I was wondering if the phenomenon was that headphones always cause flatting.
Does anyone know if headphones may also cause sharping?

And what's that about anyway? Too much level in the headphones might cause
the tympanic membrane to be pushed slightly out of shape? Would that make
the singer's voice appear higher in pitch than it really is?

I find it to be psychological rather than physical...

If a singer, particularly an inexperienced one, has too much volume from the
track, or from their vocal in the 'phones, they tend to oversing to keep up and
then often go sharp.

Or, if there is too little of the track in the 'phones, they sometimes
undersing and go flat.

It's voodoo, a black art, really, with folks who don't sing in headphones a
lot, to figure out what it is they need to feel good and stay on pitch...

But then I work with a lot of these folks....

stv