Helmholtz Number

The cut-off frequency of a duct is generally described in terms of the Helmholtz Number.

where

H_n = helmholtz number

k = wavenumber [m^-1]

a = duct radius [m]

The plane wave cut-off frequency is when the Helmholtz Number is of the order of 1.84

See also: Cutoff Frequency, Wavenumber.