EFFECT OF SPACING BETWEEN A PAIR OF SINGLE DEGREE OF FREEDOM HELMHOLTZ RESONATORS

This study focuses on the experimental and numerical evaluation of the acoustic performance and underlying physics of sound attenuation from single degree-of-freedom Helmholtz resonators, located in line as an alternative passive sound absorption device for conventional locally resonant liners. Experiments were performed using a grazing flow impedance tube with two highly instrumented Helmholtz resonators, fitted with internal microphones. The relation between the noise attenuation performance of the two resonator system and the spacing between two resonators is characterized in terms of the transmission loss, transmission coefficient and the corresponding frequency bandwidths. The change in sound pressure level (SPL) along the length of the impedance tube for different resonator spacing configurations is also analysed, thereby improving the understanding of the effect of a Helmholtz resonating cavity on the sound field in a duct. Finite element analysis is performed and validated against the experimental data, which has provided interesting results about the pressure and velocity field inside the cavity and in the impedance tube. The results demonstrate that there exists an optimum spacing where the noise attenuation performance of the resulting resonator couple is maximized.