Page 155 - The Master Handbook Of Acoustics

130 CHAPTER SEVEN

than outdoor recordings of symphonic music, which are noted for
their flatness. Clearly, symphonic and other music requires reverbera-
tion and the amount of it is a lively technical topic to be covered.
At one time reverberation was considered the single most impor-
tant characteristic of an enclosed space for speech or music. It has
shriveled in importance under the influence of important research on
the acoustics of enclosures. At the present time it is only one of several
measurable parameters that define the quality of a space.

Reverberation and Normal Modes

The natural resonances of a small room are revealed in the various
vibrational modes as described in chapter 15. It is necessary to antici-
pate this chapter a bit to understand the relationship of these natural
room resonance frequencies and the reverberation of the room.
Our introduction to the measurement of reverberation will be
through a method wholly unsuited to practical use, but as an inves-
tigative tool, it reveals some very important factors that focus attention
on the normal modes of rooms. Historically, in the Broadcasting House
in New Delhi, India, is (or was) a Studio 10 used for news broadcasts.
Measurements of reverberation time in this studio were reported by
1
Beranek and later analyzed by Schultz. The first set of measurements
were made in the completely bare, untreated room. Knowing con-
struction in India, it can be safely assumed that concrete and ceramic
tile dominated the room surfaces. The measurements were made with
sine-wave signals, and great patience and care were exercised to
obtain the detailed results.
Starting with the oscillator set to about 20 Hz below the first axial
mode, the acoustics of the room do not load the loudspeaker and a
relatively weak sound is produced with the amplifier gain turned up
full (even assuming the use of a good subwoofer). As the oscillator
frequency is adjusted upward, however, the sound becomes very
loud as the 1, 0, 0 mode (24.18 Hz) is energized (Fig. 7-1). Slowly
adjusting the oscillator upward we go through a weak valley but at
the frequency of the 0, 1, 0 mode (35.27 Hz) there is high level sound
once more. Similar peaks are found at the 1, 1, 0 tangential mode
(42.76 Hz), the 2, 0, 0 axial mode (48.37 Hz), and the 0, 0, 1 axial
mode (56.43 Hz).