An echo is set up by a reflecting wall. If an observer stands some
distance from the front of a cliff and claps his hands, or shouts, he
finds that the sound is returned to him from the cliff as an echo. So,
in an auditorium, an auditor near the speaker gets the sound first
directly from the speaker, then, an instant later, a strong repetition
of the sound by reflection from a distant wall. This echo is more
pronounced if the wall is curved and the auditor is at the point where
the sound is focused.
To cure such an echo, two methods may be considered. One method
consists in changing the form of the wall so that the reflected sound
no longer sets up the echo. That is, either change the angle of the
wall, so that the reflected sound is sent in a new direction where it
may be absorbed or where it may reinforce the direct sound without
producing any echoes, or else modify the surface of the wall by relief
work or by panels of absorbing material, so that the strong reflected
wave is broken up and the sound is scattered. The second method is to
make the reflecting wall a “perfect” absorber, so that the incident
sound is swallowed up and little or none reflected. These methods
have been designated as “surgical” and “medicinal” respectively. Each
method has its disadvantages. Changing the form of the walls in an
auditorium is likely to do violence to the architectural design. On
the other hand, there are no perfect absorbers, except open windows,
and these can seldom be applied. The cure in each case is, then, a
matter of study of the special conditions of the auditorium. Usually
a combination of the surgical and the medicinal cures is adopted. For
instance, coffering a wall so that panels of absorbing material may be
introduced has been found to work well in bettering the acoustics, and
also, in many cases, it fits in with the architectural features.