I saw an app for iphone that emits a high frequency sound when your dog barks to assist in training dogs not to bark. This would be a great application.
go to zedge.com and download the high frequency track to your sd card. no need for an app. i posted a thread a while back with it on there. it was a ringtone thread. its really freaking high pitch. it bothers younger peoples ears but elderly peeps cant hear it. kinda weird.
from G1 goto m.zedge.net .. make your way to Tmobile | G1 then in zcode input box enter ZCode of 4529174 ... I am too old to hear it but will try it out .. Also Don't know where it was stored ... have not been able to find it yet
well I downloaded it and tried it on some dogs.... not only did it not stop them from barking but they actually barked at my phone.... I must say it was actually VERY funny! here's the link for anyone else who wants to try it.... http://www.zedge.net/ringtones/280580/high-frequency-ringtone/
We recently released a simple app that does what the OP is requesting. It's called DogWhistle (Cyrket - DogWhistle). You select one of 7 frequencies which range from 15,000 Hz to 40,000 Hz and hit the button. The app generates a 5-second tone in that frequency. Early bad reviews were due to the volume of the tones being too low. We boosted the volume and the app should work fine now, although not all dogs and cats will respond to the tones (depending on the size, age, breed of the animal, and their character). The lowest tone is in the the range of human hearing...although it may not be very pleasant. The app sells for $0.99.
Found a Great Dog App. Cam2Pet. Can watch my Terra Nova, while I am out, talk to him, take a snapshot,.. Amazing this App
Theres a lot of apps out there that do that if you just search for dog whistle. Although I went ahead and made one if you'd like to use it: https://play.google.com/store/apps/details?id=com.eschoeffler.tonegenerator If you click on the doge, it emits a 25kHz sound until you click on the doge again.