Room Acoustics and Speech Perception

The acoustic speech signal received by a listener is a function of the source, distance, early reverberation, late reverberation, and noise. Specifically, it depends on the speech audibility index, which is defined here as the proportion of the combined direct speech and early reverberation (also known as early reflections) that is audible in spite of the effects of noise and late reverberation. The speech audibility index increases from 0 to 100% as the effective signal-to-noise ratio increases from -15 to +15 dB. Both reverberation and ambient noise need to be low to maintain the speech audibility index at an optimal level. The speech audibility index can be used to predict various measures of speech perception, but the results are highly dependent on the complexity of the language and the characteristics of the listener. Conditions that are tolerable for normally hearing adults in casual conversation can be difficult for adults and children in learning situations, and intolerable for persons with deficits of hearing, language, attention, or processing. Sound-field amplification can improve the speech audibility index for all listeners in a noisy room. It offers less benefit when the primary problem is reverberation and, if improperly installed, can make the reverberation problem worse. There is no good substitute for reverberation control. Audiologists have an important contribution to make in the identification and resolution of continuing inadequacies of classroom acoustics.