AMS Acoustics is a world leader in the field of speech intelligibility prediction.
The practice has a number of custom mathematical models to predict both STI and RASTI system performance. Models for both classical (Sabine) and non-classical spaces are available.
This service is particularly useful for verifying contractor's design prior to contract.
The science of speech intelligibility considers the transmission path between source(s) and receptor(s) and the subsequent degradation and corruption suffered by the speech signal en-route.
In terms of sound system design both the acoustics of the space and the system components and their disposition have a major effect.
To some extent the affect of acoustical and system factors are distinguishable. For example, the directional properties of the source (loudspeaker) from one viewpoint is an acoustical effect and the other is a system attribute. Acoustically, the directional properties affect the direct-to-reverberant ratio.
The direct component is that part of the loudspeakers output which arrives directly to the listener and the reverberant component is that part of the loudspeaker's output which serves little or no useful purpose.
Speech intelligibility increases with increasing direct-to-reverberant ratio.
This may be readily understood by considering speech communication in a large reverberant space, such as a cathedral.
At large distances between a talker and listener, intelligible communication is difficult. The reverberant sound would mask the speech syllables since the direct sound would be weak and the reverberant sound dominant. As the talker and listener move closer together, then the direct sound increases and speech communication improves. Clearly the direct-to-reverberant ratio is increasing and intelligibility improving. At very close distances, say less than 1 metre, the direct sound dominates and speech intelligibility becomes good. Intelligibility may be close to 100% even though the space has a long reverberation time.
If the distance between the talker and the listener further decreases such that the talker is able to whisper in the listeners ear, then there is virtually no audible reverberant component and the direct-to-reverberant ratio approaches infinity, and hence the intelligibility is no longer a factor of, and is not influenced by, the space.
The foregoing scenario is also applicable to public address/voice alarm systems. Different types of loudspeakers have different patterns, which are able to directly affect the direct-to-reverberant ratio.
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