334 SPEECH, MUSIC AND HEARING 



Hum 2^ is another source of noise in radio, phonograph, and sound 

 motion picture reproducing systems because practically all of these systems 

 are operated from the alternating current mains. Hum is due to inad- 

 equate filtering in the high voltage supply and inductive and capacitive 

 coupling between the power source and some part of the audio system. 



Surface noise or record scratch in phonograph records is due to dirt or 

 foreign particles in the groove and to a granular characteristic of the 

 record material. The record noise generated in a pickup which produces 

 constant output for constant velocity is proportional to the frequency for 

 a narrow frequency band of constant width. It is for this reason that 

 scratch is such a troublesome problem at the high frequencies in record 

 reproduction. 



Surface noise in film motion picture reproduction is caused by the modu- 

 lation of the light falling on the photocell, by dirt, scratches and the nat- 

 ural grain of the film. In the case of film the noise is proportional to the 

 frequency band width. Noise reduction systems increase the ratio of 

 signal to noise in film reproduction by reducing the light to the least 

 possible for the instantaneous modulation. 



Superimposed on the average atmospheric pressure are fluctuations 

 caused by thermal velocities of air molecules.^" This noise places a lower 

 limit upon the air as a transmitting medium. The ear exhibits the greatest 

 sensitivity between 1000 and 6000 cycles. The rms sound pressure due 

 to thermal noise in this frequency range is about .00005 dyne per square 

 centimeter. The maximum threshold sensitivity of the ear from Fig. 13.1 

 is .00008 dyne per square centimeter. In very sensitive ears the threshold 

 at 1000 cycles may be .0002 to .0001 dyne per square centimeter — that is, 

 of the same order of magnitude as thermal noise. For exceptionally good 

 ears a further increase in physiological sensitivity would be useless in the 

 presence of thermal noise. It is interesting to note in passing that the 

 thermal noise of the air molecules acting upon the diaphragm of a very 

 sensitive microphone may be of the same order as the thermal noise gen- 

 erated in the electrical system. 



13.19. Musical Scale. — An octave is the interval between any two 

 tones whose frequency is 2 : 1. 



A tone is a sound giving a definite sensation of pitch. 



A scale is a series of tones ascending or descending in frequency by defi- 

 nite intervals suitable for musical purposes. 



^^ Standards on Radio Receivers, Institute of Radio Engineers, 1938. 

 30 Sivian and White, Jour. Acous. Soc. Amer., Vol. 4, No. 4, p. 288, 1933. 



