Intelligibility  Conversion

%ALcons = Articulation Loss of Consonants in % ← → STI = Speech Transmission Index

 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.

Fill out the respective box and click the 'calculate' bar below.

 STI | ALcons % | ↓ | ↓ | ALcons % | STI |

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 unexceptable poor fair good excellent 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:

 STI ↓ ALcons %

ALcons and STI

For r < 3.5 × Dc at which direct/reverberation ratio D/R = −11 dB

 where 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