[linux-audio-user] Create high-pitch noises like the mosquito device

I saw the mosquito device that will annoy younger
people, but not older people:
 http://www.compoundsecurity.co.uk/teenage_control_products.html

Its pretty expensive. How would I begin to create that
from scratch? Can a sound-card generate those frequencies?

I imagine I would need a special speaker.
How about an amp?, does that also need to transmit the
higher freqs.? Is it like going from a bass amp to
a guitar amp? (and I need to keep going?)

Whats good software in linux to generate certain frequencies?

thanks for any ideas,

--
paul w