_Reflecting Boards._--The provisional cure was brought about gradually
by trying different devices suggested by the diagnosis. In one set
of experiments sounding boards of various shapes and sizes were
used. A flat board about five feet square placed at an incline over
the position of the speaker produced little effect. A larger canvas
surface, about 12 by 20 feet, was not much better. A parabolic
reflector, however, gave a pronounced effect. This reflector was
mounted over a pulpit at one end of the stage and served to intercept
much of the sound that otherwise would have gone to the dome and
produced echoes. The path of the reflected sound was parallel to the
axis of the paraboloid of which the reflector was a quarter section.
There was no difficulty in tracing out the reflected sound. Auditors in
the path of the reflected rays reported an echo, but auditors in other
parts of the Auditorium were remarkably free from the usual troubles.
The device was not used permanently, since many speakers objected to
the raised platform. Moreover, it was not a complete cure, since it was
not suited for band concerts and other events, where the entire stage
was used. Another reflector similar in shape to the one just described
is shown in Figs. 21 and 22.
[Illustration: FIG. 19. REFLECTING BOARD IN PROCESS OF CONSTRUCTION.]
[Illustration: FIG. 20. FINISHED REFLECTOR. HARD PLASTER ON WIRE LATH.]
_Sabine’s Method._--The time of reverberation was determined by
Sabine’s method. An organ pipe making approximately 526 vibrations a
second was blown for about three seconds and then stopped. An auditor
listened to the decreasing sound, and when it died out made a record
electrically on a chronograph drum. The time of reverberation was found
to be 5.90 seconds, this being the mean of 19 sets of measurements,
each of about 20 observations. The reverberation was found also by
calculation from Sabine’s equation (see Section III), taking the
volume of the Auditorium as 11,800 cubic meters and calculating the
absorbing power of all the surfaces in the room. This calculation gave
6.4 seconds. The agreement between the two results is as close as could
be expected, since neither the intensity of the sound nor the pitch
used by the author was the same as those used by Professor Sabine, and
both of these factors affect the time of reverberation.
[Illustration: FIG. 21. PARABOLIC REFLECTOR SHOWING ITS ACTION ON
SOUND.]
[Illustration: FIG. 22. PHOTOGRAPH OF PARABOLIC REFLECTOR.]
Several years later the time of reverberation was again determined
after certain changes had been made. A thick carpet had been placed
on the stage, heavy velour curtains 18 by 32 feet in area hung on the
wall at the rear of the stage, a large canvas painting 400 square feet
in area was installed, and the glass removed from the skylight in the
ceiling. The time of reverberation was reduced to 4.8 seconds. With an
audience present this value was reduced still more, and when the hall
was crowded at commencement time the reverberation was not troublesome.
_Method of Eliminating Echoes._--Although the time of reverberation
was reduced to be fairly satisfactory, as just explained, the echoes
still persisted, and were very annoying. Attempts were made to reduce
individual echoes by hanging cotton flannel on the walls at critical
points. Thus the shaded areas in Fig. 17 were covered and also the
entire rear wall in the balcony. Pronounced echoes still remained, and
it was evident that some drastic action was necessary to alleviate this
condition. Four large canvases, shown in Figs. 23 and 24, were then
hung in the dome in position suggested by the results of the diagnosis.
A very decided improvement followed. For the first time the echoes were
reduced to a marked degree and speakers on the stage could talk without
the usual annoyance. This arrangement eliminated the echoes not only on
the stage, but generally all over the house. A number of minor echoes
were still left, but the conditions were much improved, especially when
a large audience was present to reduce the reverberation.
_Proposed Final Cure._--The state of affairs just described is the
condition at the time of writing. Two propositions were considered in
planning the final cure. One proposition involved a complete remodeling
of the interior of the Auditorium. Plans of an interior were drawn
in accordance with the results of the experimental work that would
probably give satisfactory acoustics. This proposition was not carried
out because of the expense and because it was thought desirable to
attempt a cure without changing the shape of the room. The latter
plan is the one now being followed. It is proposed to replace the
present unsightly curtains with materials which will conform to the
architectural features of the Auditorium and which will have a pleasing
color scheme. At the same time, it will be necessary to hold to the
features which have improved the acoustics.
[Illustration: FIG. 23. PHOTOGRAPH OF TWO OF THE CANVAS CURTAINS IN THE
DOME OF THE AUDITORIUM. NOTE ALSO THE ABSORBING MATERIALS UNDER THE
ARCHES.]
[Illustration: FIG. 24. PHOTOGRAPH OF DOME OF AUDITORIUM SHOWING THE
CANVASES INSTALLED TO ELIMINATE ECHOES.]