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SPEAKERS 589
(6) The voice coil (speech coil, speech winding) of the dynamic speaker is of course powered by the output of the amplifier (Figure 139). It consists of comparatively few turns of wire rigidly attached to the diaphragm. It carries no core of its own, but in many speakers it moves in a slot, gap or recess provided for it in the core of the field winding.
(7) Horns are used with dynamic speakers for two reasons. One is to "load" the unit when the diaphragm is of small diameter, as explained above in connection with magnetic speakers. The other is to provide a directional characteristic, which helps in securing equal and accurate distribution of sound throughout the auditorium.
(8) The directional baffle differs from the horn principally in that it is intended to load a larger diaphragm. The neck of the directional baffle is therefore not as small as the neck of a horn, but in other respects the instrument looks enough like a horn to be mistaken for one. It helps in obtaining even distribution of sound in a large theatre. .
(9) Flat baffles are used with the larger diaphragms to obtain good reproduction of the lowest frequencies. All sound can be heard around a corner, out of sight of its source, but low-frequency sound is exceptionally nondirectional. When the speaker diaphragm moves forward it creates an area of compressed air in front of it and an area of rarefied air behind it. If the sound is of low frequency the compressed air will ooze right around the edge of the speaker into the region of rarefied air. Compression and rarefaction will cancel each other, and the low frequency sound will be lost. Cone-type speakers not equipped with horns or baffles nearly always sound "tinny" because of this loss of low frequencies. Such speakers are "baffled" by mounting them against a circular hole cut into a flat board of sufficient area. This conserves the low frequencies; it also helps to load the diaphragm, inasmuch as the air being moved, while it can slip sidewise, cannot slip backward into the area of rarefaction. The dimensions of the baffle are governed by the lowest frequency desired. The lower the f re