60 



MATTER WAVES: SOUND AND ULTRASOUND 



Noise has many components — matter waves of many frequencies. The 

 "buzz" from speech in a crowded room will center in the range 300 to 6000 

 cps. The noise generated by a wood planer has most of its energy between 

 200 and 2000 cps, while a power saw will emit noise from 50 to 6000 cps. 



Only low-pitched or high-pitched voices can be clearly understood. This 

 is the crux of the problem facing communication engineers and otologists 

 alike: to provide a sufficient sound intensity level (over background noise) 

 to the middle ear. This question is considered in more general terms in 

 Chapter 11. 



TABLE 3-1. Some Sources of Noise* 



* After Neeley, K. K., "Noise — Some Implications for Aviation," Caw. Aeronaut. J., 3,312 (1957). 

 ** Referred to 10 -16 w/cm 2 , the threshold of hearing. 



Exposure of man to high-intensity noise has several effects: change in 

 hearing acuity, and mechanical or pathological damage to the cochlea; tem- 

 porary blindness (>140 db); changes in ability to perform skilled and un- 

 skilled tasks; feelings of fear, annoyance, dissatisfaction, and nausea. Dis- 

 cussion of some of these effects follows in the next section. 



PHYSIOLOGICAL EFFECTS OF INTENSE MATTER WAVES 



The physicochemical basis of the physiological damage is fairly well 

 understood. Five facts are important to the discussion: 



(1) During the absorption of matter waves, a front of high pressure pre- 

 cedes a front of reduced pressure through the tissue. There is therefore a 

 differential pressure, or a pressure gradient, along the tissue which stretches 

 and compresses it in sympathy with the incoming wave. If the amplitude is 



