THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

228 10. Physiology of Hearing and PsychoacousticsFigure 10.7. The effect of white noise on the thresholds of detectability and intelligibilityof running speech (Hawkins and Stevens, 1950).6 dB for satisfactory communication, although the presence of speech may bedetected for S/N as low as −18 dB. If the intensity of the signal (speech) exceedsthe noise, the sign of the S/N value is plus; conversely, a negative value of S/Nindicates that the noise is more intense than the signal.Figure 10.7 maps the effect of white noise on the thresholds of detection andintelligibility of running speech. According to this figure, which was developedby Hawkins and Stevens based on extensive tests making use of running speechand white noise (Hawkins and Stevens, 1950), the threshold of intelligibility occurswhen the level of the speech exceeds the noise level by about 6 dB (S/Nof 6 dB). As the sound pressure level of the noise is increased above this value,the threshold of intelligibility is proportionally increased so that the S/N value of−6 dB remains fairly constant over a wide range of intensities. For other kindsof speech materials and different masking noises, the relationship between thethreshold of intelligibility and the level of the interfering noise may not necessarilyremain the same. At an S/N of −18 dB, running speech can be detected but notunderstandable.Other but simpler methods have been developed for measuring the effect ofinterfering noise on the intelligibility of speech. A principal method of predictingspeech intelligibility is the articulation index, or AI, which is a value that rangesfrom 0.0 to 1.0 and represents the proportion of the speech spectrum that occursabove the noise. French and Steinberg of the Bell Laboratories developed theconcept of articulation index on the basis of the assumption that most of the intelligencein speech is contained in the frequency bands between 200 and 6100 Hz.The articulation index can be calculated from the levels of the masking signal andthe speech level in the frequency bands. The contribution of each frequency bandto speech intelligibility is defined as 12 dB plus the sound level of the speech