Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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music, this reflection is not of much concern. In fact, it might even be desirable, as it makes
room sound larger.
As you can see in the graph, the overall sound energy
decays very slowly over time. Some of that sound energy can be
defined as reverberant sound. In a chamber music hall like this,
a longer reverberation time might be desirable. In a lecture hall,
a shorter reverberation time is better. You can use this impulse
response data to determine the RT60 reverberation time of the
room as shown in Figure 4.24. RT60 is the time it takes for reflections of a sound to decay by 60
dB. In the figure, RT60 is determined for eight separate frequency bands. As you can see, the
reverberation time varies for different frequency bands. This is due to the varying absorption
rates of high versus low frequencies. Because high frequencies are more easily absorbed, the
reverberation time of high frequencies tends to be lower. On average, the reverberation time of
this room is around 1.3 seconds.
Figure 4.24 RT60 reverberation time of small chamber music hall
The music hall in this example is equipped with curtains on the wall that can be lowered
to absorb more sound and reduce the reverberation time. Figure 4.25 shows the impulse response
measurement taken with the curtains in place. At first glance, this data doesn’t look very
different from Figure 4.23, when the curtains were absent. There is a slight difference, however,
in the rate of decay for the reverberant energy. The resulting reverberation time is shown in
Figure 4.26. Adding the curtains reduces the average reverberation time by around 0.2 seconds.
Figure 4.25 Impulse response of small chamber music hall with curtains on the some of the walls
Aside:
RT60 is the time it takes
for reflections of a direct
sound to decay by 60 dB.
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