Abstract
Several problems related to the small moving coil phonocardiographic transducer have been investigated. As the damping resistance of the mechanical system of the transducer, oil and air flow resistances are considered, and the characteristics of these are discussed. Especially, the relations between air flow resistance and transducer characteristics are studied in detail simulating by an electrical equivalent circuit.
The equivalent circuit for the measuring system including transducer and chest wall is discussed next, and the design method using the behavior of the cross points is investigated. On the basis of these studies, two sorts of transducers have been made for trial and the characteristics of these are also presented.
These, in comparison with the conventional piezoelectric type, offer the following merits : (1) better stability of characteristics, (2) lower output impedance, (3) possibility of extending the cable without noise generated in moving it, (4) simplified preamplifier and improved signal to noise ratio, (5) lower temperature drifts of damping resistance and smaller weight for the air flow resistance type. On the other hand, demerits include (1) effects of stray magnetic field, (2) drop in response at higher frequencies compared with the piezoelectric transducer, (3) difficulty of miniaturization because of a magnetic circuit needed.
