著者

川西 哲夫

出版者

一般社団法人日本音響学会

雑誌

日本音響学会誌 (ISSN:03694232)

巻号頁・発行日

vol.30, no.11, pp.601-607, 1974-11-01

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In the theoretical analysis of phonograph pickup, there are some researches based on the conception of one degree of freedom system, but it seems that any studies, in which the oscillating system is considered as of infinite degree of freedom have not yet been performed. In this paper the osciiating system of pickup is regarded as a simplified system shown in Fig. 1, and a theoretical analysis of an actual pickup (moving magnet type) as shown in Fig. 2, is carried out using eq. (1), thst is, the equation of motion in a bar with infinite degree of freedom. Under boundary conditions given by eq. (2) and eq (3), eq(5) is a solution of displacement for the pickup cantilever, which is needed to obtain some results, about frequency responses for example. The frequency response and the mechanical impedance characteristic are respectively calculated from eq. (8) and eq. (11), and several constants needed in the calculation are determined as shown in Table 1. Fig. 4 shows calculated freqency responses of an actual pickup, and the dotted line in the figure represents a response in case of very amall viscous damping. Fig. 5 and Fig. 6 show respectively phase angles and mechanical impedance characteristics. From these calculated results it is known that a pickup has generally three resonant points in the normal frequency range. Fig. 7 shows a measured frequency response of the actual pickup, and a true response of the vibrating system which is compensated by an electromagnetic frequency response of the pick up body (Fig. 10) is compared with the response previously calculated shown in Fig. 11. Fig. 8 shows a comparison between measured and calculated mechanical impedance characteristics. From these investigations it is made clear that the theoretical results approximately coincide with the experimental results, but it is known that this analysis does not always give satisfactory results in spite of considering the system as of infinite degree of freedom. Finaly a few shapes of vibration of the pickup cantilever are calculated, and some aspects of the bending cantilever are manifested to a certain extent. In future a study on the difference between theory and experiment will have to be accomplished.