An audio engineer's quest for reference headphones... Pt. 1

I originally posted a version of this on reddit a couple months ago before I found this community. I’ve since added/changed a couple things. Hope that’s okay!


  1. Sennheiser HD6xx
  2. Hifiman Sundara
  3. Focal Clear


I’m an audio engineer, music producer, musician, audio snob, and sometimes gamer. I’ve been lurking here for a while. This is my first ~headphone review~ … hope it starts some interesting discussions.


iMac —> Spotify / Amazon Music HD —> Universal Audio Twin MkII —> Schiit Heresy —> headphones

Reference Tracks: + some music mentioned later…

Detail Retrieval

Honestly not sure what this means. My best guess as an audio engineer would be ‘neutrality in the midrange frequencies’ because the flatter the mids are, the less masking will take place. If 2 kHz is 6db louder than it should be, it will be difficult to make out information that is directly above and below that bump. This is speculation though so I’d love to hear what other people think. I also don’t think that frequency response is everything so maybe “Detail Retrieval” ends up being a combination of everything else to some degree.

HD6xx: There is a noticeable hump in the frequency response on the 6xx somewhere in the upper lows around 200hz. I think this really takes away from the “detail retrieval” … The bass guitar articulation is definitely a little fuzzier than it is on the other two cans.

There’s also less ~sparkle~ in the airy frequencies above 10k or so.

Sundara: Slightly less detail in the mids around 1-2 kHZ than the 6xx. More detail everywhere else. The sound feels much more spread out vertically and horizontally compared to the 6xx which maybe creates more space per frequency range for details to coexist. Bass is much more articulate than the 6xx.

Clear: The Clears sit between the 6xx and the Sundara. The Sundaras sound a little brighter in a fun way and I think that exaggerates the details in the upper mids. Maybve the 2kHz recession exaggerates what is just above there in the mix. On the Clears - I feel confident that I’m hearing details as they were intended to be heard. I don’t feel like I’m missing anything. Compared to the Clears, the Sundaras are like a detail microscope that is especially interesting in busy passages. The 6xx sounds congested compared to the clears.

Speed & Dynamics

To me, “Speed and Dynamics” means “Slap Factor” or how hard does each frequency range SMACK? Is the snare drum whacking me in the face? Or is it politely tapping somewhere in the distance? Can I hear the percussive transients on the vocals? Can I hear the strums on the acoustic guitar? Are the strums on the acoustic guitar super annoying? This is a really interesting trait to me because I don’t know of any kind of ‘standard’ or ‘benchmark’ outside of personal preference. This is also probably why I have a hard time mixing on (some) headphones instead of speakers. I tend to really screw up compression decisions on some headphones because I’m making it SMACK when for whatever reason, those headphones aren’t supposed to smack…

HD6xx: Definitely the softest of the three in high frequency snappiness. Listening to simpler arrangement with less elements, there’s a healthy and familiar amount of smack going on. Once things start getting busier, it becomes harder to hear the micro dynamics (?). For the mixing nerds - It’s almost as if the headphones are acting like a VCA compressor gluing everything together subtly. I wonder if this has something to do with the ~laid-back~ quality of these / if that’s just a symptom of the frequency response. In mid-low frequencies, the 6xx may actually thump the hardest of the three.

Sundara: The Sundaras have more SNAP than SMACK. The low frequencies hit less hard on the Sundara than the 6xx but they go deeper so it’s more immersive.

Clear: The Clears have a pretty balanced smack across the frequency spectrum save high frequencies above 8k or so. Above there, although I think the frequency balance is just about perfect, it seems like there is slightly less ‘impact’ or snap than the Sundara. I wonder if this is a symptom of dynamic vs planar magnetic. I’ve never noticed this just listening to Clears casually so it could just be that the Sundara really exaggerates the dynamics in the high frequencies compared to the Clears. Could also be that I’m perceiving ‘louder’ as ‘more impactful’ …

Soundstage & Imaging

Width, Height, Depth… How wide is the sound? How tall is the sound? How far back does the sound go?This is another somewhat baffling concept to me again because I don’t know of any objective standard for comparisons. As a mixing engineer, your standard is pretty much a pair of speakers that create an equilateral triangle with your head as the third point. But different speakers seem to have different soundstages too before even addressing the impact of room acoustics on every speaker setup. I have a theory based on my experience monitoring through different speakers that “depth” has a lot do with sub bass content. Imaging, to me, means how clear is the placement of elements within the soundstage? Something I really didn’t like about 58x was the imaging. Everything felt a little garbled in space. From what I’ve gathered from reading headphone reviews, none of the headphones I’m reviewing have the vastest soundstage. It also seems as though headphones that DO have giant sound-stages are generally getting there by making sacrifices in other departments like smack and frequency response. What do you think?

HD6xx: The 6xx are really narrow. Pretty much everything sits on top of everything else towards the center except for hard panned elements which sound somewhat distant. I’ve been wondering what technically causes headphones, which are all the same design in the sense that each ear has its own source, to have different perceived widths. If anyone can explain I’d love to know!

Sundara: The Sundaras are my favorite here. I really feel like I’m inside the music in a way that I don’t with the other two. There is a depth to the Sundaras sound that I don’t perceive as readily listening to the Clears. There will be elements in a mix that really sound like they are sitting on top of one another in a problematic way with the Clears - the same mix through the Sundaras will put those elements obviously ~behind~ or ~in front~ of one another. You can check out this phenomenon listening to the vocals vs. the slide guitar in the track “Faced With Fire” by Margaux. On the Clears, the slide guitar and vocals sound, to me, like they’re fighting for real-estate on the same 2D plane. On the Sundaras, the slide guitar lands in a space behind the vocals. PS. Margaux is a dear friend of mine and I think her debut EP “More Brilliant Is The Hand That Throws The Coin” demonstrates some of the most interesting songwriting AND mixing. The EP is produced/mixed by producer Sahil Ansari.

Listening to “Fruity” by Rubblebucket, it’s me and the singer against the world. There’s someone shaking a shaker to our right, there’s horn players on either side taking turns, there’s harmonies floating above us and there’s a wobbly synth bass rocking the whole room around us. I used to work at a headphone/lifestyle shop in the East Village (NYC) called AC Gears (no longer around sadly) and we sold Hifiman products there. This was five or six years ago so the Sundara didn’t exist yet but whichever pair we had - I remember listening and thinking that I finally understood what people meant by “soundstage” for the first time. It’s as if each pair of headphones have curated a space for music to exist in. Maybe it’s brand associated placebo, but I feel like the Sundaras are demonstrating that idea more than the other two cans which are less enveloping.

Clear: The Clears are right in between the Sundaras than the 6xx. This isn’t a perfect analogy but the Clears feel more like I’m in the first row watching a very intimate show where the whole band is playing just for me. With the Sundaras I’m in the band. I would love to hear a pair of Focal headphones some day where they aren’t trying to emulate the ~speaker experience~ … From a music production standpoint, the Clears are second to none so far. Because the Clears have a less obvious sound stage, it really makes me work for it when I’m trying to create a sense of space in a mix. Then when I switch over to the Sundaras, the effect is glorious. It takes what I was trying to do with the Clears and expands on it in every direction in a beautiful way. Conversely, when I try to create a sense of space while mixing on the Sundaras, it doesn’t always carry over when I switch back to the Clears. What sounded fantastic on the Sundaras sounds especially flat on the Clears. So for music production I generally rely on the Clears. For casual listening / pure enjoyment, I almost always pick up the Sundaras which are about 1/4 the price! I’ve read that the Hifiman Anandas are similar to the Clears with a better sound stage. Would love to hear anyone’s take on this. For the record, and this will probably be controversial, I demo’d a pair of Hifiman Aryas and I didn’t like them as much as the Clears or the Sundaras. They had an enormous soundstage that made the music, to me, sound a little disjointed. The sides were too close to the front while the center was too pushed back. I also found the frequency response to be unacceptable (nasty peaks somewhere around 7kHz) without EQ which, for me, is a big no-go for something that costs well over 1k. The only thing that blew me away on the Aryas was the sub bass which is relatively non-existent on any of the headphones I’m comparing now.


Perceived Frequency Response… I have no means of measuring frequency response in any meaningful way so I’ll just give my general impressions. I’m pretty turned off by significant shifts from neutrality in headphones or speakers partly because I’m a mix engineer and I need to make decisions based off of some variation of “objectivity” and partly because I feel any big deviation in frequency response from neutral results in masking which negatively impacts “detail retrieval” . All three of these headphones are super neutral sounding in general.

HD6xx: The 6xx sound the “warmest” to me. All three headphones are generally very neutral sounding but these are definitely the most ~laid-back~ like you’re listening through your dad’s tube amp from the 70s. I don’t hear much sub bass if it’s there at all but the wide hump around 100hz shapes the frequency response in such a way that I don’t miss the sub frequencies - it doesn’t sound unbalanced without them.

Sundara: I’ve seen the Sundaras described as bright a couple times. I wonder if this is more to do with the frequency response or the presentation of the high mids. There is something ~cheap~ sounding about the the high mids to me. The the Sundaras sound like your girlfriend’s father’s car that he’s EQ’d somewhat haphazardly to sound exactly like he likes it. The treble sounds ‘boosted’ a little but it’s been done so tastefully. I called it “cheap” but I actually love it. I’ve seen reviewers use the term “grainy” and that feels about right for the Sundara’s upper mids but it’s a really nice grain that really compliments the “edge” on good recordings. I don’t like the Sundaras as much for music while I’m focusing on other things. They really demand attention because there’s so much information/detail coming at you.

When you’re looking for a place to park, if you’re like me, you turn down the volume so you can focus on finding a spot, not hitting anyone, and parking. Even while I’m writing this - I feel like the Sundaras are a little too demanding for my attention. On the flip side, if all I’m doing is listening to music, the Sundaras are glorious and I can completely lose myself in the music. I wouldn’t describe them as bright but I understand why someone would. Besides the “grain” (hopefully I’m not using that term incorrectly) in the mids, the frequency response is so killing. I feel/hear some sub bass. The low end articulation is rad. The highs sound nicely extended. I somehow only just heard about the 2017 remixes of Sgt. Pepper… The new mixes are such a wild ride on the Sundaras. I’m sure a lot of people are very mad about how much they changed the original mixes. I found the original mixes unlistenable on headphones and most less-than-ideal speaker setups. There was also a super interesting article in SoundOnSound about the hardware/software they used for the re-mixing job. I highly recommend checking it out to my fellow production geeks!

Clear: The Clears have been my reference cans since just before the pandemic started. I’ve done so much listening, producing, mixing, and mastering on them that now they just sound like the truth to me. I really trust their frequency response to translate evenly to other sources. If I’m adding a little 500 hz to the lead vocal on the Clears and it sounds right - I know it’s going to sound right on airpods, on my sister’s bluetooth speaker, on my living room hifi etc. I use speakers when mixing too in order to assess the physicality of the sound but I don’t trust my speakers like I trust the Clears. The frequency response sounds completely flat to me from just above sub bass to the highest freq’s that I can hear. If a mix has a little too much sibilance - you’re going to hear it on the Clears and it’s going to drive you nuts. There are a couple songs on my reference playlist that have potentially harsh sibilance and I have those tracks there so I can make sure I can tell that they are harsh with whatever I’m listening on. Those are some of my favorite songs anyways. Can you guess which songs those are? Apart from sounding flat - the Clears really do sound ‘expensive’. Maybe this is what people mean when they say “refined” … The only downfall for mixing on the Clears is low to sub bass. I haven’t been able to figure out how to navigate mixing the bottom of my kick drums and bass guitars without switching to speakers. For listening / enjoyment, the low end sounds great to me. But when I try to make mixing decisions in the bottom octaves it never translates quite right to other sources. I’ve found a couple threads with suggested EQ settings for the Clears. I don’t know if it’s because I let myself get too used to their normal sound, but EQing the Clears never works for me. For EQ I use SoundSource (which is an amazing app that I’ll definitely write about soon) on my mac in order to use FabFilter Pro-Q 3.


How comfortable are they when you first put them on? How comfortable are they hours later?

HD6xx: These are the clear winner in comfort. I use the 6xx for ~gaming~ pretty much just because they’re the most comfortable. There’s a reason that “gaming headphones” generally sound like ass and no one cares (or knows). Because gaming headphones don’t have to sound great to be useful. As soon as you’re focusing on the game, the sound quality (as long as there isn’t absurd masking going on) doesn’t really matter. I’ve tried all three headphones while gaming / doing other focused activities and I always end up using whatever is the most comfortable which is the 6xx of these three. It’s also worth noting that I can drive these with the output of my xbox controller. The volume is almost maxed out and I’m sure the impedance / voltage situation is not ideal for the 6xx but it still sounds and feels 10x better than any “gaming” cans I’ve heard.

Sundara: The Sundaras are not uncomfortable but they are definitely the least comfortable of the three. They almost feel like closed back headphones compared to the Clears and the 6xx. I can wear the Sundaras for a long time but I’m never going to forget that I’m wearing headphones. After listening to an album over 30 mins, my ears definitely need a break.

Clear: It’s pretty crazy how comfortable the Clears are for how heavy they are. They are so perfectly balanced on my head that I do forget I have headphones on sometimes.

This was a lot of fun. Thanks for reading!

Shoot me a DM if you’re ever looking for a mixing / mastering engineer.



Excellent review and comparison @reallyoldcob.


A very interesting read, great work! HD600 would have probably been a better pro solution than 650 though.


I’m pretty sure it’s the recessed area around 2KHz. It’s a trait that most Hifiman headphones exhibit, and I suspect it’s deliberate.


The HD-600 is the neutral reference model. I demoed the HD-650 (effectively identical to the 6XX) and bought the 600.

The Sennheiser drivers do indeed lack high end details and have limited dynamic potential.

I have no interest in wide headphone soundstages, as some music adds artificial width via effects (pseudo-surround, spatializers, etc.) If a product forced in extra width then it can never be narrow. Leave it neutral and let the creator decide about width effects.

My half-informed guess is that this follows from the shapes of the cones and the tendency of some to add cues to width as reflections or echoes. For example, Grados have a center tunnel tube that seems to echo. I sometimes hear this as noise or distortion, so it’s unwanted.

These are my everyday headphones now. I largely let the source decide and the Clear does a great job in reproducing the source (but do indeed tube them or reduce the treble sometimes).

Nice post.


Nice write-up! Nice kitty! What kind of guitar is that?

Just one comment: Most audio terms are rather subjective, but in my experience, “detail retrieval” is less about frequency amplitude (spikes, humps, dips, etc) than it is resolution of musical detail.

A good example is in well-recorded traditional acoustic jazz (for example, Miles Davis’ KIND OF BLUE). A headphone with higher resolution will let through all manner of subtle, low amplitude sonic cues (sound of fingers on bass string; sound of air blown through trumpet; the most delicate cymbals & brushwork revealed; and subtle reflections or other sonic cues inside the studio itself). A headphone with less resolution will give you the musical notes & general music flow, but omit some of those details.

I have an HD650 extensively modded by Jupiter Audio Research (JAR650), and IMO this is overall a relatively high resolution headphone, particular in the midrange. By comparison, another open-back headphone I enjoy a lot, the Philips Fidelio X2, has noticeably less resolution. They’re both terrific headphones, but resolution is one of the differences between them.

As a result, I can imagine the JAR650 being used for studio monitoring–but not the X2s.

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Definitely. I’d love to check out the hd600s at some point! The 6xx were my first foray into headphones as a means of editing and production before I was sold on the idea - so I went with the cheaper option. I’ve used Sonarworks with the 6xx and it’s actually pretty amazing. With other headphones (like the Clears) I don’t think it works at all. My issues mixing with the 6xx had less to do with frequency response and more to do with dynamics. I did two weeks of mixing on the 6xx and all of my work from that time hits a little harder than it should - the impact/slam/slap is jarring. I must have been overcompensating for the 6xx’s relative softness compared to the monitors I was used to.

Sometimes I’ll EQ +1.5 db at 2kHz with the Sundaras It doesn’t affect my perception of the soundstage besides bringing some vocals forward a step or two.


Lovely music, thanks for introducing me to it! There’s a lot of texture in this recording.

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FWIW, by several measures they’re down about 5dB or more at 2KHz.

P.S. I actually like deliberate deviations from neutral to achieve specific effects. Every headphone that I’ve liked and tried to EQ exactly to a Harman target has sounded the worse for it and usually lost whatever I liked about it. Smaller corrections like yours that make the deviations less extreme but keep the essential character of the headphone I find much more pleasing. My best example of this is my old LCD2C. That thing had an extremely recessed upper midrange, which contributed to its laid back warm/dark character. It was too much for my taste, so I usually EQ’d it up, but when I tried EQ’ing it all the way up to Harman using something like oratory1990’s measurements from Reddit, it threw off the overall balance and sounded awful.

That’s Crumbs and his twin brother Muji is elsewhere probably knocking things over.


The guitar is a super cheap “Recording King Dirty 30’s” that sounds super nice despite feeling like plastic… it gets a lot of action in the studio.

I still think that “resolution” is probably the consequence of some combination between smoothness in the frequency response and speed of the dynamics - which together will reduce masking of “details” … What is super interesting, and maybe a little unrelated, is that most of the artists I work with don’t have any means of high-resolution playback themselves. They’ll listen on whatever earpods or speakers they usually listen on and give mix feedback that way. So plenty of the artists we love have never heard, nor care to hear, the amount of detail that a lot of us are chasing - even in their own music.


:partying_face: :pray: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap: :clap:

YES! After learning to play guitar this became super obvious to me. Musicians tweak a dozen analog knobs in pursuit of ‘good tone’ and often have no means to store their settings. Nor do they care. They are in creation mode, in the mood, in the moment. It’ll be different next time, let alone in a live performance. I have limits to my audiophile equipment pursuits because I know that subtle nuances were often randomly created and incidental to a musician’s emotive communication.


Fantastic post! I read it on Reddit and I loved it then and I love it now. Any time someone takes the time to big up the Sundaras makes me happy. In fact, I believe your post might have been one of the factors that swayed me to purchase mine.

Your post has inspired me to go listen to the Super Deluxe Sgt. Pepper album again. Welcome to the community!


Off the top of my noggin…

Raal SR1a
Stax srx mk3 pro

Tyll mentioned that Aeon flow closed was a really neutral headphone.

I think Muji looks more like a William. Or maybe a Gabriel.

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Most HFM’s have a trough from 1.4 kHz to 3 kHz. Gives sense of depth but makes vocals sound like a nihilistic poetry reading in Berkeley CA circa 1956.

And yes Clears are great,

You perfectly summed up my issue with all the Hifimans I’ve tried minus the HE500. The HE400 in particular sounded asinine, but at least it had bass. But guess what, they somehow they made an even worse headphone in the 5se, it didnt even have bass below 60hz so there was no pro to it whatsoever.

Just wanted to drop my 2 cents here -
Obviously there are some “audiophile” headphones that can be considered reference (many people use the Sennheiser’s HD600 as one), but I wanted to jot down some of the headphones that are being marketed as reference-grade headphones (aimed for studio use): in no particular order

  1. Ollo Audio S4X
  2. Austrian Audio HI-X55
  3. Direct Sound headphones
  4. Telefunken THP29 (in collaboration with Direct Sound)
  5. V-Moda M200… who would’ve thought we’d ever hear “reference” and “V-Moda” in one sentence?

I’m actually interested in how these sound tbh, it definitely steps away from what they did in the past… and it seems like the OG V-Moda fans “hate” it, so I’d consider that a good sign :joy:

  1. Phonon SMB01L - this one is super exciting (and underrated)!

Ans then there are some from the Pro Audio companies as well -

  1. Neumann NDH 20
  2. Avantone Planar (brand new planar magnetic reference headphone) & their MP1 model

I think this would be it


So what, precisely, does it mean when a headphone is marketed as a reference-grade headphone for studio use? I have the Sennheiser IE 40 PRO IEM, which is marketed more as a professional than audio IEM. @ProfFalkin and I agree that it’s pretty good for audiophile use.

What are the differences, and if they’re so good, why don’t they have much audiophile following?

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What does it mean is hard to answer - could be a marketing tactic - but I truly believe that they are aimed to be used as a reference, which means that you can “trust” the headphone to truthfully represent sound “without coloration”. Do not that this is my assumption of the meaning. At the end of the day there is no such a thing as a reference headphone, I had a very recent discussion over at another thread, specifically asking the same question you asked me - and due to the way our ears work “reference” headphone is different for everybody, apparently it’s different for speakers.

Phonon does get a bit confusing because it says made by audiophiles, for audiophiles - yet in the next line it says it was made by audio engineers and is aimed towards them… could be a lost translation. But as a somebody who followed Phonon for quite a bit, I know that their true aim and market are audio engineers and Pro Audio space.




Austrian Audio