Does any of you guys have experience with generating ultrasound with a gadgeteer system?
What purpose do you want to use it for?
We used ready to use ultrasound distance sensors.
These sensors have an analog output, but digital might be available as well.
These sensors have a quite huge coil, means the greater the distance, the greater the object needs to be.
Also if the object has not an flat surface or if the surface is not perpendicular to the sensor you get larger distances. So if the surface tilts, the measured distance get bigger.
Also wind has an influence to these sensors. Light wind can change the measured distance. Strong winds can disturb the sensor in a way sot does not work anymore.
I don’t know if tem,perature has an influence as well.
Ah and of course, they wont work in space (vacuum) 
Sorry I wasnt specific enough. I want to generate ultra-sound that should go into an amplifier, and then into speakers that has freq range up to 30Khz.
The purpose is to support some research on animals reactions to that type of sound. This is in fields and in the woods.
VLSI chip on the music module has sine test mode. You can generate a signal of wanted frequency using that.
Hardware PWM should handle this. Add a bit of low pass filtering to get rid of higher order harmonics if needed. Square waves contain odd harmonics so you need to cut out anything above 3 times you fundemental frequency to get a pure tone.
Looking at page 66 in http://www.vlsi.fi/fileadmin/datasheets/vlsi/vs1053.pdf I cannot really figure out how to set the numbers for the Sine Test command to get a frequency over 11625. (Which makes sense for human audible tests). Is that right?
That sounds reasonable, can anyone guide me to example circuitry sending the pwn-signal to an external amp?