%ALCONS Calculator

Percentage Articulation Loss of Consonants is an indication of the loss of speech intelligibility that occurs in difficult acoustic environments. This calculation provides %ALCONS values for an unaided voice with a Q (or directivity) of 2, and for loudspeakers with a selectable coverage pattern. As an added bonus, the Q and DI values are calculated. The assumptions made in this calculator are; that there is 25dB of signal to noise ratio available at the listening position (low background noise); that the frequency response is uniform in critical speech intelligibility bands; and that only one loudspeaker is providing direct sound to the listening position, and that all others are primarily contributing to the reverberant field. Note that at longer reverb times the %ALCONS is very high, and this definitely affects speech communication.

What the numbers mean

For years, the acceptable maximum design %ALCONS was 15%, and as always, was based on the requirement to have at least 25dB of signal to noise ratio, and uniform frequency response in the 2-4kHz spectrum, which is the critical speech intelligibility spectrum. In the past few years, the study of speech intelligibility has led the sound industry to find 10% a more appropriate maximum for most purposes. When the information being delivered is familiar or expected, 10% is quite acceptable. In a learning environment, especially for people with hearing impairment, the target %ALCONS should be closer to 5%. Life Safety, and Voice Warning Systems have similar stringent requirements for high speech intelligibility, as they are required to deliver instructions and information that may literally be a matter of life and death. If you're involved in any project with critical sound system requirements, you should have the acoustical environment and the loudspeaker system designed by an acoustical consultant.