Articulation Index

A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded. An articulation index of 100% means that all speech can be understood, 0% means that no speech can be understood.
Articulation Index is calculated from the 1/3 octave band levels between 200Hz and 6300Hz centre frequencies. Each of the 1/3 octave dB(A) levels are weighted according to the following criteria:

  • If A-weighted 1/3 octave level lies between upper and lower limits then it will be a linear value between 0 and 1.
  • If value falls above the upper limit then result =0 for that particular 1/3 octave band.
  • If value falls below the lower limit then result =1 for that particular 1/3 octave band.
  • Multiply all of the calculated values by the AI weighting and sum all the values to get the AI in percentage terms.
1/3 Octave Centre Frequency HzAI lower level dB(A)AI upper level dB(A)AI Weighting
20023.153.11.0
25030.460.42.0
31534.464.43.25
40038.268.24.25
50041.871.84.5
63043.173.15.25
80044.274.26.5
100044.074.07.25
125042.672.68.5
160041.071.011.5
200038.268.211
250036.366.39.5
315034.264.29.0
400031.061.07.75
500026.556.56.25
630020.950.92.5
AI

Frequency (Hz)

Notes:
  • There are other methods of calculating articulation index. This particular method is that used within the automotive industry as a wind noise metric.
  • AI is designed for use where the noise is broadband in nature.
  • AI uses a limited frequency range 178Hz to 7080Hz (lower frequency of 200Hz 1/3 octave band to upper frequency of 6300Hz 1/3 octave band).
  • An AI < 0.05 is representative of very poor speech intelligibility, and an AI > 0.80 represents good speech intelligibility.
  • The Speech Privacy Index (PI) is derived from the AI.

VBA Function

 Function AIval(thrdOct, dBA)
Dim tho As Range
th = Array(200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300)
ailow = Array(23.1,30.4,34.4,38.2,41.8,43.1,44.2,44,42.6,41,38.2,36.3,34.2,31,26.5,20.9)
aiwgt = Array(1, 2, 3.25, 4.25, 4.5, 5.25, 6.5, 7.25, 8.5, 11.5, 11, 9.5, 9, 7.75, 6.25, 2.5)
′ count throught the array
I = 0
For Each tho In thrdOct
I = I + 1
For t = 0 To 15
If tho = th(t) Then
AIB = ((dBA(I) - ailow(t)) / 30) * aiwgt(t)
If dBA(I) <= ailow(t) Then AIB = 0
If dBA(I) >= ailow(t) + 30 Then AIB = aiwgt(t)
AI = AI + AIB
End If
Next t
Next tho
AIval = 100 - AI
End Function

See also: Articulation Class, Articulation Loss of Consonants, Articulation Score, Decibel, Percentage Articulation Loss of Consonants, Privacy Index, Speech Intelligibility.

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Subjects: Noise & Vibration