Engineering Acoustics. Malcolm J. Crocker

is averaged over a relatively long period (i.e. five seconds), the overall sound power level is found to be 75 dB. This corresponds to an average sound pressure level of 65 dB at 1 m from the lips of the speaker and directly in front of him or her. Converting the sound power level to sound power shows that the long time averaged sound power for men is 30 μW. The average female voice is found to emit approximately 18 μW. However, if we average over a very short time (i.e. l/8 second) we find that the power emitted in some vowel sounds can be 50 μW, while in other soft spoken consonants it is only 0.03 μW. Generally, the human voice has a dynamic range of approximately 30 dB throughout its frequency range [58]. At maximum vocal effort (loud shouting) the sound power from the male voice may reach 3000 μW.

Table 4.3 gives the long‐term rms sound pressure levels at l m from the average male mouth for normal vocal effort as given by the American National Standards 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, L_W,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.)

References

      1 1 Palmer, J.W. (1993). Anatomy for Speech and Hearing, 4e. Philadelphia, PA: Lippincott Williams & Wilkins.

      2 2 Crocker, M.J. (1974). The ear, hearing, loudness and hearing damage. In: Reduction of Machinery Noise (ed. M.J. Crocker), 43. West Lafayette, IN: Purdue University.

      3 3 Crocker, M.J. (1998). Handbook of Acoustics, 1083–1138. New York: Wiley.

      4 4 Paul, P.V. and Whitelaw, G.M. (2011). Hearing and Deafness: An Introduction for Health and Education Professionals. Sudbury, MA: Jones and Bartlett Publishers.

      5 5 Gulick, W.L., Gescheider, G.A., and Frisina, R.D. (1989). Hearing: Physiological Acoustics, Neural Coding, and Psychoacoustics, 2e. New York: Oxford University Press.

      6 6 Tysome, J. and Kanegaonkar, R. (2016). Hearing: An Introduction & Practical Guide. Boca Raton, FL: CRC Press.

      7 7 Welch, B.L. and Welch, A.S. (1970). Physiological Effects of Noise. New York: Plenum Press.

      8 8 Wada, H. (2007). The ear: its structure and function, related to hearing. In: Handbook of Noise and Vibration Control (ed. M.J. Crocker), 277–285. New York: Wiley.

      9 9 Silman,