Gert Jansens

Frame-borne surface waves in air-saturated porous media

A Rayleigh-type surface wave at an air–air saturated porous layer interface is predicted and detected experimentally in the audible frequency range. Applications of this surface wave for the metrology of sound-absorbing media are suggested.

Impedance measurements around grazing incidence for nonlocally reacting thin porous layers.

For locally reacting materials with a constant surface impedance, a classical method based on the work of Chien and Soroka [J. Sound Vib. 43, 9-20 (1975)] for measuring this impedance in situ around grazing incidence is currently used. A generalization of this work to include thin nonlocally reacting materials with a surface impedance noticeably dependent on the angle of incidence is performed. It is shown that the model by Chien and Soroka can be used, though the constant surface impedance must be replaced by the impedance at grazing incidence for the evaluation of the numerical distance. Measurements performed on a thin porous layer using this method are compared with measurements performed using the near-field acoustical holography method [M. Tamura, J. Acoust. Soc. Am. 88, 2259-2264 (1990)]. Other measurements performed on a fibrous layer are in good agreement with the predicted values of the impedance at grazing incidence.

Free field measurements of the absorption coefficient for nonlocally reacting sound absorbing porous layers

A simple asymptotic approximation with two parameters (the plane wave reflection coefficient and a correction factor) has been derived by Brekhovskikh and Godin [Acoustics of Layered Media II. Point Sources and Bounded Beams (Springer, New York, 1992)], for the spherical field reflected by nonlocally reacting surfaces. In the present work, an expression for the correction factor is obtained for the case of a homogeneous and isotropic porous layer. A free field method for evaluating reflection and absorption coefficients with this approximation is presented. The evaluation of the absorption coefficient at normal incidence is performed on a porous layer on a rigid backing, and compared to measurements performed using a two microphone technique and an impedance tube.

Reflection of spherical waves by non-locally reacting porous media

Expressions for the spherical acoustic field reflected by a porous or a fluid layer of finite thickness having a surface impedance noticeably dependent on the angle of incidence are obtained using the classical steepest descent path and the pole subtraction method. The domains of validity of both methods are defined. A simple expression that is valid for the case of thin layers, where the one intrinsic parameter is the surface impedance at grazing incidence, is obtained using the pole subtraction method.