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3-3
Background noise causes a reduction in signal-to-noise ratio over all frequencies and modulations.
Consider the comparison of the speech signal below with and without added noise:
No Noise With Added Noise
Figure 3-2. The Speech Pattern “An Emergency Has Been Reported” with Added Noise
Creating an intelligible system in the presence of background noise requires adequate signal-to-
noise ratio. In general, if the speech signal is 10 dB higher than the noise, the intelligibility loss
due to background noise is minimal.
The figure below shows degradation as a function of signal-to-noise ratio:
CIS degredation from background noise (broad spectrum)
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Signal to Noise Ratio dBA
Intelligibility (CIS Scale)
Figure 3-3. Degradation of CIS vs. Signal-to-Noise Ratio
Note: There are limits to increasing the speech signal-to-noise ratio. Above approximately
90 dB the intelligibility of speech actually decreases with increasing volume. This
implies that areas with background noise greater than 80 to 90 dB can pose a challenge to
the system designer.
Continued on next page
Influences on Intelligibility, Continued
Background Noise
CIS Degradation from Background Noise (Broad Spectrum)