Implementation of a tuning system for Helmholtz resonator arrays in HVAC ducts

Onsite tuning is essential in order to use Helmholtz resonators for HVAC ducts. However, since no tuning device is yet available for general use, engineers have avoided the use of such resonators. The objective of the present study is to develop a tuning system for resonator arrays for HVAC ducts. We used the cavity volume, the resonator neck resistance, and the neck open area as control parameters to tune the resonance frequency, resonance sharpness, and sound dissipation factor, respectively. Especially in order to control the resistance, a spiral spring with adjustable gaps between the spring plates were used. An experimental resonator array tuning system composed of six prototype tuning devices was constructed and installed in a section of duct. The 18 parameters of the six resonators were successfully tuned through a feedback approach for low-reflective ducts. However for highly reflective ducts, two key difficulties were observed. First, in the tuning process, some tuners do not work effectively at their resonance frequency regions. The other difficulty is that the insertion of a resonator significantly increases the transmitted wave pressures at certain frequencies other than the resonance frequency. The generation mechanisms of these phenomena were determined using a one-dimensional wave model constructed for ducts with side branch resonators. The effectiveness of the one-dimensional wave model in properly tuning resonators was confirmed by conducting experiments and three-dimensional boundary element simulations.