Handbook for Sound Engineers

Acoustics for Auditoriums and Concert Halls 153

speaker has, on principle, to be used for exciting the

sound field, while with musical performances it is suffi-

cient to use a nondirectional acoustic source for the first

approximation.

The majority of the room-acoustical quality criteria

is based on the monaural, directionally unweighted

assessment of the impulse response. Head-related

binaural criteria are still an exception. The influence of

the sound-incidence direction of early initial reflections

on room-acoustical quality criteria is principally known.

Since subjective evaluation criteria are still missing to a

large extent, this may also be generally disregarded

when measuring or calculating room impulse responses.

For the determination of most of the relevant criteria,

the energetic addition of the two ear signals of an artifi-

cial head is sufficient.

Just like the directional dependence, the frequency

dependence of the room-acoustical energy criteria has

also not been researched in depth, so that it is generally

sufficient at the moment to evaluate the octave with the

center frequency of 1000 Hz.

7.2.2.1 Strength Measure (G) (P. Lehmann)

The strength measure G is the ten-fold logarithmic ratio
between the sound energy components at the measuring
location and those measured at 10 m distance from the
same acoustic source in the free field. It characterizes
the volume level

(7-19)

Here, is the reference sound energy compo-
nent existing at 10 m (32.8 ft) distance with the free
sound transmission of the acoustic source.

The optimum values for musical and speech perfor-
mance rooms are located between +1 dBdGd+10 dB
which means that the loudness at any given listener’s
seat in real rooms should be roughly equal to or twice as
high as in the open at 10 m (32.8 ft) distance from the
sound source.8,9

7.2.2.2 Sound Pressure Distribution (' L)

The decrease of sound pressure level ' L in dB describes

the distribution of the volume at different reception

positions in comparison with a reference measuring

position or also for a specific measuring position on the

stage in comparison with others. If the sound energy

component at the reference measuring position or for

the reference measuring position on stage is labeled

with E 0 and at the reception measuring position or for

the measuring position on stage with E, one calculates a

sound pressure level distribution ' L

. (7-20)

It is advantageous for a room if ' L for speech and

music is in a range of 0 dBt ' L t 5dB.

7.2.2.3 Interaural Cross-Correlation Coefficient (IACC)

The IACC is a binaural, head-related criterion and

serves for describing the equality of the two ear signals

between two freely selectable temporal limits t 1 and t 2.

In this respect, however, the selection of these temporal

limits, the frequency evaluation as well as the subjective

statement, are not clarified yet. In general, one can

examine the signal identity for the initial reflections

(t 1 =0ms, t 2 = 80 ms) or for the reverberation compo-

nent (t 1 ttst, t 2 tRT 60 [see Section 7.2.1.1]). The

frequency filtration should generally take place in

octave bandwidths of between 125 Hz and 4000 Hz.

The standard interaural cross-correlation function

IACF10,11 is defined as

(7-21)

where,

pL(t) is the impulse response at the entrance to the left

auditory canal,

pR (t) is the impulse response at the entrance to the right

auditory canal.

Then the interaural cross-correlation coefficient

IACC is

G 10

Ef

Ef,10 m

------------------

©¹

= log§· dB

Ef,10 m

'L 10

Ef

Ef,0

= log©¹§·---------- dB

IACFt 1 t 2 W

pL t

f 1

f 2

³ upR^ t+Wdt

pL^2 t td

f 1

f 2

³ pR

(^2) t td
f 1
f 2

u³

= ----------------------------------------------------------------

IACCt 1 t 2 =max IACFt 1 t 2 W

for 1 ms– W +1 ms.