Speakers and Microphones



Electromagnetic effects are the moving force in the design of speakers such as the one shown in [Fig. 1]. The shape of the pulsating waveform of the input current is determined by the sound to be reproduced by the speaker at a high audio level. As the current peaks and returns to the valleys of the sound pattern, the strength of the electromagnet varies in exactly the same manner.
Fig. 1: Speaker.
Coaxial high-fidelity loudspeaker
Fig. 2: Coaxial high-fidelity loudspeaker.
This causes the cone of the speaker to vibrate at a frequency directly proportional to the pulsating input. The higher the pitch of the sound pattern, the higher the oscillating frequency between the peaks and valleys and the higher the frequency of vibration of the cone.
A second design used more frequently in more expensive speaker systems appears in [Fig. 2]. In this case the permanent magnet is fixed and the input is applied to a movable core within the magnet, as shown in the figure. High peaking currents at the input produce a strong flux pattern in the voice coil, causing it to be drawn well into the flux pattern of the permanent magnet. As occurred for the speaker of [Fig. 1], the core then vibrates at a rate determined by the input and provides the audible sound.


Microphones such as those in [Fig. 3] also employ electromagnetic effects. The sound to be reproduced at a higher audio level causes the core and attached moving coil to move within the magnetic field of the permanent magnet.
Fig. 3: Dynamic microphone.
Through Faraday's law
$$e = N d\Phi/dt$$
a voltage is induced across the movable coil proportional to the speed with which it is moving through the magnetic field. The resulting induced voltage pattern can then be amplified and reproduced at a much higher audio level through the use of speakers, as described earlier. Microphones of this type are the most frequently employed, although other types that use capacitive, carbon granular, and piezoelectric* effects are available. This particular design is commercially referred to as a dynamic microphone.

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