Speech Interference Level

Speech Interference Level (SIL) is the arithmetic average of sound pressure levels at the octave band centre frequencies of 500, 1000, 2000, and 4000 Hz. It provides a simple single-number estimate of how much a background noise environment interferes with face-to-face speech communication. Originally defined by Beranek, SIL correlates well with the distance at which reliable communication is possible for different vocal efforts. For example, at SIL = 55 dB, normal voice communication is limited to about 1 m, while raised voice extends to 2 m. The four-frequency version (sometimes called PSIL or Preferred Speech Interference Level) using 500–4000 Hz replaced the original three-frequency version. While largely superseded by more sophisticated metrics (STI, SII), SIL remains useful for quick field assessments and is referenced in ANSI S12.65 for workplace noise evaluation. It is calculated directly from octave-band noise measurements.