THE JOURNAL OF THE ACOUSTICAL SOCIETY OF JAPAN

It is discussed in this paper from the results of acoustic experiments made with three models of halls whether or not a diffusor fixed to the side wall of each model is useful for the uniform decay of a sound. The usefulness of the diffusor is judged on the basis of statistical significance by making an analysis of variation of a two-way layout regarding the standard devitation of reverberation time on two assumptions that the decay of a sound will be exponential in all places in the hall and the diffusor will be useful for the uniform distribution of a proper tone, if the sound field is in a satisfactory diffusive state. It may be fairly said from these results that a side wall diffusor is useful for the uniform decay of a sound at a level of significance of 1%, especially contributing to sounds in the low frequency range when a sound source is generally in the position where a lecturere stands in any hall.

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The mechanical impedance of the chest wall should be observed carefully in measuring the beating of the heart by means of a P. C. G. microphone (Phono cardiograph microphone. ) because various characteristics of the microphone are influenced greatly by the mechanical impedance of the chest wall itself. The author used a vibrating rod to the tip of which a small load with known mass was attached for the measurement of the mechanical impedance of the chest wall (Fig. 1). The rod was driven freely in space in one case and as it was brought in to contact with the chest wall in another case. The ratio of the driving force to the velocity of the rod and the phase difference between the two cases were derived from the result of measurement to calculate the mechanical impedance of the chest wall (Eqs. 14 and 15). The measuring apparatus is shown in Fig. 1, 2 and 5. The mechanical impedance of the chest wall was measured on the left side of the third intercostal sternum with the weight of load varied from 10 to 500 grams, the frequency range varied from 40 to 2000c/s and the contact point of the rod kept at a fixed diameter of 2 centimeters. The principal results of mesurement are shown in Fig. 9, 10 and 11. The author found that the mechanical impedance of the chest wall varied greatly with the weight of load and the stiffness compornent showed a remarkable increase under heavy load weighing 50 grams and above. The average values of the mechanical impedance of the chest wall under various weights of load are shown in Table 2. These results have led the author to the following conclusions: 1) A P. C. G. microphone should be made as light as possible for the measurement of the beating of the heart in hight frequency range. 2) The frequency characteristic of a P. C. G. microphone should rather be decided by its own weight.

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