Diaphragm Profile
A practically flat disc is far removed from the theoretical “rigid piston.” With the exception of foamed plastic, the mass, for a given rigidity, will be excessive, resulting in very low efficiency, and if the cross section is reduced, the system becomes very flexible and inefficient.
Decreasing the angle from 180° increases the stiffness enormously; concomitantly the thickness can be reduced, resulting in a lighter cone for the same degree of self support. The flexure amplitudes will be reduced, but the bell modes will make an appearance.
As the angle is reduced, it reaches an optimum value for level response at the transition frequency. There will be another angle for a maximum high-frequency response, resulting ultimately in peaking in the upper treble region. Continuing the reduction in angle, the high-frequency peak will be reduced, but the response above the peak will fall rapidly.
If instead of a straight-sided cone, the profile is curved, the “smoothness” of the overall response can be improved considerably: bell modes are discouraged and the on-axis high-frequency response is improved. The price charged for this facility is reduced low- frequency power handling capacity because, for a given weight, the curved cone is just not as stiff (and as strong) as the straight-sided version.
The most efficient shape at low frequencies is circular. Theoretical and experimental investigations have shown that an ellipse with a major–minor axis of 2 has an average of 7% lower radiation resistance in the useful low-frequency range than a circle of the same area; the loss becomes progressively greater as the shape departs still further from circular. The shape of the cross section or profile of the cone depends on the power handling and response desired.
For domestic loudspeaker systems, which must be cost-conscious, the loudspeaker size is limited to 150 to 200 mm and a frequency response of 100 Hz to about 7 kHz with, possibly, a 25-mm soft dome to accommodate the high frequencies. Straight-sided cones are usually employed when a good 2- to 5-kHz response is required and when reproduction above, say, 7 kHz may be undesirable. Curved cones improve the response above 6–7 kHz by providing an impedance viewed from the voice coil, which has a more uniformly high negative reactance and therefore absorbs more power from the high positive reactance due to voice coil mass seen looking back into the voice coil. This improvement is obtained at the expense of response in the 2- to 5-kHz region, a weaker cone structure, and reduced power handling in the extreme bass.