| Summary: | 博士 === 國立臺灣大學 === 土木工程研究所 === 84 === The aim of this dissertation is to develop an acoustic
computation theory and experimental verification techniques for
active noise control in an enclosure. A greater emphasis is put
on taking advantage of the characteristics of the sound field
in the control algorithms. A method of band summation is
proposed to simulate an impulse at a point. A band summation
(or a single band) so simmulated is then used as an input to
the sound pressure boundary integral equation for calculating
directly in time domain the impulse response (or a band
component of the impulse response). Based on the equation
developed are an exact calculation procedure (BIE) for the
direct sound and the first reflected sound, which together
comprise the beginning part of the response, and a time domain
boundary element method (BEM) for calculating the subsequent
part of the response In order to overcome difficulties
encountered in the calculation of sound pressure, which
exhibits notoriously large oscillations in both space and time
domains, it is proposed that interpolations are always carried
out on q(x,y,z,t) , a complex with the real and imaginary parts
correlated respectively to sound pressure level and phase,
instead of sound pressure p(x,y,z,t) itself, with
complexificated p(x,y,z,t) = e^{q(x,y,z,t)} . Because the
dynamic behavior of q(x,y,z,t) is rather mild although p(x,y,z,
t) is oscillatory wildly, boundary element sizes and time steps
can thereby be increased dramatically, so that both efficiency
and accuracy are enhanced to a large extent. The oscillatory
integrals which appear in the BIE and BEM procedures are
treated by two new methods, namely the Chebyshev polynomial
method (C method) and the Q method, which have been verified to
perform very well for higher frequencies and large spaces.
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