A study on the acoustic boundary admittance. Determination, results and consequences

The acoustic boundary admittance condition represents the stiffness. the ma ss and the damping behaviour of the surrounding structure. The harmonic ana lysis of small interior domains (e.g. vehicle cabin) is often carried out o nly by applying acoustically rigid boundaries since the admittances are unk nown or unreliably determined by commonly applied methods. Determination us ing an impedance tube (or Kundt's tube) does not consider the real sound fi eld; the calculation from the measured reverberation time provides an avera ge admittance (and no phase information) of the whale boundary only. Beginn ing with the definition of the complex-valued boundary admittance, a brief review of the techniques to determine boundary admittances is followed by a boundary element based method that is suited to calculate the boundary adm ittances from a known sound pressure field. Ln addition to known methods, a formulation is found where the nodal admittance is calculated by the quoti ent of a nodal particle velocity divided by the nodal sound pressure. The n odal particle velocity can be calculated from a known sound pressure field solving a Dirichlet problem. The methods are applied to three simple exampl es. Finally, other examples are given to demonstrate how the boundary admit tance can represent the fluid-structure interaction and phase angle of admi ttance influences the complex-valued eigenfrequencies. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.