Audio Engineering

Loudspeakers 687

over wavelength. This shows the degrees off the normal axis at which the attenuation
is 3, 6, 10, and 20 dB (as marked on the curves) as a function of the ratio of the piston
diameter over the wavelength of the generated sound wave.

23.10 Diaphragm Profi le .................................................................................................

A practically fl at 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 effi ciency, and if the cross section is reduced, the system becomes very fl exible
and ineffi cient.

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 fl exure 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.

100

10

(^1) 0.1 1.0 10 100
Ratio Dia. /.
20 dB
10 dB
6dB
3dB
Figure 23.9 : Directional radiation pattern with a circular piston in infi nite baffl e.