When a speaker addresses an audience, the sounds he utters proceed in
ever widening spherical waves until they strike the boundaries of the
room. Here the sound is partly reflected, partly transmitted, and the
rest absorbed. The amounts of reflection, absorption and transmission
depend on the character of the walls. A hard, smooth wall reflects most
of the sound so that but little is transmitted or absorbed. In the case
of a porous wall or a yielding wall, the absorption and transmission
are greater, and the reflection is less. After striking a number of
reflecting surfaces, the energy is used up and the sound dies out.
The reflection of sound produces certain advantages and disadvantages
for the acoustics. When it is considered that sound travels about 1100
feet a second it may be seen that a room of ordinary size is almost
immediately filled with sound because of the many reflections. In a
room 40 feet square, for instance, the number of reflections per second
between opposite walls is 1100 ÷ 40, or approximately 27. The number is
really greater than this, since the sound that goes into the corners
is reflected much more frequently than out in the middle where the
distances between walls are greater. The result is that the sound mixes
thoroughly in all parts of the room so as to give the same average
intensity; that is, the sound is of the same average _loudness_ for all
auditors, even for those in the remotest corners.
Though the reflection of sound has the advantage of fulfilling the
conditions for loudness, it introduces at the same time possibilities
for setting up defective acoustics. For instance, when the walls of the
room are hard and smooth very little energy is lost at each impact of
the sound and many reflections take place before it finally dies out.
This slow decadence of the sound, or _reverberation_ as it is called,
is the most common defect in auditoriums.
If a speaker talks in such a hall the auditors have difficulty in
understanding. Each sound, instead of dying out quickly, persists for
some time so that the succeeding words blend with their predecessors
and set up a mixture of sounds which produces confusion. The cure for
the trouble is brought about by the introduction of materials such as
carpets, tapestries, and the like, which act as absorbers of sound and
reduce the time of reverberation.
When music is played in an auditorium with a prolonged reverberation,
the tones following one another blend and produce the same effect as
that of a piano when played with the loud pedal in use. A reverberation
is more advantageous for music than for speech, since the prolongation
and blending of the musical tones is desired, but the mixing of the
words in a speech is a distinct disadvantage. When curing this defect
for halls used for both music and speaking, a middle course must be
steered, so that the reverberation is made somewhat long for speaking
and somewhat short for music, yet fairly satisfactory for both.
Going back to the consideration of the reflection of sound, it is found
that another defect may be produced, namely, an _echo_. This is the
case when a wall at some distance reflects the sound to the position
of the auditor. He hears the sound first from the speaker, then later
by reflection from the wall. The time interval between the direct and
reflected sound must be great enough to allow two distinct impressions
to be made. This time is about 1/15 of a second, but varies with the
acuteness of the observer. The farther off the wall is, the greater is
the time interval and the more pronounced is the echo. If the wall is
not very distant, the time interval is too short to allow two distinct
impressions to be made, and the effect on the auditor is then much the
same as if his neighbor at his side speaks the words of the discourse
in his ear at the same time that he gets them directly from the
speaker. In case the reflecting wall is curved so as to focus the sound
the echoes are much more pronounced. A curved wall wherever it may be
placed in an auditorium is thus always a menace to good acoustics.
There are other actions of the sound that may result in acoustical
defects. The phenomena of _resonance_, for instance, may cause trouble.
Suppose that the waves of sound impinge on an elastic wall, not too
rigid. If these waves are timed right they set the wall in vibration in
the same way that the bell ringer causes a bell to ring by a succession
of properly timed pulls on the bell rope. The wall of the room will
then vibrate under the action of this sound with which it is in tune
and will reinforce it. Now suppose a band is playing in a room. Certain
tones are reinforced, while the others are not affected. The original
sound is then distorted. The action is the same on the voice of the
speaker. The sounds he utters are complex and as they reach the walls
certain components are reinforced and the quality of the sound is
changed. This action of resonance may also be caused by the air in a
room. Each room has a definite pitch to which it responds, the smaller
the volume of the room the higher being the pitch. A large auditorium
would respond to the very low pitch of the bass drum. In small rooms
and alcoves the response is made to higher pitched tones, as may be
observed by singing the different notes of the scale until a resonance
is obtained.
Another action of sound causes the _interference_ of waves. Thus the
reflected waves may meet the oncoming ones and set up concentrations of
sound in certain positions and a dearth of sound in others.
Summing up, it is seen that the effects of sound which may exist in
a room are _loudness_, _reverberation_, _echoes_, _resonance_, and
_interference_, and that the most common defects are reverberation and
echoes. We now turn to the discussion of the methods of cure.