The understanding of speech (speech intelligibility) is directly dependent of the background
noise level, of the reverberation time and of the size of the room. For the rating of the
understanding of speech, we have different methods, where STI and %ALcons are established.
This conversion is to be regarded with some doubts,
because there are different evaluations of measurements.
|The Speech Transmission Index STI is a machine measure of intelligibility whose
value varies from 0 (Completely unintelligible) to 1 (perfect intelligibility).
|STI||0 - 0.3||0.3 - 0.45||0.45 - 0.6||0.60 - 0.75||0.75 - 1.0|
|ALcons||100 - 33%||33 - 15%||15 - 7%||7 - 3%||3 - 0%|
Since %ALcons expresses loss of consonant definition, lower values are associated with
greater intelligibility. Zero %ALcons indicates perfect clarity. It is generally assumed that the
maximum allowable value for typical paging applications is 10 %, assuming that the
environment is relatively free of masking noise. For learning environments and voice warning
systems, the desired value is 5 % or less.
We know of the characteristic nonlinear decrease of intelligibility with distance from the sound source.
There is an empirical conversion formula after Farrel Becker:
ALcons and STI
For r < 3.5 × Dc at which direct/reverberation ratio D/R = −11 dB
Dc = Critical distance
r = Distance from the nearest loudspeaker
T60 = RT (averaged over 1 and 2 kHz)
V = Volume of the room
Q = Directivity of the nearest source
M = Acoustic modifier for reverberant power = 1 is a conservative assumption
(1 + n) = Total number of equal sources
K = Listener factor ≈ 2 % for a good listener. For
r > 3.5 × Dc (Reverberant field, always worse than above)
%ALcons = 9 × T60 + K
%ALcons < 10 % intelligibility is very good
%ALcons < 15 % intelligibility is acceptable
%ALcons > 15 % intelligibility will be a problem
|Quantity to be measured||Unit of measurement||Good values|
|%ALcons||Articulation loss (popular in USA)||< 10 %|
|C50||Clarity index (not widespread)||> 3 dB|
|STI (RASTI)||Intelligibility (international known)||> 0.6|