56 



MATTER WAVES: SOUND AND ULTRASOUND 



Sensitivity, S, of a detector, or discernment per decibel of signal over back- 

 ground, is defined as 



s = log r/M t 



where A I, = I, - 1° . Sensitivity is higher the smaller is the value of A/,. 

 Usually when 6" is determined at different values of an independent variable, 

 the result is expressed as the sensitivity relative to the maximum value taken as 

 unity (S/S max ). The sensitivity of the ear is so expressed in Figure 3-4. 



moximum 

 sensitivity 



0.01 



10 100 1,000 



FREQUENCY (cycles per second; 



10,000 



100,000 



Figure 3-4. Sensitivity of Human Ear at Different Frequencies of Sound Waves. The indi 

 vidual's sensitivity curve may differ markedly from this average curve. 



THE BODY'S DETECTORS OF MATTER WAVES 



Introduction 



In this section are given an outline of the structure of the ear and a de- 

 scription of the mechanism of the sense of touch. This sketch is meant to 

 show the important general features, but does not penetrate into either the 

 depths of the mechanism nor the psychology of the resulting sensations such 

 as loudness and pitch. A very well written and concise display of the bio- 

 physics of hearing is found in the book by Stacy et a/. 6 An up-to-date survey 

 of the physiology of hearing is given by Whitfield, 7 and a masterful discus- 

 sion of biological transducers (converters of mechanical to electrical stimuli) 

 was recently given by Gray. 8 To delve deeply into this aspect of the subject 

 is, unfortunately, beyond our scope, although it is currently a very active 

 part of biophysical research. 



