You should use an electromagnetic speaker in place of a permanent
magnetic speaker wherever possible. Permanent magnet speakers are
used to perform most applications in the audio range.
Most military speakers reproduce low audio frequencies quite well, mid-band frequencies fairly well, and high frequencies quite poorly. Let’s see why.
When the speaker is constructed, only a limited number of turns may be built into the voice coil. This gives us a fixed inductance. At low frequencies, the inductive reactance of the voice coil is relatively low, and large audio currents flow. This provides a strong magnetic field around the voice coil and a strong interaction with the field of the permanent magnet. Low frequency response is excellent.
At midband frequencies, inductive reactance increases and less current flows in the voice coil. This produces less magnetic field and less interaction. Midband response is still acceptable in a properly designed speaker. At high audio frequencies inductive reactance is quite high, and little current flows in the voice coil. This results in a greatly reduced voice coil field and little interaction with the permanent magnetic field. Also at high frequencies the interwinding capacitance of the voice coil tends to shunt some of the high audio frequencies, which further reduces the high frequency response.
Frequency response of most permanent magnet speakers falls off at the higher audio frequencies. This problem is normally overcome either by the use of an expensive, specially designed speaker, or through the use of two speakers, one of which is designed to operate well at the higher audio frequencies (tweeter) and one at the lower frequencies (woofer).
As shown in the picture below, an electromagnet may be used in place of a permanent magnet to form a dynamic electromagnetic speaker . When we do this, sufficient dc power must be available to energize the field electromagnet. The operation otherwise is much the same as that of the permanent magnet type. This type of electromagnetic speaker is seldom used in military equipment.
The picture below shows a diagram of typical headphones used with equipment. The device consists of a permanent magnet and two small electromagnets through which the signal currents pass. A soft iron diaphragm is used to convert the electrical effects of the device into sound power. When no signal currents are present, the permanent magnet exerts a steady pull on the soft iron diaphragm.
Signal current flowing through the coils mounted on the soft iron pole pieces develops a magnetomotive force that either adds to or subtracts from the field of the permanent magnet. The diaphragm thus moves in or out according to the resultant field. Sound waves are then reproduced that have an amplitude and frequency (within the mechanical capability of the reproducer) similar to the amplitude and frequency of the signal currents.
As compared to permanent magnet speakers, standard headphones are considered to be high-impedance devices. Headphone electromagnets are normally wound with many turns of small wire, which provide the larger impedance. Because of the physically small size and inflexibility of the metal diaphragm, the headphones often give poor response to the lower audio frequencies. In the voice range of audio, most standard issue headphones are adequate.