Sound pressure attenuation occurs in the induction system at changes of acoustic impedance. Acoustic impedance is the ratio of acoustic pressure to induction acoustic volume velocity and therefore varies with induction system cross sectional area. Sound is attenuated by reduced cross-sectional area or area ratio
Suppose we reduce duct diameter by 40% the area ratio AR will be equal to .36 and resulting attenuation will be 4.4 dB. Although the attenuation is modest, it includes low frequency noise which is very difficult to attenuate.
In this case the noise radiation direction is opposite of mean air flow direction. When the air flow encounters an abrupt cross-sectional area expansion the duct air flow will be significantly impacted. A reduction of 64% cross-sectional area will result in the loss of 40% of maximum pressure loss which is significant.
Pressure loss of the cross-sectional area reduction can be reduced from 40% to 10% but using an optimized tapered diffuser with 4 degree walls.
Additional pressure drop can be recovered by shaping right angled duct bends.
Optimized duct has same flow restriction as conventional duct but attenuates noise 4.4 dB at all frequencies.
Optimized duct Conventional duct
Noise Attenuation with Resonators
Helmholtz Resonators are composed of a neck that conducts plane waves and a bottle that generates non-plane waves . They attenuate noise at their resonance frequency and are typically designed with a simple resonator equation. Effective neck length however is not linear and must be estimated with 3-dimensional acoustic analysis.
The optimum placement of a side-branch resonator is at a high pressure zone in the system. High pressure zones can be analyzed analytically or experimentally.
Dual-neck resonators are side-branch resonators that do not follow the Helmholtz equation. These resonators are used in restricted space environments where Helmholtz resonators perform poorly. Dual-neck resonators are designed with 3-dimensional acoustic analysis.
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