Headphone THD levels and EQ

[Jean-Paul Petrosyan]

Hi! I’m sorry if I’m posting this in the wrong section of the forums, but it seemed like the most appropriate one to me. And excuse me for this probably quite foolish question.

There’s a headphone I’ve been looking at for quite some time now that is attractive to me due to certain FR characteristics (as well as price and purported build quality), even though the overall tuning is… questionable.

I’m used to EQing my head/earphones, which is something I do with every pair I get anyway, so getting a set that basically requires EQ to sound good doesn’t seem like an issue to me in and of itself.

However, to my understanding, where drastic adjustments may be required, distortion can become a concern.

And, alas, this particular headphone doesn’t seem to perform well in that regard. Moreso, it seems like I might have to make significant boosts to the exact frequencies that show elevated distortion. But… might this impression be wrong?

Here I have to admit that I’m pretty much clueless in these matters still, and have a pretty feeble grasp on many related basic, yet crucial concepts. One of them being normalization.

When the frequency response graph is normalized to the target or compensation curve at different frequencies, the relative levels difference between the two changes across the frequency spectrum and hence EQ adjustments that you would make to match the former to the latter also differ in gain values. For example, if I go by default normalization setting and try to match the midrange to DF HRTF with a -1 dB per octave tilt, I have to boost ~250 Hz by ~7 dB and ~1500 Hz by 6 dB, while cutting 3000-8000 Hz by ~4 dB. But if I normalize to 500 Hz, I have to just cut the 3000-8000 Hz region by ~8-9 dB.

Intuitively, to my confused mind it seems like the first approach would lead to an increase in THD in the 150-2000 Hz range, while the second would only lead to a decrease in THD in the 3000-8000 Hz range without affecting the distortion in the low mids. But that doesn’t actually make much sense, does it?

The THD levels should end up the same in both cases, when normalized to the same SPL at the same frequency (say 500 Hz), right?

When the measurements for THD level are performed at various SPLs like 95 dB and 105 dB for example… Do those values typically refer to SPL at a single reference frequency like 1 kHz or 500 Hz, or do they represent a weighted average across the entire frequency range?

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The headphone’s FR graph is normalized differently between the measurements in the blog article and on the squig.link page, which seems to be one of the main sources for my confusion.

Is the THD measurement from the article more representative of the second picture, not the first - is that right? As far I understand, that would be the case if it was performed with a SPL of 95 dB matched at a single fixed frequency - 1000 Hz or 500 Hz (like the frequency response graphs from the same article would suggest?).

In short… Am I going to have a considerable increase in THD in the 200-2000 Hz range after equalizing the Austrian Audio Hi-X20 to conform to DF HRTF with a 1 dB/Octave tilt (or Harman 2018 filters - the difference should be relatively small in this range). Should I boost the 200-200 Hz range, or rather cut the 3000-8000 Hz range to achieve that goal - this should not matter?

Thank you very much for your time, patience and attention!

 

[AnalogSteph]

And excuse me for this probably quite foolish question.

Not foolish at all, actually.

There’s a headphone I’ve been looking at for quite some time now that is attractive to me due to certain FR characteristics [...], even though the overall tuning is… questionable.

So let me get that straight: You both like and dislike the stock frequency response at the same time? #logic

If you have to EQ things anyway, you might as well get something that is comfortable and has performance characteristics that lend themselves to EQ.

And, alas, this particular headphone doesn’t seem to perform well in that regard. Moreso, it seems like I might have to make significant boosts to the exact frequencies that show elevated distortion. But… might this impression be wrong?

For the most part, I would say it is... the only exception I noticed would be the ~1.5 kHz area.

The THD levels should end up the same in both cases, when normalized to the same SPL at the same frequency (say 500 Hz), right?

Yes. This only really matters in terms of digital headroom required, and even then any EQ worth its salt is going to have a pre-gain/pre-cut setting. So ultimately you can pretty much pick whichever reference level seems most convenient in terms of required number of PEQ bands and such, and then keep your levels in check with pre-cut.

When the measurements for THD level are performed at various SPLs like 95 dB and 105 dB for example… Do those values typically refer to SPL at a single reference frequency like 1 kHz or 500 Hz, or do they represent a weighted average across the entire frequency range?

That's a really good question. This may very much depend on who performs the measurement.

 

[Jean-Paul Petrosyan]

So let me get that straight: You both like and dislike the stock frequency response at the same time? #logic

If you have to EQ things anyway, you might as well get something that is comfortable and has performance characteristics that lend themselves to EQ

Thing is, barring distortion profile and level, that headphone's FR seems to lend itself to EQ quite well. Despite the overall tonal balance being quite disproportionate and upper treble having limited extension, going by both B&K 5128 and GRAS KB501X measurements it's FR seems to mostly lack sharp peaks and/or dips in the midrange and even treble (which are present in most other closed back headphones I've been looking at, like HD620S, FiiO FT1, Focal Azurys and Lensys, DT770 and Sony MDR M1) that I'd much rather not have to live with and which would be problematic to address with EQ. It also seems to have good channel balance in the upper mids and treble, as well as good fit consistency/low fit related variation (at least compared to something like FT1). EQ-ing treble can be a pain even when it graphs smoothly and the channel balance is tight, but when it's really uneven and varies significantly b/w channels and seatings, it becomes nigh impossible. And when you can't achieve more or less perceptually even upper mids and treble, it becomes so much harder to balance it with the remaining frequency range to achieve satisfactory sound...

That's a really good question. This may very much depend on who performs the measurement.

So there's no widely accepted conventional standard on this most people are most likely to default to in this regard? I wanted to ask the person who performed the measurements about this, but, alas, to my utter frustration I was unable to register on that platform. That would have helped to clear my confusion...

I think one of my main issued might be not understanding dB normalization that squig.link tool defaults to, as opposed to frequency normalization.

To my understanding, the frequency response graphs in the articles seem to be normalized to frequency. Looking at them, the 200 to 2000 Hz range on HD600 and Hi-X20 is reasonably close to each other and the compensation target.

But looking at them on squig.link that belongs to that same person, where they're normalized differently, the gap is way more drastic.

Lets say I wanted to match Hi-X20's midrange to that of HD600. Looking at them might give off an impression that I would need to make a considerably more significant boost to the former's lower mids to achieve that. It would need ~ 7 dB boost around 300 Hz and ~ 5 dB boost around 2000 Hz, exactly where the distortion spikes are located, as well as a ~3,5-4 dB boost from 450 Hz to 1500 Hz. Which would lead to an increase in distortion produced in that range. Without EQ, Hi-X20's seems to still have less distortion in the mids than many hybrid IEMs utilizing BA driver, to which I'm pretty accustomed to, so it wouldn't be all that concerning if not for an impression that I would need to make significant boosts in that range via EQ.

But that impression may be false, as I understand it.

Looking at the graphs in the articles, it would only need a ~ 4,5 dB boost at 300 Hz and a ~3,5 dB boost at 2000 Hz, though with more significant cuts to bass, upper mids and treble if I wanted to match across the entire frequency range. That might still lead to an increase in distortion, but not as drastic as it might have seemed when looking at the graphs on Squig.

But maybe if one normalization method and reference is more representative of the wide-band sound perception of an average human than the other, there might be a meaningful difference in practice between those approaches.

At the very least, it will be with regards to digital negative gain that should be applied to compensate for positive gain EQ adjustments and retain headroom. Here are two preliminary EQ profiles, they're somewhat crude and don't match perfectly in their results. But the difference in required negative digital gain seems to be more drastic that the differences in frequency levels across the range between the resulting curves would suggest.

This should have real implications in terms of source gear performance: analog amp stage power/gain requirements, as well as dynamic range, noise level etc (to my understanding).

Are there in terms of headphones performance, though?