Impact of air gap thickness on acoustic metamaterials with internal simple expansion chamber structure

The air gap thickness plays a significant role in enhancing sound absorption. This study explores the role of air gap thickness in enhancing sound absorption in 3D-printed acoustic metamaterials (AMMs) featuring a Simple Expansion Chamber (SEC) structure. Nine samples (B1 to B9) were designed and then 3D-printed with varying SEC lengths (2-18 mm) maintaining an area expansion ratio of 16. An air gap (5-30 mm) was introduced between each sample and the rigid backing. The Sound Absorption Coefficient (SAC) was measured using the Impedance Tube Apparatus. Results showed a notable increase in peak SAC from 0.57 to 0.96 with a 5 mm air gap for a sample with a 2 mm long SEC. However, this effect did not replicate across other samples, suggesting the peak SAC was unaltered by further variation of the air gap. Additionally, the peak absorption frequency exhibited an exponential reduction with increasing air gap thickness. This research illuminates the nuanced relationship between air gap thickness and the acoustic performance of 3D-printed materials, offering insights for optimizing sound absorption in diverse applications. Keywords: Air gap thickness, Simple Expansion Chamber, Area expansion ratio, Sound Absorption Coefficient, Sound Transmission Loss