Rauf,

Checking the above mentioned buzzer companies will give you some good frequency targets. High tones pleasing to the ear are usually around 2200 to 2800 hz and low tones around 800 if I recall correctly. Most phone audio runs from roughly 600 to 3000 hz so you want to stay within this range if you want humans to hear it.

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When I was in university 25 years or so ago, I built myself a S-100 computer. I designed a sound effects board based on the General Instruments AY3-8910 or something like that. I wirewrapped it up and experimented writing code and experimenting with sound. It was a great assembly language project.

One day while experimenting, I noticed that our cat, a Blue Persian, reacted to one of my sequenced sound effects. I soon deduced that one of the high frequency tone patterns were tickling his eyes and he'd try to stick his jaw out like a scuba diver to clear pressure in its ears... as a scuba diver, that's how I interpreted his reastion. It looked like the cat was gagging and about to be sick.

Of course I didn't torment the cat but I did use it to trick visitors. I'd assign the tone to a function key on my computer and as visitors were being introduced to their first peek of a home computer, I'd subtly convince one of the visitors that our cat appeared to be allergic to them. Whenever the target picked up the cat, I'd press the key and the cat would start its act and of course no one else would appear to trigger the same reaction. :)

aka J