The conventional stethoscope operated on the principle of transmitting sound via air pressure changes produced by a vibrating diaphragm. The diaphragm moves and pushes air, pressure increases, and the pressure wave travels up the tubes to the listener's ears.
The problem with this method is that tubing forms an unintentional and undesirable filter which cuts certain frequencies and attenuates sound. So you get faint, muffled murmuring sounds.
The Thinklabs capacitive Electromagnetic Diaphragm (EmD), vibrates in exactly the same way as an acoustic stethoscope, but a completely new sensor was created to sense the vibration.
A capacitive plate is placed behind the diaphragm with a high voltage charge, producing a very intense electric field (blue lines). As the diaphragm vibrates back and forth, the distance between the diaphragm and plate changes, changing the electric field intensity. This is sensed as a voltage change, translating the mechanical vibration into an analogous audio signal.
"I play a small-bodied Santa Cruz - extraordinary sound!"
"The diaphragm is the fundamental element that gives a stethoscope its sound. I play acoustic guitar - you can add an electronic pickup, but take away the wood resonance and you get an electric guitar - a very different sound.
Likewise, eliminating the diaphragm and using a vibration sensor, such as a piezo-electric transducer, you lose the diaphragm sound. I wanted to keep the diaphragm "timbre", just as I love the sound of an acoustic guitar.
I investigated various sensor methods and had many failures, as you'd expect in research. But one method performed beautifully - using capacitive sensing, where the diaphragm acts as one plate of a capacitor. It's really tricky getting such circuits to work well, but I was working in my garage in the middle of winter. The Colorado humidity was almost zero - exactly what you need to make this technology work in a rough prototype. It must have been 15 Farenheit in there, but sitting in the cold, with headphones on, listening to my heartbeat, I knew something was working."
- Clive Smith, One Designer, Thinklabs Founder
The audio signal can then be amplified, processed and reproduced using all the advanced technology available today for producing excellent audio. Air tubing, with all its problems, is replaced by advanced electronics, with all its precision and performance.