Katz's Corner Episode 23: A Big Step Forward

I was planning on presenting the next episode in my "How Insensitive" series, but that'll have to wait till next time because a wonderful new toy has just arrived straight from the technical geniuses in Hong Kong. Today we're going to introduce you to a powerful new and affordable investigative tool: The MiniDSP EARS Headphone Jig, costing $179 USD. MiniDSP has been manufacturing audio-related DSP products since 2009. Led by charismatic Frenchman Tony Rouget, MiniDSP has made some innovative and affordable audio amplifiers, digital equalizers, DACs, calibrated measurement microphones and measurement tools.

I've got two of their equalizers working, one in Studio A, my Mastering Room, as a backup for my Acourate EQ/crossover, and one in Studio B as a monitoring EQ for the mixing room. Professionals have shied away from monitor EQ for a long time, and for good reason. Over the decades EQ has been more abused than properly used, but measurement techniques have come out of the dark ages, so with proper knowledge accompanied by proper acoustics, an EQ can be the final tweak on a precision monitoring room.

KatzCorner_Ep23_Photo_MiniDSP

Fig. 1 and Fig 1A: Mini DSP EARS calibrated test jig for headphone measurement

Likewise, it should be possible to equalize headphones to a target response, and to that end, MiniDSP has produced the EARS headphone measuring jig shown above, and a new companion DSP-based headphone amplifier, the HA-DSP ($325 USD). I've already put EARS to good use. In upcoming Katz's Corners I will describe EARS in detail, its strengths and weaknesses, and intended use. Today, I've put it to good use right away to help generate even better and more precise corrections for my Stax 007 Mk2 and my Audeze LCD-4 headphones. To paraphrase the folks over in Hong Kong, we're living in exciting times to have an affordable headphone measurement jig.

Bob's First EARS-aided Headphone EQs
We're not in Headphone Nerd Nirvana yet—as we're far away from being able to produce an accurate target response curve that we can use with EARS measurements for many reasons that I'll go over here and in future episodes. But in this episode we will produce some EQs that sound very, very good. So first it's time for some number-crunching, followed by (hopefully) some pleasurable listening! I used the EARS jig to measure my two great reference headphones, the Audeze LCD-4 and the Stax 007 Mk2. Since both these headphones are already very linear, we should hopefully succeed with a simple EQ shape.

Previous to getting the EARS, the EQs that I've produced by ear and reported these past months do sound very good. Let's see if measurements will allow me to make even better EQs. These measurements are corrected only with my raw capsule responses, provided specially for me by the folks at MiniDSP. They do not include any other kind of compensation curve. So the measurements should look "funny", they should look kind of like what we would expect from concha, pinnae and to some degree ear canal response, and they do.

KatzCorner_Ep23_graph_Fig2

Fig 2: Mini DSP EARS measurements, microphone response correction only: Audeze LCD-4 (red), Stax 007 Mk2 (turquoise). Average of 5 positions x 2 ears averaged together. Smoothed 1/12 octave.

Having two excellent open-backed planar headphones is a real asset. This allows me to compare what I already hear against what I can measure, and to examine common characteristics of the two phones. There is a lot of correlation between what I see and what I have heard. For example in Fig. 2 above, below 60Hz we observe that the Stax response is 2 to 2-1/2dB below the Audeze—which confirms my thoughts that the Stax is a bit deficient in the bass. Since the Audeze has excellent bottom end, it makes a good reference to shoot for. The Stax EQ I made by ear already has a bass boost, but now I can refine its shape since the EARS indicates the frequencies I need to work at and how much to boost to bring it in line with the Audeze.

The two headphones have very similar, nearly flat response from 70-500Hz, which indicates the quality of both these planar drivers. Above 500, the Stax response is more irregular, with +/- 3dB swings compared to only +/-1dB swings from the Audeze until about 4kHz. The Stax dip centered around 2.5kHz, where the Audeze is quite flat, clues me in to listen for presence issues, and I decided to implement a bit of compensation for that dip. But I left the other Stax irregularities alone for now.

We should expect a concha-based resonance in the upper midrange, but the narrow band goose in the Stax measurement circa 4.2kHz is troubling; I've not noticed it in previous listening, but recently after listening to a wide variety of "less pure" recordings on Tidal Masters, I'm suspecting that this 4.2kHz goose is real and needs to be tamed. There is an analogous rise in the Audeze, but with much wider Q and not as strong. I decide for the moment to ignore this feature, but watch out for it as it may be excited by certain pop material with strong information in the 4.5kHz range, especially female vocals, especially Linda Ronstadt!

I think the dip in the Audeze circa 6.5kHz is part of the EARS concha and will eventually make it into the EARS target response, as it resembles a dip measured by Tyll on the HATS, but at a slightly different frequency, as we will see in figure 5 in a moment. I hesitate to EQ this range up, as it could ruin the warmth of the Audeze. From 10 to about 15kHz the Stax "air" exceeds the level of the Audeze, and since I already know the Stax is a bit too bright in this region, I decide to do a little dip here. From 15k up the two cans are quite similar.

I have another pair of Stax pro headphones, made from pro elements placed in an SR-5 black and gold headshell. So I will measure them at another time to see whether the narrow 4kHz anomaly appears on that can.

Here are the "semi-automatic" EARS-aided eqs that I came up with, implemented in Equilibrium (Figures 3 and 4). Equilibrium is a very powerful plugin equalizer with many detailed adjustable shapes that allow me to create complements to the measurements.

KatzCorner_Ep23_graph_Fig3

Fig. 3: Stax "semi-automatic" EQ produced by examination of MiniDSP EARS measurement and integrating previous listening experience

Take a look at the Stax bass EQ, for example, with just one band in Equilibrium I was able to counteract the headphone's weaknesses, including complementing the slightly elevated response circa 60Hz and the little dip circa 40. Ordinary equalizers would have required three bands to implement that complex shape. To refine that shape, I used a technique: start with a dip and try to match the shape of the measurement, and then convert that to a boost. Next, the Stax did not sound quite warm enough to me, being a little hooked on the Audeze, so I added a bit of sugar with an 0.2dB boost at 250Hz.

Equilibrium provides a Butterworth shape, which looks like a flat top, which proved the perfect complement to the 2.5kHz issue. On listening to my reference recording of Lindsey Webster, the new 2.5kHz boost restored the missing presence and matched up the Stax's midrange to the more open quality of the Audeze, which I had deemed more correct. After some more careful listening, I reduced that 2.5kHz boost from +1 to +0.5 as it better matched the Audeze and the loudspeakers. Lastly, a little rolloff from 12 to 20kHz took the slightly "Hi-Fi" tizzy cymbals back in line. While listening, I reduced the strength of the 2.5kHz and 20kHz corrections compared to the EARS predictions and feel that my new EARs-aided Stax EQ sounds considerably better than I was previously able to engineer solely by ear.

KatzCorner_Ep23_graph_Fig4

Fig. 4: LCD-4 "semi-automatic" EQ produced by examination of MiniDSP EARS measurement and previous listening experience

Long ago I've concluded that the LCD-4 is an excellent headphone which really only needs some correction at extreme high frequencies to sound even better. So I decided to use only one EQ band. Figure 4, the LCD-4 semi-automatic EQ that I came up with after examining its EARS measurements, uses just a single high shelving boost in Equilibrium. Notice that the EQ I ended up with only has 1.2dB of boost by the time it reaches about 12kHz, although the EARS measurement tells us that we might need 6dB to match the Stax. I started listening with 6dB, which sounded very wrong. Even 2dB HF boost sounded just wrong to me with this headphone. It thinned out the critical lower midrange by the yin and yang effect, and made vocal fundamentals sound too thin. This reinforces that EQ'ing headphones is both an art and a science, and that the magnitudes predicted by our measurements are probably far more than we actually need.

Furthermore, I decided to make this band linear phase because when listening, I could hear the phase shift of a minimum phase EQ which was bringing the cymbals artificially closer to the ear, destroying the depth. An FIR equalizer like Equilibrium can make any or all bands be linear phase if we wish. Changing the high end to linear phase preserved the depth and in this case sounded more natural to me.

These two new EQs bring the best characteristics of the Stax over to the Audeze and vice versa. The transient impact of the Audeze, which was already great, has been improved. Snare drums pop, the sound is more lively and real in the Audeze. In the Stax, the improved 2.5kHz presence sounds quite right, I'm very glad that the EARS measurement clued me into that issue. The combination of the improved bass and treble makes the Stax an even more sweet headphone, yet it retains its electrostatic clarity and speed. It's now impossible to casually identify which is the electrostatic and which the planar-magnetic headphone, except for the physical weight of the Audeze.

Stax Polarity
I also discovered, with the aid of EARS, that the Stax were inverted, at least through the Mjolnir KGSS amplifier. In this comparison of the LCD-4 and 007 step responses (Fig. 2A), you can see that the LCD-4 pulse is "upward going" while the 007 is downward. My ears are not particularly sensitive to absolute polarity, but I certainly want to reproduce it correctly. I can perform the polarity inversion in JRiver, Acourate Convolver, or better, by rewiring the Mjolnir amplifier, so I took it to the bench and reversed the polarity coming into the attenuator, a simple operation that does not even require soldering. Perhaps this will improve the impact of bass drum, bass, and the depth of instruments—but it will be subtle. We cannot say if the cause of the problem is the Mjolnir Amplifier or if Stax headphones themselves have been inverted for all time! I'll do further investigation of the amplifier itself. Let's see what Spritzer has to say about this as well.

KatzCorner_Ep23_graph_Fig2A

Fig 2A: Mini DSP EARS measurements, step response. Audeze LCD-4 through Audeze Deckard amplifier(dark color), Stax 007 Mk2 through Mjolnir KGSS Carbon amplifier (pink). Notice that the 007's polarity is incorrect.

Now On to Tyll
Here's a comparison of Tyll's measurements of my serial number LCD-4 with his very expensive HATS jig versus my $179 EARS (Fig 5). The bass ranges are very similar, within about 1dB. Tyll's jig definitely shows greater excursions and resonances than the EARS from 400Hz on up. Another interesting difference is that many of the HATS ear-shape-related peaks and dips have been shifted akHz or more upward in frequency by the EARS. Still, if the EARS manufacturing QC proves consistent (we have yet to check on this), then this may become a valuable measurement jig suitable on its own merits.

KatzCorner_Ep23_graph_Fig5

Fig. 5: LCD-4 measured by Tyll with HATS (turquoise). By Bob Katz with MiniDSP EARS (red). Average of 5 positions x 2 ears averaged together, 1/12 octave smoothing. Each measurement is corrected using microphone capsule information furnished by manufacturer.

Tyll has generated his own first stab at a headphone target response which he has derived mathematically using measurements produced by his HATS in Harman Audio's calibrated listening room. He measured the flat-response loudspeakers at Harman with the HATS. Read Tyll's blog about it here. He hasn't been able to listen to this proposed target, so my job today is to create a filter that makes my headphones conform with his target. I'd also like to compare it with my own semi-automatic EARS EQ. Since this blog post, Tyll has done a bit of manipulation and set his Target to flat from 200Hz on down. I think this is a good idea, or for bass freaks, a tetch of bass boost that could easily be done with a user-controlled Baxandall EQ on top of the headphone EQ.

Here are the steps I went through to create filters for the LCD-4 that we can easily A/B compare:

For Tyll's HATS EQ:
1) Smooth and average Tyll's LCD-4 HATS measurements, average the L&R channels to a single value measurement. Performed in Room EQ Wizard (REW), Fig. 6. I think the ears react to tonality on a wider basis than the fine features we can measure, but the degree of smoothing needed is a matter of opinion. I'm currently using 1/12th octave smoothing "to be safe", but my experience shows that 1/6 octave is probably not overdoing it.

2) Calculate an EQ correction. Start with Tyll's target, which is the response he would expect a perfect headphone to measure on the HATS (orange trace in Fig. 6). Attenuate Tyll's LCD-4 measurement to yield 0dB center in the bass range (fat green trace in Fig. 6). Divide Tyll's target by Tyll's LCD-4 HATS measurement (trace arithmetic). This yields Tyll's computed LCD-4 EQ (red curve in Fig. 6). Performed in REW.

KatzCorner_Ep23_graph_Fig6

Fig. 6: Tyll LCD-4 measurement (fat green trace). Tyll target curve (orange trace). Tyll's calculated EQ (red trace). Measurements are average of both ears x 5 positions.

The "error" inherent in the LCD-4 would be the inversion of the EQ (red trace), that is, how far off the measurement is from the ideal response. The goal of the EQ should be an accurate-sounding headphone, we hope! Notice that this EQ is quite a complex shape that is impossible to exactly replicate in any standard equalizer. The only way to accurately produce such a complex EQ is by using convolution, that effectively has an infinite number of bands and can replicate any shape. In fact, Tyll's complex EQ would not have been possible to implement without the unique convolution technology that I am using. Convolution is an available option in JRiver Media Center and in certain special software processors like Acourate Convolver.

Acourate is a powerful analysis tool that can do about everything which REW can do plus much more. Acourate can import Tyll's calculated EQ as a text file and convert it to an impulse response that can be used in a convolver. So we can play this EQ in Acourate Convolver or the convolver built into JRiver Media Center. Dave Gamble, developer of Equilibrium, is considering being able to import an impulse response and turn it into an EQ setting, which would be a great challenge. Perhaps Equilibrium's 32 bands would be enough to approximate the EQ.

Convolution technology has only come of age with today's faster, multi-core computers. Some of the dedicated DSP chips (Analog Devices SHARC, for example) are fast enough to perform some small degree of FIR convolution, but you really need a multicore Intel chip to perform multichannel convolution with a large enough FFT frame to yield accurate low frequency response without ripple. A six-core Intel i7 is fast enough to allow me to run 12 audio channels in Acourate Convolver at 192kHz sampling with a ridiculously large 131,072 sample impulse width. This stresses the computer to the point where it's only practical to run a couple of applications at once. I need 12 channels to simultaneously compute 5.1 surround with a two way crossover to the loudspeakers and two different headphone EQs and I use a 16-channel Lynx AES/EBU interface to feed all of these destinations.

3) Export the predicted EQ as a text file from REW. Import this EQ into Acourate and convert it to a windowed impulse response at all the different sample rates and then into a filter that can be played in JRiver and/or Acourate Convolver. Actually, six different filters, one for each of the popular sample rates, since a good convolver should operate without any sample rate conversion.

For Bob's EARS EQ:
Convert Bob's semi-automatic Equilibrium "EARS EQ" into an impulse response that can also be used in Acourate Convolver. This is performed by playing a Dirac pulse through Equilibrium, capturing that pulse to a wav file, then windowing it into a filter in Acourate. In the future, Dave Gamble, author of Equilibrium, will allow direct export of Equilibrium's impulse response to a WAV file which could be used in a Convolver.

The Proof is in the Listening
Now, finally, we can listen and compare Bob's EARS-based semi-automatic EQ to Tyll's HATS-based correction! The difficulty in comparing two such disparate EQs is the loudness and the headroom issue. My EARS EQ only needs 1.2dB of digital attenuation to prevent any potential overload, probably much less since there is no acoustic music with full scale level information above 10kHz. But to be safe, let's say 1.2dB of attenuation. However, Tyll's EQ has up to 15dB of boost. To be safe, I'll attenuate it digitally by 15dB. It's a floating point correction file and will be dithered to 24 bits on the way out of either JRiver or Acourate Convolver. And I'll need to attenuate the EARS EQ until its perceived loudness matches that of Tyll's EQ.

OK, finally, I got to listen, and to be honest, I didn't need to listen to more than one piece of music to realize that there is something very wrong with Tyll's EQ. It sounds harsh, very thin, far too much 3.5kHz and far too bright. I tried my own variation on Sean Olive's Harman curve a while back, and it did have 12 or 13dB boost circa 4kHz. It sounded a bit wrong, but not at all as strident as Tyll's version, and I'm not sure of the reasons, but the inflection points are very critical, and if there is any kind of math error or measurement error, this type of curve can go very wrong. For example, even one dB error along the way with desired 3.5kHz rise can make the difference between accurate and harsh. So, it's back to the drawing board, guys.

Back to the Drawing Board?
Yes, we need to derive Nirvana, a headphone Target Curve, but I think it will have to be done with art, science, sweat, toil, and tears. As we saw from my experience with the EARS, the magnitudes predicted by measurement, especially above 200Hz, seem to be much stronger than our ear/brain demands. It's the wrong thing to expect simple frequency response measurements to match the response of distant loudspeakers in a room to that of transducers located inches from the eardrums.

Instead we need to interpret frequency response based on the ear's perception that transients in an earphone are far louder than they are in a room with loudspeakers. Increased transients make a sound louder and brighter. We'll have to weight the response measurements differently: neither free field nor far field response adequately predict how we react to headphones. Researchers should not be surprised at this conclusion, as we already know that nearfield loudspeakers need a different EQ curve than mid- and far-field.

We should praise Tyll for his amazing effort—there are a lot of things to conquer and it ain't easy, folks. I suggest that we go back to the drawing board and use a different approach to measure calibrated loudspeakers with the HATS. I invite Tyll to take HATS down to Florida. Tyll, we've got a spare bed for you, no problem! Here we will remeasure using a time-based FFT approach, find the frequencies and the amplitudes, and even so, expect to need far less correction for headphones than what the in-room measurement predicts, whether or not we perform anechoic windowing. To repeat: the psychoacoustics of a set of transducers located inches from the eardrum exaggerate transients and thus perceived high frequency response. The science has not progressed to the level of the art, so there will be a lot of human judgment involved.

In fact, Tyll has attempted to conquer a very thorny problem: measuring the response of a pair of calibrated loudspeakers using a dummy head. Tyll's method simply measured the level of a set of sine wave frequencies, which would exacerbate modal effects in the room, especially at low frequencies. Furthermore, his method is unable to separate the loudspeaker response from the room response, although the ear/brain is able to separate these at frequencies above the bass region. The ear/brain integrates the room with the loudspeakers at low frequencies, but listens almost anechoically at high frequencies, almost exclusively to the direct sound from the transducers. This means there is a discrepancy between ordinary measurements and the actual psychoacoustics of loudspeaker listening. I believe this quandary has only been solved by psychoacousticians such as Jim Johnston and designers such as Uli Brueggemann, inventor of Acourate. This is why I chose Acourate software to perform my room correction.

Traditional power response measurements with pink noise are out, because they do not separate the room from the loudspeaker. I think we do need to judge the frequency response above about 200Hz in a near-anechoic manner, especially since the headphone experience will be near-anechoic. Furthermore, we should only play the left or right speaker or we'd get comb filtering. The next issue is that a dummy head has two ears separated by the body of the head, so sound from the left speaker arrives at the right ear attenuated, delayed and colored. How do we integrate the perceived binaural frequency response of the two ears?

With Acourate, I can sum the impulse response of the left ear with the delayed response from the right ear in the time domain, but we really don't know how the brain judges frequency response of these combined signals. Does the brain judge frequency response in mono? Is it legitimate to perform a 100% time domain sum of left and right eardrum for our purposes? And then how do we reconcile this with the 100% separation of a pair of headphones? I suspect that perceived sum of left and right ear will be less bright than Tyll's single ear measurement produced. I asked psychoacoustician Jim Johnston about all these conundrums, who replied that this is a very hard problem for which he does not have an immediate answer! Back to the drawing board? How about "back to the psychoacoustic laboratory!"

Simplifying The Process
Life would be a lot simpler if we can create a headphone EQ in a standard equalizer—then we can export biquad coefficients directly to an outboard digital equalizer which can be inserted into our system via SPDIF. Many outboard equalizer brands are supported directly within REW, with some models having up to 20 filter bands. It is possible to automatically create an EQ in REW that can be exported to a digital equalizer, but only with a simple target designed for loudspeakers, not the complex shape that a headphone EQ would require. I can manually dial in my simple Equilibrium settings into REW, but a more complex EQ such as Tyll's could not be replicated with only 10 bands and I'm not sure if even 20 bands are sufficient. So for the moment a convolver is our best solution and JRiver is quite an affordable and powerful media player at $49.98. Fans of Tidal streaming can play Tidal's master stream with these EQs with a little work: I play Tidal's master MQA stream using Tidal's desktop app, at 44.1, 48, 88.2 or 96kHz from a Motu interface on a Mac and feed this via SPDIF into another interface on a PC where I can apply a headphone or loudspeaker EQ in Acourate Convolver. There is reportedly a way to play Tidal's app through WDM into JRiver but I have not explored it.

Acourate Convolver costs 126 euro, and can connect to JRiver or external sources. The catch is that AC only accepts its proprietary file format, which has to be exported from Acourate software, that costs 286 euro. That's an excellent price for a full-featured analysis program, but hard for hobbyists to justify. I'm happy to provide my readers wav filter files suitable for JRiver's convolver and AC files suitable for Acourate Convolver at no cost, but only for the headphones that I have already equalized.

Audeze has created a VST plugin called Reveal, which simplifies listening with a correction filter, but only for Audeze headphones. I'll be reviewing Reveal in the near future. Sonarworks purports to have commercialized and simplified the correction process, and they provide a headphone measuring service. I've been resisting Sonarworks for a long time on general principles, but no doubt at some time in the future I'll evaluate their approach, after I succeed in learning how to do it well myself, and showing you all how to do it, too!

(Editor's Note: Ha! Heck of a can of worms you've opened up, Bob. Love that you've decided to dig into it.

I'll be receiving a MiniDSP headphones test jig in the near future and intend to start my investigation by simply measuring a number of headphones and comparing the measurements with my HATS. I'm thinking the first step for me is to see if the differences between those measurements is constant or if it changes with various headphones. That should be interesting in and of itself.

Thanks for the invite, Bob, sure would like to take you up on it, we'll have to see if budget and/or time allows. In truth I think my time is best spent reviewing headphones in the way I always have. But I sure would like to find a target response curve that will be useful with the upcoming on-line headphone measurement and comparison tool. It will have the capability to use a variety of compensation curves so even if we don't get it right at the start we can always add new curves later.

To some extent, I think feedback from InnerFidelity readers will be the most important input in developing a target response. I intend to use my preliminary curve and then allow readers to comment on where they think the curve needs attention. It seems to me that it really is the attentive subjective listening experience of many people that will allow useful adjustments as opposed to some objective method for deriving a target response. We'll see.

Regardless of its absolute accuracy, I think the appearance of this affordable gadget will be a boon to the community as it will allow enthusiasts to compare headphone measurements in an apples-to-apples manner. I encourage readers who have an interest to follow along here as Bob and I play with this tool, and I'd also like to point out this terrific thread at superbestaudiofriends.org where some very experienced folks are beginning to compare the device with their personal measurement rigs. Great stuff!)

COMMENTS
thefitz's picture

It's so good, it doesn't detect any of the flaws certain headphones have on every other measurement rig.

detlev24's picture
Quote:

[...] after I succeed in learning how to do it well myself, and showing you all how to do it, too!

My initial approach will be to measure the Sonarworks individual calibration of my HiFiMAN HE-500 + the average one for the Focal Elear. Their target follows a very well translated 'B&K 1974 optimum Hi-Fi curve' and this is certainly a good point to start with. I will then try to recreate it on my RME ADI-2 Pro as accurately, as possible - to be independent from any software/-station.

At this point, let me quote "MC" of RME, who encountered some issues regarding measurements of IEM with the EARS: "We did the first tests and EARS basically works good. [...] But there is one really stupid problem with this system: the silicon ear is not modeled after a human ear. It is not possible to insert professional in-ears as the space is missing that their housing intentionally uses (to fix the in-ear mechanically). [...]"
https://www.forum.rme-audio.de/viewtopic.php?id=26346 #4

I hope miniDSP will fix it soon!

Furthermore, good crossfeed [as by RME, SPL, Meier Audio etc. - the latter is available as foobar2000 component(!) ] requires some more attention.

detlev24's picture

In case of an individual headphones calibration, which usually does correct for the L and R channels separately [#channel_imbalance]:

Crossfeed would naturally need to be applied before the FR correction curves, in the audio chain!

Regards

detlev24's picture

Following an example with foobar2000, of the DSP chain logic for individual headphones calibration [L + R channels follow their separate correction curves]:

https://picload.org/view/ddlprapa/individual_eq.png.html

RME's crossfeed is inspired by the 'Bauer stereophonic-to-binaural DSP'. For detailed information on both the implementations:

http://www.meier-audio.homepage.t-online.de/crossfeed.htm
http://bs2b.sourceforge.net/

Enjoy!

mnaganov's picture

The shift in Tyll's HATS vs EARS can be explained by different artificial ear canal lengths, which shift See for example this MSc thesis on ear canal modelling: http://lib.tkk.fi/Dipl/2008/urn012834.pdf, p. 44.

Which brings us to the question—if such variation exists between ear simulators, what about your own ears? It's not a secret that people's ear canal shapes are often different between the left and right ear. Not mentioning the variation of pinna shapes and sizes.

Thus, headphones equialized for a simulator will not be as flat for a real human. Ideally, what we need is to measure at the eardrum in each ear for this individual.

David Griesinger (ex. Lexicon / Harman researcher) shared a lot of experience on the topic of accurate binaural recordings, and it all comes to the need for individual EQ. Actually, he has another approach for it that is based on loudness matching, see https://youtu.be/1VaKZr208Dc.

Unless people do mastering like Bob, they don't really need to have a flat headphone response, simply using bass and treble tone controls seems to provide much more excitement. In Sean Olive's studies, people were always adding some bass and treble while listening to headphones equalized to Harman Target Curve. So why not just use the tone controls only?

detlev24's picture

We certainly cannot debate about personal taste but if somebody wants to listen to music as close as to what the mastering engineer's intentions were - then calibration to one of the common standards [B&K 1974, HARMAN target(s), Dirac Live etc., Bob Katz's target // btw., those are not flat or neutral!] makes sense. The personal flavor always can be added, since there does not exist 'the only one right way' to EQing.

Another example: the same instrument, identically tuned, will sound different to a listener concerning its surrounding space. In other words, the same orchestra playing the same symphony will sound differently in different concert halls [i.e., everything the same - just different rooms]. It is true, that if we split our body into L and R, our complementary body parts are never the same (ears, eyes etc.) but our brain compensates for differences. The mentioned orchestra will sound natural to one even if hearing on one ear differs significantly from the other ear [i.e., the acoustic space created by the orchestra, influenced by the room and surrounding the head will not adapt to anatomical needs]. Of course, it might be fun to have artificially added energy by compensating for anatomical "flaws"; but that would no longer be accurate/natural and rather be a subjective preference [e.g., elder people generally tolerate or like more energy in the high frequencies better than younger individuals - due to the normal loss of hearing with age].

About Audeze Reveal.
Is my impression right, that it offers only an average calibration profile to a certain series of headphones? If so and regarding the high variances amongst the same headphone model(s), i.a., in FR - independently of the price(!) - encountered so far; that might turn out not to be a sufficiently accurate offering for audio professionals + individual calibration might still be needed.

Regards

Bern L.'s picture

Thank you for the preliminary work in this area. As a user of Sonarworks (with the Sennheiser HD800), I think this is one area that has an extreme reward/investment for the headphone owner. I could never quite get the EQ manually tweaked to my satisfaction.

Bern

mindbomb's picture

Rather than try to hew to some arbitrary standard, I would be interested to see what would happen if you tried to capture a binaural impulse response of your speakers with the EARS and used that with a convolver and equalized out the headphone response. With additional tweaking with listening tests, I wonder how close you can get to replicating your speakers? Probably have to stuff some things in between the EARS to represent the head.

mnaganov's picture

You need a full head and torso for that. See Tyll's explanations here: https://www.innerfidelity.com/content/headphone-measurements-explained-f...

zobel's picture

All that matters in the whole scheme of things, especially for the vast majority of people who don't need or want to tweak the frequency response of their headphones, is...what headphones sound best to YOU. As mnaganov (above) points out, no measurement device will be anything but a ballpark approximation of what individuals hear...(for many reasons too many to start listing here)...finding cans that sound good to you first, and just tweaking tone controls, if desired, will get you down the road further than anything else.

Personally, I prefer to find headphones that are musical sounding, with no obvious faults, such as exaggerated or absent highs, lows, or uneven peaky response, that are full range, low distortion, dynamic, or in other words, authentic sounding.

No matter how much is said about finding a perfect frequency response curve for headphones, one doesn't exist. 'Accuracy' however, as measurable in waterfall plots, distortion analysis, and seen in square wave measurements, in my opinion, is the most valuable measuring stick, for practical consideration, in determining quality of reproduction...assuming the tonal balance is right for the individuals ears. Subjective judgments, like listening fatigue, imaging, and most importantly, gut involvement with music are not only of primary concern, but can not be addressed simply by frequency response considerations. Resonances,and smoothness of response weigh heavily, ...as do some headphones themselves :-) ...and IM and harmonic distortion distortions can be the most important factors in achieving the elusive goal of delivering the reality of the recording, and hopefully therefore,the beauty of the music in all of it's original inner detail and macro and micro dynamics.

Oh, they have to fit your head and ears too.

detlev24's picture

Some very valid points! Thank you. Yes, maybe a little too much attention is given to FR only; which is not exclusively defining audio fidelity: at least THD+N should find an equally high consideration.

This leads me to the Sonoma Model One, which apparently has a 'good' FR without EQ, but its THD+N is way too high (see measurements), inducing audible coloration already at moderate listening levels. This performance [there is no tube amplification involved] is unacceptable for any headphone that comes close to a US$1000 price tag - this electrostatic should be 5x better at least! ;) But hey, that's just my 50¢.

detlev24's picture

Some very valid points! Thank you. Yes, maybe a little too much attention is given to FR only; which is not exclusively defining audio fidelity: at least THD+N should find an equally high consideration.

This leads me to the Sonoma Model One, which apparently has a 'good' FR without EQ, but its THD+N is way too high (see measurements), inducing audible coloration already at moderate listening levels. This performance [there is no tube amplification involved, that would possibly explain] is unacceptable for any headphone that comes close to a US$1000 price tag - this electrostatic should be 5x better at least! ;) But hey, that's just my 50¢.

mnaganov's picture

Fully agree. People often confuse the needs of records production vs sound reproduction. When making recording it's very important to have as uncolored monitors / room / headphones as possible, and this is where all those equalizations and corrections are crucial. For listening, it matters much less.

People often say "I want to hear exactly what the producer wanted me to hear, so I need similarly equalized equipment." But again—with other body, other room, different headphones pair no one can guarantee that this will actually happen. I would really like to relax the standards when listening. If the music is good, and well recorded, you can enjoy it even when it is played through mobile phone speakers. And badly produced record will sound like garbage even on a hundred thousand dollars setup.

detlev24's picture

For listening, it matters much less.

Not really. The total quality of the sound transducers used is of utter importance for reproduction, since loudspeakers/headphones should not function as musical instruments. Reproduction of exclusively the playing material's content is essential, as pointed out by zobel. Of course, the production quality of at least ~90% of all the material available is the biggest issue of today - but that is nothing new and unfortunately, it will not change anytime soon - due to too low consumer demand.

If you want to learn about the differences between studio and home listening environments, the following PDF contains some information (see "2. Listening Conditions") which is the basis of today's acoustical engineering and valid for stereophonic sound, likewise.
https://www.tonmeister.de/foren/surround/texte/SSF_01_1_E_2002_v2a.PDF

[...] you can enjoy it even when it is played through mobile phone speakers.

Enjoyment is a subjective perception; so no and yes. For example, those speakers distort heavily over their whole frequency range and already at moderate listening levels [limitation by physical laws]. Can one enjoy it? Probably yes, under certain circumstances and/or if one does not know better (as so often in life). Until now, I did not touch FR but yes, playing a pipe organ recording; harp, bass, contrabassoon, tuba – or electronic music – etc. via mobile phone speakers can be enjoyable. Most probably though, if one does not know 'the real thing'. Either one cannot hear what is there [limitation by FR of the equipment] or it distorts heavily.

Human anatomy [you called it 'body'] is never part of the calibration process for loudspeakers - and should not be for headphones, either! [That is probably impossible for earphones, though.] Everybody will have their own perception of what sounds natural; just as I cannot be sure your perception of any color is the same as mine [e.g., "red" for you might be what my brain processes as "green"].
Furthermore, regarding headphones the surrounding room is not an issue at all. Standards need not be relaxed but need to be defined for headphones!

You do not want to spend money and time in accurate music reproduction? That is fine. Actually, you are lucky as long as you are happy. ;) However, I do not know any person who went back, once they jumped on this exclusive train.

mnaganov's picture

Don't get me wrong. I'm not advocating for using bad or improperly set up equipment for casual listening. What I'm trying to say is that obsessive FR correction, especially for headphones maybe not what people's efforts should be dedicated to.

Human anatomy [you called it 'body'] is never part of the calibration process for loudspeakers - and should not be for headphones, either!

Here I disagree. As for speakers—yes, they can be objectively tested in an anechoic chamber for resonances and distortions, then multiple speakers can be set up in a room to yield the same sound power, FR, etc for a certain listener position(s) according to measurements.

But then, you change the volume level (or play another recording that uses a lower sound level)—and your perception of the frequency response changes due to non-linear nature of human hearing. Human ears and brain still doesn't perceive the reproduced sound the same way as measurement equipment does.

And for the headphones, involving human anatomy and psychoacoustics is even more important because headphones skip some of the sound transformation phases that sound from speakers comes through. IEMs skips the most, over-ear headphones skip interaction with head and shoulders.

For any headphones it's a known issue that measured or perceived FR varies greatly depending on how they are positioned on the head, and how good the sealing is. That's why headphone measurements are always averaged over multiple seatings.

And then again—you have just calibrated the headphones for a dummy head, but it's not the same as yours. Thus, some personal tweakings are still needed (and Bob mentions that).

detlev24's picture

At a certain point, a model has to be defined with the best approximation to the theoretical optimum and that is where standardization is so important. Btw, everything is just an approximation. Nobody would ever claim that loudspeakers, not even in a "perfect" setup, sound exactly like the original source in its optimum environment [e.g., an orchestra in a concert hall] to the same listener. Yet, it serves its purpose very well! Perfection is simply not possible.

For headphones, I agree: head, shoulders, clothes etc. are influential but naturally omitted parameters. Nevertheless, it simply is impossible to calibrate for those on a universal scale! Therefore my previous comment, that "The personal flavor always can be added, since there does not exist 'the only one right way' to EQing.". Furthermore, we shall not forget that headphone EQ is mainly done with reference to EQed loudspeakers [e.g., HARMAN target]. An approximation to an approximation! In case of Sonarworks, individual calibration provides the closest match to what I hear on my calibrated set of full-range studio monitors [if at the same target FR]; and those sound closest to the mental model I have acquired throughout the years 'in the real world'. Sonarworks is by far not perfect, but the result is quite impressive and good enough for headphones. To my knowledge, they use a calibration process to a flat FR with translation to the 'B&K 1974 optimum Hi-Fi curve' - technically speaking they try to imitate the same acoustic space around the ears that calibrated loudspeakers create; and *not* at the eardrum. Trained listeners, in comparison to a reference loudspeaker setup, do final tuning // and crossfeed needs to be individually applied in any case. #psychoacoustics

Howsoever; head, shoulders, clothes etc. will mainly influence the high midrange and high frequencies due to the relatively short wavelengths in these frequency bands, right?

mnaganov's picture

For understanding headphone frequency measurement and how different parts of body affect different frequencies there is an excellent post by Tyll: https://www.innerfidelity.com/content/headphone-measurements-explained-f...


technically speaking they try to imitate the same acoustic space around the ears that calibrated loudspeakers create

I haven't heard of this approach. Do you have any references to materials where they explain it?

detlev24's picture

I haven't heard of this approach. Do you have any references to materials where they explain it?

Maybe you can find some hints in their FAQ [ https://sonarworks.zendesk.com/hc/en-us ] but I doubt it.

I received this information when I contacted their support - prior to my first individual headphones calibration 2 years ago - but of course, they did not give any insight in the process to protect their expertise.

Without question, it would be very nice to learn about this approach; which turned out to work quite well! :)

gzost's picture

I'm a (relatively recent) True-Fi user.

My experience so far is that this significantly improves my listening experience.

As a rule, across several headphones, this mostly eliminates the choosing-a-headphone for the track/current musical style. Things just sound more correct across the board - instruments are more natural, imaging improves, listening becomes more relaxed since the background frequency correction that my hearing/processing attempts is reduced. There are also some electronic music tracks where my biological frequency correction had no basis for guessing what would be correct and which I now understand much better structurally.

The differences the correction makes varies by headphone. Generally, when switching between corrected and uncorrected, the corrected version is much preferable and just sounds more natural.

Specifically: Sennheiser HD598's finally have bass, much cleaned treble, and for me are completely transformed. With a pair of Fostex T50RP mk3, the difference is smaller, but still significant. I bought a pair of individually calibrated Fostex TH-x00. The standard correction curve for these helps, but the inidividual one is quite a bit better - and both clearly beat the uncorrected version. With a pair of Superlux HD681 vs. the better cans, the limits of what frequency correction can achieve become clear: What is there is OK, but a lot of information just isn't. With tracks with lots of subtle spatial cues/microdetail, there were instances in testing where I was wondering whether I was listening to the same music. With Sennheiser HD380, the boxy sound due to the cup reflections is still there.

Downsides?

It's possible that the processing introduces some artefacts. I'm not schooled in listening for these in general, and with all of the headphones I've used with True-Fi, the benefits to me have greatly outweighed any small possible losses.

I also now consider getting the T50RP measured - so your finances may suffer.

JMB's picture

Normal hearing is done with our ear canal and what ever their from is our brain is compensating for their shape, length etc. (our built in frequency correction). With the exception of IEMs that function is not changed by wearing headphones. The contribution of the outer ear will be to modified by wearing headphones (even just having your hands around the ears will do that). So I do not understand why a simulated ear canal is part of the measurement system - it only complicates the comparison of different measurements.

amartignano's picture

You have to give the headphones' driver a realistic acoustic load. Doing this you have to put the microphone inside the simulated ear canal, wich change the response. Then you have to correct this to "virtually return" to the freq response before entering the canal, but with proper acoustic load. It's the ever lasting problem of physics: when you measure, you perturb the system you're measuring.

amartignano's picture

Moreover, in my opinion the measurements have to be corrected only for the variation introduced by the simulated/artificial ear/ear canal, and NOT for a "target response" based on average preference. My preference is not the average, and I want to see the unadulterated measures (corrected for the objective physical variations introduced by the head phantom), not corrected for a supposed "preferred by the most response".

JMB's picture

Any simulated ear will not represent one owns ear:
materials are different (especially in is vibration damping parameters)
human ears are variable (even between left/right)
The ear canal is a resonator but one which is compensated by the brain.
Any measurement setup has to be calibrated so in this case this should not only include the the microphone but the entire setup but how that calibration should be done is not clear to me. Maybe difference to a standard headphone instead of absolute measurements are to only precise way to compare headphones

mtmercer's picture

I hope innerfidelity will review Sonarworks Reference 4 Systemwide. I find I really like Reference 4 Systemwide. I only listen to my HD800s and Focal Elears with it. If you have a supported headphone that has known frequency response issues, I recommend it. The scenarios where I find limited value is for the HD650 and HD600 as they are pretty close to neutral and thus corrections are only minor. The other limitation is some headphones do not EQ well (i.e. Phillips SHP9500 - not good). I use Reference 4 Systemwide instead of True-fi as Reference 4 Systemwide allows selection of linear phase filtering. True-fi only uses minimum phase filtering. I would like to know how much practical difference there is between an average profile and a headphone specific profile.

I think Sonarworks Reference 4 (Headphone only) is actually a great value considering the costs of everything else in this hobby.

Note my use case is listening to music for personal enjoyment. I do not do any mixing or mastering. Thanks!

Bern L.'s picture

I too found the HD600 did not really benefit much(I'm using Reference 3)...but it really changed the HD800 for the better. I am using the average profile.

Bern

detlev24's picture

Reference 4 has an additional zero latency filter and the plugin is MIDI mappable.

However, the average calibration profiles are identical for both versions!

mnaganov's picture

If you don't send your headphones for a personal calibration to Sonarworks, then Toneboosters Morphit may be another good alternative. They have measurements for much wider range of headphones. One drawback however is that they only exist as a VST / AU plugin, so if you want to have the correction applied systemwide, you'll have to use something like Audio Hijack Pro in order to hook it up to system audio output. And they implement correction using a recursive (IIR) filter (in Reference 4, this mode is called "zero latency"), which introduces some phase shifts.

detlev24's picture

The scenarios where I find limited value is for the HD650 and HD600 as they are pretty close to neutral and thus corrections are only minor.

I do not fully agree; see the following examples of the averaged HD650. You really prefer a neutral/flat target to the 'B&K 1974 optimum Hi-Fi curve'? Nothing wrong with that, but for pure listening purposes the latter usually is the better reference [Bob explained in an earlier of his blog posts why not flat; but you can also read via https://www.bksv.com/doc/17-197.pdf how it arose]. Even EQed to a flat FR, the HD650 [similarly the HD600] takes big advantage especially in the sub bass and high midrange regions; whereas accuracy in the sub bass region will be limited by THD+N becoming audible latest around 90dB SPL. Of course, you need to choose tracks that contain significant information in the corrected FR bands; otherwise the effect will be negligible! Furthermore, channel imbalance seems to be an issue with this series of headphones [see Tyll's measurements] and thus individual calibration, which i.a. provides L + R channels correction - instead of an average between the both - would have a bigger impact.

[The following frequency response is translated to what loudspeakers measured in an anechoic environment would show.]
BLUE = measured average frequency response (of many samples)
RED = target frequency response [B&K 1974]
GREEN = correction
https://picload.org/view/driwwggi/hd650_1.png.html

MAGENTA = corrected/final frequency response
https://picload.org/view/driwwggw/hd650_2.png.html
https://picload.org/view/ddlllwal/hd650_flat.png.html [Flat target = 0db line]

[...] some headphones do not EQ well (i.e. Phillips SHP9500 - not good)

The reason lies most probably within the previously mentioned: technical limitations with less sophisticated gear. [SHP9500 measurements at http://en.goldenears.net/31990 ]

Regarding your question: additionally to the above indicated, individual calibration will account for the specific FR of your(!) headphone sample; since variances are significant even amongst the same models and their magnitude is independent of a unit's price [see Audeze LCD-4 or Focal Utopia]. Furthermore, accuracy of the correction is increased from +/- 3dB to +/- 0.9dB, and that certainly is within the audible range. Btw, miniDSP 'EARS' is calibrated [the individual file needs to be pre-loaded] to +/- 1dB of accuracy from 20Hz - 20kHz, which is impressive and basically identical to what Sonarworks offers!

I never used True-Fi; but I am interested in your opinion where - for pure listening purposes - you find yourself limited by a minimum phase setting? Another, to the average listener probably irrelevant, difference is that with any system-wide application there are limitations due to the nature and functionality of the driver a specific OS requires. [ https://sonarworks.zendesk.com/hc/en-us/articles/214211325-Systemwide-vs... ] and [ https://sonarworks.zendesk.com/hc/en-us/articles/212037245-What-are-filt... ]

Regards

mtmercer's picture

Thanks for the info provided above.

Regarding "I never used True-Fi; but I am interested in your opinion where - for pure listening purposes - you find yourself limited by a minimum phase setting?"

I have to admit, I do not think I can actually hear much of any difference between filter types for Sonarworks. I am confident it is my confirmation bias at work. But indulging my confirmation bias in this case makes me happy :) For my dac however I can hear a difference between minimum phase and linear phase and I much prefer linear phase. That has caused me to just try to standardize on linear phase.

For the driver question, I do not use the Reference 4 plug-in so I do not know if there is any audible difference between the plug-in and the Systemwide 4 virtual audio cable. Cheers!

detlev24's picture

I am glad to read you are happy. :)

The following is not in relation to your input directly. However, the blog posts are recent and might as well be of interest to others who use digital filters:

1) https://archimago.blogspot.com/2017/12/howto-musings-playing-with-digita...
2) https://archimago.blogspot.com/2018/01/musings-more-fun-with-digital-fil...
3) https://archimago.blogspot.com/2018/01/audiophile-myth-260-detestable-di...

Enjoy the music!

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