A device for measuring small displacements as an application of the frequency modulation technique has been widely used for determination of the mode of vibration. However, there are many factors to be discussed associated with the device such as the limit of the air gap between vibrating body and pick up pole, the maximum or minimum measurable displacement, the amount of error, and so on. In this paper these points are discussed and the following results have been obtained; 1) the ratio of air gap (D) to radius of pick up pole (b) has to be selected at 0. 1〜0. 6, 2) the lower limit of measurable displacement is 2×10^<-5> m/m (at S/N=20dB), and the upper limit is 0. 6αb (where α is determined from the allowable maximum distortion), and 3) the maximum total error is about 6% (at b=1. 5 m/m).
To calculate the distribution of sound pressure level in a room given by a simple source of sound lacated in it, Beranek's formula or Ishii's formula has been practically used. However, in a large room, it has been often found that the calculated values of sound pressure level are not in good agreement with the observed ones, except when the mean absorption coefficient of the room is exceedingly small. Then, to obtain the applicable limit of these formulae, a formula has been derived geometrically which gives the distribution of sound pressure level in a room, and the values calculated from these three formulae have been compared with the observed values in an auditorium and a broadcasting studio. It was found that the values calculated from Beranek's formula and Ishii's formula show an error of 2-4. 5 dB at a distance greater than 10 m from the sound source in an ordinary large room whose mean absorption coefficient is 0. 2-0. 3, whereas the values calculated from the formula derived here show a good agreement with the observed values. When the absorption coefficient of a room is smaller than 0. 05, all values calculated from these three formulae agree well.