Loudspeakers 691interference patterns that result in some cancellation of the emitted sound in the mid
upper frequencies, say 800–1100 Hz. This effect can be mitigated by using a highly
damped anulus, with the object of absorbing as much as possible of the “ back wave. ”
Expanded polystyrene is the favorite material for these diaphragms, although expanded
polyurethane has met with some success. An extension of this principle is exemplifi ed
where the diaphragm is almost the full size of the front of the cabinet (say 24 1 8
inches). In this case the diaphragm, even at low frequencies, does not behave as a rigid
piston. The overall performance is impossible of any mathematical solution and must be
largely determined experimentally, but the lower bass (because of multiple resonances)
is, in the opinion of its advocates, “ fruity ” and “ full! ” It has been developed to use two
or even three voice coils at strategic places on the diaphragm. For synthesized noise it is
possible, but in the writer’s opinion, for “ serious ” music listening, it adds nuances to the
music never envisaged or intended by the composer.
Vacuum-formed sheet thermoplastic resins have become very popular. Their mechanical
stability is excellent, they are nonhygroscopic, and repeatability (a very important facet
when mass producing units in hundreds of thousands) is several orders of magnitude better
than paper cones. However, there is a price to pay: most of them contain a plasticizer,
which increases the internal mechanical losses in the structure, and hence the magnitude of
diaphragm resonances is reduced. However, under user conditions, dependent on electrical
power input and thus operating temperature, they tend to migrate. This results in a changed
cone (or dome) shape, and because the internal mechanical loss is reduced, the frequency
response is changed. In extreme cases, especially with small thin diaphragms, cracking has
occurred, but it must be emphasized that with a correctly designed unit operating within its
specifi ed power and frequency limits, these “ plastic ” diaphragms (especially those using
specifi ed grades of polypropylene) give a cost-effective effi cient system.
23.13 Soft Domes
For use at medium and high frequencies, the “ soft dome ” system has found favor. It
consists of a preformed fabric dome with integral surround and usual voice coil assembly.
It is very light and its rigidity can be controlled by the amount of impregnant, but the
beauty of the concept is that the damping can be adjusted by the quantity and viscosity
of the “ dope ” applied to the dome. Responses fl at ±1 dB to 20 kHz are standard, even on
inexpensive mass-produced units!