Trapped acoustic resonance touch switches

The wonderful Appliance Design magazine has a nifty article about a touch switch that uses trapped acoustic resonance. This is a little different than the acoustic pulse touchscreen I described back in August 2007. The switch plate can be made of half0inch thick stainless steel so this is a great technology for outdoor or vandal resistant switches. The key paragraphs:

The principle of trapped acoustic resonance rests upon the fact that a material capable of supporting shear and torsional mechanical waves at ultrasonic frequencies can have those waves trapped, or localized, by creating specific features or geometries in the material. The cylindrical regions designated to trap the ultrasonic energy serve as resonant cavities that are set into motion by ultrasonic transducers emitting in the area of 1 MHz.

The ultrasonic waves induce a twisting motion, which is called the trapped torsional mode, and confines the motion to the cylindrical cavity. In doing so, the ultrasonic vibrations initiate a resonance in the cavity, which is analogous to ringing a bell. As with a bell, the resonance from the cavity will decay, or ring down, at a predictable rate, depending upon its material and geometry. Touching the outside surface of a resonant cavity has a damping effect that accelerates the rate at which the resonance decays, just as someone touching a struck bell damps the vibrations and hastens the end of the ringing sound. The damping occurs when the surface is touched by a finger, gloved finger, or any acoustic-absorbing object.

Check out the article as well as the whole magazine. It always has some good system-level related content.