688 Chapter 23
If instead of a straight-sided cone, the profi le 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 effi cient 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 profi le 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.
23.11 Straight-Sided Cones.............................................................................................
The most important parameter affecting the performance of a loudspeaker is “ cone
fl exure. ” Because real materials are not infi nitely rigid and have mass, the velocity of
propagation through the material is fi nite. The cone is driven at the apex and the impulse
travels outward toward the periphery where it is refl ected back to the source. At particular
frequencies when the distance to the edge are odd quarter wavelengths, the returning
impulse will be 180° out of phase and tend to cancel; conversely, when the distance is
multiples of half wavelengths they will augment—under these conditions the system can
be considered as a transmission line, and theoretically (and to some extent, practically),
if the outer annulus were made resistive and of the correct value, the line would be
terminated and no refl ections would occur [see Figure 23.10(a) ].