2014 年 1 巻 5 号 p. DR0052
To prevent noise generated by devices such as compressors, generators, or motors, a noise-proof cover is usually installed around them. It is also installed to protect ultra precise devices from ambient noise. Because of space considerations, the noise radiating device or ultra precise devices to be protected from outer noise are generally situated at the center of the cover. However, this often lowers the performance of the noise-proof cover at some frequencies because of the occurrence of its inner cover acoustic modes. To solve these problems from the standpoint of the cover's dimensions and the occurrence frequency of the inner acoustic mode in the cover, which protect devices within the cover from outer ambient noise, we used a simplified one-dimensional Transfer Matrix Method (TMM) to investigate the most effective arrangement of acoustic tubes on the inside of the cover to restrain the acoustic mode. The results show that the most effective arrangement depends on the width of the target frequency range. If the target frequency range is plus-minus 10 to 17 % around the peak caused by the acoustic mode, for example, the most effective arrangement is one in which tubes 1/4 as long as the longest length of the cover edge are set at both ends and at the center along the longest direction of the cover. Finally, the effect of the proposed structure was investigated by numerical acoustic calculations using the Boundary Element Method (BEM) and validated by experimental measurements. The BEM results correspond well to those of the experiment; the experimental results show that the sound pressure level is reduced about 6 dB over all of the frequency range around the original peak frequency plus or minus 50%.
