I've searched high and low through the ferrite magnetics catalogs and this list for an answer and can't find data on ferrite core permeability below 10kHz. My application is for audio use and I need to know how the permeability of the core drops off with falling frequency.
Ideally the permeability will drop rapidly at 300Hz, with some flexibility in the frequency. I imagine core losses will be small in this region since a lower permeability results in lower hysteresis losses. Also I suspect the core air gap will not play much of a role.
I know I came across this information many years ago but am having a problem finding it again.