I tried 6780khz, 13560khz and 27120khz and heard nothing but normal background noise. BTW, I think that diathermy devices are on 27120khz. Back in June-July 2020 I spent a month in a nursing home after a bicycle vs van accident (I was 74 YO, now 76) and getting a plate in my right shoulder in the hospital. I had to undergo diathermy for my swollen right hip during that month. I'm much more careful now.