328 SPEECH, MUSIC AND HEARING 



14,000 cycles. A good radio transmitter and receiver in the broadcast 

 band will cover a frequency range of from 40 to 8000 cycles and a volume 

 range of 50 to 60 db. These frequency and volume ranges will cover the 

 speech area of Fig. 13.7, but will not cover the orchestral music area. For 

 the latter case some form of volume compression must be used. 



13.13. Absolute Amplitudes and Spectra of Speech, Musical Instru- 

 ments and Orchestras 20,21,21^^ — 'pj^g average and peak outputs of speech 

 and musical instruments are of importance in the design of all types of re- 

 producing equipment. For example, the average power output involves 

 such factors as the heating of the voice coil, the heating of audio power 

 transformers, etc., while the peak power output fixes the overload point 

 of the system. 



The ratio of the average sound pressure per cycle to the average total 

 pressure of the entire spectrum for speech, various musical instruments 

 and orchestras is shown in Fig. 13.10. The ratio of the peak pressure to 

 the average pressure of the entire spectrum for speech, various musical 

 instruments and orchestras is shown in Fig. 13.11. The peak or total 

 power output can be computed from the pressure and the distances follow- 

 ing the procedure as outlined in Sec. 11. 3D. 



13.14. Effect of Nonlinear Distortion upon the Quality of Reproduced 

 Speech and Music 22,23,24^ — jj^ ^j^ ideal reproducing system the elements 

 are invarient with respect to the time. However, in practical systems the 

 elements are nonlinear. These elements introduce nonlinear distortion. 

 Some idea of the effect of nonlinear distortion can be obtained from a 

 study of the masking curves. From these data it will be seen that the 

 higher order harmonics are noticeable at much lower levels than the lower 

 order harmonics. Furthermore, as the high frequency range is increased 

 the effect of the harmonics is more noticeable. In the complex waves of 

 speech and music, sum and difference tones are also an important phase of 

 the problem of nonlinear distortion. 



Tests of music reproduction, on a system with uniform response from 

 45 to 8500 cycles at a peak level of 80 db, have indicated that 5 per cent 

 second harmonic and 3 per cent third harmonic are noticeable on a direct 

 comparison with a system having less than 1 per cent total distortion. In 



2° Sivian, Dunn and White, Jour. Acous. Soc. Amer., Vol. 2, No. 3, p. 330, 1931. 



21 Sivian, L. J., Jour. Acous. Soc. Amer., Vol. 1, No. 2, Part 2, p. 1, 1930. 

 21^ Hathaway, J. L., Electronics, Vol. 12, No. 11, p. 29, 1939. 



22 Stevens and Davis, " Hearing," p. 200, John Wiley and Sons, New York. 



2^ Olson and Massa, " Applied Acoustics," P. Blakiston's Son and Co., Phila- 

 delphia. 



24 Massa, P., Proc. Inst. Rad. Eng., Vol. 19, No. 5, p. 682, 1933. 



