Handbook for Sound Engineers

156 Chapter 7

(7-31)

Based on the comparison of subjective examination
results with a maximum possible intelligibility of sylla-
bles of 96%, the STI values are graded in subjective
values for syllable intelligibility according to Table 7-1
(EN ISO 9921: Feb. 2004).

7.2.2.8 Articulation Loss, Alcons, with Speech (Peutz,
Klein)

Peutz^17 and Klein^18 have ascertained that the articula-
tion loss of spoken consonants Alcons is decisive for the
evaluation of speech intelligibility in rooms. Starting
from this discovery they developed a criterion for the
determination of intelligibility:

(7-32)

where,

rLH is the distance sound source-listener,

rH is the reverberation radius or, in case of directional
sound sources, critical distance rR,
RT 60 is the reverberation time in seconds.

From the measured room impulse response one can

determine Alcons according to Peutz,^17 if for the direct

sound energy one applies the energy after about 25 ms

to 40 ms (default 35 ms), and for the reverberation

energy the residual energy after 35 ms

(7-33)

Assigning the results to speech intelligibility yields

Table 7-2.

Long reverberation times entail an increased articu-

lation loss. With the corresponding duration, this rever-

beration acts like noise on the following signals and

thus reduces the intelligibility.

Fig. 7-10 shows the articulation loss, Alcons, as a

function of the SNR and the reverberation time RT 60.

The top diagram allows us to ascertain the influence of

the difference LR (diffuse sound level) –LN (noise level)

and of the reverberation time RT 60 on the Alcons value,

which gives ALconsR/N. Depending on how large the

SNR (LDLRN) is, this value is then corrected in the

bottom diagram in order to obtain AlconsD/R/N. The

noise and the signal level have to be entered as dBA

values.

The illustration shows also that with an increase of

the SNR to more than 25 dB, it is practically no longer

possible to achieve an improved intelligibility. (In

praxis, this value is often even considerably lower, since

with high volumes, for example above 90 dB, and due

to the heavy impedance changes in the middle ear that

set on here as well as through the strong bass emphasis

that occurs owing to the frequency-dependent ear

sensitivity.)

7.2.2.9 Subjective Intelligibility Tests

A subjective evaluation method for speech intelligibility

consists in the recognizability of clearly spoken

pronounced words (so-called test words) chosen on the

Figure 7-9. STIPa signal in frequency presentation.

Table 7-1. Subjective Weighting for STI

Subjective Intelligibility STI Value

unsatisfactory 0.00–0.30

poor 0.30–0.45

satisfactory 0.45–0.60

good 0.60–0.75

excellent 0.75–1.00

Magnitude (Full IR) Magnitude

stipa.etm

(c)EASERA

STI X 15+

30

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

Alcons 0.652

rLH

rH

©¹§·--------

2

| RT 60 %

Table 7-2. Subjective Weighting for Alcons

Subjective Intelligibility Alcons

Ideal intelligibility d3%

Good intelligibility 3–8%

Satisfactory intelligibility 8–11%

Poor intelligibility >11%

Worthless intelligibility >20% (limit value 15%)

Alcons 0.652

Ef–E 35

E 35

| ©¹§·--------------------- RT 60 %