Room modes with regard to speech intelligibility in classrooms: comments on Knecht et al. (2002).
OBJECTIVE
To comment on the article "Background Noise Levels and Reverberation Times in Unoccupied Classrooms: Predictions and Measurements" by Knecht, Nelson, Whitelaw, and Feth (2002).


METHODS
Schroeder frequencies to determine the transition from individual resonances to many overlapping normal modes are calculated and discussed.


RESULTS
Schroeder frequencies ranged from 67 Hz to 141 Hz.


CONCLUSIONS
Knecht et al. could have validly measured reverberation time at 250 Hz in all classrooms and at 125 Hz in the majority of classrooms because, above the Schroeder frequency, measurement of reverberation time is not significantly affected by room modes. Furthermore, Knecht et al. selected 5 different positions in each classroom for measurement of reverberation time, but in small volumes more measurement positions must be selected due to the effects of room modes compared with large volumes.
P Jafari
PJ
Shalkouhi
D000162:Acoustics; D006801:Humans; D012574:Schools; D013061:Speech Acoustics; D013065:Speech Intelligibility