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3. Typical carbon microphones have a sensitivity of Tup ≈ 500 mV/Pa, which corresponds to 500 mV for a 94-dB sound. Several disadvantages should be mentioned though. First, carbone microphones produce many upper harmonics with a power of up to 25% of the fundamental harmonic. Second, they generate quite a bit of internal noise, and, finally, they are power consuming. 6. Other examples of controlled couplers are the foil-strain gauge (a controlled resistor), the piezotransistor (a pressure-sensitive transistor), and the compressed-air loudspeaker in which an electromagnetic valve controls a stream of compressed air to generate sounds of up to 160 dB.

The circuit diagrams are given in Fig. 8. The characteristic frequency for the mechanic oscillator is ω0 = √ 1 . 10) This relationship becomes evident by looking at the analogue electric circuit and noting that the two capacitances are serially linked. Consequently, the effective mass is m1 m2 . 9 shows a simple cavity resonator with two finite compliances, otherwise known as an acoustic oscillator . Such closed-cavity resonators are, for example, applied for calibration of microphones because they are well insulated 3 Recall that in electrical terms the loop equation is u n = 0, with n = 1, 2, 3, · · ·, meaning that by completely circling a mesh we end at the same electric potential.

They cancel each other out upon arrival because of their different travel distances, making the receiver insensitive to this direction ˆ Sound impinges frontally. Now all waves propagate in phase to the microphone and add up there upon arrival. 18 illustrates the complete directional characteristics for two receivers of different length. 4. It should be noted that the slit is often covered with fabric with a flow resistance that varies along the tube. This compensates for losses during wave propagation in the tube.