If these are just pure tones, you can use a package like Cool Edit or Sound Forge to generate a wave file of a specific frequency. You can even generate mixed tones if you like.

Having done this myself, I can attest that this does work quite well. I even posted the MP3 files at one point (although I think I took them down recently). I also discovered that you can't MP3-compress an 18khz sine wave and expect it to sound proper...

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Tony Fabris