Sec. 1-2] 



MECIIAXICAL IX PUT T I! AXS Dl'CERS 



s:; 



change of the applied steady-state load of 1 kg. The method has als< i 

 been used by the same authors for the measurement of steady-state 

 air pressure surrounding a piezoelectric transducer. 



The high-frequency limit is imposed by the mechanical resonance 

 of the piezoelectric element plus associated mountings. The fre- 

 quency response of piezoelectric transducers may have spurious 

 peaks. Some are due to different modes of oscillations in the crystal, 

 others to vibrations of the transducer mounting, resonances in 

 attachments, cables, cooling tubes, etc. Such resonances are usually 

 of a lower frequency and can be eliminated by careful mounting. 



String--^ 



M 



t 



r-*~ 



P 



£ 



-o 



e 



Fig. (1-2)65. Oscillating-string transducer, 

 schematic diagram. 



The piezoelectric transducer is simple and rugged, suitable for 

 many applications where sufficient mechanical force is available. 

 The phonograph crystal cartridge is, at times, an excellent means for 

 experimental work in the frequency range from 20 to about 20,000 

 cps; the output voltage from such cartridges can be as high as 

 several volts. The piezoelectric transducer has a high output 

 impedance and requires a high input impedance of the subsequent 

 stage. Its greatest disadvantage is its lack of response to steady- 

 state ("d-c") displacements or forces. 



1-26. Miscellaneous Transducer Systems 



a. Vibrating -string Transducer. The natural frequency of a string 

 of the length L and the cross-sectional area a held between two fixed 

 suspensions, Fig. (1-2)65, is 



-J- 



2ZA s 



(1) 



where F is the force which holds the string taut and causes a tensile 

 stress a ( = Fja) in the wire, and s is the density of the wire material. 

 The force F also causes the wire of the original (unstretched) length 



