Impulse measurements of impedance and propagation constant compared to rigid‐frame and dual‐wave predictions for foam

Acoustic impulse techniques are used to obtain measurements of propagation constant in three different foams and to determine both the characteristic impedance and the normal impedance at the surface of a hard‐backed layer over the frequency range from 200 Hz to 10 kHz. A comparison is made of existing rigid‐frame and dual‐wave models of sound propagation in porous media, and predictions from both models are compared with experimental measurements. Both models give good agreement with the lower flow resistivity foam measurements, however, the dual‐wave model better predicts the overall results for a foam with a flow resistivity of 25100 N s m⁻⁴, implying the need of the slow wave mode. The position of the maxima in the layer impedance results are particularly sensitive to the correct choice of tortuosity. An emphasis is placed on how the two models compare as the frame material becomes rigid and these ideas are used to consider the implication of a previous claim about the existence of a slow wave in foams.