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Intelligibility Conversion

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The comprehension of speech (speech intelligibility) is directly dependent on the ambient noise level, the reverberation time, and the dimensions of the room. Various methods, including Speech Transmission Index (STI) and percentage of Articulation Loss of Consonants (%ALcons), serve as intelligibility conversion tools, enabling the evaluation of speech intelligibility based on specific acoustic parameters.

Speech Transmission Index (STI)

The Speech Transmission Index STI is a quantitative metric to assess speech intelligibility. It is a numerical value that ranges from 0 (completely unintelligible) to 1 (perfect intelligibility). The STI is calculated using the Modulation Transfer Function (MTF) of a pink noise in seven octave bands and examines how well the modulations of the signal are preserved.[NTI]

Percentage Articulation Loss of Consonants (%ALcons)

ALcons is defined as the percentage of consonants that are not acoustically correctly understood. The values can range from 0% (excellent speech intelligibility) to 100% (unintelligible).[AL]


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.

The %ALcons can be calculated by:

\%ALcons = \frac{ 200r^2T_{60}^2(1+n) }{VQM} + K

where

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.

Relation of STI and %ALcons

  • [TAB]
  •   Intelligibility  |  unacceptable  |          poor          |           fair           |         good         |      excellent
  • 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%
Relation between STI and %ALcons values.


  • [TAB]
  • 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

Intelligibility Conversion

These two metrics can be converted into one another, allowing researchers and practioners to interpret results from different methods. For the intelligibility conversion, the following two formulas can be used:

STI=1-0.46\cdot\log(ALcons)


ALcons = 10^\frac{1-STI}{0.46}


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

ALconsSTI


This conversion is to be regarded with some doubts, because there are different evaluations of  measurements.

Because of the characteristic nonlinear decrease of intelligibility with distance from the sound source, Farrel Becker developed an empirical conversion formula:[FB]

ALcons=170.5405\cdot e^{-5.4919 \cdot STI}


STI=-\frac{\ln( {ALcons \over 170.5405 })}{5.4919}


ALconsSTIEmpirical


  • [REF]
  • NTI Speech Transmission Index: https://www.nti-audio.com/en/support/know-how/how-do-we-measure-speech-intelligibility-sti
  • [AL]%ALcons: https://itwissen.info/en/articulation-loss-of-consonants-audibility-ALcons-120021.html#gsc.tab=0
  • [FB] Farrel Becker, A do-it-yourself guide to computing the speech transmission index: https://www.prosoundtraining.com/2010/03/17/a-do-it-yourselfers-guide-to-computing-the-speech-transmission-index/
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