If you're looking for a frequency-division style device, the BatBox Baton is relatively low-priced.
|18-32 kHz||signals displeasure/danger/etc, OR anything associated with displeasure/danger/etc|
|32-96 kHz||signals pleasure|
|~40 kHz||a pup that's isolated, to make it easier for the mother to locate it|
After experimenting, I found that phonemes that generate the most ultrasound are the fricatives. The stop phonemes often have some ultrasound as well, but not as strong and not reliably. (eg. "map", "kiss")
I wrote a program to find words that have a high number of ultrasound components. I then spoke them through the bat detector, and selected ones that had a fairly unique signature in the 30-50 kHz range. (a unique rhythm, or a unique series of fricatives+stops).