Microphone carrier

Process chain for production of the microphone carrier for a hearing aid... 

FIGURE 9.14 Microphone carrier for a behind-the-ear (BTE) hearing aid (graphics courtesy of HARTING Mitronics)... 

The MID microphone carrier is part of a behind-the-ear (BTE) hearing aid. There are three microphones in all, positioned and contacted on the MID. The MID also makes the electrical connections between the microphones and the mainboard. Precision positioning of the microphones is essential for correct operation of the hearing aid, because this is what enables directional audio perception for the wearer (i.e. the ability to identify noises as coming from in front or behind). 

Ia early telephoaes, souad (voice) waves caused a carboa microphone s resistance to vary, thus varyiag the current flowing ia a series external circuit. This d-c curreat could thea be used to regeaerate voice waves ia a receiver. Two wires were required to carry a single coaversatioa. With time, telecommunications traffic was eacoded oa a-c carriers, at first usiag ampHtude or frequeacy modulatioa, and more recently pulse code modulation. 

Since capacitors do not pass dc current, special steps must be taken to set the charge stored on each capacitor and protect the circuit from drift due to leakage currents, carrier generation, etc. The resistors in Fig. 6.1.8 labelled R s serve this purpose. Additionally, measurements are usually not performed at dc, but rather at some carrier frequency /m, typically a few 100 kHz or MHz. The higher frequencies are needed especially with CMOS circuits to reject the electronics own low frequency flicker noise which often extends to the low MHz. The only exception are applications in which the signal itself is modulated or free from dc content, such as resonant gyroscopes [1] or microphones. Even in those cases modulation is often used. 

Filtration is provided by a C2A1 canister attached to the mask by a corrugated hose the canister is housed in a specially designed canister carrier. The M42 integrates with the combat vehicle filtration protection system. The M42 also has a dynamic microphone that integrates with the combat vehicle via a microphone cable.19,20... 

Photoacoustic spectroscopy involves the modulation of IR radiation at a frequency in the acoustic range. The radiation strikes a sample and is subsequently turned into heat energy by nonradiative deexcitation processes (i.e., excitation is loss by thermal motion and not by the emission of energy). Heat-generated thermal waves then propagate to the sample surface and facilitate the rapid expansion and contraction of a carrier gas. A microphone is used to sense the change in pressure in the enclosed cell. 

The directional positioning of the microphones is the single most important benefit of MID technology for this particular application. It dispenses with a second carrier that would have to be connected by wires to the mainboard. 

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