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Effect of acoustic environment on the sensitivity of speech transmission index to source directivity.

Article Excerpt
Introduction

Speech transmission index (STI) is an objective indicator of speech intelligibility derived from the measurement of the modulation transfer function of a system (International Electrotechnical Commission, 2003). It may be used in assessing room acoustics, sound reinforcement systems, telecomunications systems, and other systems in which speech is conveyed to people. For a measurement often a loudspeaker would be used as the initial sound source for the test signal--and this loudspeaker should have similar directional characteristics to those of a human talker. However, in practice loudspeakers that closely match human speech directivity may not be available, so the question arises of how sensitive a measurement would be to deviations from ideal directivity. In this paper we consider the potential influence that sound source directivity has on STI in terms of source-receiver distance, room volume, room reverberation time, and background noise level.

Previous studies by Bozzoli, Bilzi and Farina, (2005a), Bozzoli, Viktrorovitch and Farina, (2005b) and Stewart and Cabrera, (2007) have explored the issue of directivity of STI sound sources through physical measurement. By testing a variety of loudspeaker enclosures approximating the human form in various room acoustical contexts, Bozzoli et al., (2005a) found that directivity appeared to have little effect on STI measurements in normal rooms (classrooms), but did have a substantial effect in a car cabin. Bozzoli et al., (2005b) found that the measured average directivity of ten talkers was a good match for the directivity of a Bruel and Kjaer 4128C head and torso simulator. However, one issue with this is that a head and torso simulator is very expensive and somewhat fragile, and so is not necessarily a practical solution to STI measurement. Stewart and Cabrera, (2007) examined the effect of mouth size on the directivity of a head and torso simulator, finding a substantial effect at some frequencies. When the effect of mouth size on STI measurements in real rooms (a lecture theatre and a meeting room) was tested, there was little effect, with only a modest variation in STI in one of the positions tested.

These measurement-based studies are limited by the effort involved in making physical measurements, and so only offer a glimpse of the range of possible scenarios. Hence, the present paper takes a theoretical appoach to the question.

Model of STI

According to Houtgast, Steeneken, and Plomp, (1980), the modulation transfer function of a room acoustical system, m(F), can be estimated from equation 1, based on principles from statistical room acoustics. The modulation transfer function is formed by a set of moduation reduction coefficients (i.e., values that enumerate the extent to which the modulation depth of a modulated signal is reduced by the acoustic system) for a range of modulation frequencies (F), which for STI measurements span 0.63 Hz to 12.5 Hz in 1/3-octave intervals. In STI measurements, this modulation transfer function is calculated for each of seven carrier frequency octave bands, from 125 Hz to 8 kHz.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

Here [Q.sub.t] is the directivity factor of the talker, and [Q.sub.l] is the directivity factor of the listener. In emulating STI measurements [Q.sub.t] is the directivity of the measurement loudspeaker and [Q.sub.l]=1 (an omidirectional measurement microphone is used). For...

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