As we saw with the unaided talker, the direct sound level is what we're most interested in when we're trying to achieve maximum speech intelligibility. This gives us the best immunity to background noise in the room and the most effective delivery of critical information content in the 2-4kHz bands. The plot below shows the direct sound level of eight ceiling speakers on a 10' ceiling. A higher ceiling would allow us to reduce the speaker count, a lower ceiling requires more speakers to achieve uniform coverage.
The total range of this plot is 75dB to 92.5dB, with the variation over the seating area being 7dB.
Add in the room
This plot shows the direct sound plus the reverberant sound level in the room with a 1.2 second reverberation times. The total range of the display is 95.8dB to 97.5dB which shows that the reverberant level has filled a great deal of the variation in direct level. As we saw with the unaided speech plots, the uniform reverberant level doesn't necessarily help us out in improving speech intelligibility since the reverberant sound field also moderates the level variation of any noise source in the room.
With the reverberation time reduced to 0.6 seconds, we've increased the variation between minimum and maximum level to 89.5dB to 94.2dB, but we've done that while we've improved the direct to reverberant sound level ratio, which definitely helps intelligibility.
Improving Intelligibility, Not Sound Level
What we're really trying to achieve with a speech reinforcement system is a high direct sound to revereberant sound ratio, so that everyone hears direct sound at a significantly higher level than the reverberant sound level. By using the speech reinforcement system we're effectively moving the students closer to the teacher so that they are all effectively seated within 8 feet of the teacher.
This plot shows the point where the rubber meets the road, this is the Percentage Articulation Loss of Consonants (%ALCons) plot which shows the loss of consonants with a 1.2 second reverberation time. Note that the range is from -2% to -10%, with the best speech intelligibility performance ocurring directly under the loudspeakers.
With the reverberation time reduced to 0.6 seconds, the %ALCons is vastly improved. The total range of variation is -0.2% to -5%, and the area in which we achieve better than -2%ALCons encompasses the entire seating area.
A speech reinforcement system in a classroom provides much more uniform sound coverage of the teacher's voice, as well as A/V source material. If the teacher wears a microphone, their voice will not change in level when the face the blackboard or face the class. This is also a benefit to teachers with softer voices who may have to strain their voices to be heard in the back row of a classroom. This is often a problem in the summertime when windows may be open and outside noise is present, or in the situation where HVAC systems are noisy, or the students are involved in moderately noisy activities.
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