Note that the theoretical Q or Directivity index of a loudspeaker is very theoretical. Real live loudspeakers tend to vary significantly from theory, and one manufacturer's 90x40 loudspeaker is not often the same as another's. Loudspeakers also vary widely in directivity with frequency. Where a speaker may actually have a Q of 12.8 at 2kHz, it is likely that that the directivity value would drop to a Q of 4 at 500Hz, and as low as a Q of 1 at 125Hz. This is why we have only evaluated the 2kHz value for critical distance and the related %ALCONS. A thorough acoustical and sound system design would include the prediction of expected sound system behaviour across the entire audio bandwidth, taking into consideration the variations in loudspeaker coverage and variations in reverberation time. This is where computer modelling is very handy.

Also note that with loudspeakers, more is not necessarily better. In very reverberant spaces, adding more loudspeakers can dramatically reduce intelligibility. It is very easy to get into a no-win situation, and acoustical treatment is the only course of action to deliver speech intelligibility.

This is the reverb time you would expect to see in an unfurnished room with gypsum wallboard walls and ceilings, a concrete floor plus the carpet, ceiling tile, acoustical treatment and audience added. If the basic room materials are different than described, the resulting acoustical characteristics will be very different than predicted here.

These would be the criterion values for reverberation time

Application	125Hz	500Hz	2000Hz
Classroom	0.9 sec	0.6sec	0.6sec
Church or theatre for speech or amplified music	1.3 sec	1.0 sec	1.0 sec
Church or theatre for music	1.8-2.0 sec	1.5 - 1.8 sec	1.5 - 1.8 sec
Convention facility	1.8 sec	1.5 sec	1.5 sec
Gymnasium for teaching	1.8-2.0 sec	1.5 - 1.8 sec	1.5 - 1.8 sec
Small arena (500 -2000 seats)	2.75 sec	2.0 sec	2.0 sec
Large arena (2000+ seats)	3.25 sec	2.75 sec	2.75 sec

Why does excess reverberation affect speech intelligibility?

Critical distance is defined as the distance from a sound source located in a reverberant field, where the direct sound is equal in level to the reverberant field level. Beyond this distance from the source, there will be no further drop in sound level, as the room's reverberant field is defined as having a uniform sound level (in fact that's how to tell when you're in the reverberant field). Critical Distance is affected by the directivity of the source. With an optimum signal-to-noise ratio (a complete lack of background noise) and short reverb times, you could still understand speech beyond critical distance, but the optimum signal-to-noise ratio is rarely encountered. With long reverb times Critical Distance becomes the limiting distance for speech intelligibility.

This is the expected %ALCONS (or Percentage Articulation Loss of Consonants) for an unaided talker and the loudspeaker selected above. This refers to the percentage of consonants that will be lost or misunderstood in speech, and it is in these consonants that intelligibility lies. The maximum acceptable %ALCONS for speech communication would be 15%, as long as there is a minimum of 25dB of signal to noise ratio, and the spoken information is being delivered at a pace suitable for the acoustic environment. A more suitable maximum %ALCONS for familiar speech would be 10% (this would also apply to life safety systems, voice warning or fire evacuation page systems). For school and education environments or in public spaces where people may not speak english as a first language, a suitable maximum %ALCONS would be 5%. As you try different values, note the effect of reverberation time, speaker selection, number of speakers, and listening distance on %ALCONS. If you are seeing values in excess of 100% the listener distance is beyond 3.16 times critical distance. More detailed information on %ALcons can be found in Appendix X on page 639 in Sound System Engineering by Don and Carolyn Davis