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Scattering Filters at the Cylinder-Cone Junction

As derived in §C.18.4, the wave impedance (for volume velocity) at frequency $ \omega $ rad/sec in a converging cone is given by

$\displaystyle Z_\xi(j\omega) = \frac{\rho c}{S(\xi)} \frac{j\omega}{j\omega-c/\xi}$ (C.149)

where $ \xi$ is the distance to the apex of the cone, $ S(\xi)$ is the cross-sectional area, and $ \rho c$ is the wave impedance in open air. A cylindrical tube is the special case $ \xi=\infty$ , giving $ Z_\infty(j\omega) = \rho c/S$ , independent of position in the tube. Under normal assumptions such as pressure continuity and flow conservation at the cylinder-cone junction (see, e.g., [302]), the junction reflection transfer function (reflectance) seen from the cylinder looking into the cone is derived to be

$\displaystyle R(s) = -\frac{c/\xi}{c/\xi - 2s}$ (C.150)

(where $ s$ is the Laplace transform variable which generalizes $ s=j\omega$ ) while the junction transmission transfer function (transmittance) to the right is given by

$\displaystyle T(s) = 1 + R(s) = -\frac{2s}{c/\xi - 2s}$ (C.151)

The reflectance and transmittance from the right of the junction are the same when there is no wavefront area discontinuity at the junction [302]. Both $ R(s)$ and $ T(s)$ are first-order transfer functions: They each have a single real pole at $ s=c/(2\xi)$ . Since this pole is in the right-half plane, it corresponds to an unstable one-pole filter.


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``Physical Audio Signal Processing'', by Julius O. Smith III, W3K Publishing, 2010, ISBN 978-0-9745607-2-4.
Copyright © 2014-06-11 by Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University
CCRMA