PROPERTIES OF MATTER WAVES 



51 



Amplitude and Intensity 



There is a minimum pressure and power of matter waves below which the 

 ear cannot detect the wave. This value is about 0.0002 dynes/cm 2 , an ex- 

 tremely small value because the ear is very sensitive. The corresponding 

 power or intensity limit is ~10 9 ergs/cm 2 sec, i.e., ^lO" 16 w/cm 2 ! This 

 value places its sensitivity very close to the threshold of the power in heat 

 motion, and thus very close to the minimum background agitation of matter 

 in our environment. The maximum amplitude the eardrum can stand, with- 

 out certain irreparable damage resulting, is ~200 dynes/cm 2 . Therefore, the 

 range of sensitivity of the ear is phenomenally high, one to a million. It is 

 the most sensitive at 1,000 cps. 



The sense of touch, particularly on the fingers and tongue, is not nearly 

 so sensitive, but responds down to much lower frequencies. 



To our knowledge, man has no detection apparatus for frequencies above 

 about 20,000 cps. However, there is some evidence that ultrasound can 

 penetrate to the brain and cause psychological aberrations, which may or 

 may not be a result of organic damage. 



One of the most convenient ways of generating matter waves of controlled 

 frequency is by means of the vibrating crystal. Certain crystals are piezo- 

 electric — that is, they expand or contract if an electric voltage is applied to 

 contacts with two different crystal faces (Figure 3-2). The amount of the 



(o) 



£l_ 



+ V volts 





crystol 



(b) 



applied voltage, V 



(c) 



time 



radiating, 

 vibrati ng 

 surfoce 



-target 



beamed 

 ultrasound 



crystals 



Figure 3-2. About Piezoelectric Crystals: (a) Voltage difference is applied between two 

 opposite faces, (b) The length changes as the applied voltage is changed, (c) Varying volt- 

 age, V, gives varying length, y. (d) Concave radiator concentrates matter waves on a target. 



