Electret Microphones

Microphones measure sound pressure. Inside an KA microphone is a thin flexible diaphragm, an electrically charged plate, and an amplifier (Fig. 1.). The output voltage is proportional to changes in the small separation between the diaphragm and the charged
plate (Fig. 2).

As sound pressure inside the front cavity increases, the diaphragm is pushed closer to the plate. As the pressure decreases, it moves further away. The motion of the diaphragm produces a small electrical signal that is amplified by a miniature circuit inside the microphone.

FIG. 1.: Cross section of an KA EM microphone.	FIG. 2.: EM diaphragm and electret.

The sensitivity of a typical KA microphone is shown in Fig. 3. in units of dB relative to 1 Volt per 0.1 Pascal. The microphone components can be modified to make it more or less sensitive at lower frequencies or to reduce, increase, or shift the peak at higher frequencies.

	FIG. 3.: EM sensitivity in dB relative to 1 volt per 0.1 Pa.

Balanced Armature Speakers

The speaker converts an electrical signal into sound. A cross section of a typical KA speaker is shown in Fig. 1. The basic components of the speaker are: a coil of wire, a metal U-shaped reed called the armature, a pair of permanent magnets, a drive rod, and a diaphragm.

The coil and armature act as an electromagnet. An alternating current in the coil causes the polarity of the armature to switch back and forth from north to south. The free end of the armature bends slightly up and down as it is attracted alternately to the top and bottom magnets (Fig. 2). The diaphragm, pulled along by the drive rod, pumps air in and out of the speaker. The mechanical motion of the armature is thus converted into sound.

The sound output of a typical KA speaker is shown in Fig. 3 in units of dB SPL (sound pressure level) relative to
20 Pa. The speaker components can be modified to produce more or less output at lower frequencies or to reduce or increase the higher frequency peaks.

Fig. 1: Cross section of a KA EH speaker	Fig. 2: The motor of the speaker has a coil, an armature, and a pair of permanent magnets.

down as it is attracted alternately to the top and bottom magnets (Fig. 2). The diaphragm, pulled along by the drive rod, pumps air in and out of the speaker. The mechanical motion of the armature is thus converted into sound.

The sound output of a typical KA speaker is shown in Fig. 3 in units of dB SPL (sound pressure level) relative to
20 Pa. The speaker components can be modified to produce more or less output at lower frequencies or to reduce or increase the higher frequency peaks.

	Fig. 3: EH output in dB SPL to a nominal 0.7 mA AC current drive.