Noise interference with speech is ususlly a masking process. The background noise increases our threshold of hearing, and, as a result, we may hear only a few or perhaps none of the sounds necessary for tsatisfactory intelligibility.
Because of the annoyance of interference with speech and also because noise interferes with work where speech comunication is necessary, a noise rating based on the speech-interference level is frequently useful. Environmental Engineers should know how to improve speech communication in a noisy place. To do this, it is useful to evaluate the speech interference level of a noise.2
Experimenters have found that nearly all the information in speech is contained in the frequencies between 200 to 6000 Hz. To simplify the calculations of speech interference levels, only the sound pressure levels in three octave bands are used. This is called the three-band preferred octave speech-intergerence level (PSIL).2
PSIL = (Lp500 + Lp1000 + Lp2000)/3 dB
Lp500 - the octave-band presssure level at 500 Hz
Lp1000 - the octave-band pressure level at 1000 Hz
Lp2000 - the octave-band pressure level at 2000 Hz
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