148 Chapter 7

a very spatial and reverberating sound in a large cathe-
dral, whereas one expects a dry sound in the open. Thus
deviations from this experience are regarded as being
bothersome. A listener seated in the front section of a
concert hall expects a clearer sound than one seated in a
rear section. On the other hand, however, he wants to
enjoy an optimally balanced acoustic pattern on all
seats, as he has grown up with the media and mainly
postprocessed sound productions that are independent
of the room, and thus acquired auditory expectations,
which do not allow an evaluation of the objectively
existing room.
The evaluation of speech is generally a bit easier,
since optimum audibility and clear intelligibility are
desired here in an atmosphere that is not influenced by
the room or electroacoustical means. Perhaps with the
exception of sacral rooms, the spaciousness generally
does not play such an outstanding role in this regard,
whereas sound volume and intelligibility are all the
more important.
Numerous room-acoustical criteria were defined in
order to clarify the terms applied for the subjective and
objective assessment of a spoken or musical perfor-
mance. In the following we have listed a relevant selec-
tion of them, in which context one should note that there
is a close correlation between the individual criteria.
One single optimally determined parameter may not at
all be acoustically satisfactory, because another param-
eter influences the judgement in a negative way. For
example, the optimum value range of center time and
definition can only be evaluated with a subjectively
correct estimated reverberation time. The guide values
of the reverberance measure are valid only if the clarity
measure C 80 is in the optimal range.

On principle, the room-acoustical quality criteria can
be subdivided into time and energy criteria. The main
type of use—speech or music—then determines the
recommendations for the guide values to be targeted.
With multi-purpose halls (without available variable
measures for changing the acoustics), a compromise is
required that should orient itself on the main type of use.

7.2.1 Time Criteria

7.2.1.1 Reverberation Time RT 60

The reverberation time RT 60 is not only the oldest, but
also the most best-known room-acoustical quantity. It is
the time that passes after an acoustic source in a room
has been turned off until the mean steady-state
sound-energy density w(t) has decreased to 1/1,000,000

of the initial value w 0 or until the sound pressure has

decayed to 1/1.000—i. e., by 60 dB

. (7-1)

Thus the time response of the sound energy density

in reverberation^1 results as

(7-2)

The steady-state condition is reached only after the

starting time tst of the even sound distribution in a room

(approximately 20 sound reflections within 10 ms) after

the starting time of the excitation^2

(7-3)

where,

tst is in ms,

V is in m³ (ft^3 ).

The defined drop of the sound pressure level of

60 dB corresponds roughly to the dynamic range of a

large orchestra.^3 The listener, however, can follow the

decay process only until the noise level in the room

becomes perceptible. This subjectively assessed param-

eter reverberation time duration thus depends on the

excitation level as well as on the noise level.

The required evaluation dynamic range is difficult to

achieve even with objective measuring, especially in the

low frequency range. Therefore, the reverberation time

is determined by measuring the sound level decay in a

range from 5dB to 35 dB and then defined as RT30 dB

(also RT 30 ). The initial reverberation time (IRT

according to Atal,^2 RT15 dB between 5dB and 20 dB)

and the early decay time (EDT according to Jordan,^2

RT10 dB between 0 dB and 10 dB) are mostly more in

conformity with the subjective assessment of the dura-

tion of reverberation, especially at low-level volumes.

This also explains the fact that the reverberation time

subjectively perceived in the room may vary, while the

values measured objectively according to the classical

definition with a dynamic range of 60 dB or 30 dB are,

except admissible fluctuations, generally independent of

the location.

Serving as a single indicator for the principal charac-

terization of the room in an occupied or unoccupied

state, the reverberation time is used as the mean value

between the two octave bandwidths 500 Hz and

wRT 10

6–

= w 0

wt w 0

• 
  

  6 logRT------ -t

  
  

w 0

• 
  

  13.82RT------ -t

  
  

tst= 1 } 20.17 } 0.34 V