Resonance

Author: N.H. Crowhurst

The property of resonance plays an important part in audio. For music, it is a property much sought after, but for musical reproduction, it is something to be avoided. Every musical instrument has its own variety of resonator. In stringed instruments the strings vibrate at a resonant frequency that fixes the note played, and the resonances in the body of the instrument control the overtone structure. Pipes and horns use a resonant column of air, in which the weight and elasticity of the air particles themselves (and the dimensions of the pipe or tube) control the frequency of vibration. Reed organs use vibrating reeds where the natural frequency of the tongue fixes the tone with resonators that give the desired timbre.

Double resonance in the marimba

With all these musical instruments, it is a family of resonance patterns that control the production of tones peculiar to that instrument. In the xylophone or marimba, a vibrating piece of wood initiates the tone, which is reinforced and given its fullness of tone by a resonant tube, in which an air column is set in vibration by the feeble vibrations from the wood bar.

In audio, however, resonance is taboo. We want to hear the original instruments* tones and timbres, not some extra effects contributed by the audio system. Unfortunately, it is not possible completely to eliminate resonance from anything with moving parts, as a microphone, loudspeaker, or phonograph pickup.

A telephone diaphragm is a simple steel disk of metal that vibrates according to the currents in the earpiece. It also has its own simple resonance (much like a miniature drum). This tone tends to predominate, giving the quality of speech peculiar to telephone reception. Choice of a suitable resonant frequency (in the construction of the instrument), makes for maximum intelligibility on speech, but music does not sound very good over a telephone.

Everything indicated here can cause resonance

In loudspeaker design, every effort is made to minimize resonance effects, but there are still plenty left. The best that can be done in a good design is to spread the resonances so they all have as little effect as possible. Some manufacturers, from time to time, make the claim of having produced the perfect loudspeaker, with no "tone" of its own. So far, none of these claims has been true, and future claimants should be checked very carefully.

The loudspeaker is not the only part of an audio system that is subject to resonance problems. They afflict microphones, phono cutters, pickups, and even some electrical circuits. As a result, it is difficult to be certain that the very simple statements that we have made are readily true.

We said that a microphone converts acoustic vibrations into electrical waves. It does. But how faithfully does it do so? Are the electrical waves an accurate copy of the acoustic vibrations? This is very difficult to be sure of, because resonances in almost any microphone will augment some frequencies and diminish others. We know this happens, but by how much?

Determining this is complicated by the fact that rooms in which we try to measure things also have a resonance of their own - standing waves have the same effect as resonances in microphones or loudspeakers; they emphasize some frequencies, and by contrast they diminish others. Thus even if we know about the microphone, how much of the acoustic vibration picked up is due to the original sound wave and how much is augmentation due to standing waves?

Resonance problems may be caused by microphones, phono cutters, pickups, etc...

In view of the extreme difficulty in getting rid of undesirable resonances, it is difficult to make measurements against an absolute standard. It is relatively easy to compare two pieces of equipment, either with instruments or by listening. When you know how much they differ, the question remains as to which one is nearest right. Listening can offer a guess, but different people's guesses do not always agree.

The thing we need to establish is a standard sound field as a basis for comparison. This means an area in which the intensity of sound vibration itself is accurately known. Anything that involves the use of mechanical movement to measure movement of air particles brings in the problem of resonance, which will make the method give different answers at different frequencies. For this reason, standards of wave measurement use devices that do not depend on transmitting the movement to mechanical parts to measure it.