I'm using a procedure that I made up. I'm certain I'm not the first person to think of this, but I'm pretty sure I am the first to post about it online. I'd like to share my technique here in case anyone finds it helpful, or someone blasts holes in my methodology

The results all match my observations though, so I think I have it right. My method works better for planar magnetic drivers, since their impedance curve more closely resembles resistance. For dynamic drivers it's still an accurate enough estimate to be useful, and will typically lean more toward the worst case.

It's pretty simple really. Grab a pair of cans, and on your digital multimeter's lowest setting, touch one lead to the ground on the plug and the other to the tip. Note the reading, then move the second lead to the plug's middle ring.

Take the difference between the two drivers, then divide it by the driver's impedance. For instance, my AKG Q701 measures 62.2 ohms on one side and 61.4 on the other, so the difference is .8. Divide .8 / 62 to get a percentage, 1.29% or .0129. Now multiply that by the intended listening level. I use 85 db, but it doesn't make much difference if you use 80 or 100. So .0129 x 85 db = 1.09 db. Therefore my Q701 has a rather large 1 db imbalance, which is indeed audible.

(I read that 1 db is intended to be the smallest perceptible change in volume, which seems about right to me. I received an ATH-EC700 today, and my measured .8 db imbalance has been driving me nuts!)

A couple of things I feel I should note:

*I've tested every pair I own, and I've noticed nearly every "high end" pair is matched to 1/3 of a db or less. The worst is an old Pioneer SE L40, and after removing the parallel resistor in the earcups I got 16 ohms on one side and 14 on the other! Yikes!

*Most multimeters will only display three (and a "half") significant digits, so I'm not sure what you can do for anything over 200 ohms. It's possible that tenths of an ohm might be negligible for numbers that high. I don't own any to test.

*Multimeters can't measure impedance, only DC resistance. Yes, DC is bad for headphones, but on the lowest setting a multimeter isn't going to provide anything near what it would take to do any damage, even on sensitive earphones. I don't own any BA drivers though and I haven't tested one, but I don't think there's any plausible chance of damaging them either. If it can play music at an audible level, it can handle the one second it takes to measure.