Standing Waves

Author: N.H. Crowhurst

As a result of this standing wave pattern, the normally definite sense of direction that enables us to tell where sounds come from is lost. This can be verified with an oscillator, amplifier, and loudspeaker, or by just getting an instrumentalist to play a single long note. The sound seems to fill the room. If you have an impression that the sound comes from one direction (with your eyes shut) you are probably wrong, and if you move your head slightly, it will seem to come from a different place.

The effect of standing waves. Cancellation occurs along the lines. The + and - signs represent points of maximum intensity (when there is pressure at the points marked +, there is rarefaction at the points marked -)

If your head is placed where a maximum intensity occurs by your right ear, and the left ear is at a point of less intensity, the sound will seem to come from your right side. If both ears are on a line of maximum intensity, or are at equal intensity, the sound will seem to be in front of you. And if both ears receive pressure at the same intensity, but one receives a pressure wave when the other receives a rarefaction wave, and vice versa, the impression of direction is confused.

Standing wave patterns take time to build up and, what is more important in acoustics, also take time to die away (decay). When the vibration causing the standing waves in the water ceases, or when the tone in the room stops, the vibrations all over the area die out gradually, rather in the way that a swinging pendulum comes to rest when nothing continues to drive it.

When the sound source is removed, standing waves die out gradually.

This dying away occurs in a fraction of a second in an ordinary living room. Most of us are so used to the brief presence of these patterns that we do not normally notice it. In a large hall or auditorium, however, or in a stadium with walls all around it, the time required for sound to die away can be quite noticeable.

The time required for sound to die away after the originating tone ceases is called the reverberation time of the room or building. (Reverberation is the name given to "echo" when it is not sufficiently separated from the original sound to be noticed as a separate repetition.)