Handbook for Sound Engineers

Acoustics for Auditoriums and Concert Halls 159

VSNR is the influence factor of the useful level (speech

level) Lx and of the disturbance level Lst according to

Fig. 7-14,^6

The correlation shown in Fig. 7-15 can also be
derived between articulation loss and syllable intelligi-
bility. For low reverberation times, syllable intelligi-

bility is almost independent of the articulation loss. An
inverse correlation behavior sets in only with increasing
reverberation time. It is evident that with the usual
Alcons values between 1% and 50%, syllable intelligi-
bility can take on values between 68% and 93%

(meaning a variation of 25%) and that for an articula-
tion loss <15% (the limit value of acceptable intelligi-
bility) the syllable intelligibility VS reaches always,
independently of the reverberation time, values over
75% which corresponds roughly to a definition measure
of C 50 >4 dB.
This correlation can also be seen in Fig. 7-16, which

shows the correlation between measured RASTI-values
and articulation loss Alcons. One sees that acceptable
articulation losses of Alcons<15% require RASTI values
in the range from 0.4 to 1 (meaning between satisfactory
and excellent intelligibility). Via the equation

(7-35)

it is also possible to establish an analytical correlation
between the two quantities. In good approximation this

relationship may be used not only for RASTI but for
STI as well.

7.2.2.10 Clarity Measure (C80) for Music (Abdel Alim)

The clarity measure C 8021 describes the temporal trans-

parency of musical performances (defined for an octave

center frequency of 1000 Hz) and is calculated from the

tenfold logarithm of the ratio between the sound energy

arriving at a reception measuring position up to 80 ms

after the arrival of the direct sound and the following

sound energy.

Figure 7-14. Syllable intelligibility factor VS/N as a function of
speech sound pressure level LS and noise pressure level LN.

VR 610

6– ts

ms

©¹§·------ -^

2

• u 0.0012

ts

ms

= – ©¹§·------ - 1.0488+

RASTI 0.9482 0.1845–= ln Alcons

Figure 7-15. Syllable intelligibility IS as a function of the

articulation loss Alcons. Parameter: reverberation time

RT 60. Preconditions: approximate statistical reverberation

behavior; signal-to-noise ratio (SNR) t 25 dB.

Figure 7-16. Relationship between Alcons values and

RASTI values.