Basic concepts and formulation of the speech interference level (SIL) measure are discussed and the implications of the use of SIL to measure aircraft cabin environment are analyzed. Intelligibility tests with both words and phrases indicate that serious interference with speech can be demonstrated by adding supposedly unimportant frequencies to the SIL criterion masking band. Innocuous effects are also demonstrated by masking with low frequencies alone. Conclusions are drawn that because of the complex nature of a propeller-driven aircraft acoustic environment, SIL criteria, without qualification, are unacceptable for measuring either intelligibility or comfort.
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