Loud enough for me to tell it was playing, but nowhere near as loud as 17 and down.
I remember the hearing tests for RAF Pilot Selection, where they played various notes in your ears (random tones, random ear) where you had to click a button when you heard a tone. Sounded not too dissimilar to these tones.
For extra freakout, try playing two tones at the same time.
(, Wed 19 Nov 2008, 17:32, Reply)