Acoustic metamaterial design for levelling the impact of double-wall resonance on sound insulation

This paper presents two solutions employing locally resonant metamaterial to level the mass-air-mass resonance impact on the sound insulation. The first operates on the cantilever beam resonance, and the second uses masses vibrating in flexural mode cut out from the additional panel. Both structures are mounted between two lightweight, honeycomb cardboard panels with a double-wall resonance of 420 Hz. Solutions were analysed numerically for their vibration and acoustic properties and measured in the reverberation chamber, resulting in information about the dispersion curve, effective dynamic mass, and sound insulation. The analytical results of Sound Transmission Loss (STL) and the experimental measurements of diffused-field Sound Reduction Index (SRI) proved the existence of sound-insulation enhancement. The local rise in SRI resulted in an increase of broadband Weighted SRI up to 5 dB.