Speech Interference Levels

Reference data and engineering information about speech interference levels for miscellaneous applications.

speechinterferencelevels

Overview

Engineering reference data for Speech Interference Levels in miscellaneous.

Key Formulas

Unit Conversion

$y = x \cdot k$

Multiply by conversion factor.

Linear Interpolation

$y = y_1 + \frac{(x - x_1)(y_2 - y_1)}{x_2 - x_1}$

Estimate between two known points.

Percentage

$p = \frac{\text{part}}{\text{whole}} \times 100\%$

Part as fraction of whole.

Variables

Symbol	Description	Unit
$x$	Input value	—
$y$	Output value	—
$k$	Conversion factor	—

The Speech Interference Level (SIL) is calculated as the arithmetic average of the sound pressure levels (SPL) in three octave bands: $\text{SIL} = \frac{SPL_{500\,\text{Hz}} + SPL_{1000\,\text{Hz}} + SPL_{2000\,\text{Hz}}}{3}$

This metric is specifically chosen because the 500 Hz to 2000 Hz range is critical for speech intelligibility.

Voice Levels and Maximum Distance

The following table summarizes the typical voice effort required and the approximate maximum distance for intelligible speech at various SIL levels, based on common engineering diagrams.

4 rows

Approximate maximum distances for intelligible speech (50% of words understood) as a function of background SIL and voice effort.
Voice Type	Typical Voice SPL(dB)	Max Dist. @ SIL 60(m (ft))	Max Dist. @ SIL 70(m (ft))	Max Dist. @ SIL 80(m (ft))
Normal	60	3.6 (12)	1.8 (6)	0.9 (3)
Raised	70	7.2 (24)	3.6 (12)	1.8 (6)
Very Loud	80	14.4 (48)	7.2 (24)	3.6 (12)
Shouting	90	28.8 (96)	14.4 (48)	7.2 (24)

Source: engineeringtoolbox.com

Worked Example: Voice Selection

Problem: The background noise has a measured SIL of 75 dB. Two people are 6 meters apart. What voice effort is required for communication?

Solution:

From the table (or diagram), for a Very Loud voice at SIL 75 dB (interpolating between SIL 70 and SIL 80), the maximum intelligible distance is approximately 5.4 m.
Since the required distance (6 m) is greater than 5.4 m, a Very Loud voice will be insufficient.
The individuals must therefore use a Shouting voice level (typical SPL ~90 dB), which provides a maximum distance of ~7.2 m at this SIL.

Unit Conversion Note

For engineering calculations, note the standard conversion: $1\,\text{ft} = 0.3048\,\text{m}$ This is critical when converting distances from imperial to SI units.