Diffuse field measurements of locally resonant partitions

Abstract

Noise control at lower frequencies is becoming increasingly important with housing densification, as this frequency range is where the threshhold of the human ear is at its highest and sound insulation is the most challenging and expensive. Meta-materials offer a novel approach to achieving sound and vibration management through the use of panels with internal resonant structures with geometries sub-wavelength. 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 desirable sound transmission loss (TL) performance spectrums. Designs with the targeted TL characteristics were then examined in detail under FEA and plane wave impedance tube testing and samples were fabricated using industry-standard materials and processes. This paper focuses on the acoustic testing of large scale LRS samples at low frequencies under diffuse field conditions. Locally resonant partition wall systems were designed to target frequency regions where poor TL is expected such as the mass air mass region (MAM) and the coincidence frequency region (CF). Samples from between 2m2 to 10m2 were tested with variations in system arrangements such as mass and geometry. Results are presented here for 2:4m2 samples which showed significant TL improvements with approximately 20dB improvement above that of a conventional panel over bandwidths in the order of 300Hz. Comparisons were made between, numerical predictions and plane wave experimental results. The resulting systems have the potential to provide significantly higher transmission loss at low frequencies than conventional wall systems of similar size and weight.