what kind of core for audio SMPS?

[capslock]

For either a 12 V primary, with or without galvanic insulation, or a 230 V with insulation, what is the optimum core (toroid or EI, air gap or not, ferrite or insulated iron powder)?

I notice that most commercial implementations use EI ferrite. Not sure about air gap.

What is best practise here?

What are relevant design criteria?

Thanks