Basic Audio is a free introductory textbook to the basics of audio physics and electronics. See the editorial for more information....


Author: N.H. Crowhurst

Normal sound vibrations are very small - so small that the movement of a loudspeaker diaphragm is not visible, except at the lower frequencies. The only reason that sound can be transmitted so efficiently with such small movements is that it uses the natural transmission speed of the air. Nonetheless, the magnitude of movement of the particles diminishes with distance traveled (recall the inverse-square law). Although hearing covers a very wide range of intensity variation, sound will only carry a certain distance before it becomes inaudible.

Early acoustic amplifiers

From earliest times man has had this problem to overcome. Long before electronic amplifiers became possible, some kinds of acoustic "amplifiers" were used. Actually these were not amplifiers in the true sense of the word - they did not increase the sound power but merely conserved what power was available. The ear trumpet, for example, collects a larger area of the sound wave and thus increases the intensity at the earpiece. The megaphone concentrates sound at the sending end, to restrict it within a narrow angle. The sound in front of the megaphone is louder, but you can hear less than normal in all other directions. The speaking tube is more efficient; it virtually prevents any sound escaping at all, so that all the power is conveyed along the tube. In this way, sound can be transmitted for considerable distances. The reflector board over the speaker's platform serves a purpose similar to the megaphone by making use of sound that otherwise would escape upwards.

An early attempt at real amplification, (the pneumatic amplifier) was entirely acoustic. Sound vibrations striking a diaphragm were used to operate a valve, somewhat like a reed, through which air under pressure was driven. The amount of air passed by the reed was controlled, or "modulated" by the sound vibrations reaching the diaphragm, and produced a more intense replica of the original sound.

The amplification given was rather crude. The apparatus was far convenient than modern electrical, or electronic amplification. The "microphone" and "loudspeaker" had to be mechanically coupled, and thus close together. It could not be used for as many purposes, and its quality was rather rough, to say the least.

Pneumatic amplification

Nonetheless, this acoustic device had one thing in common with any real amplifier. Extra power had to come from somewhere. In that case it was air under pressure supplied from a suitable pump. In electrical amplification, extra electrical power is added and later converted to sound. Very small electrical impulses, or waves, put into an amplifier, control a larger amount of power taken from a battery, power line, or some suitable source and give a large amount of audio power to drive a loudspeaker or other transducer.

Last Update: 2010-11-03