Experimental evaluation on acoustic impedance and sound absorption performances of porous foams with additives with Helmholtz number

Porous foams may be classified into various sub-categories basing on their different microstructures nature. Each type of foams is involved with different acoustic damping capabilities and physical properties. In this work, we experimentally study and compare the acoustic damping performances of different types of porous foams with different densities and constitutions. A sample from each type of porous foams is tested in an acoustic impedance tube. Here the sound absorption coefficient, the reflection coefficient and the acoustic impedance are defined to quantify the noise damping performances of these samples over a broad Helmholtz number He range. The effects of 1) the porosity, 2) the materials density and 3) the thickness of the materials are examined one at a time. For this, a total of 13 samples are experimentally tested and comparison is then made. It is found that increasing the material density does not lead to an improvement on noise damping at Helmholtz number He below 0.17. In addition, increasing the thickness of the same samples gives rise to dramatic different noise damping behaviours at low Helmholtz number range (<0.35). However, the sound absorption coefficients of different thickness samples at higher He follow the similar trends. The thickness is shown to be affect a little on the noise damping performances. Ceramic fibre sample is shown to involve with a more broad effective He range than that of the samples of the normal concrete and polyurethane foam. To further enhance the noise damping performances, triphenylene is introduced an additive and combined into the pure silicone material. The additives effect is then evaluated. It is found that the sound absorption coefficient is increased dramatically, as the triphenylene is applied. The maximum absorption coefficient is increased from approximately 0.2 to 0.87. In general, the present work sheds light on how to optimize the acoustic damping performances of porous foams.