User Guide

Sound System Design Reference Manual
General Requirements for Speech
Intelligibility
The requirements for speech intelligibility are
basically the same for unamplified as for amplified
speech. The most important factors are:
1. Speech level versus ambient noise level.
Every effort should be made to minimize noise due to
air handling systems and outside interferences. In
general, the noise level should be 25 dB or greater
below the lowest speech levels which are expected.
However, for quite high levels of reinforced speech,
as may be encountered outdoors, a noise level 10 to
15 dB below speech levels may be tolerated.
2. Reverberation time. Speech syllables occur
three or four times per second. For reverberation
times of 1.5 seconds or less, the effect of reverberant
overhang on the clarity of speech will be minimal.
3. Direct-to-reverberant ratio. For reverberation
times in excess of 1.5 seconds, the clarity of speech
is a function of both reverberation time and the ratio
of direct-to-reverberant sound.
In an important paper (8), Peutz set forth a
method of estimating speech intelligibility which has
found considerable application in sound system
design. The Peutz findings were compiled on the
basis of data gathered over a period of years. The
data and the method used to arrive at the published
conclusion are clearly set forth in the paper itself.
The conclusions can be summarized as follows:
1. In practice, the articulation loss of
consonants can be used as a single indicator of
intelligibility. Although the original research of Peutz
was in Dutch speech, the findings seem to be equally
applicable to English.
2. As would be expected, the researchers found
wide variations in both talkers and listeners.
However, a 15% articulation loss of consonants
seems to be the maximum allowable for acceptable
speech intelligibility. In other words, if articulation loss
of consonants exceeds 15% for the majority of
listeners, most of those people will find the
intelligibility of speech to be unacceptable.
3. Articulation loss of consonants can be
estimated for typical rooms. Articulation loss of
consonants is a function of reverberation time and
the direct-to-reverberant sound ratio.
4. As a listener moves farther from a talker
(decreasing the direct-to-reverberant sound ratio)
articulation loss of consonants increases. That is,
intelligibility becomes less as the direct-to-
reverberant ratio decreases. However, this
relationship is maintained only to a certain distance,
beyond which no further change takes place. The
boundary corresponds to a direct-to-reverberant ratio
of -10 dB.
Figure 6-11. Measurement of sound system gain and delta (∆)
6-11