Breakdown of classical models in highly damped ultrasonic propagation.

The acoustic behavior of ordinary porous foams with moderate flow resistivities (σ∼103-104 Pa · s · m-2), commonly used in noise control applications and featuring relatively simple microstructures, is well understood within the framework of the Johnson-Allard-Champoux models. For cases requiring greater precision, these models can be refined further using the extensions proposed by Lafarge and Pride. In this work, using free-field ultrasonic (∼100 kHz) and guided low-frequency (∼100 Hz) time-domain measurements, we demonstrate a possible significant inadequacy of these models when applied to foams engineered to exhibit anomalously small viscous and thermal characteristic lengths and marked by exceptionally strong intrinsic absorption. This does not necessarily call into question the underlying local equivalent-fluid theory, although scattering effects may emerge. However, scattering effects may emerge when applied to foams engineered to exhibit anomalously small viscous and thermal characteristic lengths and marked by exceptionally strong intrinsic absorption. Our findings open avenues for extending existing models to more accurately capture and experimentally probe complex pore geometries, potentially enabling the assessment of scattering effects not included in the equivalent-fluid description, and for designing more effective, broadband sound-absorbing structures that achieve strong net absorption through controlled reflection, particularly at low frequencies.