### ASA 124th Meeting New Orleans 1992 October

## 1pPAb3. Frequency and time domain modeling of acoustic liner boundary
conditions.

**Donald B. Bliss
**

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Linda F. Peretti
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**
**
*Dept. of Mech. Eng., Duke Univ., Durham, NC 27706
*

*
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A general model of the acoustic boundary condition for an absorptive
liner, including bulk reaction effects, is developed. The boundary condition is
an extension of the traditional impedance boundary condition modified to
include the effect of gradients along the absorptive surface. Using a
bulk-reacting porous material as an example, a boundary condition of this type
can be constructed that is applicable over the entire frequency range, provided
the liner damping is sufficiently high. The boundary condition contains
frequency-dependent coefficients, in addition to the normal incidence
impedance. These coefficients can be calculated theoretically or measured
experimentally. A measurement procedure is described. The accuracy of the
boundary condition is demonstrated by comparison with the exact solution for
some simple cases. The boundary condition is converted from frequency domain to
time domain by calculating the appropriate impulse response behavior. The
various impulse response functions are derived by Fourier transform methods,
and their behavior is discussed. The impulse functions exhibit behavior
associated with viscous relaxation and wave reflection. Once the impulse
response functions are known, a response to an arbitrary pressure field can be
constructed. Future extensions and applications of the bulk reacting boundary
condition are described briefly. [Work supported by NASA.]