystal earphones of the Japanese type still out there and working today, and if you have a chance to listen to one you might be surprised and how smooth and deep the bass from one of these gadgets can be. You can make your own Rochelle Salt! Here is the recipe!

But Rochelle salt has a problem or two: It is heat sensitive, and past some temp near boiling it is ruined. Even more important, the material is hygroscopic, which means it absorbs water easily, rendering it inoperative. Despite these limitations, the cheap crystal elements produced in the millions by Japan inc in the 60s keep transducing away. Perhaps it is time to revisit the Rochelle salt design, and see if it has any advantages over the conventional microphone of today.

3 comments:

Graham Spice said...

I'd be interested to see if you were able to get their frequency response up to snuff. Seems like most engineers that saw a freq. resp. chart of ~60 to 10k might back off even if the mic sounded great for most situations. ;)

Anonymous said...

what about encapsulating the crystal in epoxy or polyurethane resin? or just in a screw top test tube, a whole tube full, contacts at each end...

Anonymous said...

People who read charts instead of listening are responsible for some bad sound I think. Do you think your ears are "flat"??????