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Alcons means Percentage Articulation Loss of Consonants = %ALcons. This machine measure of intelligibility is closely associated with the TEF sound analyzer. It is computed from measurements of the Direct-to-Reverberant Ratio and the Early Decay Time using a set of correlations defined by SynAudCon, and is specified in percent. See also Intelligibility (communication).

Consonants play a much more significant role in speech intelligibility than vowels. If the consonants are heard clearly, the speech can be understood more easily.

Since %ALcons expresses loss of consonant definition, lower values are associated with greater intelligibility. 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.

Alcons is the measured percentage of Articulation Loss of Consonants by a listener. % Alcons of 0 indicates perfect clarity and intelligibility with no loss of consonant understanding, while 10% and beyond is growing toward bad intelligibility, and 15% typically is the maximum loss acceptable.

The %Alcons method is widely used by acoustical consultants (particularly in the United States), but it has significant drawbacks. First, it is based on measurements in a single one-third octave band centered on 2 kHz; all other frequencies are ignored, so the system’s frequency response must be verified in some other way for the %Alcons score to be meaningful.

Moreover, the method does not account for many factors that can dramatically affect intelligibility, including signal-to-noise ratio, the background noise spectrum, distortion, late reflections or echoes, system frequency response, compression, non-linear phase, equalization and acoustic power. %Alcons measurements of sound systems therefore often yield overly optimistic scores. Where reverberation or strong, late-arriving reflections are the primary problem, however, they can sometimes be more useful and accurate than RASTI.

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