Dynamic Loudspeakers

A dynamic loudspeaker should be mounted in a cabinet or in a baffle if it is to operate satisfactorily, particularly at low frequencies. When the cone moves out, a condensation is produced on the front side and a rarefaction occurs on the rear side. The air accordingly flows around the edge, neutralizing the pressure difference and hence largely preventing sound radiation at that frequency. The baffle corrects this by providing a long path from the front of the diaphragm around to the back, thus preventing neutralization until after considerable sound has been radiated. The dimensions of the baffle should be such that the shortest air path between the front and the back of the cone is at least one-fourth the wavelength of the lowest note to be reproduced. Thus, for a 50-cycle note the wavelength would be approximately (1125 x 12)/50 = 270 inches. One-fourth of this is 67.5 inches or about 5.6 feet for the distance from the front around the baffle to the back of the cone.

From the standpoint of frequency response alone, a dynamic loudspeaker mounted in the center of a large wall would be ideal. However, since much sound is radiated from the back of the cone, this would result in low efficiency. Dynamic speakers are often mounted in a cabinet, the back of which is left open. The cabinet then acts as a baffle and greatly influences the radiated sound. When it is necessary to enclose the back of the cone entirely, the box should be as large as possible and should be lined with hair felt or other sound-absorbing material to prevent the box from causing serious distortion. Distortion is caused in two ways: First, the reflected waves from the unlined walls strike the cone and interfere with the proper operation of the speaker, thus impairing its output. Second, the box is mechanically resonant for certain frequencies and if unlined is more likely to be set in undesired resonant vibration.