Low Frequency Noise Radiation from Stacks and Its Abatement

This paper introduces a new concept which can easily reduce sound reflection coefficient at a duct open end. When we make a duct side wall next to open end of appropriate perforated plate or porous material, sound reflection at that open end is remarkably reduced and the radiated sound is also reduced. Here this phenomenon is analyzed theoretically and proved experimentally. This technique is useful and applicable to reduce acoustic resonant intensity in the duct such as stacks of plants, mufflers of engines and so on. As the result, it can reduce the radiating sound from the duct open end.