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HANDBOOK OF PHYSIOLOGY ^^ CIRCULATION I 



TRANSDUCERS 



The ideal microphone should have a \oltage output 

 that is a faithful and undistorted image of the pressure 

 variations at the input. The response sliould be inde- 

 pendent of frequency in the pertinent range. The 

 signal-to-noise ratio should be favorable. Noise is 

 created by thermal agitation of molecules in the 

 resistive components of the microphone or of air 

 molecules against the diaphragm. Self-noise of a 

 microphone may be of the order of 20 db, a not 

 inconsiderable factor. 



Microphones used in phonocardiography operate 

 either a) indirectly, with a clo.sed air chamber between 

 the skin surface and the diaphragm of the microphone, 

 and the conversion of the displacement at the skin 

 surface into a pressure which acts on the diaphragm; 

 or /)) directly, with the application of an element of 

 microphone, such as a button or bar, directly to the 

 skin surface. In the Groom microphone the skin 

 functions as one plate of a capacitance transducer. 



In the indirect method, the shape, volume, and 

 size of the air chamber are important because cavity 

 resonance can distort the response at the frequency of 

 natural resonance of the cavity. If the volume of a 

 cylindrical cavity is kept large enough (around 10 

 cm^) with a diameter-height ratio of 3 : i the response 

 characteristics remain linear up to the resonance 

 point well above 1000 cps. 



Transducers in phonocardiography can be further 

 classified as a) displacement and h) dynamic. Both the 

 crystal and the capacitor microphone transduce dis- 

 placement into emf, since the static pressure developed 

 in the closed chamber is a function of the displacement 

 of the skin which it overlies. The emf generated in the 

 dynamic microphone (e.g., the "moving coil" micro- 

 phone and the piston-phone) is proportional to the 

 velocity of the moving element; that is, the dcri\ative 

 of displacement is sensed by the dynamic microphone. 

 The response of the dynamic microphone increases 

 with frequency. This characteristic can be advan- 

 tageous in phonocardiography, since part of the 

 equalization, or low-frequency attenuation, is accom- 

 plished at the transducer level. [A dynamic micro- 

 phone has been used with the Sanborn Twin-Beam 

 and Stethocardiette phonocardiograph, and the 

 Elema phonocardiograph (20), as well as others.] 



Microphones can be classed according to the 

 physical principle underlying their function as trans- 

 ducers. For its transducer properties the capacitor 

 microphone depends on variations in the capacitance 

 of a condenser, one plate of which is free to move 



with the sound v'ibrations. An example is the Groom 

 microphone in which the skin surface is itself the 

 movable plate of the condenser element. A second 

 example is the Altec microphone. Advantages of the 

 Groom microphone are /) reduced influence of 

 ambient noise, 2) minimal loading of the precordium, 

 and j) high sensitivity with favorable signal-to-noise 

 ratio. Disadvantages are /) relatively large size 

 which makes for difficulties of placement in .some 

 areas of the chest, 2) lack of the possibility of acoustic 

 filtration, and j) difficulties in calibration. Advan- 

 tages of the Altec- microphone (e.g., 21 BR 150) — in 

 which inert components such as glass, gold, stainless 

 steel, and Mycalex are used — are temperature 

 stability, permitting, for example, sterilization with 

 the hot air oven, and stability to mechanical shocks. 

 Acoustic filtration of low-frequency components can 

 be used; it can be calibrated; it withstands high 

 pressures with minimal distortion. Most important 

 is its "flat" frequency response (amplitude propor- 

 tional to pressiu'e) in the pertinent range and its 

 reasonable size and weight minimizing loading of the 

 precordium. High humidity has ill effects and caution 

 must be exercised when it is used in the presence of 

 explosive anesthetics. 



Examples of piezoelectric transducers are the 

 crystal (Rochelle salts, sodium potassium tartrate) 

 microphone and the ceramic (barium titanate) 

 microphone. The latter is the type employed in intra- 

 cardiac phonocardiography. Disadvantages of these 

 transducers are vulnerability to mechanical and 

 thermal injury and to the ill effects of himiidity. An 

 advantage is low cost. Ethylene oxide can be used, 

 rather than heat, in the sterilization of some intra- 

 cardiac catheters. 



The electrodynamic, or mo\ing-coil, microphone 

 consists of a permanent magnet and a coil mounted 

 on a diaphragm that moves with the \ibrations under 

 study. Disadvantages are the relati\ely large size and 

 weight, and a tendency to pick up hum. The "vari- 

 able-reluctance" microphone (13) can also be termed 

 generically an electrodynamic microphone. 



The piston-phone is useful mainh- in the calibration 

 of other microphones and in other situations requiring 

 sound generation. 



Microphones for use in phonocardiography are 

 calibrated in terms of a constant pressure applied in a 

 uniform manner to the diaphragm of the microphone. 

 The sensiti\ity is customarily expressed in terms of 

 output below I \olt per microbar of pressure applied 



- Altec-Lansing Corp., Beverly Hills, Calif. 



