The Mechanism of Hearing as Revealed Through Experiment on the Masking Effect of Thermal Noise.

Considerable information concerning the dynamical properties of the hearing mechanism can be obtained from physical measurements on audition. In fact, probably more precise information can be obtained by such measurements than from data obtained by animal experimentation, or from data on post mortem sections of human temporal bones. It is well known that when a person is immersed in, a noise his ability to hear other sounds is decreased. The noise is said to produce a masking effect. Before describing the experiments on the masking effect of thermal noise I will first describe the method of obtaining thermal noise and how it is quantitatively defined. In an electrical conductor there is a statistical variation of the electrical potential difference between its two ends which is due to the thermal agitation of the atoms, including the electrons. This fluctuating voltage may be amplified by means of a vacuum tube amplifier. Such an electrical disturbance is called an electrical thermal noise. If this electrical thermal noise is sent through a telephone receiver, or a loud speaker, it is then converted into acoustical thermal noise. Such an acoustical thermal noise was used in the masking experiments discussed in this paper. Let If be defined as the intensity per cycle of thermal noise using 10-16 watts* per square centimeter as a unit of intensity. If AI is the intensity due to the thermal noise in the band of frequencies between f and f + Af, then AI If= . (1) Af