Microphones

Both microphones and loudspeakers are transducers—electromechanical devices for converting sound waves into electrical signals and vice versa. Microphones sense small changes in sound pressure through motion of a thin diaphragm. Cone loudspeakers create changes in pressure through the motion of a diaphragm driven by a coil of wire, immersed in a magnetic field. Since both microphones and loudspeakers operate in a similar manner, microphones can be used as loudspeakers and loudspeakers as microphones. Even the human eardrum can act as a loudspeaker.

The most common types of microphones in use are: 1) dynamic, 2) condenser, 3) electret, 4) ceramic, and 5) ribbon. All microphones consist of a diaphragm, which moves back and forth in response to changes in pressure or velocity brought about by a sound wave, and electronic components that convert the movement into an electric signal. Microphones are characterized by a sensitivity, which is the open circuit output voltage produced by a given pressure, expressed in decibels re 1 V/Pa. A one-inch diameter instrumentation microphone might produce 54 mV for an rms pressure of 1 Pa, yielding a sensitivity of 20 log [(54 mV) / (1 Pa)] [(1Pa)/(1 V)] =- 25 dB. Note that 1 Pa is the sound pressure that corresponds to the 94 dB sound pressure level generated by standard piston-phone calibrators.

A dynamic microphone, illustrated in Fig. 4.1, operates on the same principal as a loudspeaker. A diaphragm moves in response to the changes in sound pressure and is mechanically connected to a coil of wire that is positioned in a magnetic field. The induced current, produced by the motion of the coil, is the microphone's output signal. Both the diaphragm and the coil must be very light to produce adequate high-frequency response. Most dynamic microphones produce a very low output voltage; however, since the electrical output impedance is low, the microphone can be located relatively far away from the preamplifier. Dynamic microphones are rugged and are primarily used in sound reinforcement applications, where low fidelity is good enough. One manufacturer of dynamic microphones used to demonstrate its product's toughness by using the side of it to pound a nail into a block of wood.

A condenser microphone, in Fig. 4.2, consists of a thin stretched stainless-steel diaphragm that is separated from a back plate by a narrow air gap. The two parallel plates

Figure 4.1 Dynamic (Moving Coil) Microphone (Rossing, 1990)

Sound pressure on the diaphragm causes the voice coil to move in a magnetic field.

Figure 4.1 Dynamic (Moving Coil) Microphone (Rossing, 1990)

Sound pressure on the diaphragm causes the voice coil to move in a magnetic field.

Figure 4.2 Condenser Microphone (Rossing, 1990)

A metal diaphragm is one plate of a capacitor. As it moves the changing capacitance modulates the voltage at the preamplifier.

Figure 4.2 Condenser Microphone (Rossing, 1990)

A metal diaphragm is one plate of a capacitor. As it moves the changing capacitance modulates the voltage at the preamplifier.

become a capacitor when a DC polarizing voltage, typically 150 to 200 V, is applied. Motion of the diaphragm generates an electrical signal by varying the capacitance and thus the voltage between the plates. These microphones are very sensitive and accurate and have excellent frequency response characteristics. They are less rugged than dynamics and require a source of the polarizing voltage.

An electret microphone, in Fig. 4.3, is another form of condenser, which is sometimes called an electret condenser. It includes a thin polymeric diaphragm, where the polarizing voltage is not externally applied but is built into the polymer so that it is permanent. Otherwise the microphone operates in much the same way as the condenser does.

The ceramic microphone, in Fig. 4.4, has a diaphragm that is mechanically coupled to a piezoelectric material. A piezoelectric generates a voltage when strained. Many such materials exist such as lead zirconate titanate, called PZT, barium titanate, and rochelle salt. These microphones are more rugged than the capacitive types, are less sensitive, and do not require an external polarization voltage.

A ribbon microphone, sometimes referred to as a velocity microphone, works by suspending a thin metallic foil in a magnetic field. Figure 4.5 shows an example. The conducting ribbon is light enough that it responds to the particle velocity rather than the pressure. Since the ribbon is open to the back and shielded on the sides by the magnet, these microphones have a bidirectional polarity pattern. Ribbon microphones are very sensitive to moving air

Figure 4.3 Electret Condenser Microphone (Rossing, 1990)

A thin metallized plastic, diaphragm is tightly stretched across a perforated backing plate. The holes in the back plate couple to an air cavity.

Plastic Diaphragm

Figure 4.4 Ceramic Microphone (Rossing, 1990)

Sound pressure on the diaphragm causes deformation of the crystal, generating an electrical signal.

Piezoelectric Crystal

Figure 4.5 Ribbon Microphone (Rossing, 1990)

A lightweight ribbon diaphragm moves in a magnetic field, thus generating an electrical signal.

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