Hi;
I had built a working sonic screwdriver once, a few years back.
It’s common knowledge that sound/vibration can effect physical structure:soldiers must break step before crossing a bridge, an opera singer can break a wine glass by hitting the proper note, and a helicopter can easily shake a house from a distance overhead. So it seemed to me to be just a matter of frequency and power.
To test the theory I used an old box from a disused piezo-speaker that had a pair of screws on its top as well as a standard stereo speaker. The speaker was connected to a medium-powered stereo amplifier that was being fed by a simple audio sine wave generator.
At a certain frequency the screws on the box began to turn fast and easily until they reached the bottom; at a higher frequency they slowly unscrewed themselves. This was resonance at work. Going one step farther, I placed the speaker up against a solid wood door and adjusted the frequency upward; at a certaain point the key in the door turned itself to the right although not with enough force to open the lock. The key could even be turned when the speaker was some distance from the door (basic Mechanical Engineering 101). At one point I even managed to get dinner plates to spin on the sound head. That was cool.
Once I knew the theory was right, I had to see if I could a practical screwdriver. I tried several ideas but what worked best was a standard piezo buzzer from Radio Shack. Driven at the right frequency and at high enough voltage, it could easily turn the screws on the ox from three to four inches. This was great but at 143 dB, this didn’t work too well for quiet breaking and entering at 3:00 in the morning; not to mention it hurt any bystanders. I decided that this had to work at a near ultrasonic frequency, say 13 kHz. This was a frquency most people could only barely hear. By itself, this frequency didn’t turn the screws too well (resonance effects end at 10 kHz) but when either frequency or amplitude modulated, that worked the same as being used at an audible frequency. It could still be heard, but only because of the secondary modulation—if set for 10 Hz, one would hear the 10 Hz beat and not the 13 kHz even though it was mouch louder.
The final problem to be solved was the size. The SS took a lot of power. One couldn’t use big batteries or carry a battery pack; tat was tacky. The solution came when some company produced a neat little IC that could easily boost 3 volts to 9 VDC. Instead of big batteries, I only needed two sub-C to get all the power the SS needed. When it was finished, the sonic screwdriver was about 11 inches long; a little shorter than the 5th Doctor’s screwdriver (I had a chance to ask Peter Davidson about it once). Made out of aluminum and copper, it was a thing of beauty. Its only design flaw was the on/off switch: I could never get that like on TV. :-)
Eventually, somehow, it got lost...
‘Doc
I'm going to have to check my posts twice as hard when you're around...