But it sounds backwards. Sound stops being adiabatic
when thermal conduction sets in. Low frequencies are
associated with long wavelengths, and since thermal
conduction is diffusive I would expect _less_ conduction
and _more_ adiabatic behavior at low frequencies, for
plane waves and such.
Diffusion makes progress like the square root of time.
OTOH if we're talking about standing waves in a heavy
copper organ pipe, then OK, low-frequency waves will
exchange heat with the walls of the pipe. But this
will be horribly dependent on the details of the
situation; if somebody puts this into a tabulation
of "properties of the air" I'm not going to believe it.