Composite acoustic system for low-frequency sound absorption.

Absorption of low-frequency sound is a difficult engineering problem because long-wave sound energy is poorly dissipated. Traditional sound absorbing materials such as porous materials, microperforated sheets or sound absorbing wedges have a poor low-frequency sound absorption performance. In this work a porous composite absorbing plate connected with a resonant cavity is developed to absorb low frequency sounds. The special developed sound-absorbing plate is rigid and consists of a more fibrous layer (up to approx. 3.0 mm thick) and a more plastic, thin polymer surface layer (up to approx. 0.5 mm thick). The walls of resonant cavity are rigid and smooth. By changing the angle between the absorbing plate and the direction of the incident sound wave and changing the length of the cavity, it is possible to create acoustic systems with a given level of sound absorption and in a given low-frequency range. The larger the angle, the maximum absorption occurs for sounds of lower frequencies. As the cavity length increases, the range of maximum absorption occurring at the resonant frequency shifts towards lower frequencies and the maximum value of the sound absorption coefficient increases. The results are compared with composite variant without a polymer surface layer.