An experimental investigation of cavity noise control using mistuned Helmholtz resonators

The major objective of this paper is to demonstrate higher noise reduction when an acoustic cavity is coupled to a combination of Helmholtz resonators (HRs) tuned to slightly different frequencies. An acoustic cavity, whose first two modes are close to each other, is selected for carrying out experiments. These are carried out to understand the behaviour of the coupled Helmholtz resonator-cavity system; the cavity is excited using two speakers and sound pressure is measured using two microphones on a rotating boom. When a resonator is coupled to a cavity, the acoustic natural frequency to which it is tuned, splits into two modes, one below and one above the original frequency. When multiple resonators are used, it is demonstrated that higher noise reduction can be achieved by mistuning them to the new split frequencies rather than tuning all of them exactly to the first cavity mode. Similarly, higher noise reduction is seen when resonators are placed near edges/corners of the cavity walls than at the centres of the boundary walls. In all these cases anti-nodal locations are selected for superior noise reduction. A detailed study has been carried out and guidelines for tuning resonators for superior noise control is proposed.