Study on direct sound reduction structure for reducing noise generated by vibrating solids

A direct sound reduction structure is proposed as a new method for reducing noise generated by vibrating solids. The proposed method involves directly attaching a hard sound absorbing material with a honeycomb structure to the surfaces of vibrating solids to provide a moderate amount of back air space. Using a one-dimensional acoustic-field model, the fundamental sound reduction characteristics are investigated theoretically and experimentally, and the effectiveness of the proposed concept is verified. In the theoretical analysis, an analytical model with a sinusoidally excited rigid-frame sound reduction structure is considered. In the experiments, a one-dimensional acoustic tube is used to determine the sound reduction ratios for both stationary excitation and non-stationary impulsive excitation as a function of frequency. Furthermore, in order to verify the practical usefulness of this approach, an experimental study on sound reduction in a three-dimensional acoustic field is carried out using either a rigid piston plate surrounded by a baffle or a flexible flat plate with different vibration modes as vibrating bases. The results indicate that the sound reduction ratio has a quasi-periodic form that depends on the thickness of the back air space and the sound frequency, and a sound pressure reduction of approximately 80 percent (−14 dB) is observed around the minima. In addition, the average reduction in the frequency range 0.5–5 kHz is approximately 40 percent (−4.4 dB). The results obtained in this study are expected to act as useful indices for designing a nearly optimum sound reduction structure if the target frequency is known in advance.