The really dangerous part is you can't tell until it starts damaging your hearing, and sometimes not even then, depending on decibels. You won't hear the sound

The really dangerous part is you can't tell until it starts damaging your hearing, and sometimes not even then, depending on decibels. You won't hear the sound, so you're depending on it meeting your pain threshold to be warned that something is happening. If it's "merely" 100db or so (a loud drill), you won't feel a thing, and could be exposed for hours without noticing. These are the lasers of hearing. Without really good studies to back up the safety of this amount of energy, I might just end up avoiding all starbucks.