NOISE IN REPRODUCING SYSTEMS 



333 



changes in complexity up to a pitchless sound such as thermal noise. 

 Work^®^ has been carried out on the subjective measurement of timbre. 

 However, the subject of timbre is more complex than that of loudness 

 and pitch. 



100 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 13.13. The minimum perceptible change in intensity level of pure tones as a function 

 of the frequency for various sensation levels. Sensation level is level above threshold. 

 (After Fletcher " Speech and Hearing," D. Van Nostrand Co.) 



13.18. Noise in Reproducing Systems. — Static, hiss, scratch, surface 

 noise and hum are terms which have been introduced to describe various 

 kinds of noises produced by reproducing systems. A few of the types of 

 noises in reproducing systems will be discussed in this section. 



In radio broadcasting systems random noise is produced by thermal 

 agitation ^'^ in conductors, and the small shot effect ^^ in vacuum and fluc- 

 tuating noise is due to atmospherics and man made interference. The 

 energy produced by thermal noise and the small shot effect is proportional 

 to the width of the frequency band. The noise level in a quiet residence 

 is about 30 db. See Sec. Y1.1F. If the noise generated in the broadcasting 

 system is below 30 db it will not be noticed. Of course, the effects of at- 

 mospherics and man made static can be reduced by increased power. 

 Accentuating the high frequency response in transmission and introduc- 

 ing complementary equalization in the receiver will reduce the noise, since 

 the noise is proportional to the band width. For the same reason, fre- 

 quency modulation also reduces noise in radio reproducing systems. 



"^^^ Seashore, C. E., " Psychology of Music," McGraw Hill Book Co., New York. 



27 Johnson, J. B., Phys. Rev., Vol. 32, No. 1, p. 97, 1928. 



28 Schottky, W., Ann. d. Phys., Vol. 57, p. 541, 1918. 



