Incident Sound Wave

to be transported away from the interaction site to dissipate. Little attenuation seems to occur as a result of induced motion of the fibers (Mechel and Ver, 1992).

A lower (isothermal) sound velocity within a porous material also contributes to absorption. Friction forces and direction changes slow down the passage of the wave, and the isothermal nature of the process leads to a different equation of state. When sound waves travel parallel to the plane of the absorber some of the wave motion occurs within the absorber. Waves near the surface are diffracted, drawn into the material, due to the lower sound velocity.

In general, porous absorbers are too complicated for their precise impedances to be predicted from first principles. Rather, it is customary to measure the flow resistance, f, of the bulk material to determine the resistive component of the impedance. The bulk flow resistance is defined as the ratio between the pressure drop A P across the absorbing material and the steady velocity us of the air passing through the material.

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