Monday, December 04, 2006

The Microphone part 6

The Piezoelectric Microphone was never really used in broadcasting. But the iea behind them is nifty, and they are used in a lot of radio hardware. As driven elements, piezoelectric devices are used as telephone receivers, acoustic transducers and record cutters. They are used for small loudspeakers, although dynamic loudspeakers give much better results. Later they were used in ceramic phonograph cartridges that were thankfully both cheap and accurate. It made record players cheaper to maintain and easier to build. The ceramic or crystal microphone was invented 1933 by the Astatic Corporation when two short wave geeks C. M. Chorpening and F. H. Woodworth (W8MJM, and W8AHW respectively) found that they could make a microphone out of Rochelle salts (Sodium Potassium Tartrate). They found that when a sound wave struck these kins of crystals, they vibrated and generated an electrical current. By 1933, Chorpening and Woodworth incorporated a manufacturing and sales buisness for their line of pizeoelectric Crystal Microphones, Crystal Phonograph Pickups and Recording heads.

The physics go back much further of course. It was long known that certain crystals, notably tourmaline, would attract light objects when heated. This was the pyroelectric effect: an electrical polarization caused by heating. The brothers Pierre and P.J. Curie studied this, and discovered the production of electrical polarization when a crystal was strained, in 1880. (They also found the inverse in 1881) NOTE: it is often suggested Pierre's fire Marie was involved as well since she was the one with the physics degree.

FACT: while dying of radiation poisoning, Pierre was trampled to death by horses. Also interesting is that the other man (see below) most responsible for the commercial uses of the pizeoelectric effect also used to date Marie... Was she secretly behind both mens'work?

Rochelle salt first became more than a curiosity around 1917, when it was used by Paul Langévin in ultrasonic acoustic transducers for the submarine detection. This was the first practical application for pizeoelectric devices. What we now call sonar was first developed in 1917 in France. Langvin's detector consisted of a transducer, made of thin quartz crystals carefully glued betweentwo steel plates, and a hydrophone to detect the returned echo. By emitting a high-frequency chirp from the transducer, (a ping) and measuring the amount of time it takes to hear an echo from the sound waves bouncingoff an object, one can calculate the distance to that object.