As already stated, a chaos of sound was set up when an observer in
the Auditorium spoke or shouted or clapped his hands. Both echoes and
reverberations were present and could be heard in all parts of the
room, though the echoes seemed to be strongest on the stage and in
the balcony. The prospects for bettering the acoustics were not very
encouraging. Luckily, the cure for the reverberation was fairly simple,
since Sabine’s method gave a definite procedure that could be applied
to this case. The cure for the echo, however, was yet to be found. It
was first necessary to find out which walls set up the defect.
The attempt to locate echoes by generating a sound and listening with
the ear met with only partial success. The ear is sensitive enough but
becomes confused when many echoes are present, coming apparently from
every direction, so that the evidence thus obtained is not altogether
conclusive. It became apparent that the successful solution lay in
fixing the attention on the sound going in a particular direction and
finding out where it went after reflection; then tracing out the path
in another particular direction, and so on until the evidence obtained
gave some hint of the general action of the sound.
[Illustration: FIG. 4. WATCH AS SOURCE OF SOUND, BACKED BY A CONCAVE
REFLECTOR.]
The first step in the application of this principle was to use a
faint sound which could not be heard at any great distance unless
reinforced in some way. The ticks of a watch were directed, by means
of a reflector (Fig. 4) to certain walls suspected of giving echoes.
Using the relation that the angle of incidence equals the angle of
reflection, the reflected sound was readily located, and the watch
ticks heard distinctly after they had traveled a total distance as
great as 70 to 80 feet from the source.
In a later experiment, a metronome was used which gave a louder
sound. It was enclosed in a sound-proof structure (Fig. 5) with only
one opening, so that the sound could be directed by means of a horn.
This method was suggested by the work of Gustav Lyon in the Hall of
the Trocadero at Paris,[17] where a somewhat similar arrangement was
used. The method was successful and verified the observations taken
previously.
[Illustration: FIG. 5. METRONOME AS SOURCE OF SOUND.]
[Illustration: FIG. 6. ARC-LIGHT AS SOURCE OF SOUND.]
Though the results obtained with the watch and metronome seemed
conclusive, yet the observer was not always confident of the results. A
further method was sought, and a more satisfactory one found by using
an alternating current arc-light at the focus of a parabolic reflector
(Fig. 6). In addition to the light, the arc gave forth a hissing sound,
which was of short wave length and therefore experienced but little
diffraction. The bundle of light rays was, therefore, accompanied by
a bundle of sound, both coming from the same source and subject to
the same law of reflection. The path of the sound was easily found by
noting the position of the spot of light on the wall. The reflected
sound was located by applying the relation that the angles of incidence
and reflection are equal. The arc-light sound was intense and gave the
observer confidence in results that was lacking in the other methods.
To trace successive reflections, small mirrors were fastened to the
reflecting walls so that the path of the reflected sound was indicated
by the reflected light. A “diagnosis” of the acoustical troubles of the
Auditorium was then made by this method.
It should be noted here that the arc-light sound is not the same as the
sounds of music or speech, these latter ones being of lower pitch and
of longer wave length. It was, therefore, a matter of doubt whether the
results obtained would hold also for the case of speech or music. Tests
made by observers stationed in the Auditorium when musical numbers
and speeches were rendered, however, verified the general conclusions
obtained with the arc-light.
It should be pointed out in this connection that there is an objection
to applying the “ray” method of geometrical optics to the case of
sound. It is much more difficult to get a ray of sound than it is to
get a ray of light.[18] This is due to the difference in the wave
lengths in the two cases. It appears that the waves are diffracted,
or spread out, in proportion to their length, the longer waves being
spread out to a greater extent. The short waves of light from the sun,
for instance, as they come through a window mark out a sharp pattern
on the floor, which shows that the waves proceed in straight lines
with but little diffraction or spreading. Far different is it with
the longer waves of sound. If the window is open, we are able to hear
practically all the sounds from outdoors, even that of a wagon around
the corner, although we may be at the other end of the room away from
the window. The longer sound waves spread out and bend at right angles
around corners, so that it is almost impossible to get a sound shadow
with them. Furthermore, in the matter of reflection, it appears that
the area of the reflecting wall must be comparable with the length of
the waves being reflected. In the case of light, the waves are very
minute, hence a mirror can be very small and yet be able to set up a
reflection; but sound waves are of greater length, the average wave
length of speech (45 cm.) being about 700 000 times longer than the
wave length of yellow light (.00006 cm.), hence the reflecting surface
must be correspondingly larger. An illustration will perhaps make
this clearer. Suppose a post one foot square projects through a water
surface. The small ripples on the water will be reflected easily from
the post, but the large water waves pass by almost as if the post were
not there. The reflecting surface must have an area comparable with
the size of the wave if it is to cause an effective reflection. Relief
work in auditoriums, if of small dimensions, will affect only the high
pitched sounds, i. e., those of short wave length, while the low
pitched sounds of long wave length are reflected much the same as from
a rather rough wall. It is also shown that the area of the reflecting
surface is dependent on its distance from the source of sound and from
the observer; the greater these distances are the larger must be the
reflecting surface.[19]
These considerations all show that the reflection of sound is a
complicated matter. The dimensions of a wall to reflect sound, or of
relief work to scatter it, are determined by the wave length and by
the various other factors mentioned. It should be said with caution
that a “ray” of sound is reflected in a definite way from a small bit
of relief work. We must deal with _bundles_ of sound, not too sharply
bounded, and have them strike surfaces of considerable area in order to
produce reflections with any completeness.
[Illustration: FIG. 7. LONGITUDINAL SECTION SHOWING THE CHIEF
CONCENTRATIONS OF SOUND, THE DIFFRACTION EFFECTS BEING DISREGARDED.]