Articulation Loss (%ALcons)
Definition
Articulation Loss (%ALcons)
Articulation Loss of Consonants (%ALcons) measures the percentage of consonants lost or misunderstood by listeners in a space. Lower values indicate better intelligibility. The metric bridges room acoustics and listener perception, predicting speech clarity from physical measurements of reverberation, distance, and loudspeaker directivity.
%ALcons = 200 × D² × RT60² / (V × Q) (Peutz equation — D = distance in m, V = volume in m³, Q = directivity factor)
%ALcons was developed by V.M.A. Peutz in the 1970s as a practical metric for predicting speech intelligibility in rooms. It calculates the percentage of consonants a typical listener would fail to hear correctly, based on measurable room acoustic parameters. Consonants carry the majority of information in speech, so their loss directly impacts comprehension.
The Peutz equation relates %ALcons to four key variables: the distance from source to listener (D), the reverberation time (RT60), the room volume (V), and the loudspeaker directivity factor (Q). As distance increases or reverberation time grows, %ALcons rises — meaning more consonants are lost. Larger room volume and higher directivity (more focused loudspeaker pattern) reduce %ALcons by improving the direct-to-reverberant ratio.
In practice, %ALcons below 10% is considered "good" intelligibility, while above 15% is "poor." Values above 25% indicate nearly unintelligible speech. The metric was widely used in sound system design before the STI standard (IEC 60268-16) became dominant. While STI has largely superseded %ALcons in formal standards, %ALcons remains valuable for quick estimates and intuitive understanding — a "12% loss" is more immediately meaningful to non-acousticians than an "STI of 0.62."
SonaVyx can derive %ALcons estimates from its STI measurements using the empirical relationship %ALcons ≈ 170.5405 × e^(−5.419 × STI). For direct measurement-based estimates, the RT60 from reverberation time analysis and SPL from the sound level meter provide the inputs for the Peutz equation. AcousPlan's RT60 calculator can predict %ALcons at the design stage.
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