Material
Hard impregnated or filled pressed calendered papers are used when loudness efficiency and apparent high-frequency response are important. The impregnant is usually a hard thermo-setting resin. The radiation response provides very little dissipation in direct radiator cones, hence by using paper having low internal flexural losses, the transmission line is made to have strong resonances. The transient response of this type is necessarily poor, as noncenter moving modes of the cone are unappreciably damped by the motor unit. Soft, loosely packed, felted cones are used when some loss in the high-frequency response can be tolerated and a smooth response curve with reduced transient distortion is required. The apparent loudness efficiency of high loss cones of this type is anything up to 6 dB lower than that of low loss cones of similar weight.
In an effort to overcome the intransigencies of paper cones, resort has been made to other materials. Light-weight metal (aluminum alloys, etc.) immediately springs to mind because of its stability, homogeneity, and repeatability but, because of the very low internal frictional losses, strong multiple resonances occur in the upper frequencies. A diaphragm of, say, 250 mm in diameter made from a 0.1-mm-thick aluminum alloy with a total mass of 40 g will show a “ruler” level response up to approximately 2 kHz when multiple resonances occur. These are extremely narrow band (in some cases only 1 or 2 Hz wide) with an amplitude of anything up to 40 dB and an effective Q of several hundred. Putting a low-pass filter cutting very sharply at, say, 1 kHz does not eliminate shock excitation of these resonances at low frequencies and the result is a “tinny” sound.
Reducing the cone diameter and making the flare exponential reduce this effect and also places the resonant frequency a few octaves higher, but does not entirely eliminate the problem. Using foamed plastic materials (and sometimes coating the surfaces with a metal to form an effective girder structure) has met with some success. There are problems associated with the solid diaphragm in that the different finite times taken for the sound wave to travel directly from the voice coil through the material to the front and along the back edge of the diaphragm to the anulus and then across the front cause interference 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 exemplified where the diaphragm is almost the full size of the front of the cabinet (say 24 X 18 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 specified power and frequency limits, these “plastic” diaphragms (especially those using specified grades of polypropylene) give a cost-effective efficient system.