The Speaker Diaphragm

Author: N.H. Crowhurst

Why the speaker diaphragm is a cone

Next, why the shape of the diaphragm - a cone? Again the requirement of rigidity with lightness of weight. Try waving a piece of cardboard or paper in a large sheet, and you will find it is impossible to move any air rapidly back and forth - wherever you hold the paper or card. But take a cone of paper and hold it by the center, where the coil drives a loudspeaker cone, and you will be able to push air back and forth with it much more easily, although it is just as light, as the flat sheet. The conical shape gives rigidity.

But how big should the cone be? This depends on the wavelength of the sounds we want to make. If the size of the diaphragm is smaller than a wavelength, the air tends to run round the edges instead of going back and forth with the diaphragm. If the diaphragm is large compared to the wavelength, the air will not have time to dodge around it during the passage of each wave.

Cone size and frequency

Wavelength varies inversely with frequency. The low-frequency tones have long wavelengths, whereas the high-frequency tones have short wavelengths. This means that we need a large diaphragm for the lower frequencies, while a smaller one will serve for the high frequencies. Of course, a large diaphragm will also move air at the higher frequencies, unless it is too heavy to be driven effectively at the higher speeds. For this reason, many installations use two or more speakers of different sizes, each of which handles only the band of frequencies that it serves best: large speakers (woofers) for the low frequencies, medium-sized speakers (squawkers) for the mid-range, and small speakers (tweeters) for tne high frequencies.

Because of the 'size' of the wave at low frequencies, a diaphragm has to move a lot of air to make sound. If you look at a loudspeaker diaphragm, you will see that it moves quite a long way for the low frequencies, although at higher frequencies, it radiates sound without visible movement. At these higher frequencies, however, the air load on the diaphragm is so great that the voice coil can hardly move. The high frequencies thus are not radiated.

Differences between low and high frequencies

The reason for this difference in movement can best be understood by thinking of wavelengths. To create a pressure change, air has to be pressed into a given space. When the wavelength is small (as at high frequencies), the space momentarily requiring the additional air is small, and the pressure can be increased by very small (but very rapid) air movement. When the wavelength is large, the area of increased or reduced pressure is large and requires the movement of a bigger of air to achieve it.