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If your microphone had no diaphragm, how much better could it sound?

Every microphone used in pro audio has a diaphragm. Suppose that a new technology could detect sound in a non-invasive way. Surely that would be much more accurate than the mics we now have?

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Think of all the different types of microphones that are in common, and uncommon, use for capturing sound traveling through air...

  • The dynamic mic, where a diaphragm moves due to the pressure, pressure gradient or velocity of air molecules and in turn moves a coil located within the field of a magnet.
  • The capacitor mic, where a diaphragm moves due to the pressure, pressure gradient or velocity of air molecules. The diaphragm forms part of a capacitor and a signal is generated according to the principal of voltage equals charge divided by capacitance.
  • A ribbon microphone, where a thin metallic or metalized ribbon forms a diaphragm and is also part of a 1-turn coil, operating in a manner similar to the dynamic microphone.
  • A carbon granule microphone, such as the transducer formerly used in telephone handsets. Sound waves strike a metal diaphragm which is in contact with a quantity of carbon granules. The vibration agitates the granules leading to a variation in their resistance, which leads to a variation in an electric current passing through them.
  • The piezo microphone - a diaphragm is in contact with a material that generates electricity when bent. Motion in the diaphragm leads to bending of the material which in turn leads to a signal being generated.

Do you notice one thing that is in common with all of these types of microphone? Yes, they all have a diaphragm. In every case, sound must contact and move a diaphragm before it can be turned into a signal.

The Heisenberg Uncertainty Principal

Yes, I thought that would make you sit up and take notice. A generalized form of this idea states that the process of taking a measurement disturbs the object or phenomenon being measured.

So the act of placing a diaphragm in a sound field changes the characteristics of that sound field.

This means that however accurate a microphone is, it is always limited by the fact that the presence of the diaphragm disturbs the sound field. A lighter diaphragm would obviously be better but, even so, it is impeding the motion of air molecules. Its acoustic impedance is different and therefore a barrier to the transmission of sound from air to diaphragm... there are probably a whole host of reasons why a diaphragm is in general a 'bad thing'.

Clearly the limitation here is that vibrational energy traveling in air is being transferred to vibrational energy in another medium. Surely it would be better to be able to measure the motion of air molecules without asking them to give up their energy?

Suppose for example that it were possible to color air molecules so that they were visible. Now, their motion could be tracked by light sensors - no intervention necessary. Or if air molecules could be made radioactive, perhaps a radioactivity sensor could track their motion.

Rather than leaving this as a flight of fancy, it really does seem that we have exploited the possibilities of the diaphragm to the absolute limit. To progress microphone technology further, a different approach will be necessary that is not invasive to the air molecules that are carrying the sound wave.

So how could it be done?

By David Mellor Monday February 21, 2011
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