DECIBEL SCALE 287 



of the scala tympani and out at the round window. The resulting pres- 

 sure gradient across the scala media may cause the tectorial membrane 

 and organs of Corti to bend about their inward edges where they are 

 supported by the inward edge of the internal cochlear bony shelf. 



The aural membrane, composed of the supporting basilar membrane, 

 loaded at its upper surface by the Corti organ and tectorial membrane, 

 and bounded at its lower surface by the tissue lining of the scala tym- 

 pani, contains the necessary mechanical elements to act as a highly 

 damped mechanism responding to the impulsive forces passing over it. 



The Helmholtz Resonance Theory of Hearing 



The conception of the cochlea as a resonating response mechanism 

 will always be identified with the name of Hermann von Helmholtz. 

 Helmholtz states definitely in the third edition (1870) of his work, 

 Die Lehre von den Tonempfindungen, that " any exciting tone would set 

 that part of the membrane into sympathetic vibration, for which the 

 proper tone of one of its radial fibers, that are loaded with the various 

 appendages already described, corresponds most nearly with the exciting 

 tone; and thence the vibrations will extend with rapidly diminishing 

 strength on to the adjacent parts of the membrane." 



This statement indicates that what Helmholtz pictured was the divi- 

 sion of the basilar membrane into vibrating areas, with regions of 

 maximum intensity of vibration at their centers, and not the selective 

 vibration of independent fibers of the basilar membrane. Before an ade- 

 quate theory of hearing can be developed, the contributions of the aural 

 mechanism to the subjective phenomena of loudness and pitch must be 

 explained. 



The counterpart of loudness is the intensity or pressure presented to 

 the ear by the medium in which the ear is immersed. Loudness to 

 intensity is not a 1-to-l relation; it is an approximate logarithmic rela- 

 tion. A decibel notation has been adopted to express these logarithmic 

 magnitudes. 



Decibel Scale 



In the study of the relation between the strength of a stimulus and the 

 amount of the experienced sensation, it is found within restricted limits, 

 that an appreciable change of the sensation is brought about by a corre- 

 sponding change in the stimulus. Thus an increase in the experienced 

 sensation (AS) is proportional to the increase in stimulus (A/). If the 

 just-perceptible differences in sensation are assumed to represent equal 

 amounts of sensation, it is found that it takes less stimulus to produce 



