After re reading the posts concerning this subject, perhaps is there a method to create a large voltage swing but at the desired frequencies?
I borrowed a unit that supposed to deter rats, dogs deer etc (it has a knob that selects what you want to deter)
I took it apart and from what I can tell it's a 556 connected to a piezo via a couple of transistors and an inductor that has 4 leads coming from it. Thinking that maybe just go with two seperate sound producing elements (an 8ohm speaker and a piezo tweeter( one of the last two I posted links to)
the 8 ohm speaker for 2 - 8 khz frequencies and the piezo for the 15-20khz frequencies. Thinking that if I use an inductor then perhaps a circuit to vary the resonant frequency of the inductor?
in the book I purchased it has similar where a 3 pos dip switch is used to produce 3 different frequencies (25khz, 16khz and 12khz all using two 1mh inductors.
will post schematic as soon as I get one drawen up.