PHYSICAL MEASUREMENTS OF AUDITION 165 



tones are masked. It is a common experience of one working with 

 complex sounds to have the low frequencies always gain in prominence 

 as the sound is amplified. 



The question naturally arises, Does the same interfering effect 

 exist when the two tones are introduced into opposite ears instead of 

 both being introduced into the same ear ? The answer is No. 

 Curves showing the results in such tests are shown in Fig. 10. For 

 comparison the results for the case when in tones are both in the 

 same ear are given by the light lines. Take the case of 1,200 and 

 1,300 cycles. It is rather remarkable that a tone in one ear can be 

 raised to 60 units, that is, increased in intensity one million times, 

 before the threshold value for the tone in the other ear is noticeably 

 affected. If the 1,300-cycle tone were introduced into the same ear 

 as the 1,200-cycle tone, its loudness would need to be shifted 40 units, 

 corresponding to a 10,000-fold magnification in intensity above its 

 threshold intensity in the free ear before it can be heard. It is seen 

 that if one set of curves is shifted about 50 units it will coincide with 

 the second set. This strongly suggests that the interference in this 

 case is due to the loud tone being transmitted by bone conduction 

 through the head with sufficient energy to cause masking. The 

 vibration is probably picked up by the base of the incus and trans- 

 mitted from there to the cochlea in the usual way. There is other 

 evidence * which I shall not have space here to discuss, which in- 

 dicates that the effective attenuation from one ear to the other is 

 approximately 50 units. 



Theories of Audition 



With these facts in mind, we are now ready to discuss the theory 

 of hearing which will best account for them. I will refer briefly to 

 just a few of the principal theories of hearing which have been pro- 

 posed. The sketch shown in Fig. 11 gives a diagrammatic picture of 

 the internal ear. In the Helmholtz theory, as first formulated, it is 

 stated that the organ of Corti located between the basilar membrane 

 and the tectorial membrane act like a set of resonators which are 

 sharply tuned. Each tone stimulates a single organ depending upon 

 its pitch. Later this theory was somewhat modified as it was thought 

 that the resonant property might reside in one of the membranes 

 in the cochlea. 



* See paper by Wegel and Lane soon to be published in the Physical Review 

 entitled "The Auditory Masking of One Pure Tone by Another and its Relation to 

 the Dynamics of the Inner Ear." 



