Cutoff Frequency
The Pipe Acoustics interface assumes plane wave propagation. This means that it cannot model the propagation of the higher order modes that can propagate above their cutoff frequency
f
c
. In a rectangular pipe of cross-section width
w
i
and height
h
i
, the cutoff frequency is
In a pipe of circular cross section (with radius
a
), the cutoff frequency is
where
α
'
mn
is the
n
th zero of the differential of the Bessel function
J
'
m
(
x
) or order
m
. The first few values are
α
'
01
= 0,
α
'
02
= 3.83,
α
'
11
= 1.84, and
α
'
21
= 3.05 (see
Ref. 1
and
Ref. 5
for further details).
