A comparative study of speech intelligibility metrics and the derivation of optimum reverberation time and maximum background-noise level for classrooms

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DOIResolve DOI: http://doi.org/10.4224/20331303
AuthorSearch for: ; Search for:
TypeTechnical Report
Series titleInternal Report, Institute for Research in Construction, National Research Council Canada; Volume 778
Physical description79 p.
Subjectspeech intelligibility in classrooms; optimum reverberation time for classrooms; maximum background noise for classrooms; classroom acoustics; room acoustics; Classroom acoustics; Speech security
AbstractA comparative study of speech intelligibility metrics that take into account sound refections in the room and the background noise has been developed based on the assumption of a d iffuse sound field in the room Under this assumption the room reverberant response results in an exponential decay with a damping constant inversely proportional to the reverberation time. This allows analytical formulas to be obtained for each of the speech intelligibility metrics considered in this study, and provides a common basis for this comparative study. These formulas were applied to three sizes of a rectangular shaped classroom, inside which the sound source was a human voice without amplification. The background noise was taken into account in these formulas by a noise-to-signal ratio written in terms of the noise level and the vocaloutput level at 1 meter in front of the talker. Correlation between the metrics and speechintelligibility obtained by the respective authors, using different types of articulation testing material, are also presented and applied to the classrooms under study. Relationships between some of the speech intelligibility metrics were also established. It was then possible to verify that for each noise-to-signal ratio, the value of each speech intelligibility metric is maximized for a specific reverberation time, which was considered to be the optimum reverberation time. It was found that for very quiet classrooms, the optimum reverberation time predicted by all the speech intelligibility metrics is 0.1 - 0.3 s. Where there is more ambient noise, optimum reverberation times are larger. The range of reverberation times for 100 % speech intelligibility was found to vary somewhat among the dzfferent speech intelligibility metrics. This is due to different articulation tests used, and also due to a rather arbitrary way of establishing cut-off values forthe metrics that use a subjective speech intelligibility scale. However it is possible to obtain 100% speech intelligibility with reverberation times of at least 0.4 - 0.5 s, and these reverberation times may be preferable in rooms with moderate ambient noise levels. For some metrics the reverberation time can be even higher, while still giving 100% speech intelligibility. The present study suggests an "ideal" and an "acceptable" maximum background-noise level for classrooms of 25 dB and 20 dB respectively below the voice level at 1 meter in front of the talker.
Publication date
PublisherNational Research Council Canada. Institute for Research in Construction
AffiliationNational Research Council Canada; NRC Institute for Research in Construction
Peer reviewedNo
NRC number9670
NPARC number20331303
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Record identifierc1f8f2f9-c9bd-40d4-b92c-8d1c1317f57b
Record created2012-07-18
Record modified2016-10-03
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