Silencer design for awning windows: Modified Helmholtz resonators with perforated foam

In order to dissipate sound which passes through an open awning window, we have developed a hybrid absorber made of a modified Helmholtz resonator with perforated foam. Generally, acoustic absorbers are divided into two major categories. The first one is porous materials which dissipate the acoustic wave in a visco-inertial form and heat. The second category is systems which absorb the energy of the acoustic wave by resonance. The aim of this work was to design a hybrid absorber which incorporates the benefits of the two categories for both high frequencies (conventional foam) and low frequencies (resonator). After describing the concept and the principles, we present the prototype which was built and tested under laboratory conditions between two reverberant rooms to attenuate the sound when an awning window is open. The prototype allows reduction of noise transmission (up to 6 dB) at low and high frequencies.