Enhancement of the low frequency performance of thin, film-faced layers of foam by surface segmentation

While the control of high frequency noise by the use of porous treatments is relatively straightforward, it is correspondingly difficult to control low frequency noise owing to the general requirement that a porous layer should be a significant fraction of a wavelength in depth. Thus, it is of interest to develop low frequency sound absorbing treatments that are relatively compact by the use of surface treatments. Here, we illustrate an approach, in which a surface film is cut, to allow the incident sound to communicate directly with the air within the foam behind the film facing. The present work was inspired by standing wave tube measurements of film-faced foams. It was found that measurements of unfaced foam layers, or of foam layers with a loosely attached film layer could be reproduced well by existing theories. However, when a film was bonded to the surface of the foam layer, the results did not conform to the expected values, and showed an anomalous absorption peak at relatively low frequencies. Upon careful examination of the experimental arrangement, it was noticed that there was a narrow gap around the circumference of the film facing: i.e., there was a leak around the edge of the sample. When that leak was sealed by using a bead of silicone sealant, the anomalous low frequency peak disappeared, and the absorption was then as expected for a continuous film-faced layer. Thus, it was apparent that the low frequency absorption peak was due to the presence of the thin gap around the circumference of the foam layer