Google search says 15DB is half way between a leaf falling and leaves rustling so should be able to hear it from a few feet away if there’s no other sounds around you but only just. What’s equally important is what kind of sound it is, some fans are more annoying than others, and that 15DB is obviously when idle, not under load which they don’t mention which is concerning.
Honestly though I’m surprised by the amount of cooling in this thing, it’s half of the enclosure. I thought Apple chips were so much more efficient that they didn’t need so much. Hopefully it’s just overkill so it will stay quieter, we shall see.