Capacitor microphone

In modern devices the flexible membrane is part of a capacitor with the other plate fixed. The pressure rise causes a corresponding change of the capacitance, which can be converted to an AC voltage (Fig. 4.76b). This sensitive detector, which is essentially a capacitor microphone, is now widely used in photoacoustic spectroscopy (Sect. 6.3) to detect the absorption spectrum of molecular gases by the pressure rise proportional to the absorption coefficient. 

Barium titanate has many important commercial apphcations. It has both ferroelectric and piezoelectric properties. Also, it has a very high dielectric constant (about 1,000 times that of water). The compound has five crystalline modifications, each of which is stable over a particular temperature range. Ceramic bodies of barium titanate find wide applications in dielectric amplifiers, magnetic amplifiers, and capacitors. These storage devices are used in digital calculators, radio and television sets, ultrasonic apparatus, crystal microphone and telephone, sonar equipment, and many other electronic devices. 

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. 

While microelectronics can be described as the fabrication of electrical components like transistors, diodes, resistors and capacitors on a semiconductor substrate, mostly silicon, MEMS and MOEMS are using the manufacturing technologies of microelectronics to fabricate mechanical and optical structures as well as sensing or actuating devices. Typical examples are pressure sensors, microphones, acceleration and angular-rate sensors, magnetic compasses, inkjet heads, micro-scanners, micro-fluidic devices, biosensors, etc., to name some. 

A microphone has a similar symbol but is unrelated, of course. Microphones can be electromagnetic like loudspeakers, or they can be variable capacitors ("electret type"), or "piezoelectric."... 

The key component of an LOAS instrument is the spectrophone. Its construction depends critically on the state of aggregation of the sample. The SP for gas samples consists of a gas cell with input and output windows for laser radiation and a microphone, fixed on an inside or outside wall of the cell. As in the case of MAS with SDLs. the sensitivity can be improved by the use of multipass SPs. Plane capacitor or electronic microphones are the most widely used. The sensitive element of these microphones is a thin (1-10 pm) elastic membrane made of Mylar. Teflon, or metallic foil. The membrane serves as one electrode of a dielectric capacitor (membranes made of dielectric materials have metallic coatings). Thus, the acoustic vibrations of a gas mixture in the cell can be directly converted to an electrical signal. Commercial microphones with sensitivities of 5-50mV/Pa are utilized in routine applications of LOAS specially constructed and optimized ones are used for ultrasensitive analyses. 

Fig. 5.39. Schematic diagram of a parallel-plate capacitor in which one plate is essentially rigid whereas the other plate is a deformable film. The configuration has potential for function as an acoustic microphone. A characteristic feature of the configuration is that, For any voltage greater than a particular critical voltage, the system has no stable equilibrium configurations.

Figure 2 illustrates the fundamental configuration of the acoustic and electrical systems of an ECM. The omnidirectional condenser microphone forms a capacitor... 

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