PHYSICAL MEASUREMENTS OF AUDITION 



163 



quency if the frequencies of the two tones are near together. When 

 the two tones are very close together in pitch the presence of the 

 masked tone is perceived by the beats it produces. This accounts 

 for the sharp drop in the curves at these frequencies. A similar 

 thing happens for those regions corresponding to harmonics of the 

 masking frequency. In the charts for the 200- and 400-cycle masking 

 tones these drops are not shown inasmuch as they were small, but 

 in an accurate picture they should be shown. 



Monaural Masking 



10 40 60 80 

 Loudness of Fp 



20 40 60 80 100 

 Loudness of Fp 



20 40 60 

 Loudness of Fp 



20 40 60 80 100 

 Loudness of Fp 



20 40 60 80 

 Loudness of Fp 



Fig. 9 



10 40 60 80 

 Loudness of Fp 



In Fig. 9, these results are shown plotted in a different way. The 

 abscissae represent the loudness of the primary tones whose frequency 

 is indicated at the top of each of the charts. The amounts that the 

 threshold is shifted are plotted as ordinates as in the previous figure. 

 For example, in Chart 1, the results are shown for a masking tone of 

 200 cycles. The curve marked 3,000 indicates the masking effect of 

 a 200-cycle upon a 3,000-cycle tone. It is seen that the loudness 

 of the low pitched tone can be raised to 55 units before it has any 

 interfering effect upon the high pitched tone. For louder values 

 than this it has a very marked effect. It will be noticed that in nearly 

 all of the charts the curves for different frequencies intersect. This 

 leads to some rather interesting conclusions, regarding the percep- 

 tion of a complex tone. For example, consider the curves for a mask- 

 ing tone having a frequency of 400 cycles. Assume we have a complex 

 tone having three frequencies of 400, 300 and 200 cycles with relative 

 loudness values of 50, 10 and 10, respectively. The ear will hear only 



