Institute [59] for both one‐third‐octave and one‐octave bands. Although approximately 80% of the energy in speech lies below 600 Hz (including most vowels), it is in the higher frequencies that most consonants have most of their energy. These low‐energy transient consonants contribute to the intelligibility perceived. For example, it has been found [60] that if speech is passed through a high‐pass filter having a cutoff frequency of 1000 Hz then 90% of the spoken words can be understood. However, if the same speech is passed through a low‐pass filter, then a cutoff frequency of 3000 Hz is required to produce the same percentage word intelligibility. Speech sounds below 200 Hz and above 6000 Hz do not significantly contribute to intelligibility but they do add to the natural qualities of the voice [57]. Calculation of the intelligibility of speech is discussed in Chapter 6.
Table 4.3 Male voice speech sound pressure levels +12 dB at 1 m from lips for both one‐third‐ and one‐octave bands. These levels represent the speech peaks that contribute to intelligibility. The voice peak sound power levels, LW,pk, can be evaluated by adding 10.8 to the above values as shown for octave bands.
| Center frequency, Hz | Lp,pk (one-third-octave) | Lp,pk (octave) | L W,pk |
|---|---|---|---|
| 200 | 67.0 | ||
| 250 | 68.0 | 72.5 | 83.3 |
| 315 | 69.0 | ||
| 400 | 70.0 | ||
| 500 | 68.5 | 74.0 | 84.8 |
| 630 | 66.5 | ||
| 800 | 65.0 | ||
| 1000 | 64.0 | 68.0 | 78.8 |
| 1250 | 62.0 | ||
| 1600 | 60.5 | ||
| 2000 | 59.5 | 62.0 | 72.8 |
| 2500 | 58.0 | ||
| 3150 | 56.0 | ||
| 4000 | 53.0 | 57.0 | 67.8 |
| 5000 | 51.0 |
Since speech is emitted from the mouth, it is not surprising to find that the acoustic radiation from this small aperture set in a larger object (the head) is subject to fairly strong directivity effects. These directivity effects become more marked at high frequencies. Figures 4.26 and 4.27 show the relative A‐weighted sound pressure levels for the human voice in the horizontal and vertical planes, respectively. These experiments were conducted by Chu and Warnock in 40 adults, 20 male and 20 female [61]. Directivity effects can become important for audience members seated at the end of the front rows of an auditorium, since they will receive considerably less of the direct sound at high frequencies. This can considerably reduce the intelligibility of speech.
Figure 4.26 Directivity patterns for the human voice in a horizontal plane.
(Source: From Ref. [61] with permission.)
Figure 4.27 Directivity patterns for the human voice in a vertical plane.
(Source: From Ref. [61] with permission.)
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