Frequencies used are generally within a 1 khz - 12 khz band.
Hearing aids can also function as maskers by amplifying external sounds.
Many people find that tuning a regular FM radio to an empty frequency and listening to the static beneficial. Another popular method is to run an electric fan. In the latter case it was important to properly adjust the hearing aid before attempting masking. The effect residual inhibition has is to cause the tinnitus sound to partially or completely disappear for a few mins. I was also tested for ri at the University of Washingtons' Tinnitus Clinic in 1986, but I was not to be so fortunate as others at either place I tried.