Background noise frequencies that interferes with speech.
Speech Interference Levels - SIL - are arithmetic average sound pressure levels at the 500, 1000 and 2000 Hz center frequencies - and a measure of the degree to which background noise interferes with speech.
1 ft = 0.3048 m
The diagram above indicates the SIL and the distance at which a person would have talk in a normal, raised, very loud or shouting voice to be understood.
Example - Use of Voice with Background Noise
If the background noise - sound pressure level - is 75 dB and the distance between two persons is 6 m - they have to raise their voice to more than a very load level according the diagram above.