Tyco 579-769 No Noise With Added Noise, Degradation of CIS vs. Signal-to-Noise Ratio

Influences on Intelligibility, Continued

Background Noise	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:

CISDegradatione tion from backgroundBackgroundnoiseNoise(broad(Broadspectrum)Spectrum)

	1
Scale)	0.95
	0.9
	0.85
(CIS
	0.8
	0.75
Intelligibility
	0.7
	0.65
	0.6
	0.55
	0.5
	-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

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.

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