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Engineering ToolBox > SIL - Speech Interference Levels
Speech Interference Levels - SIL - are the arithmetic average of sound pressure levels at the 500, 1000 and 2000 Hz center frequencies and is a measure of the degree to which background noise interfere with speech.

The diagram above indicates the SILs and distances at which a person would have talk in a normal, raised, very loud or shouting voice to be understood.