I'm afraid I'm not sure. But I do have an idea to how to do this.
Essentially you could rip out a voice modulator from a current voice current modulating toy like one of those Transformer helmets. Inside it, you should be able to modify the frequency and wavelength by altering the voltage if you were able to change out the the resistor for a potentiameter and the power source that ranges between the buzzers minimum and maximum power limits.
Of course there does exists schematics online to how you can build your own. It's just a matter if you're willing to go to a radioshack to get the parts and also if you know how to read the schematics and use a soldering iron.
Well I hope this helped.
When in doubt, just take the next small step.
With that next step take a deep breath. It calms the mind.
When the mind is calm you realize the best is still yet to come.