The Use of a Horn

Author: N.H. Crowhurst

The best diaphragm- or cone-type loudspeaker is a very inefficient transducer : only a small part of the electrical power delivered to it is converted into sound waves. An average efficiency figure for modern units is about 10%; a poor one may be 5% efficient or less; a good one may reach 20%. Thus even the best throws away four-fifths of the power that it gets. The main reason for this inefficiency is the difference in the density of the material of which the diaphragm is made and that of the air. Because of this big difference, the voice coil spends much more of its driving force moving the diaphragm than the air in contact with the diaphragm.

A horn improves acoustic efficiency

Using a horn is a way to improve this situation. The method by which a horn improves acoustic efficiency can be illustrated by two devices: a speaking tube and a megaphone. In a speaking tube, sound is propagated without the wave being allowed to expand. For this reason, the intensity does not fall off according to the inverse-square law, but reaches the other end of the tube almost undiminished.

The megaphone, in addition to concentrating the sound into a narrower angle, intensifies the sound in that angle. Without the megaphone, sound comes from the mouth in a sudden transition from a narrow aperture (the mouth) to "free" space. The megaphone, by tapering off this transition, enables the mouth to radiate more sound with less effort.

To transmit a certain amount of air or sound wave through a small hole requires the air to be forced further (and harder) than in a large hole.

Putting a diaphragm over a smaller hole increases the effort to move the air and reduces diaphragm movement, improving efficiency in converting voice-coil force to air movement.

It requires more force to move air in a narrow channel than it does in a wider channel, because individual air particles in the narrow channel have to move farther to allow the same quantity, or volume movement. For this reason, placing a diaphragm opposite a channel that is smaller in cross section area will enable the voice coil to spend more of its energy in driving the air, wasting less in driving the diaphragm. This is like putting a speaking tube to the diaphragm. If we couple the tube to a megaphone, we shall have a more efficient sound radiator than the diaphragm by itself. The narrow opening makes the diaphragm move more air, and the megaphone effects a transition more gradually, so that this greater movement is used, instead of being lost.