hearphone and ultrasonic silent

[Shannon]

You can get good results with VERY low volumes on headphones using ultrasonics. You are not supposed to use ultrasonics on headphones at all, however, not just because of the possibility of hearing damage from high volumes, but from accidentally turning the volume to zero. As someone recently discovered, using ultrasonics with headphones at too high a volume can also trigger tinnitus. So if you do use headphones with an ultrasonic, I agree with Q - 1, 2 or 3 out of 25 (or the equivalent) will work just fine.