EFFECT OF MASKING ON LOUDNESS 295 



These results have an important bearing on the electrical reproduction 

 of musical and vocal selections from phonograph records and the loud- 

 ness levels at which loud speakers should be used. It is well recognized 

 that in the reproduction of music by electrical devices the original quality 

 of the selection can be duplicated only if the reproduction is made at the 

 same intensity level as the rendition. If reproduction is made at an 

 intensity below the intensity level of rendition, the selection heard 

 appears to have lost its low-frequency tones, and the selection sounds 

 thin and unimpressive. Reproduction at too high an intensity level is 

 experienced as a dull rendition because of the overemphasized low tones. 



Equal-Loudness Contours 



The experienced loudness varies not only with the intensity but also 

 with the frequency of the source. This condition was first mentioned 

 in connection with the threshold of audibility curve. To show how the 

 intensity level varies with frequency when the loudness level is main- 

 tained constant, the results of experiments obtained by Fletcher and 

 Munson [1933] must be examined. They found that two tones of differ- 

 ent frequencies are not heard equally loud unless they are presented to 

 the ear at different intensity levels. The precise relation between loud- 

 ness and frequency was discovered by plotting what are called equal- 

 loudness contours, that is to say, by determining at what intensities 

 tones of different frequencies are experienced as equal in loudness to a 

 standard tone (1000 cycles) at various intensities. These contours are 

 shown in Fig. VII-19. 



The lowest contour, marked zero, is the same as the threshold curve 

 for minimum audible pressure shown in Fig. VII-6. The curve marked 

 40 indicates the loudness level (not loudness) in decibels of the 1000-cycle 

 tone above threshold. All frequencies lying on this curve must be raised 

 to intensity levels indicated by the magnitudes of the ordinates, so that 

 they are experienced as having the same loudness as the 1000-cycle 

 reference tone. 



Effect of Masking on Loudness 



The loudness of a tone of constant intensity and frequency is modified 

 by the presence or absence of other tones. The natural inclination of a 

 person is to raise the pitch of his conversation and shout when sur- 

 rounded by the noise of a machine shop or of a running airplane engine, 

 or when conversing in large indoor social gatherings. 



In these situations the low frequencies of the general noise mask the 

 high frequencies of the voice very effectively. To compensate for this 



