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This paper concerns a method of reducing noise level in a mail processing machine at low cost. A stack part of the mail processing machine is one of the main noise sources. This is because an installation area of the stack part is large and is widely open so as to take out mails, that a noise is emitted towards an operator. The purpose of this study is to propose a method to reduce a noise level of a stack part at low cost as an alternative for sound-absorption material. A broadband noise is generated from a mail processing machine as various kinds of mails are conveyed. Also, information such as installation environment and housing size is usually not provided at the design stage. So, we need a method of reducing noise level to derive a design parameter from a certain range of prediction. Based on the characteristics of a stack part of a mail processing machine, we examined a method using a back opening of a stack part to reduce noise that is amplified by internal resonance. The reduction of the sound at the front evaluation point is examined by a model with a front and a back opening which has a trade-off between decreasing internal sound and increasing diffracted sound. The fact that a sound pressure level at the front evaluation point reduced with a back opening was confirmed by using experimental equipment. We have applied this approach to mail processing machine, and experimental value roughly corresponded to calculated value. The reducing noise level of a slit-like back opening was-3dB with white noise, and it had the same effect as a sound-absorption material.We have proposed a low cost noise reducing method as substitute for a sound-absorption material that can be used at a design stage.

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