HBK Headphone Measurement Talks from Head-Fi and Sean Olive

[metal571]

Pad wear can make a big difference on headphones, for instance this is what Oratory measured on one of my K702's with old vs new pads on the same headphone, he said to imagine the red line measurement was inaccurate below around 60Hz and it instead would track the same difference to the green line without crossing it (a measurement error below 60Hz for the red line), so it's basically a 2dB Low Shelf from 1000Hz, so the older pads make it sound warmer (more bass) which would be similar for your AKG headphones you're talking about, so you'd have to take pad wear into consideration between the two headphones (and anyway Q701 and K701 are not the same headphone so not sure how you can compare bass levels in terms of concluding Chinese vs Austrian differences):
View attachment 158197
There also can be quite a lot of unit to unit variance between at least the K702 headphones, so that's another variable that could affect your Chinese vs Austrian AKG comparisons - ie make it less valid potentially, following is the frequency response of my two Chinese K702's with new pads:
View attachment 158198
So I think it's not easy for you to drill down to say it's a Chinese vs Austrian thing or even how the headphone specifications may or may not have changed through the years - there's too much variation due to pad wear & unit to unit variation to draw such fine conclusions in my view.

I was simply pointing out my experience with the units I owned. It is exactly for the reasons you just pointed out that it is not possible to paint every K701, Austrian or not, as more bass light or the same bass level wise as every Q701 of course. I probably wasn't clear enough.

Incidentally, to this day I still have no idea what the exact difference is between K701 and K702 other than colorway and removable cable. Unit variation alone is enough to make graphs seem to skew one as brighter than the other and vice versa.

 

[Robbo99999]

I was simply pointing out my experience with the units I owned. It is exactly for the reasons you just pointed out that it is not possible to paint every K701, Austrian or not, as more bass light or the same bass level wise as every Q701 of course. I probably wasn't clear enough.

Incidentally, to this day I still have no idea what the exact difference is between K701 and K702 other than colorway and removable cable. Unit variation alone is enough to make graphs seem to skew one as brighter than the other and vice versa.

Right, I see where you're coming from. About K701 vs K702, I was curious about this a few months ago & did a comparison of K702 vs K701:

I did this comparison before Oratory measured my 2nd unit of K702, so for K702 this is an average of my Unit 1 K702 & Crinacle's K702 measurement, which is then compared to Crinacle's K701 measurement. They're very close, within unit to unit variation of my K702 it seems! I think the K701 have flat pads though don't they? I know of course the K702 have angled pads. (Ha, I don't want to accept they're the same headphone!)

 

[ADU]

Depends on which K701 it is. I specifically sought an early Austrian made K701. Changes to production over the years and use of different foam under the pads are highly likely. I know for absolutely certain that what my ears heard between my Chinese made Q701 and my Austrian K701 was a noticeable difference in bass quantity. Graphs are graphs of a *single* unit. They cannot represent every revision made to a model that has been manufactured for over a decade now.

Howdy, and thanks for your clariification on this, metal571.

Since you've used these (and are undoubtedly much more familiar with their design than I am), I wondered if you might have any thoughts/impressions on the discrepancies in the K701 measurements on the two different rigs shown below, specifically in the lower frequencies. And whether that might possibly be due either to the angled shape of the pads, or possibly some leakage around the cheek or other areas on the 5128.

Maybe the pads on the K701 are stiff enough that they would not seal so well on the more curved/bumpy surface of the 5128 mannikin head. And perhaps that could explain at least some of the difference on this particular headphone. Or perhaps the swivel mechanism on the cups could be a factor?

I'm assuming that these measurements were made using the same pair of K701's btw. Probably within in a close time frame (iow, not long enough for add'l pad wear to accumulate between the two readings). So this is why I ask, in spite of your observations about the different manufacturing locations, and possible revisions.

 

[Deleted member 16543]

Hello @Mad_Economist ,

We are not speaking of an earless system. We have our own ears.

I read your previous posts and the linked AES Floyd E, Toole paper, “THE ACOUSTIC AND PSYCHOACOUSTICS OF HEADPHONES”. My impression from the Floyd E, Toole paper that the artificial ear becomes more important with the addition of variables beyond Frequency Response alone.

Let me point out that when we are talking about target curves we are speaking of Frequency Response and transfer functions not spatialization or other variables. Harman research tells us that the most important variable is Frequency Response.

I do not believe that the Harman Target Curve addresses anything other than Frequency Response. An artificial ear need not apply. We do not need a HATS, we are not creating 3D artificial space.

Thanks DT

If this is not fun I will stop.

Let me ask just some simple questions that immediately pop into my mind every time I read a post like yours:
Why a flat surface? What makes a flat surface the 'right' one? Why not a hemisphere, or a concave surface, or any other of the infinite possible surface choices that contour to the headphone pads?
Actually, while we're at it, what makes the headphone pads touching the surface a necessary requirement?
Why not a measurement mic 1m far from the pad, speaker style?

 

[DualTriode]

Let me ask just some simple questions that immediately pop into my mind every time I read a post like yours:
Why a flat surface? What makes a flat surface the 'right' one? Why not a hemisphere, or a concave surface, or any other of the infinite possible surface choices that contour to the headphone pads?
Actually, while we're at it, what makes the headphone pads touching the surface a necessary requirement?
Why not a measurement mic 1m far from the pad, speaker style?

Hello,

Why a flat surface?

It is simple, easy, reliable, uniform, Floyd E, Toole did it and GRAS sells a GRAS 45 CA flat world version test fixture. The other type surfaces you mentioned would also work.

Why touching the surface?

A seal is required; sound is a pressure wave, more so at low frequencies. If there is no seal, pressures do not build up and SPL is attenuated. Same story for one meter away. Inside the headphone cup sealed to the flat world test fixture sound does modulate the pressure inside.

Look up the GRAS microphone I referenced it is a pressure type microphone.

Thanks DT

 

[Deleted member 16543]

O

Hello,

Why a flat surface?

It is simple, easy, reliable, uniform, Floyd E, Toole did it and GRAS sells a GRAS 45 CA version test fixture. The other type surfaces you mentioned would also work.

Why touching the surface?

A seal is required; sound is a pressure wave, more so at low frequencies. If there is no seal, pressures do not build up and SPL is attenuated. Same story for one meter away. Inside the headphone sealed to the flat work test fixture sound does modulate the pressure inside

Look up the GRAS microphone I referenced it is a pressure type microphone.

Thanks DT

Ok then.
Build two rigs, one with a flat surface and one with any of the other surfaces that, in your opinion, would also work.
Then EQ a few headphones with both rigs.
You will find that the correction EQ is not only different (that's the obvious part), but also that for different HPs the delta between the correction EQs is not the same, but rather it varies depending on the HPs in consideration.
How do you explain that?

 

[DualTriode]

I build the flat one you build the other.

I will test them both.

I would expect there to be different curves for the different shapes but not so much with the different headphones.

Thanks DT

 

[Mad_Economist]

Still, my question stands: why not a univocal transfer function between the two rigs? At least for iems, since we seem to at least agree on the fact that the pinna imprints directional information to the sound waves hitting it.

Because a single dimensional transmission still has impedance interactions - the two systems have different ear Z, and in the case of IEMs, the acoustic impedance of the ears is actually likely to be fairly significant.

We are not speaking of an earless system. We have our own ears.

Yes, but you cannot make a measurement of a headphone with an earless microphone system and extrapolate the eardrum behavior on your head is my point...

My impression from the Floyd E, Toole paper that the artificial ear becomes more important with the addition of variables beyond Frequency Response alone.

If there are variables beyond frequency response of note in headphones, that's...within the domain of contentious science. The artificial ear (both the impedance simulator and the pinna) is highly significant for frequency response behavior, howevwer. You can make some generalizations about how an earless, coupled microphone measurement will differ from a measurement with a pinna, but each headphone-ear system is distinct in how it differs from the headphone coupled to a microphone...

 

[Deleted member 16543]

I build the flat one you build the other.

I will test them both.

I would expect there to be different curves for the different shapes but not so much with the different headphones.

Thanks DT

I know that's what YOU would expect, but it's not what happens.

You can ask @solderdude. He can get accordance only up to a certain frequency (I believe 1 kHz or so, if I remember correctly).

 

[Deleted member 16543]

Because a single dimensional transmission still has impedance interactions - the two systems have different ear Z, and in the case of IEMs, the acoustic impedance of the ears is actually likely to be fairly significant.

Sure, but if there was no directional information imparted by the canal at all, would you not expect the different Z to be different always by the same amount?

 

[Mad_Economist]

Sure, but if there was no directional information imparted by the canal at all, would you not expect the different Z to be different always by the same amount?

Yes, it is, but the IEM has a source impedance, and the effect on transfer is the result of both source (IEM) and load (ear) Z.

 

[Deleted member 16543]

Yes, it is, but the IEM has a source impedance, and the effect on transfer is the result of both source (IEM) and load (ear) Z.

I remember you being on the side of skepticism when it comes to load impedance meaningfully affecting the response at the capsule. Maybe I don't remember correctly, maybe you changed your mind. Doesn't matter..
Am I interpreting what you're saying correctly if I summarized it like this:

Shape of canal is important, but not because directional behavior of the wave inside of it as much as the difference of source/load impedance pairs, which is unique for each headphone model?

 

[Mad_Economist]

I remember you being on the side of skepticism when it comes to load impedance meaningfully affecting the response at the capsule. Maybe I don't remember correctly, maybe you changed your mind. Doesn't matter..

For circumaural designs, that's correct - I have never observed substantial apparent impedance interactions with over-ear headphones, which is most of what I measure.

For IEMs, (a section of) the ear is the entire air volume they're interacting with, and they're much higher impedance sources, so it's conceivable to me that there's an effect there. Particularly, some conversation I had with Vince Rey at B&K regarding the low-frequency behavior of the 5128 with sealing IEMs has pushed me towards the position that, for very small sealing microdrivers, ear Z matters (which matches with Oratory1990, who is much more experienced with sealed in-ear stuff than me, being much more interested in ear Z than I am ).

Am I interpreting what you're saying correctly if I summarized it like this:

Shape of canal is important, but not because directional behavior of the wave inside of it as much as the difference of source/load impedance pairs, which is unique for each headphone model?

Yes, with the caveat that a straight tube which achieved the same Z would be equivalent, if the IEM coupling and etc. were the same. This obviously isn't going to be the case in practice, and that's likely more work than making a geometrically accurate canal and using their four resonator ear sim, so I can see why B&K decided to have their cake and eat it too.

 

[ADU]

Some 5128 measurements made by the SoundGuys, for y'alls viewing pleasure. I think a couple of these were posted earlier, but not all of em...

AKG K371
Audeze Mobius
AudioTechnica M40X
Beyerdynamic DT-770 (80-ohm)
Beyerdynamic DT-880 (250-ohm)
Bose NC 700
Bose QC 35-II
Sennheiser HD 280 Pro
Sennheiser HD 6XX

And one of my more recent attempts at defining a target headphone curve for the 5128 rig, based on using the 5128 DF curve and sound power data. For possible comparison with the above...

Here is also an early version of the SoundGuys 5128 target, which I believe is based on averaging several good sounding 5128 headphone measurements together...

https://mk0soundguyshosprmrt.kinstacdn.com/wp-content/uploads/2021/07/sg-calc-jpg-1024x672.webp

Fwiw, the other target curves used on the above SoundGuys graphs do not appear as accurate or reliable to me.

I will point out, it isn't that hard to make a target for the 5128, but yes, if you're looking for something in the vein of the Harman work, there is no better alternative to the 45CA and its GRAS family for the moment (which is why, memory serving, I recommended it over the 5128 some time ago).

Where there is a will, there is usually a way.

 

[Deleted member 16543]

For circumaural designs, that's correct - I have never observed substantial apparent impedance interactions with over-ear headphones, which is most of what I measure.

For IEMs, (a section of) the ear is the entire air volume they're interacting with, and they're much higher impedance sources, so it's conceivable to me that there's an effect there. Particularly, some conversation I had with Vince Rey at B&K regarding the low-frequency behavior of the 5128 with sealing IEMs has pushed me towards the position that, for very small sealing microdrivers, ear Z matters (which matches with Oratory1990, who is much more experienced with sealed in-ear stuff than me, being much more interested in ear Z than I am ).


Yes, with the caveat that a straight tube which achieved the same Z would be equivalent, if the IEM coupling and etc. were the same. This obviously isn't going to be the case in practice, and that's likely more work than making a geometrically accurate canal and using their four resonator ear sim, so I can see why B&K decided to have their cake and eat it too.

Got it. So if for circumaural HPs the effect is, in your opinion, negligible, I expect you think that an even lower source impedance like that of a speaker would be even more negligible, right?

But there has to be some kind of effect, be it the canal impedance or the turbulent behavior of air particles inside the canal, or who knows what else. Differences have been measured, with speakers at different positions, and attributing all of them to positioning errors, especially when particular attention has been given to minimizing it, seems like a sweeping inconvenient data under the rug kind of thing, to me.
Besides, if canal impedance doesn't matter that much for circumaural HPs, a universal transfer function among rigs (just for that type of HPs) would have been found. But it wasn't. It seems like the same 'different difference with different HPs' factor is still the outcome.
Any idea why?
This is just an academic interest for me, by the way. I think the B&K approach of having their cake and eat it too is the most reasonable. Why worry how good you can mimic the behavior of the canal with conventional shapes, when you can make an anatomically accurate one? Time for them to move on to finding that target response. Enough, already.
Anyway.. time for bed. Always good talking to you.

 

[ADU]

Pad wear can make a big difference on headphones, for instance this is what Oratory measured on one of my K702's with old vs new pads on the same headphone, he said to imagine the red line measurement was inaccurate below around 60Hz and it instead would track the same difference to the green line without crossing it (a measurement error below 60Hz for the red line), so it's basically a 2dB Low Shelf from 1000Hz, so the older pads make it sound warmer (more bass) which would be similar for your AKG headphones you're talking about, so you'd have to take pad wear into consideration between the two headphones (and anyway Q701 and K701 are not the same headphone so not sure how you can compare bass levels in terms of concluding Chinese vs Austrian differences):
View attachment 158197
There also can be quite a lot of unit to unit variance between at least the K702 headphones, so that's another variable that could affect your Chinese vs Austrian AKG comparisons - ie make it less valid potentially, following is the frequency response of my two Chinese K702's with new pads:
View attachment 158198
So I think it's not easy for you to drill down to say it's a Chinese vs Austrian thing or even how the headphone specifications may or may not have changed through the years - there's too much variation due to pad wear & unit to unit variation to draw such fine conclusions in my view.

But some of what I'm saying here might be relevant for what @ADU is observing in the discrepancies of measurement he's pointed out.

As I appended to my last message above to metal571, I think that would probably depend on the time frame between the measurements, and whether they were done with the same pair of headphones.

I'm guessin that Harman probably made the two measurements shown below using the same pair of AKG K701's though. Probably over a fairly short space of time. If that is the case, then different amounts of wear, or the other issues related to revisions or manufacturing that metal571 touched on above would probably not be the issue in this particular case... Probably.

 

[pozz]

Got it. So if for circumaural HPs the effect is, in your opinion, negligible, I expect you think that an even lower source impedance like that of a speaker would be even more negligible, right?

But there has to be some kind of effect, be it the canal impedance or the turbulent behavior of air particles inside the canal, or who knows what else. Differences have been measured, with speakers at different positions, and attributing all of them to positioning errors, especially when particular attention has been given to minimizing it, seems like a sweeping inconvenient data under the rug kind of thing, to me.
Besides, if canal impedance doesn't matter that much for circumaural HPs, a universal transfer function among rigs (just for that type of HPs) would have been found. But it wasn't. It seems like the same 'different difference with different HPs' factor is still the outcome.
Any idea why?
This is just an academic interest for me, by the way. I think the B&K approach of having their cake and eat it too is the most reasonable. Why worry how good you can mimic the behavior of the canal with conventional shapes, when you can make an anatomically accurate one? Time for them to move on to finding that target response. Enough, already.
Anyway.. time for bed. Always good talking to you.

Only partially related. One thing to consider is that if wave propagation inside the eardrum was directional, then its transduction to the middle ear and onwards should have psychoacoustic effects. But there is no biological mechanism for detecting directional waves in the ear canal. Timing, sure, and magnitude. Even if you're right, the evolved mechanism seems to discard that additional information (which limits the variation from person to person).

 

[Mad_Economist]

But there has to be some kind of effect, be it the canal impedance or the turbulent behavior of air particles inside the canal, or who knows what else. Differences have been measured, with speakers at different positions, and attributing all of them to positioning errors, especially when particular attention has been given to minimizing it, seems like a sweeping inconvenient data under the rug kind of thing, to me.

Frankly, I don't think it would seem that way to you if you'd actually tried to do these sorts of in situ canal probe measurements. And if the canal is to be attributed significance at low frequencies, you need to contend with Middlebrooks and explain why under his method, which minimizes potential errors by measuring at two points simultaneously, there isn't anything of note under 14k...

Besides, if canal impedance doesn't matter that much for circumaural HPs, a universal transfer function among rigs (just for that type of HPs) would have been found. But it wasn't. It seems like the same 'different difference with different HPs' factor is still the outcome.
Any idea why?

Pinnae differ, and the ways that pinnae interact with a given headphone are individual to the pinna and the headphone (in the same way that the difference between my 0 azimuth, 0 elevation free field HRTF and yours, and the difference between my 45 degree azimuth, 45 degree elevation free field HRTF and yours is not necessarily the same). If you measure the response of circumaural headphones at blocked canals (both on humans and mannequins), you will still see variation which cannot be compensated with a static function.

 

[DualTriode]

I know that's what YOU would expect, but it's not what happens.

You can ask @solderdude. He can get accordance only up to a certain frequency (I believe 1 kHz or so, if I remember correctly).

Lots of firmly held opinion here, with an extra helping of emotion.

Evidence?

Thanks DT

 

[DualTriode]

O

Ok then.
Build two rigs, one with a flat surface and one with any of the other surfaces that, in your opinion, would also work.
Then EQ a few headphones with both rigs.
You will find that the correction EQ is not only different (that's the obvious part), but also that for different HPs the delta between the correction EQs is not the same, but rather it varies depending on the HPs in consideration.
How do you explain that?

Hello,

You have something fouled up in the thought process here.

We are going to start with a headphone, I have an AKG 712. Let’s put this on my GRAS 45 CA-9 test fixture and equalize it as close as possible to the Harman Target curve. At this point the equalization is locked with an armed guard posted.

Next we put this headphone with the fixed equalization on my flat earth test fixture with calibrated pressure microphones and sweep it with the now fixed equalized APx555 generator output.

We will now have a new flat earth target curve calibrated to the freshly calibrated flat earth test fixture.

Now we take a second set of headphones and place them on the flat earth test fixture and equalize them to the new flat earth test curve.

My premise is that this equalized second set of headphones will test very close to the Harman target curve on my GRAS 45 CA-9 test fixture.

Wrap your head around that.

Thanks DT