The sound absorption performance of an absorber composed of a baffled micro-perforated panel (MPP) backed by a cavity is investigated in this paper. As a continuation of our previous work focusing on the normal incidence case, a three dimensional model is established which allows the consideration of oblique and diffused sound impingement on an absorber of finite dimension. The extreme case having a large panel dimension is used to approximate an absorber of infinite dimension, which is compared numerically with Maa's formula to show the validity of the model. It is observed that the grazing modes of the backing cavity can be efficiently excited under the grazing incidence. These grazing modes contribute significantly to the total sound absorption, which contrasts with the case of normal incidence. Analysis shows the underlying physics of the sound absorption based on modal coupling, which differs significantly when incident angle varies. Hence for an absorber in a diffused field, multiple modes would dominate the sound absorption, leaving large rooms for optimal design of the absorber towards better absorption performance.
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