DISPERSION OF SOUND 299 



Graph A shows the response frequency characteristic very close to the 

 receiver and, therefore, indicates the direct sound output. The direc- 

 tional characteristics of this receiver were uniform over an angle of 120°. 

 The sharp variations in response frequency characteristics taken in other 

 parts of the room are due to the reflected sound. The direct sound energy 

 density and the reflected sound energy density are approximately equal 

 at a distance of five feet from the receiver for the average living room and 

 average reproducer. It is interesting to note that the response frequency 

 characteristics taken in various positions in the room have the same shape 

 as that taken very close to the receiver. The reverberation time charac- 

 teristic of this room was quite uniform with respect to frequency, there- 

 fore, the reflected sound does not vary appreciably with frequency since 

 the output of the receiver is independent of the frequency. See equation 

 12.6. 



The response frequency characteristics upon the ears. Fig. 13.1, to be 

 considered in Sec. 13.4, show that corresponding to the intensity of a 1000 

 cycle note there is an intensity at another frequency that will sound as 

 loud. These characteristics show that if the sound is reproduced at a lower 

 level than that of the original sound it will appear to be deficient in low 

 frequency response. In general, the reproduction level in the home is 

 much lower than the level of the original reproduction. In order to com- 

 pensate for the low frequency deficiency, the volume control in most radio 

 receivers and phonographs is designed so that the low frequency response 

 is accentuated in an inverse ratio to the relative sensitivity of the ear in 

 going from the original level to the lower level of reproduction. This type 

 of volume control is termed an acoustically compensated volume control. ^^ 



K. Radio Receiver Operating in an Automobile. — The loud speaker in an 

 automobile is usually placed in one of the following three positions: in the 

 header (above the windshield), in the instrument panel, and on the fire 

 wall or dash. The header position gives somewhat better distribution of 

 high frequency response in the back seat than the other two positions. 

 However, the low frequency response of a loud speaker mounted in the 

 header is usually attenuated due to the small volume behind the loud 

 speaker. The dash or fire wall position gives good distribution of high 

 frequency response in the front seat but not as good distribution in the 

 back seat. The low frequency response in this position can be made very 

 good by employing a large loud speaker case or by venting the back of the 

 case into the engine compartment. Sometimes a combination of a low 

 frequency dash loud speaker and a high frequency header or instrument 



18 Wolff and Cornell, Electronics, Vol. 6, No. 2, p. 50, 1933. 



