November 19, 2008

Sound Tactics

Dynamic or moving coil microphoneI always wondered, I do even now, as to how the diaphragm of a loudspeaker vibrates, when sounds of different frequencies, as in a musical excited it. Everyone knows how the diaphragm vibrates in response to an alternating current. Just wind a few turns of enameled (insulated) wire around the tapering end of a cone of paper, place it on a permanent magnet and touch both the ends (the ends should be rubbed off of their enamel at their ends) to the positive and negative terminal of a battery, you will see it vibrate. Its response to a pure sine wave is easily predictable; it will vibrate back and forth as a function of simple harmonic motion.

The case becomes problematic when many different frequencies are present at the same time. Will the diaphragm vibrate ‘on the whole’ as an algebraic summation of those mechanical waves, or will different coordinates (parts) of it vibrate independently?

I personally think that the latter choice is the actual response. When a person beats a drum, it will produce different frequencies depending on where it is struck. Thus some parts of the loudspeaker will be more sensitive than others, for a particular frequency. In a similar way, the vibrations in the eardrum produce a wave on the basilar membrane (BM) of the internal ear; the location of peak amplitude of that wave being a function of frequency.

In electronics, sound to electricity transducers is called microphones. The loudspeaker described above can also function as a microphone: a dynamic or moving coil type microphone (see picture). Electret or condenser micElectret or condenser type microphones (picture given) employ two capacitor plates, mechanical vibrations bring one plate of this capacitor near the other making a change in capacitance. This is mechanical to electrical transduction is done by the BM in the cochlea of our internal ear.

Our body is a gadget par excellence. We hear music or words when those sound waves strike our eardrums. Like the diaphragm, it has excellent damping action, the oscillation stops almost as soon as sound stops. This ensures that it is ready for the next wave. I do not know how the tympanic membrane (TM) vibrates to those complex sounds, topology wise. Anyway, the TM then transfers its kinetic energy to the ‘oval window’ via an ossicular chain (in the middle ear) consisting of 3 tiny bones: malleus, incus and stapes. Stapes transfers this vibration to the cochlea (in internal ear), a spirally shaped tunnel consisting of 2.75 turns. What happens next is high level electronics, physics and mathematics, the body resorts to for processing and interpreting sounds. It performs cube roots, logarithms, compares phases and time lags and many complex functions.

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