322 



SPEECH, MUSIC AND HEARING 



it will be seen that for a 50 cycle tone the intensity required to reach the 

 threshold of hearing is 250,000 times that required for a reference 1000 

 cycle tone. 



13.6. Change of Pitch with Loudness. — Frequency of a sound wave 

 is the number of cycles per second executed by the particles of the medium 

 in which a sound is being propagated. Pitch is that subjective quality of 

 a sound which determines its position in a musical scale. Pitch may be 

 measured as the frequency of a pure tone having a specified sound pressure 

 which seems to the average ear to occupy the same position in a musical 



100 200 



FREQUENCY 



500 1000 



CYCLES PER SECOND 



2000 



Fig. 13.4. Contours of constant loudness level. Curves show the amount by which the 

 pitch of a pure tone of any frequency is shifted as the tone is raised in loudness level from 

 40 to the level of the contour. For example, a 100-cycle tone will be changed ten per cent 

 downward in pitch if raised from a loudness level 40 to a loudness level 100, but a SOO-cycIe 

 tone will be changed only two per cent for the same loudness level increase. (After Snow.) 



scale. Thus it will be seen that there is definite distinction between fre- 

 quency and pitch. For example, a tone of a fixed frequency of a few hun- 

 dred cycles decreases in pitch as the intensity is increased. The change ^ 

 in pitch with loudness is shown in Fig. 13.4. 



13.7. Masking ^'^ — The reduction of the ability of a listener to hear 

 one sound in the presence of other sounds is known as masking. In testing 

 the masking properties of a sound, pure tones are generally used as the 

 masked sound. The number of decibels that the threshold level of a pure 



3 Snow, W. B., Jour. Acous. Soc. Amer., Vol. 8, No. 1, p. 14, 1936. 



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



^ Fletcher and Munson, Jour. Acous. Soc. Amer., Vol. 9, No. 1, p. 1, 1937. 



