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Disagreement between STI and STIPA Measurements Due to High Level, Discrete Reflections

Objective measures of intelligibility, speech transmission index (STI), and speech transmission index for public address systems (STIPA) often form the basis for sound system verification. The reported work challenges the accuracy of both measures when encountering high level, discrete reflections. Tests were carried out in an anechoic environment with artificial reflections added between 0 and 500 ms. Discrepancies were found to occur above 80 ms due to synchronization between modulation frequencies and reflection arrival times. Differences between STI and STIPA of up to 0.1 were found to occur for the same delay condition. Results suggest STIPA should be avoided in acoustic environments where high level, discrete reflections occur after 80 ms and STI should only be used alongside other verification methods.

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