STI or its derivatives (RaSTI and STIPa) have become the internationally accepted methods for acoustically measuring the potential intelligibility performance of a sound system. However, in practice, many of the measurements carried out in the field, to either verify or ascertain sound system and Voice Alarm intelligibility performance, are often based on flawed techniques. The paper examines a number of common problems found to affect measurement accuracy. The paper also highlights conditions under which STI and STIPa inherently appear to incorrectly predict intelligibility performance. In particular it is shown that the currently available commercial software programmes and instrumentation fail to correctly predict the performance of sound systems exhibiting irregular or band limited frequency responses when they are operating in reverberant environments under quiet (i.e. high signal to noise ratio) conditions.
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