I use ultrasonics over headphones to keep from interfering with others and because I have them on during most of the day since people leave me no other choice. The most important thing is to really calibrate the volume. Start by playing the masked track. Set it to a comfortable volume, preferably a bit on the softer side. Now, while keeping that volume, switch to the ultrasonics. Use FrequenSee to ensure the level is about the same.
SubClub ultras are slightly louder than masked, possibly because they only have voice and no noise. (EDIT: Correction, the water of the masked track is actually about 10dB above the ultra according to my spectrum visualizer)
And always stay mindful of the volume. You won’t hear if it accidentally raised the volume, so ensure that won’t happen and maybe switch to the masked every so often (or use FrequenSee) just to be safe. If you get a sudden headache or your ears start beeping as if you’ve just been to a concert, keep your headphones off for a while.
As for the effectiveness of volume using masked, keep them at a level where you can still fall asleep. Most people can’t fall asleep above a certain volume, so stay below it. That in mind, keep the volume as high as is comfortable for you. Imagine someone talking you you during a noisy party. You may need to tell them to speak up or you’ll miss every other word. Same goes for masked subs. If the volume is too low, you might miss a few words here and there and your mind will have to figure out what they are through deduction. Combine that with the fact that the headband may move away from the ears while sleeping (my SleepPhones are never over my ears anymore in the morning) and you might want to keep the volume high enough.
@Anonymanas I’m looking forward to that explanation.