This thread is a spin off from the CSD thread. Some HF'ers had asked that I take a non-linear distortion measurements (harmonic + intermodulation + whatever else.)
So what got me thinking is this hypothetical question: "What exactly does 1% harmonic distortion mean at 50Hz @90db?". Well, IMO, it means "not very much." The reason it means very little is that we don't know the exact nature of that distortion. It's best to show via pictures. The following three graphs below show the frequency spectrum of three headphones (HD800, HP1000, and LCD3) being fed a 50Hz sine wave signal so that it measures 90db.
The extra crap that is seen (anything other then the 50Hz spike) is distortion. There are some caveats to these graphs: the sub-bass 20-40Hz consists of a lot of environmental noise - it should be ignored. The noise floor is fairly high - you can definitely see extraneous crap up high in the spectrum and in the bass and lower midrange regions.
Our main focus will be on harmonic distortion, which would be multiples of 50Hz.
HD800. For anyone who's heard the HD800, it is a very good performer in the bass department. Good extension. A little dry sounding. The 3rd harmonic (150Hz) accounts for most of the distortion of the 50Hz signal. It's -40db down. The 2nd harmonic (100Hz) is over -45db down from the fundamental. Overall very good, especially for an open headphone.
HP1000. Anyone who's had the opportunity to hear the HP1000's bass knows it's pretty slamming. Maybe even a bit muddy. That's what second order distortion sounds like. There is quite a lot of second order distortion (100Hz), with it being less than -30db down from the fundamental.
LCD3. Everyone who has heard the LCD3 knows how amazingly loud and clean the bass on these headphones can be. I've wanted to take this measurement for a long time because I thought it would be perfect and indeed it is. We can't even see any distortion at the 2nd harmonic! This is really amazing. The 150Hz spike is environmental noise (I don't know why it's there tonight), so effectively 2nd order and 3rd distortion of the 50Hz signal at 90db is not measurable - it's below the noise floor of the system and environment.
I'm going to keep things simple for a bit and just focus on 50Hz sine waves. I'll throw up a few more measurements of some headphones I have lying around.
Edited by purrin - 2/23/12 at 10:35pm