HD-650 frequency dead spot

Hi, I've had my Sennheiser HD-650's for a couple of months and have clocked up about 150hrs of use; roughly 30 hours with my Oppo BDP-83 / Denon AVR-2310 set-up and 120 hours with an Asus Xonar Essence STX...

I ran a 24bit/92kHz 20-20kHz Stereo frequency sweep off a AIX Records Audo Calibration Disc (Blu-ray) and heard a dead spot in the treble area; the test tone quickly fades to nothing then back up again ... I found some mp3 left/right 20-20kHz sweeps over the Internet for playback with my STX card using foobar2000; Again the dead spot is there.

With the foobar spectrum visualization I know this dead spot occurs within the 8kHz range in both left & right channels; dropping at just over 8kHz and back up again just bellow 9kHz [8.3 to 8.7kHz area] ... Are my HD-650's faulty? Is this inherent to the 650's?

Thanks ... Paul

Thanks for the quick replies; even knowing that the gap is there hasn't detracted from my enjoyment of listening to music with the HD-650's and it's not something that I've noticed under normal listening ... as suggested I'll ask a friend to confirm what I'm hearing, or not.

Listened again to the sweeps at a much higher volume this time and there is a definite gap. The STX pci-e audio card has a bright signature so the upper mid's and treble sound stronger, but I can here the slight drop then a fast fade to nothing and back up again for the last little bit.

Shame I don't have a way of taking measurements, it's these little mysteries that add an extra touch of enjoyment to the hobby...

Only speakers ... I moved the computer into the front room so I could connect the STX card to the Denon amp. Playing the 20 -20kHz sweep through the front speakers I get an unbroken tone all the way through, plug the 650's in and the gap is there.

It's a definite dead spot rather than a dip in output from the drivers.

I'll query Sennheiser over it and see what they recommend.