Ed -
Did you consider putting the discs in at an angle instead of
perpendicular to the longitudinal direction? I've wondered whether
having baffles at angles wouldn't increase the chaos of waves - i.e.
increase attenuation/cancellation. Maybe even a tube where baffles are
inserted from alternate sides. E.g.

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Joe Hull
Bellevue (Seattle), Washington
Cozy MkIV #991 (working on Engine & Electrical & Finishing)
http://www.maddyhome.com/canardpages/pages/JoeHull/index.html