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Headphone Measurements Using Brüel & Kjær 5128 HATS

amirm

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This is an investigation thread into feasibility of ASR adopting the Brüel & Kjær Type 5128 "high-frequency" Head and Torso Simulator (HATS). It is on kind short-term loan from the company. As configured, it costs around US $41,000. This is quite a bit more money than competing solution. The premium comes from the 5128 benefiting from a more accurate representation of our hearing system. How much difference this makes in real life is something we need to determine in short order before I have to return the unit in four days.

Not that it matters but I like the somewhat friend/crash dummy look of the 5128 :)
Brüel & Kjær Type 5128 HATS Headphone Measurement.jpg


There is an external CCLD signal conditioner model 1704 which can operate on internal battery for USB. Using the latter, I saw a noise spike in upper band of the audible spectrum. Not an issue for frequency response measurements but will impact distortion data. So for that, I will be using it on battery.

One of the challenges with this fixture is that in higher frequencies it invalidates and is compatible with other measurement systems that are compliant with the so called "711" standard. Sadly, Harman target curve is not yet available either but we need something to know what the measurements mean. Without such, we would have no idea what is the property of the headphone, the measurement fixture or both.

I have been working with our resident headphone metrologist, ahd headphone technology researcher, @Mad_Economist, who has crunched a bunch of spreadsheets and models to provide two different potential target curves, the Harman one and the diffused field as provided by BK. Here it is in his words:

----------------
This file should hopefully contain everything needed to understand how this works and nothing not needed for that. To give a narrative account of how this worked, to start with I interpolated Oratory1990's Harman target data into the same divisions as the B&K data (R40 or 1/12 octave bands). I then subtracted the flat in-room baseline from the 2013 paper from the final preferred targets of the 2013, 2015, and 2018 papers to get the adjustments which were preferred on average by the listener panels.

In past, I'd have just added these to DF, since the DF-HRTF isn't that far away from the response you'd see in-room, but since I'd already done the work of making a spreadsheet with Chris Struck's methodology for in-room HRTF approximation, I decided to use that. I took the directivity data for the Revel F208 from your measurements of it, the RT60 and volume of the Harman listening room from Olive 2009, and the free and diffuse field HRTF data from the files you sent me, and plugged them into the calculator I made for Struck 2013's in-room HRTF/target inference.

As Sean has described the in-room measurement as consisting of an average of +30 degrees, -30 degrees (330 degree FF HRTF), and 0 degrees in past
index.php


I approached the in-room flat response of the 5128 two ways: first, I did an in-room response for each of the three angles and averaged them together ("Average of B+C+D" in workpage "5128 approximations of Harman"); IMO this is the closest approximation of what Todd actually did, since using three different positions gives triple weight to the indirect sound. However I additionally averaged the three free field HRTFs prior to processing through the Struck calculator to give a picture of what the flat response would be without factoring the reflections three times over ("In room flat from average of 30, 0, 330 deg FFs" in workpage "5128 approximations of Harman"). Unsurprisingly, this response was less smooth in the higher frequencies and preserved more HRTF features in fine detail.

I then applied the preference adjustments to these two "in-room baselines", and compared the results with the original Harman targets - this is in workpage "5128 Harman vs Harman". The notch like features of the 2018 adjustments strike me as odd and I have not seen any justification of them in detail from Olive, so I opted to present both the 2018/current versions and versions based on the much simpler filters in the 2015 paper in the "5128 targets" workpage.

If there's anything unclear or which you'd like elaboration on - or if you have any feedback - please let me know! I'm sort of taking this as it comes...
-------------------

I am very appreciative of his work around the clock in the last couple of days to compute this data. As he notes, the information is included in the zip file.

Note: these are draft measurements to test the fixture and for discussion. They are not formal measurements. I have performed no averaging of multiple measurements, nor playing with many headphone positions. I have a ton of headphones to measure and want to get the data out quickly. Stereo measurements are provided which should give some idea of variability although transducer differences will compound that issue.

The thread is for technical discussions only. Please don't voice objections, complaints, etc. This is a collective effort to evaluate this fixture and nothing more.

I will be measuring headphones one by one as I process them. If there is feedback on other things to do, I can include them in the ones not yet measured.

Let's see if we can figure this out everyone! Appreciate all the help in advance.
 

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amirm

amirm

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As promised, here is the HD-650 results:

Sennheiser HD-650 headphone BK 5128 Measurements.png


The two channels as measured are in red and green. They are pretty close to each other which is good.

We have to targets to compare it to. First one is in dashed blue and that is what @Mad_Economist has computed per intro. The other is BK supplied diffused field (dashed purple).

Re-measuring the same but having the Audio Precision software equalize the measurements using inverted Harman computed one we get:

Sennheiser HD-650 headphone BK 5128 Compensated Measurements.png


This indicates too little bass below 100 Hz or so. There is also lack of energy between 2 and 6 kHz as noted.

Inversely, there is too much around 7 to 10 kHz. Using a much earlier measurement that varied somewhat from this, I made this EQ:

Sennheiser HD-650 headphone BK 5128 Equalization Measurements.png


I was impressed by how much improvement it made. There was bass that I had never heard from HD-650 due to those two shelving filters. At normal listening levels, and across a couple of clips, I did not detect distortion either. Boosting though may cause digital clipping and there may be problems with other content.

The other two filters made impressive difference in bringing out detail and getting rid of bit of high frequency harshness (at least this is what I remember from a couple of days ago).

This is my confirmation method by the way on whether 5128 measurements are correct. If we act on them with equalization and the sound improves, then they must represent the truth.

Lots more experimentation is needed to determine the above but thought I share what I have for discussion. No, I have not tried Oratory EQ. Just no time. In general i am not a fan of many filters for equalization which he seems to be using. Real filter implementations have ringing and other artifacts that keep them from working as good as it seems on paper.

Anyway, the mission begins! :)
 
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amirm

amirm

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Here is Hifiman HE-400i. I am not annotating these graphs in the interest of time:

1597529908163.png


Compensated:

1597529963949.png


What do we think? Is that peaking between 6 and 10 kHz real?
 

MZKM

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Re-measuring the same but having the Audio Precision software equalize the measurements using inverted Harman computed one we get:

index.php
To compare against other Harman compensated graphs:
Oratory1990
154B9409-8789-4F66-AEB9-739DBF48397D.jpeg


RTINGS
A91F45A0-8B25-4BED-BCCF-59ECF58069AD.jpeg


Crinacle
graph 7.png


So yeah:
Bass starts to lose presence starting @ ~150Hz
~200Hz is emphasized

They all also show a deemphasis in the treble, but it varies with frequency.

We also of course don’t know unit-to-unit consistency.

____________

This is my confirmation method by the way on whether 5128 measurements are correct. If we act on them with equalization and the sound improves, then they must represent the truth.

People should be aware of that, especially with the in-ear target, the Harman target may not be your personal target. To check, you can EQ based off the Harman compensated measurement and use this perceptual sine sweep file from AudioCheck (check out the rest of his site too), if any frequency band sounds louder/softer, that means you still need to fiddle with the EQ as your personal target isn’t an exact match (and/or unit-to-unit variance).
 
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amirm

amirm

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Sony MDR-7506

1597530610803.png


I matched levels at 200 Hz as you see. Strange that the two channels don't match at any other frequency!

Here is the compensated:

1597530774687.png


This is how the Sony did in Harman research:

1597530796292.png


Note that the vertical scale is much more compressed (80 dB versus my 50 dB). We once again have that peaking between 8 and 10 khz which is not in Harman measurements in the same manner. Are we getting more accuracy or false reading???
 
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amirm

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AKG N700NC which produced one of the highest preference scores and compliance with Harman target:

1597531267327.png


And Harman curve compensated:

1597531284094.png


Bass response matches harman curve quite well giving us confidence there. Above 3 kHz though, response shadows Harman but at elevated levels. I should do some EQ listening tests to see if it sounds better with that region reduced.
 

Mad_Economist

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To compare against other Harman compensated graphs:
Oratory1990
View attachment 78335

RTINGS
View attachment 78336

Crinacle
View attachment 78338

So yeah:
Bass starts to presence starting @ ~150Hz
~200Hz is emphasized

They also show a deemphasis in the treble, but it varies with frequency.

We also of course don’t know unit-to-unit consistency.
Note that the only sites publishing truly Harman data are those with GRAS 43AGs or other GRAS gear - Oratory's private measurements, Brent Butterworth's measurements if you compensate them, Keith Howard/HeadphoneTestLab, Speakerphone/ClarityFidelity. Crinacle and RTings' "Harman" targets are also extrapolated.
 

Mad_Economist

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@amirm to check, which of the four "Harman-based" targets from my spreadsheet are you using here? The overall level around the treble is a major differentiation point of the approaches - my default suggestion would be the "approximated Harman target drawn from one in-room orientation consisting of a three FF-HRTF average, 2015 preference adjustments", FWIW.

Edit: As a reference point for folks who don't want to open the spreadsheets, these are the six targets included in the .xlsx:
targets.png
 

MZKM

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Is the compensation different per ear? Once compensated I am seeing differences in channel matching, like in post #6 where you say you level matched at 200Hz, yet in the compensated graph they are no longer level matched at 200Hz. Or are you not level matching between charts?
 
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amirm

amirm

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AKG K92. The two channels were massively out of balance. Wonder if this is behind people saying imaging changes between headphones. I compensated for it:

1597532098261.png


Compensated:

1597532130999.png


What a mess! I don't like listening to these at all but they are comfortable to wear.
 
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amirm

amirm

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@amirm to check, which of the four "Harman-based" targets from my spreadsheet are you using here?
Oh, I hope I am not using the wrong ones! These are the file names for the two you gave me:
MAD DF-HRTF
Mad calculated DF target (I am calling this "Harman").
 

Mad_Economist

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Oh, I hope I am not using the wrong ones! These are the file names for the two you gave me:
MAD DF-HRTF
Mad calculated DF target (I am calling this "Harman").
If you aren't using the files from the most recent .xlsx, they're the wrong ones - I mean, the DF is still the same, but the Harman target won't be. My apologies for making that insufficiently clear in our DM, that likely accounts for some of the delta we're describing here.

Should I get them into the AP format and attach them here?
 

LearningToSmile

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Very interesting measurements. The channel mismatch is somewhat drastic at times.

I wonder if EQing will result in better listening experience every time(though I can't come up with a reason why wouldn't it). I might be in the minority, but my own experiments in EQing my headphones to match the harman curve using oratory1990 presets resulted in a deeply unpleasant experience.
I was impressed by how much improvement it made. There was bass that I had never heard from HD-650 due to those two shelving filters. At normal listening levels, and across a couple of clips, I did not detect distortion either. Boosting though may cause digital clipping and there may be problems with other content.
This is something I'm interested in, would it be possible to measure distortion after EQ? I wonder if you could really just EQ every headphone with significant bass roll off with no significant downsides.
 

Cahudson42

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Personally, I can't see from all his how you can possibly decide to spend $41000 on this, or not.

I'm not sure even another week would provide enough info - maybe. But why not ask? If not, return it, explore alternatives - then revisit.
 
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