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.
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.