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594 RICHARDSON'S BLUEBOOK OF PROJECTION
139, where only a single speaker is used for all frequencies.
Another method of controlling frequency response is to use a number of speaker units of the same type, or of similar type, in contrast to the different types shown in Figure 144, but to provide these units with baffles of special and sometimes complicated construction, each baffle reproducing only a selected range of sound.
Troubles in Speaker Units
(14) Damage to the voice coil, to the diaphragm or to the speech coil may either impair the quality of sound obtained from a speaker, or render it entirely inoperative.
The voice coil may burn out, but this trouble is not very common. However, prolonged and excessive volume, especially at low frequencies, may cause it. Open circuit in a voice coil is rather more likely to be' caused by a break at the point where flexible leads are joined to it. The continuous vibration of the coil may break that connection. In some makes of speakers a break at that point can be re-soldered, in some it cannot. In some the construction of the leads is such as to make a break of that kind impossible.
It is a peculiar and interesting fact that in some makes of speakers excessive field current will ruin the voice coil. This has been known to happen where the construction of the unit is such that the voice coil moves in a slot or gap provided for that purpose in the core of the field. Excessive current through the field coil overheats both that coil and its core. When the slot in which the voice coil moves becomes too warm, the cement or glue that holds it to the diaphragm softens and sticks to the sides of the slot, and the coil tears.
The field coil, on the contrary, is commonly wound of heavy wire, little subject to open-circuit, and in a good speaker very seldom gives trouble unless excessive voltage is applied. It is the field coil supply, rather than the winding itself, that needs watching.
The diaphragm, as a moving part subject to great and continuous strain, is the most delicate part of the speaker.