An experimental and theoretical programme was undertaken to
measure and predict the attenuation of plane and spiral modes
within a cylindrical and annular duct. The duct was lined with a
partitioned absorber designed to act as a locally reacting surface.
A detailed duct modal theory was evolved for the cylindrical duct
and used to compare with the measured results. A thin annulus .
theory was adopted for the annular duct which made use of an
existing computer programme originally written for the
rectangular duct problem. (Ref 1-3).
The experimental work was conducted using a siren rig and also,
in order to obtain greater detail and reliability, a loudspeaker
rig was built and used extensively for the m = 0 and m = 1 modes.
These results confirm, within the limits of experimental accuracy ..
that the theoretical approach used is a valid one at least under
the prevailing laboratory conditions of zero mean air flow and
low sound pressure levels. Excellent agreem ent was obtained
between theory and experiment for the cylindrical duct. In the
case,-· of the annular duct the comparison was less satisfactory
but prOvided at least qualitative agreem ent.
The principle observed effects are the increase of attenuation
rate with increasing mode number and decreasing cut-off
The thesis is written wit ha. bias towards the problems of the
aero-engine industry and includes a brief account of present
day absorption technology in this field.
A dissertation submitted as a requirement for the degree of MSc (by research).