Noise reduction in a large enclosure using single, dual and ensconced Helmholtz resonators

Helmholtz resonators reduce the noise level especially at the low frequency end of the sound spectrum. This study explores the amount of reduction for different configurations of Helmholtz resonators coupled to a relatively large-sized enclosure through detailed experiments. The experiments were performed in a similar to 1.2 m(3) enclosure. The eigenfrequencies of the enclosure were calculated theoretically and verified experimentally. Thereafter, a tunable piston-cylinder-type Helmholtz resonator was designed to target a particular frequency, which was coupled to the enclosure volume. The location of the resonator was chosen to lie close to the anti-nodal plane. The results obtained showed a substantial decrease in sound pressure level. A study was also made with two Helmholtz resonators with identical resonance frequency and mounted at separate anti-nodal planes; this configuration showed a still larger reduction in the sound pressure level. Resonator placed inside the cavity volume was further explored to increase the applicability of the Helmholtz resonator; encouraging results were obtained for two cases considered here