A term generally used to describe aerodynamic noise produced when a gas flows within a duct or when the gas exits the duct, hence flow noise. This is a typical problem observed in the exhaust systems of internal combustion engines and high engine rotational speeds and hence high gas flow rates.
There is a balance between the attenuation provided by a reactive silencer, such as an expansion chamber, and the flow noise that is generated at the discontinuities.
Low noise level in air moving systems may be achieved by careful attention to the following design features that can be applied to exhaust systems, air handling ducts and fans:
- Minimize the air flow requirement. Note that noise increases as a fifth power of flow speed.
- i.e. the total sound power radiated by turbulent flow in the region of a solid body increases by 18 dB for every doubling of flow velocity.
- Look at the flow speed distribution as areas of high speed flow will generate very high noise levels whilst overall you may be only requiring an average velocity that is much lower and so much lower noise levels.
- Minimize the pressure head required at the fan.
- Design for a minimum loss air flow path with larger cross sections, no unnecessary restrictions or direction changes; duct surfaces.
- Operate the impeller at its maximum efficiency and minimum tip speed.
- Design the vanes to minimize vortex shedding e.g. backward curved blades for larger fans. Be sure the number of blades is adequate.
- Isolate each structural component from its neighbour as far as possible, e.g. motor, fan, and ducting.
- Use the longest possible maximum radius guide and turning vanes in the ducts. Stiffen ductwork to avoid panel resonances, and use acoustic linings or coatings where possible.