Experimental evidence of surface waves above thin porous layers of thickness varying from 1.8 to 6 mm of plastic foams having a porosity close to one is presented. Phase velocity measurements at 40 kHz indicate the presence of waves propagating slower than homogeneous plane waves. The pole of the reflection coefficient related to these waves is detected by near-field holography at 20 kHz. The different experimental results are compared with predictions obtained by replacing the air inside the layers by an equivalent fluid, and using methods developed in the context of electromagnetism to localize the poles of the reflection coefficient.
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