Abstract
It is convenient to have a device and a method of generating single cut-on
modes in cylindrical hard-walled waveguides or at least in laboratory
models of such systems. This allows to examine, among other things,
properties of various active and/or passive elements inserted in a
cylindrical duct by testing them in conditions when the incident (input)
wave comprises only one cut-on mode and determining the reflection and
transmission coefficients for single selected incident modes. As it has
been already demonstrated by the present authors, it is possible to
generate single cut-on modes in a circular duct using a small (although
increasing with mode order) number of acoustic monopoles arranged properly
on a duct cross-section and driven with appropriate acoustic volume
amplitudes and phases. Laboratory models of such sources are proposed in
this paper and results of tests verifying their directional properties are
presented. The other technical issue relating to practical utilization of
the proposed method is the possible error introduced by the apparatus used
for scanning the acoustic field inside the duct model. It is shown that
insertion of the measuring probe changes the total energy radiated into
the free space only by a fraction of a decibel.
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