Introduction to Acoustics

of the sound fields. Consequently, it is obviously necessary
to apply one objective parameter for each of these
subjective aspects.
It is also clear that different listeners will judge the
importance of the various subjective parameters differently.
While some may base their judgment primarily on
loudness or reverberance others may be more concerned
about spaciousness or timbre.
For these reasons, one should be very sceptical when
people attempt to rank concert halls along a single, universal,
one-dimensional quality scale or speak about the
best hall in the world. Fortunately, the diversity of room
acoustic design and the complexity of our perception
does not justify such simplifications.
9.3.9 Recommended Values
of Objective Parameters
In view of the remarks above, it may be regarded risky
to recommend values for the various objective acoustic
parameters. On the other hand, there is a long tradition
of suggesting optimal values for reverberation time T
as a function of hall volume and type of performance.
Besides, most of the other parameters will seldom deviate
drastically from the expected value determined by
T. As we shall see later, this deviation is primarily influenced
by how we shape the room to control early
reflections. Designing for strong early reflections can
increase clarity, sound strength and spaciousness. Current
taste regarding concert hall acoustics for classical
music seems to be in favor of high values for these factors
in addition to strong reverberance. This could lead
to suggested ranges for the various parameters in small
and large halls, as shown in Table 9.1. These values relate
to empty halls with well-upholstered seating, assuming
Acoustics in Halls for Speech and Music 9.3 Subjective and Objective Room Acoustic Parameters 313
that, when fully occupied with musicians and audience,
the T values will drop by no more than say 0.2 s. The reason
for relating the values to the unoccupied condition
is that it is seldom possible to make acoustic measurements
in the occupied state and almost all reference
data existing regarding parameters other than T are from
unoccupied halls. On the other hand, suggesting wellupholstered
seats is a sound recommendation in most
cases, which will ensure only small changes depending
on the degree of occupancy. This will also justify
extrapolation of the unoccupied values of the other parameters
to the occupied condition, as mentioned later
in this section.
The values listed in the table were arrived at by
first choosing values for T ensuring high reverberance,
while the correlative values for EDT, G and C stem
from the diffuse field expected values; but these have
been slightly changed to promote high clarity and high
levels. As this is particularly important in larger halls,
it has been suggested to use an EDT value 0.1s lower
than T (= EDTexp) andC values 1 dB higher than Cexp
in a 2500 m 3 hall, but 0.2 s less than T and 2 dB higher
than Cexp, respectively, in the 25 000 m 2 hall. Likewise,
it is suggested to use G values 2 dB less than Gexp in
the small halls; but only 1 dB less than or even equal to
Gexp in large halls. For halls of size between the 2500
and 25 000 m 3 listed in the table, one may of course
interpolate to taste. Please note that, in general, G is
found to be 2–3 dB lower that Gexp (Sect. 9.5.6) sothe
G values suggested here are actually 1–2 dB higher than
those found in “normal” halls.
In the occupied condition, one may expect EDT, C
and G to be reduced by amounts equal to the difference
between the expected values calculated using T for the
empty and occupied conditions respectively. In any case,
Table 9.1 Suggested position-averaged values of objective room acoustic parameters in unoccupied concert halls for
classical music. It is assumed that the seats are highly absorptive, so that T will be reduced by no more than 0.2swhen
the hall is fully occupied. 2.4 s should be regarded as an upper limit mainly suitable for terraced arena or DRS halls with
large volumes per seat (Sect. 9.7.3), whereas 2.0 s will be more suitable for shoe-box-shaped halls, which are often found
to be equally reverberant with lower T values and lower volumes per seat
Parameter Symbol Chamber music Symphony
Hall size V/N 2500 m 3 /300 seats 25 000 m 3 /2000 seats
Reverberation time T 1.5s 2.0–2.4s
Early decay time EDT 1.4s 2.2s
Strength G 10 dB 3dB
Clarity C 3dB −1dB
Lateral energy fraction LEF 0.15–0.20 0.20–0.25
Interaural cross correlation 1 − IACC 0.6 0.7
Early support STearly −10 dB −14 dB
Part C 9.3