Development of locally resonant structures for sonic barriers

The intrusion of unwanted sound is a matter of increasing concern worldwide as growing sound pollution impacts human health, productivity and wellbeing. Lower frequency noise is particularly important, as this is where the human ear is most sensitive. Irritating acoustic intrusion increasingly occurs in buildings, yet sound insulation at low frequencies is challenging and expensive. Meta-materials off er a relatively new approach to achieving sound and vibration isolation. This approach is being used to develop panels with internal resonant structures that are only a few centimetres thick yet strongly interact with acoustic waves. These structures can yield significantly greater transmission loss than conventional insulation systems. Numerical models based on networks of single-degree of freedom oscillators were used to understand how the components of the locally resonant structure (LRS) can be manipulated to generate sound transmission loss (STL) performance spectrums. Designs with the desired STL characteristics were then examined in detail and samples were fabricated using industry-standard materials and processes. This paper focuses on the acoustic testing of these LRS samples at low frequencies. Comparisons were made between, numerical predictions and experimental results (small scale (plane wave) to large scale (diffuse field) conditions). The highest performing network arrangements combined layers of resonators with multiple resonances to increase system bandwidth. At frequencies below 1 kHz the samples yielded large attenuation gains with peaks of 80dB under normal incidence, and good correspondence to modelling predictions. In diffuse field conditions the samples still showed significant STL improvements above that of a conventional panel over bandwidths in the order of 300 Hz. The resulting systems have the potential to provide significantly higher transmission loss at low frequencies than conventional wall systems of similar size and weight.