2008 年 2 巻 1 号 p. 342-353
This paper describes the estimation method of transient sound radiating from a structure excited by random impact forces and the noise reduction design of a coin storage box of a coin counting machine. To represent the random impulses, the probabilistic model of random impact forces is derived by defining the collision position and time of impulses as probability variables and using the probability density functions of the variables. The probability density functions of the collision position and time are assumed as Gaussian probability distribution. The vibration displacement of the structure at each time step is represented through time-averaged function with respect to the above probability variables. The transient sound pressure is obtained by using a Rayleigh integral equation and the sound power is evaluated by integrating the sound pressure distribution in frequency domain. To check the accuracy of estimation based on the probabilistic model, the sound pressure and power radiated from the structure are measured. It is confirmed that the estimated frequency responses agrees well with the measured values in low frequency domain and that the proposed probabilistic model is effective for estimating the random impact sound. Next, using the above probabilistic model, the structural modification to reduce the sound power from the box is carried out. In this structural modification, straight dimples to cross the antinodes of the dominant mode of sound power are added. As a result of sound power measurement, it is confirmed that the proposed method is applicable to noise reduction design.
