Helmholtz resonator (HR) is a device for noise reduction. It is effective in reducing noise that has a dominant frequency component. However, in some other frequencies, it makes the sound pressure increase. Therefore, it is necessary to reduce the effective sound pressure (root-mean-square, ESP) in a certain frequency range and it is one of effective methods that acoustic material (AM) is inserted into a HR for damping. In this study, the relation between resonant frequency of HR and ESP, and the relation between amount of AM and ESP are investigated through a numerical analysis and experiments. As results, appropriate resonant frequency, size of HR and amount of AM inserted are revealed. The guidelines for designing HR with the use of AM to reduce ESP are obtained.

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Helmholtz resonator is a device for noise reduction. It is effective for noise that has a dominant frequency component. However, in some other frequency components, Helmholtz resonator makes the sound pressure increase. In some cases, it is necessary to reduce the effective sound pressure. In this study, acoustic material is inserted to Helmholtz resonator for damping, and the resonator is applied to a sound tube for a basic study. As a result, the SPL is reduced using acoustic material. However, it is considered that the optimal reduction effect has not been obtained and it is necessary to change the resonant frequency of Helmholtz resonator. Next, the optimal value of the resonant frequency and the damping ratio of Helmholtz resonator is found, using identification of modal parameters and the theoretical formula of dynamic absorber's FRF.

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圃場が小区画で分散しがちな府県におけるトウモロコシを始めとする長大型作物の収穫・調製作業の省力化と省人化を目的として細断型ベーラ及び自載式ラッパを開発した。1条刈フォレージハーベスタを直装した44kWのトラクタの後方に細断型ベーラをけん引し, 自載式ラッパ用に22kWのトラクタを用いてトウモロコシとソルガムの収穫・調製作業に供試した結果, 作業者2名で収穫から密封までの作業が可能となった。細断ベールの乾物密度はトウモロコシで平均205k9/m^3,ソルガムで平均153kg/m^3であり, その発酵品質も良好であった。また, 損失は梱包損失及びラップ損失を合わせて3%未満に抑えることができた。

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Electric vehicles (EVs) generate different cabin noise from internal combustion engine vehicles (ICE) during low-speed driving. In order to identify the cause of the interior noise of EVs at low-speed (30km/h) and to reduce the noise, a Helmholtz resonator made of pulp molds was fabricated and its noise reduction effect was evaluated. First, a Helmholtz resonator to reduce the cavity resonance of the tire is investigated by numerical calculation. Through numerical calculations, it was demonstrated that a stable noise reduction effect could be achieved by installing multiple Helmholtz resonators. Next, we fabricated a Helmholtz resonator using pulp molds as the material and installed multiple resonators in the tire. In the experiment, the resonators were able to reduce the tire cavity resonance by 13.8 dB to 9.7 dB. When driving tests were conducted with these tires installed, the noise at the ear position of the front seats in the cabin was reduced by 11.3 dB. From these results, we were able to identify that tire cavity resonance is the cause of the cabin noise when the EV is running at low-speeds. In addition, a driving test in which the same number of Helmholtz resonators as installed in the tires were installed on the cabin ceiling resulted in an 8.1 dB reduction in cabin noise. This demonstrated that it is possible to reduce the noise caused by tire cavity resonance even with parts inside the vehicle.

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