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.

Continued on next page

3-3

Page 31
Image 31
Tyco 579-769 specifications No Noise With Added Noise, Degradation of CIS vs. Signal-to-Noise Ratio