Between 38 and 60 HZ You can vary the tone with the software...simple... Find your tone. You need a pretty decent bass speaker with the correct frequency response.
One 56 HZ Plus or minus specific frequency will make you puke.
It's best to download the high quality wav sounds. Apparently though you have to listen for up to 45 minutes in a silent dark room. I have a friend that swore by that stuff. I never tried it though because I have no patience.