The Helmholtz type of hydraulic silencer is known as one of the most practical silencers. The silencer consists of a neck and a cylindrical vessel. A distinctive characteristic of the Helmholtz of hydraulic silencer is that the effective range of attenuation frequency is very limited. The maximum attenuation performance can be obtained at the resonant frequency because the Helmholtz resonance phenomenon is utilized. Therefore, it is very important to estimate the resonant frequency precisely at the design stage. In the design case that the length of cylindrical vessel becomes close to the diameter, i.e. the ratio of the length and diameter is nearly equal to 1.0, the resonant frequency of the existing model begins to deviate from the experimental results and this disparity increases with a decrease in the ratio of length and diameter. The purpose of this research is to clarify the influence of geometry on the attenuation characteristics of a Helmholtz type hydraulic silencer. In particular, a new mathematical model is proposed, that considers the flow for the radial direction of cylindrical vessel in the design case where the ratio is small. The transmission loss characteristics of the Helmholtz type hydraulic silencer are investigated theoretically for both the proposed model and existing model. These models are evaluated by experimental examinations in which the ratio is varied.

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