This totally freaked me out. I started off with 8-10-12kHz, could hear those without a problem. Then 14Khz - nothing. I played the file like 5 times and couldn't hear a thing. I thought great, I am officially deaf, at least I know why my mixes suck so hard. Then I said fuck it and clicked on the 21 Khz and I could hear that! Weak but it was definitely there. I was like WTF? Then I started going backwards and could hear 19-15 Khz allright. WTF? That's not normal is it?