Telephone Noise

Overview

Engineering reference data for Telephone Noise in acoustics.

Key Formulas

Speed of Sound

$c = \sqrt{\gamma R T}$

Speed of sound in an ideal gas.

Sound Level

$L = 10 \log_{10}(I/I_0)$

Decibel level.

Wavelength

$\lambda = c / f$

Wavelength = speed / frequency.

Variables

Symbol	Description	Unit
$c$	Speed of sound	m/s
$L$	Sound level	dB
$\lambda$	Wavelength	m
$f$	Frequency	Hz

Speech Interference Levels (SIL) Thresholds

Telephone conversation quality is directly related to the background noise level, measured in Speech Interference Level (SIL). The general acceptability thresholds are:

Condition	SIL (dB)	Approx. dBA
Satisfactory	< 65	72
Difficult	65 - 80	72 - 87
Impossible	> 80	87

The relationship between SIL and A-weighted sound level (dBA) can be approximated for typical noise spectra. A common engineering conversion is:

$$L_{A} \approx SIL + 7 \, \text{dB}$$

Where:

• $L_{A}$ is the A-weighted sound pressure level in dBA.
• $SIL$ is the Speech Interference Level in dB.

This 7 dB offset reflects the weighting applied to lower frequencies in the A-weighting curve, which are less critical for speech intelligibility.

References

• Original Source