The experimental study of coupled cavities and Helmholtz resonators at low Mach number

Abstract

Ventilation systems in buildings are essential for good indoor air quality. However, there is a challenging problem that annoying noise can be generated by the ventilation system, which is a primary source of noise inside a building. The noises produced by ventilation systems will propagate along ductwork and enter the occupied regions of a building. If they are not attenuated properly, they will lead to poor indoor acoustical quality, which will result in lower productivity and health issues finally. In present study, the investigations are mainly focused on the sound power transmission loss (TL) of coupled cavities in close proximity and Helmholtz resonators with and without flow. In the first part of this thesis, a coupled cavity system, which was made by offsetting two cavities along a rectangular duct, was investigated for its acoustical performance with and without a low Mach number flow. The experimental results show that the offset creates a prominent sound transmission loss peak. The larger the offset distance, the stronger the sound transmission loss. In the presence of flow, the mean flow reduces the sound transmission loss, especially when the flow velocity exceeds a threshold. An effective aerodynamic length scale for the coupled cavity region under different sound excitation level is established. In the second part, measurements of single Helmholtz resonators of eight different geometries were conducted to investigate the effect of different passive suppression techniques on the flow-excited acoustic resonance of cavities. Finite element method was conducted to validate the model. Results show that the finite element method gave predictions which basically agree with experimental observations. The proposed geometrical variation at the neck of resonator is proven to be effective methods for improving the sound attenuation performance in the presence of flow. It is found that when the flow velocity U exceeds a certain threshold, tonal amplification can be found at frequencies below 900 Hz. Most of these frequencies agree with the prediction by the Rossiter feedback frequency formula. The geometry of Helmholtz resonator with semi-circle edges in the outlet of the neck is the best sound attenuating setting around 670 Hz when flow velocity reaches 12m/s. In the third part, sound power transmission loss of the coupled Helmholtz resonators were investigated with and without flow. The measurements of single, coupled, three co-planar, and four co-planar resonators were conducted. For a single resonator, the obtained TL decreases with increasing air flow velocity. Two TL peaks are obtained when there are two coupled resonators. The neck width of resonators can be tuned close to each other to construct a broad attenuation bandwidth. Three co-planar coupled resonators provide two or three TL peaks within the working frequency range. The sound attenuation frequency range of the four co-planar coupled resonators is the widest. In present study, the sound power transmission loss of coupled cavities and Helmholtz resonator are examined in details. The physics of the aerodynamics and the aeroacoustics found in this investigation can deepen the current fundamental understanding on the subject of flow-structure-acoustics interaction. In addition, the high performance broadband resonance-based silencers with relatively small static pressure drop is extremely useful inside buildings.