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Speech intelligibility: background
A widely used value to quantify speech intelligibility is the Speech Transmission
Index (STI). It is a nondimensional quantity varying between 0 and 1 according to
the evaluation scale indicated below.
The speech intelligibility level has to be at least “fair” in all places and under all
circumstances: STI ≥ 0.45 Because of the importance of the intelligibility of sound
messages in a railway station, the objective for most areas on the platform is at
least the level “good”: STI ≥ 0.60.
Speech intelligibility levels are largely dependent on the reverberation time of the
space and on background noise levels at the position of the listener. The speech
intelligibility for an ideal diffuse sound field can be calculated by means of the
reverberation time T [s] and the ratio of signal to noise S/N [dB] at the place of
the listener. This last value expresses the db value with which the background
noise exceeds the useful signal of a speaker at the position of the listener. Figure
12 indicates the relation in two ways and illustrates two important methods to
optimize speech intelligibility levels.
The speech intelligibility level increases with a reduced reverberation time and
an increased signal/noise ratio. A primary requirement for a decent speech
intelligibility level is therefore a fairly short reverberation time, even at very high
S/N values. Consequently, there has to be sufficient sound absorption in the
space. This is all the more important for a railway station environment where a
fair S/N ratio is not always guaranteed. A somewhat longer reverberation time can
sometimes compensate for this flaw.
The second requirement for a decent speech intelligibility level is a sufficiently
powerful amplification system, capable of surpassing background noise levels.
This is achieved by adapting the power of the amplifier to the existing background
noise. Of course this is only possible within a certain range. It is for instance
pointless to increase amplification to a level that exceeds the noise level of a
passing train. The amplification is usually automatically adjusted in a few fixed
steps to a previously determined maximum level.
0 0.2 0.45 0.6 0.85 1 STI
unintelligible poor fair good excellent evaluation
fg. 11 Relationship between the STI and the subjective evaluation of speech intelligibility.
fg 12.1 Relation speech intelligibility/reverberation time of a space for different signal/noise ratios.
fg 12.2 Relation speech intelligibility/signal to noise ratios, for reverberation times of a space.
Speech intelligibility (STI) Speech intelligibility (STI)
Signal-noise level (dB) reverberation time (s)
excellent
good
fair
poor
bad
excellent
good
fair
poor
bad