ACOUSTICAL INSTRUMENTS 



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corresponding auditory range is indicated by the cross-sectioned area. 

 It will be noted that when operated at its full frequency range even 

 this relatively sensitive microphone is incapable of translating practi- 

 cally sound of intensities as low as the ear can hear. 



This instrument, which in a similar form is used as a commercial 

 microphone, is several inches in diameter and so is not without effect 

 on the sound field. Where a certain amount of operating range and 

 sensitivity may be sacrificed, as in many acoustic measurements, it is 

 possible to construct this instrument in a much smaller form. 



Capillary and Magnetostriction Microphones 

 Besides the electrostatic and electromagnetic methods of translating 

 the mechanical pressures of a sound wave into corresponding electrical 

 potentials, there are other electromechanical phenomena which may 

 be applied for the purpose. Outstanding among these are the capil- 

 lary electrometer, magnetostriction, and piezoelectric action. 



When a potential is applied at the interface between an electrolyte 

 and mercury the surface tension is changed. If the mercury is in a 

 capillary tube the change in surface tension will result in a change in 

 position of the mercury ; conversely when a force tending to move the 

 surface is applied, there will be a resulting change in potential across 

 the interface. This phenomenon has been applied in the design of 

 microphones. One form of construction of such an instrument is 

 shown in Fig. 3, taken from a paper by Latour.^'' The instrument 



ELECTROLYTE 



Fig. 3 — Latour capillary microphone. 



appears to have been used but little up to the present time and there 

 seems to be very little in the literature regarding its performance. 



3» Compt. Rend. 186, 223 (1928). 



