Okay, I spent time over the past few days measuring total harmonic distortion. Actooly, THD+Noise vs. Frequency.
In these measurements a tone is swept through he frequency range. After it comes back from the head, the fundamental tone is filtered out; the remaining audio will be the distortion components (2nd, 3rd, 4th harmonics, etc) and any noise in the system.
There's a couple of things to be aware of as you view the graphs:
- Each curve on the graph uses a tone of a different amplitude (10dB apart). Typically the higher the volume, the more distortion. But remember that each curve represents the percentage of distortion relative to the reference tone level. As the reference tone goes down in level, the noise in the system goes up as a proportion of the reference tone. Therefor, you'll notice that in the higher frequencies of the graphs, where noise dominates over harmonics, the curves cross, as the noise is an ever larger percentage of the reference tone as it gets lower in level.
- Because we are measuring the amount of energy contained in the harmonics of the fundamental tone as it is swept, once we get beyond about 5kHz the measurements become ever more meaningless. At 5kHz for example, the second harmonic is at 10kHz, third at 15kHz, and the fourth at 20kHz. Because the system begins to loose response above 20kHz, the amount of harmonic distortion is being artificially reduced by the system bandwidth. This can be seen in the higher reference levels where the distortions are extraordinarily low in the high frequencies. Also at 7kHz the AP automatically limits the measurement to 2nd and 3rd harmonics only.
Sennheiser HD600
Sennheiser HD800
AKG K702
The data rather surprises me. The lowest curve in these graphs is 90dB in two cases; 85dB in the other; that's still a fairly strong volume level. These curves show that good headphones have fairly low levels of distortion, unless being driven to loud levels. That the K702 behaves poorly at 110dBSPL is really not too much of a problem because that's a ridiculously high level to be blasting your ears with. In fact, your ears are probably distorting worse than the headphones at that point as your acoustic reflex will have kicked in and the dominant noise source will be the muscles in your ears tensely telling you to TURN IT DOWN!!!
Another thing about these graphs: driving a headphone amp into, or near, clipping in order to measure it is one thing; driving headphones into clipping to measure them is Russian Roulette with the headphones. As I measured the K702 at 110dB I though for sure it was going to blow up. So the highest two curves of the graphs (110dB and 105dB) I would be very hesitant to impose on headphones on a regular basis.
Looking at the graphs we can see they rise as they go lower in frequency. At the moment I'm assuming this distortion is the driver diaphragm approaching its excursion limits and going non-linear. Since all the headphones above showed some characteristic distortion at 100dB, I figure we can use that as an upper limit for the moment.
Since these pretty good headphones were behaving quite well by the time they reached 90dB (the curves are fairly flat meaning distortion products are basically at or below the measurement noise floor) there's not much reason to test at lower volumes.
So making the measurements at 90dB and 100dB seems a good start. Let's see if it yields some good information.
Here's a graph of a bunch of different headphones measured at 90dB and 100dB. (You'll notice I didn't test above 7kHz as it becomes somewhat unmeaningful as mentioned previously.
Things I see (this is a lot of guesswork):
- The HD600 measures best of all the headphones in terms of low distortion. I'll go out on a limb here and say that the perception of the "Sennheiser Veil" may, at least in part, be due to folks not being used to hearing the smoothness of such low levels of distortion.
- The at the low end 650 and 800 do very well, the D5000 is pretty good followed shortly thereafter by the DT880; in the high frequencies they perform similarly, with the remarkable exception of a significant distortion peak centered just above 2kHz, possibly cone break-up of some kind.
- The 800 has a markedly different shaped curve, which might be indicitive of it's toroidal diaphragm design that it so fundamentally different than other more traditional drivers of this group.
- The small supra-aural PX200 had a VERY hard time putting out sound at these levels. It struggled mightily even at 90dB.
- The cyan (aqua) colored trace of the cheap HD201 headphones show a sharp distortion peak at about 205Hz. Again, I assume this is a resonant cone break-up.
At this point I switched over to looking at distortion spectra. I was curious if I could see a marked change in distortion spectra between, say, 170Hz and where the distortion peak occurred at 205Hz on the HD201.
In the graph below a single tone at 90dB is played at 170Hz (yellow) and 205Hz (green). The series of peaks to the right of the fundamental are the byproducts of non-linear distortions and occur at multiples of the fundamental tone. if you look carefully, you can see that the even order harmonics (2nd 4th 6th) harmonics of the 170Hz (yellow) are much lower in level than the even harmonics of the 205Hz signal. This indicates to me that the spike in the HD201 THD vs F data is an exitation of an even order vibrational mode in the driver diaphragm.
I'm really happy with how much information I got out of the THD vs F graphs, and I think I'd like to include them in the final test sequence. OTOH, I had planned to include distortion spectra data, but after seeing the significant difference on the 201 between 170Hz and 205Hz I'm beginning to wonder if it's a fair measure. Since a single frequency would be chosen to make the test, and because the harmonic series may be markedly different at different frequencies there is no way to make the test indicative of the whole character of the cans. A single (or even a few) distortion spectra measurements might be misleading.
However, because all the cans show marked distortion at the low end, I did run some tests displaying distortion spectra at 50Hz. I did the test at both 90dB and 100dB for the cans that I've been playing with. I'm don't think it delivered any where near the rich amount of info the THD v F graphs did, IMHO. See what you think:
50Hz @ 90dB
50Hz @ 100dB
The intention at the moment is to construct a standardized series of tests to characterize headphones for public display. It seems to me that it would be impossible to select one frequency and amplitude to produce distortion spectra that would be clearly characteristic of the headphones under test. I feel the THD+noise vs. F do a much better job of that.
Once we get past the first hurdle of developing a standardized test and have tested the bulk of the headphones needed, there will certainly be room and time to make closer and individualized measurements of headphones to draw out some finer detail on a case by case basis.
At the moment I'm thinking that a 90dB THD+noise Vs. F is definitely needed. I'd like to do the measurements at 100dB, but I worry about some headphones not possibly becoming damaged, and it's certainly not an "approved" listening level. OTOH, it's nice to test the limits of things; seeing the curves at both 90dB and 100dB give a very good indication of power handling capability.
Looking forward to your comments on distortion measurements.