324 



SPEECH, MUSIC AND HEARING 



The sensation levels of the fundamental at which the various harmonics 

 first become detectable/'^ are shown in Fig. 13.6. The subjective effects 

 of the harmonics generated in the ear are more pronounced at the lower 

 frequencies. Furthermore, the harmonics appear at a lower level at the 

 lower frequencies. 



FREQUENCY IN CYCLES PER SECOND 



Fig. 13.6. The level above threshold at which harmonics are generated in the ear at various 

 frequencies. (After Wegel and Lane.) 



13.9. Effect of Phase Relations Among the Harmonics ^^o". — The 



phase of a harmonic affects the threshold of perceptible distortion as well 

 as the quality of a complex sound. This statement contradicts the so 

 called Ohm's Auditory Law; that the ear tends to analyze the compounds 

 of a complex sound regardless of the phase relations. There is a definite 

 phase relation which will produce the greatest loudness and another which 

 will produce the least loudness. For example, a harmonic in the actuating 

 sound may re-enforce or cancel an aural harmonic. 



13.10. Modulation ^2' ^^. — Amplitude, phase, or frequency modulation 

 refers to a change in the amplitude, phase or frequency of a sound wave. 

 Suppose that an oscillator is connected to a loud speaker, Fig. 11.6. If the 

 volume control is varied, the result is amplitude modulation. If the fre- 



7 Wegel and Lane, Phys. Rev., Vol. 23, No. 2, p. 266, 1924. 

 ^ Fletcher, " Speech and Hearing," D. Van Nostrand Co., New York. 

 ^ Chapin and Firestone, Jour. Acous. Soc. Amer., Vol. 5, No. 3, p. 173, 1934. 

 10 Lewis and Larsen, Nat. Acad. Sci., Vol. 23, p. 415, 1937. 

 " Stevens and Davis, " Hearing," p. 203, John Wiley and Sons, New York. 

 12 Seashore, C. E., " Psychology of Music,' p. 33, McGraw Hill Book Co., 

 New York. 



^^ Stevens and Davis, " Hearing," p. 225, John Wiley & Sons, New York. 



