Transmission and reflection characteristics at normal incidence have been measured for 1‐cm‐thick foam layers of 98% porosity in the frequency range 10–200 kHz. The ’’boundary frequency’’ is determined from these measurements. Below this frequency, (1) the transmitted field has no irregularity, (2) the transmission loss is proportional to the square root of frequency, (3) the reflection is specular, and (4) the reflected wave consists of two components which are reflected from the front and back surfaces of the layer and interfere with each other. The boundary frequency is 60 to above 200 kHz, for the layers with cell densities of 14 to 50 cells/in. It is found that, for each cell density, the boundary frequency is almost equal to the frequency at which one‐half a wavelength of sound is equal to the average diameter of cells.