Séminaire du Centre Lyonnais d'Acoustique (Labex CeLyA) en comodalité : Optimal damping of sound in ducts

Prof. Mats Åbom
The Marcus Wallenberg Laboratory for Sound and Vibration Research, KTH (Stockholm, Sweden)

Résumé : cliquer ici

The topic of optimal damping or rather maximum propagation damping in locally reacting lined ducts is a classic problem. The original solution based on mode merging (“an exceptional point”) and valid for no flow was proposed by Cremer (1953). The resulting optimal impedance is therefore referred to as the “Cremer impedance” and is normally computed for the lowest “zero” mode. Later the solution was extended to also include a plug flow assuming the so called Ingard-Meyer boundary condition. At low frequencies this solution implies a negative real part of the impedance, e.g., an active boundary. For the upstream case the solution also fails to produce maximum damping for the lowest mode, instead the second and third modes are merged. As recently shown these issues can be resolved by instead using the classical boundary condition, which for sufficiently low frequencies is the best choice. This paper will discuss this as well as other aspects of the Cremer impedance, including a possible “best strategy” for low frequency damping in a lined duct.