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Author Topic: (Harmonic distortion discussion) SPLIT from original Beyer DT880-2003 thread  (Read 3986 times)

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stv014

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Re: (Harmonic distortion discussion) SPLIT from original Beyer DT880-2003 thread
« Reply #20 on: September 18, 2013, 08:11:40 PM »

BTW I think your DT-880 measurements are messed up too.

Well, that is not impossible, maybe the microphone "corrects" the distortion by having second order distortion with inverse phase relative to the headphone drivers. At high SPL and high frequency, the measurement definitely does become microphone limited (if I move the microphone away from the drivers, so that it does not receive as high SPL, the distortion drops significantly). However, the result seems to be fairly consistent with changing frequency, level (graphs will be shown soon), and even with different headphones I still get low distortion.

One thing I cannot guarantee is that the SPL is correct. I simply used Tyll's sensitivity measurements as a reference, so that I only needed to measure voltage, which I can do accurately enough.

Quote (selected)
You are getting way too low second harmonic for the DT880 driver or any dynamic driver. In my years of experience, I don't recall seeing any driver have a second harmonic at -68db at any reasonable volume.

It is actually fairly consistent with the HeadRoom distortion measurements, at least as far as the 2nd harmonic is concerned. For some Sennheiser headphones, HeadRoom in fact shows even lower distortion.

Quote (selected)
I know Tyll's IF measurement ~0.3% @1kHz is THD+noise, but THD+noise is typically only a little be higher than THD by itself, not a magnitude higher.

0.3% THD+N at 90 dB SPL is 39.5 dB distortion + noise SPL. Without A-weighting (and THD+N measurements are usually unweighted), it can plausibly be entirely noise. Especially if the possibility of inaccurate SPL (due to the headphone not having perfectly flat response) is taken into account, so it may even be a few dB less than 90.

As an example, the THD+N (unweighted 20 Hz to 20 kHz) of the same tone that I plotted earlier to show the -68 dB distortion is about 1%. Of course, that was recorded with a crappy Behringer measurement microphone in a "normal" room, but even with better equipment it is not easy to get the noise at 90 dB SPL to under 0.1% without weighting.

The fact that the InnerFidelity graphs so often show less THD+N at 100 dB SPL than at 90 dB also makes it likely that the lower SPL measurement is actually dominated by noise.

Quote (selected)
Subtract the noise, and the THD is likely to be ~0.15 to 0.25%, with the second order distortion being similar to THD (since second order dominates), not your 0.04%.

Of course, if there is actual high THD, then adding the noise will not increase it as much. But the point is that even the noise alone can already produce a relatively high THD+N, so it masks any really low THD. Even for your example of 0.25->0.3% increase, 0.166% noise is needed.
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ultrabike

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Re: (Harmonic distortion discussion) SPLIT from original Beyer DT880-2003 thread
« Reply #21 on: September 18, 2013, 08:54:15 PM »

One thing I cannot guarantee is that the SPL is correct. I simply used Tyll's sensitivity measurements as a reference, so that I only needed to measure voltage, which I can do accurately enough.

Maybe that is part of the problem. There is sensitivity variance in same make/model headphones. Consider Innerfidelity Beyer 70 2 sets of measurements which hit 90 dB SPL at 0.099 Vrms (0.03 mW), and 0.073 Vrms (0.02 mW). I think that's about 1.76 dB, but 2 headphones is a small sample space to get a feel for the variations. Furthermore, some headphone models might have more or less variation.

It is actually fairly consistent with the HeadRoom distortion measurements, at least as far as the 2nd harmonic is concerned. For some Sennheiser headphones, HeadRoom in fact shows even lower distortion.

I know Tyll's IF measurement ~0.3% @1kHz is THD+noise, but THD+noise is typically only a little be higher than THD by itself, not a magnitude higher.

I think HeadRoom displays the distortion contribution of a single 500 Hz signal, while I think Innerfidelity (Tyll's newer approach), Marv and some others use sinusoidal sweeps to derive THD+N. In these cases THD+N might not just be the contribution of a 500 Hz tone (missing the 250 Hz, 125 Hz and so on contributions which may or may not be negligible). Here is a hopefully useful Linky.

Cheers.
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stv014

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Re: (Harmonic distortion discussion) SPLIT from original Beyer DT880-2003 thread
« Reply #22 on: September 18, 2013, 09:01:54 PM »

I have created some THD vs. frequency graphs:

No smoothing
Smoothed version (still affected by noise)
Frequency response

A DT770 measurement is also included for comparison. Note that this headphone is slightly (2-3 dB) more sensitive, so the SPL is higher, even though the voltage of the sweeps was the same.

The frequency range is limited to 22-6000 Hz. Treble is not shown, because I do not have suitable equipment for making usable headphone measurements in that range. Also, there is in fact less bass roll-off, partly because the microphone adds some roll-off too (a few dB at 20 Hz IIRC), and partly due to the lack of a good seal (in the best case, I get up to +6 dB bass boost on the 770). Distortion and FR graphs were generated from the same sweeps, but the former have even lower usable frequency range, because above 2 kHz D3 already falls outside the relatively flat 6 kHz bandwidth. THD products up to the 3rd or 5th harmonic are included, depending on the SPL. Distortion was calculated as total power of harmonics / total power of fundamental + harmonics (so it can never be higher than 100%).

Edit: here is also an older measurement of the same DT880. This shows somewhat more distortion, and some microphone placement related problems in the >1 kHz range. The difference could be caused by a number of factors like positioning/seal, pad wear, amplifier (it had much higher output impedance then), or even burn-in.  :)p17
« Last Edit: September 18, 2013, 09:41:23 PM by stv014 »
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stv014

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Re: (Harmonic distortion discussion) SPLIT from original Beyer DT880-2003 thread
« Reply #23 on: September 18, 2013, 09:19:35 PM »

Maybe that is part of the problem. There is sensitivity variance in same make/model headphones. Consider Innerfidelity Beyer 70 2 sets of measurements which hit 90 dB SPL at 0.099 Vrms (0.03 mW), and 0.073 Vrms (0.02 mW). I think that's about 1.76 dB, but 2 headphones is a small sample space to get a feel for the variations. Furthermore, some headphone models might have more or less variation.

Yes, that is quite possible. Although my -10 dBV level assumes 94 dB/mW sensitivity, which is already 2 dB worse than the manufacturer's specification. But it could still be quieter by a couple of dB.

Quote (selected)
I think HeadRoom displays the distortion contribution of a single 500 Hz signal, while I think Innerfidelity (Tyll's newer approach), Marv and some others use sinusoidal sweeps to derive THD+N. In these cases THD+N might not just be the contribution of a 500 Hz tone (missing the 250 Hz, 125 Hz and so on contributions which may or may not be negligible). Here is a hopefully useful Linky.

If it is an actual sine sweep (only one frequency playing at any time), like what I have used, then it should not really make a difference. For multi-tone tests where multiple frequencies are played at the same time, I think the frequencies are chosen such that the distortion products will not overlap (which might not be easy once IMD is also taken into account).
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ultrabike

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Re: (Harmonic distortion discussion) SPLIT from original Beyer DT880-2003 thread
« Reply #24 on: September 18, 2013, 11:48:55 PM »

Yes. The sweep seems logarithmic and more than likely will and should not overlap. Besides the sensitivity variations and SPL settings I'm not sure. Maybe the number of FFT bins and/or windowing. Could also be rig differences like you said.

The thing that bothers me about noise (THD+N vs THD) being an issue though is that in Marv's plots, D3 and D4 are very low at 1 kHz... unless this noise is affecting even harmonics only at 1 kHz which brings it back harmonic distortion instead of noise floor.
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stv014

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Re: (Harmonic distortion discussion) SPLIT from original Beyer DT880-2003 thread
« Reply #25 on: September 19, 2013, 10:18:35 AM »

The thing that bothers me about noise (THD+N vs THD) being an issue though is that in Marv's plots, D3 and D4 are very low at 1 kHz... unless this noise is affecting even harmonics only at 1 kHz which brings it back harmonic distortion instead of noise floor.

I was referring only to Tyll's graphs regarding THD+N vs. THD. Obviously, when an FFT plot shows the actual distortion, then it is not just noise. I do not doubt that Marv's plots really show high distortion. On the other hand, a typical Tyll's plot like this HD650 one

shows low THD+N at 100 dB SPL (only about 0.1% in the midrange), and the graph becomes noticeably "fuzzy" when the THD+N level is low. This makes one suspect it is in fact mostly noise above 300 Hz, and at the presumably less demanding 90 dB SPL the driver actually has less than 0.1% THD.
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Solderdude

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Re: (Harmonic distortion discussion) SPLIT from original Beyer DT880-2003 thread
« Reply #26 on: September 19, 2013, 10:00:04 PM »

I really do agree with Marv's remark about the drivers being different.
The DT880 2003 might have very different drivers (and thus measurements ?) as newer versions as one expects progress over the years and while bringing out different looking models over the years.

For all we know ALL measurements are (very close to) being correct and we are talking of different headphones with the only real common factor of both having 'Beyerdynamic DT880' on it.

THD will always show values (slightly) higher than the highest harmonic peaks and they will always be FAR above the noise limit of headphone amplifiers but might very well be near those of microphone amps.
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