Introduction to Acoustics

9.3.1 Reverberation Time
Reverberance is probably the best known of all subjective
room acoustic aspects. When a room creates too
much reverberance, speech loses intelligibility because
important details (consonants) are masked by louder,
lingering speech sounds (the vowels). For many forms
of music, however, reverberance can add an attractive
fullness to the sound by bonding adjacent notes
together and blending the sounds from the different
instruments/voices in an ensemble.
The reverberation time T which is the traditional
objective measure of this quality, was invented 100 years
ago by W. C. Sabine. T is defined as the time it takes
for the sound level in the room to decrease by 60 dB
after a continuous sound source has been shut off. In
practice, the evaluation is limited to a smaller interval
of the decay curve, from −5dB to −35 dB (or −5dB
to −25 dB) below the start value; but still relating to a
60 dB decay (Fig. 9.5), i. e.:
(t−35) − (t−5)
Acoustics in Halls for Speech and Music 9.3 Subjective and Objective Room Acoustic Parameters 307
T = 60 dB
. (9.1)
(−5dB) − (−35 dB)
In this equation, t−x denotes the time when the decay
has decreased to XdB below its start value, or, if we let
R(t) represent the squared value of the decaying sound
pressure and shut off the sound source at time t = 0:
� �
R(t−X)
10 log10 =−X dB . (9.2)
R(0)
With the fluctuations always present in decay curves, T
should rather be determined from the decay rate, A dB/s,
as found from a least-squares regression line (determined
from the relevant interval of the decay curve).
Hereby we get for T:
60 dB
T =
A dB s
= 60
s . (9.3)
A
Ways to obtain the decay curve from the impulse response
will be further explained in the section on
measurement techniques.
Due to masking, the entire decay process is only perceivable
during breaks in the speech or music. Besides,
the rate of decay is often different in the beginning and
further down the decay curve. During running music or
speech, the later, weaker part of the reverberation will
be masked by the next syllable or musical note. Therefore
an alternative measure, early decay time (EDT)has
turned out to be better correlated with the reverberance
perceived during running speech and music. This pa-
Sound level (dB)
80
70
60
50
40
30
20
10
60dB
Reverberation time
1.9s
Measurement
range (30dB)
0
0 1
2
Time (s)
Fig. 9.5 The definition of reverberation time (after [9.4])
rameter, like T, also measures the rate of the decay; but
now evaluated from the initial part, the interval between
0and−10 dB, only. Thus,
EDT = 6(t−10) or EDT =
60
A(0 dB→−10 dB)
s .
(9.4)
The detailed behavior of the early part of the reverberation
curve is influenced by the relative levels and
distribution in time of the early reflections, which in
turn vary depending on the positions of the source and
receiver in the room. Likewise, the value of EDT is often
found to vary throughout a hall, which is seldom the
case with T.
In spite of the fact that EDT is a better descriptor
of reverberance than T, T is still regarded the basic
and most important objective parameter. This is mainly
due to the general relationship between T and many
of the other room acoustic parameters and because
a lot of room acoustic theory relates to this concept,
not least diffuse field theory, which is the basis for
measurements of sound power, sound absorption, and
sound insulation. T is also important by being referred
to in legislation regarding room acoustic conditions in
buildings.
Talking about diffuse field theory, it is often of
relevance to compare the measured values of certain
objective parameters with their expected values according
to diffuse field theory and the measured or
calculated reverberation time. As diffuse field theory
predicts the decay to be purely exponential, the distribution
in time of the impulse response squared should
Part C 9.3