I got curious earlier today and had a listen to some test-tones on some Sennheiser HD25-1 headphones. Like you do.
These are the headphones that we supply with all our PA systems for troubleshooting and quickly checking recordings are happening, and their signals clean as can be. They sound quite flattering when listening in “hifi” or “walkman” situations and I’ve always liked them for this, especially where significant isolation is required from external sound sources. Trouble is, it seems they don’t do so well in the very high frequencies, which I noted this morning while listening to the 30th Anniversary remaster/re-recording of Oxygene. I thought it was just my ears or setup, but the sparkle I remembered from the same recording played on my home “hifi” was all but gone. That’s what got me doing a sweep with some test-tones, and that’s what had me doing a quick Google search, which turned up this plot, which very much explains what I thought I was hearing.
In live mixing, much of the “air” of a vocal or instrument will be up past 10KHz, and if the headphones are audibly struggling at 13KHz, it’s likely they’ll not be doing what they should be even as low as 10KHz. If these are used for monitoring, one would likely be thinking everything sounds a bit dull and cranking up HF from 10KHz up on pretty much every channel. And I regularly see our guys mixing with our headphones as a reference doing exactly that, taking the paint off the walls in the room in the process. My HD25SP’s have a similar response, and it explains why anything I mix on headphones tends to end up sound far too bright when played on other systems.
One answer might be “get better headphones” – all well and good, but these are absolutely no substitute for listening to, and mixing for, the room you’re actually standing/sitting in, rather mixing for the silent space between your own ears. Everybody hears the room, only you hear your cans.