🔶 Ollo S4X Reference Headphone

This is the official thread for the Ollo S4X Reference Headphone

This thread is for discussion and reviews.

  • Type: Over Ear
  • Amp needed: No
  • Open Back
  • Dynamic Driver
  • 32 Ohm Impedance

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Ollo Product Website


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Who is Ollo Audio?

Originating from Slovenia, Ollo Audio is a fairly new company in the headphone space. Focusing specifically in the field of professional audio engineering use, Ollo Audio has been manufacturing headphones ever since Ron Gulič - the founder - decided to officially start making headphones under the company name… and let’s say that that was when his wife was disturbed while sleeping. Ollo was founded back in 2015, however, it wasn’t until 2016 that they would create their first ever headphone prototype. Ever since, Rok and his team would go on to release three different headphone models in total - the S4, S4R, and the S4X, but also the product that inspired Rok to start the company - Play2ME, a haptic subwoofer that acted as an extension for the headphones. Play2Me was a product that was aimed to fill in the gap that Rok felt headphones couldn’t - the chest feeling you get from the subwoofers. This product would later be discontinued, although there seems to be an upcoming MKII.

Either way, from what I can see, Ollo Audio seems to be mainly focusing on their headphone line-up, as though they released the claimed “flat out of the box” S4X model - a headphone that is aimed to be used as a reference in the professional space. Ollo Audio listened to the feedback from the existing customers, and they fixed some of the issues that were present in the previous two models. Since the company consists of a small team of just 6 people, I believe they have more time to focus and listen to the feedback. This gives me hope that they will release an even further improved headphone model that is based on the feedback from the S4X. Overall, they are on the right path of figuring it out as a new company to the market.


This is a lengthy review that is looking at the S4X from a very technical perspective. I purposely chose to approach the review with a technical approach instead of my standard music listening approach because of the unique nature of the S4X. A unique headphone deserves a unique approach - I don’t think an audiophile approach is suitable or valuable for a headphone like this. The review was made with the target audience (pro audio industry) in mind.

I wrote the review over the course of two months - this also included research.

I formed the review in a way that would let the average reader get a better understanding of judging a headphone of this nature from a technical perspective. A lot of effort was put in to this review, and I hope that it opens a different perspective on headphones, especially “reference” headphones. It was not an easy job, and I hope it was worth it for the reader.

I am not receiving any payment nor financial benefit from Ollo Audio.

Thank you.

Unboxing experience

Raw and simple. No fancy presentation or anything like that, it’s quite straight to the point. You are greeted with rather organic packaging, environmentally friendly packaging that is composed of cardboard and paper.

Formal format of what’s inside (excluding papers, manuals, warranty, individual FR graph):

1x S4X headphone
1x Cable
1x Carrying pouch
1x 6.3 mm adapter

Build quality & design

It’s not so often that you see a wooden studio headphone. Usually when I hear “studio headphone”, I think of old-school Sony’s, or some Beyer’s (Beyerdynamic).

The Ollo S4X seems to be following a very simple design, but when you look into it, it’s clear that many parts were carefully thought out. I personally think that Mr. Rok definitely had longevity in mind when he was designing his headphones. This is simply due to the fact that every part of this headphone is easily replaceable - not only are the spare parts are widely available (you can order them from Ollo’s official website), but they are also quite accessible in terms of pricing. Every part is easily replaceable and can be replaced by the owner (instead of sending the product to the manufacturer’s service). You can replace every part at home, a rather large advantage if you ask me - sound engineers will probably be using this headphone as a tool, and they don’t have time to waste.

The headband construction is relatively minimalistic - it consists of a solid frame, a “bolt”, a headphone yoke (ear-cup holder) that is hidden inside the wooden enclosure, and that’s it.

The frame is made out of stainless spring steel (AISI 301). The nautical leather (faux leather) headband is attached to the frame with the same bolt that holds together the headphone yoke. It has a self-adjusting suspension system - the headband is stitched to an elastic piece (you cannot see it because it is hidden under the faux leather headband piece). Interestingly enough, this suspension system is quite efficient in doing its job, and I believe it’s mainly due to the lightweight (350g) nature of the S4X, but also due to the right clamp force. Now, what I mean by “efficient” is that I can very easily adjust the “size” and it will stay in place, which means that you can very accurately adjust where you want the ear-cups to sit on your head without worrying that the headphones will slide down. I had my doubts at first, but was proved wrong shortly after.

And of course, the ear-cups. Made out of solid wood, the ear-cups are the accent of all Ollo Audio headphones. It’s worth noting that all headphones from Ollo Audio are handcrafted, which explains some rough edges around the hole cutouts on the ear-cups. Thanks to the bolt that holds the yokes, the ear-cups have full 360˚ of pivot (horizontal rotation). Unfortunately the ear-cups do not have any tilt (vertical rotation), and this is something that I would like to see in the future models.

Smell & Wood

I have worked with wooden headphones before, but I have never encountered a headphone like the S4X. They smell like freshly processed wood, and everybody who is familiar with wood knows how lovely that smell is. For some reason, no other wooden headphones had this authentic smell - which I believe is due to the fact that Ollo headphones are handcrafted and are probably freshly made from real wood.

The finish is quite unique and dependent on lighting. While not in direct light, it has an oak finish - picture this as dark chocolate, and while in direct light, it has a gold finish with a gorgeous sheen.


Perhaps not the best part of the S4X is the cable. There are some good parts about it, and some not so good parts. Let’s start off with the good - the 2.5mm connectors’ housings are made of nice aluminum, while the housing for the 3.5mm jack is made of a different kind of metal (more polished). The jack’s housing also features a screw-on system for the 6.3mm adaptor, and has a spring protector which will keep the cable safe from bending damage.
The bad part starts from the Y-splitter. The Y-splitter itself is made out of rubber, which isn’t a big deal breaker, but the red part of the cable is. It looks much more like a cable you would find on some equipment from a workshop rather than a headphone cable. The braided part of the cable didn’t bother me as much (it’s good thickness and good quality), but I just don’t like the look & the feel of the material. I definitely think Ollo should change out the cable for the next model, perhaps use a full rubber cable instead - but keep the metal housings for the connectors and the jack.
Measuring at around 2 meters, I should mention that it has more than enough length.


With a unique combination of custom made ear-pads and a self-adjustable headband, I am surprised to say that these two result to a very comfortable headphone. As mentioned before, the headband has a very good elastic which keeps it in place. The hybrid ear-pads themselves are very comfortable, and the pleather used is of high quality. I’m glad that Ollo Audio decided for a hybrid design, because I cannot imagine how anything other than velvet would’ve felt on this pair.

The fit of the pads is very snug. I personally really like this as though it is a perfect fit for my ears. Very cozy! However, I am certain that people with larger ears will have a semi on-ear fit. This is definitely something to keep in mind.

All of this being said, I experienced no signs of discomfort after wearing the S4X for over 3 hours. This should mean that you can use them for long-listening sessions.


The whole deal about the S4X is the claimed “flat frequency response” out of the box. Now, the previous models have been marketed similarly, but people found the claim to be incorrect. Unfortunately, this has been assumed for the S4X as well… while nobody actually took the time to take any measurements for the S4X. First of all, there is a great reasoning behind the pricing of this headphone. You are not paying 400 euros for nothing. This time around Ollo Audio entirely lived up to its claim by having a flat frequency response, or as Ollo Audio calls it “brutally honest”.

The Approach

Audiophile headphones can be tested through testing tracks and music, but how do you test how "reference-grade” a headphone is? This is a question I asked myself before writing this review. I wanted to find the answer, so I proceeded to do some researching. After asking some community members we all came down to the conclusion that there is no convenient way to test how “reference-grade” a headphone sounds. To simply put it - there is no “true” answer.
I mentioned audiophile headphones before, so what is an “audiophile headphone”? In short, I believe it is a headphone that is intended to be used for music listening. These headphones are usually colored and have specific sound enhancements. In other words, the headphone has a character and an intentionally colored frequency response.

Let’s go back for a moment, what is “coloration”? The simplest way to put it would be to say that it is a method of altering the sound, it has some sort of enhancement or recession in the frequency spectrum. Coloration is used to satisfy a certain signature, a certain goal and user base - elevating lower frequencies is one of the most common colorations, reducing high frequencies is another. This is not the “pure” form of sound, it has been changed thanks to tuning, this is something that you do not want as a sound engineer, the main reason is because you are not hearing sound in its “true” form. What a professional wants is a tool that they can trust, something that is as free of bias as possible.

Coloration is an alteration of sound, that’s one thing - but interpretation is another. Alteration of sound is an option, it is a conscious decision the manufacturer makes to tune a product, however, interpretation is something that depends on the person. If you have been in this hobby for long enough, you should be aware of the fact that we all hear differently, and this is due to interpretation of sound that differs from person to person. Interpretation of sound is dependent on quite a few factors, some of them are: pinna, concha, ear canal - these three variables alone can significantly impact sound. Interpretation and perception are both elements that are individual and personal, this is what makes our ears subjective, and in no way can you change this. You cannot change your ears, you are born with them. I have theorized a “solution” that isn’t quite convenient, I will touch on that later.

Why can’t you test how reference-grade a headphone sounds? First of all, you can test how flat the frequency response is technically, this would be with the help of an ear simulating measurement systems… but a frequency response can only tell so much. Due to the fact that our ear structures are so different from each other, and the fact that we perceive sound differently, to achieve a reference-grade sound signature you would have to do it individually. Why is this? Well, it all has to do with the fact that you do not actually hear the original frequency response. What the person hears is a different and altered frequency response than the “original” one, this is what you call HRTF. This frequency response varies from person to person, and this is why it makes it impossible to make sure that we all hear the same. Remember the three factors I named before? Each one of them affect the sound in a unique way. If you were to take the pinna as an example, can you imagine how much the pinna differs with different ethnic groups? This is the limitation of ear simulating measurement systems, because I am pretty sure manufacturers do not tune their headphones differently in different parts of the world. If you search “ear resonance” in Google, you will be able to find how different parts of our ear and body affect sound. You can see that the pinna slightly boosts frequencies from 2kHz - 5kHz, the concha has a sharp boost of 5kHz frequencies, and the ear canal & eardrum boost the 2.5kHz frequencies. The question is, how much do these ear resonances vary from person to person? If the answer is not by a lot, then it means that we can have a good idea of how a headphone will sound. This is only if we simulate these resonances in the tuning phase (basically what an ear simulator does).

This being said, when I say “you can’t test how reference-grade a headphone sounds”, I am just referring to an approach that could be applied on a commercial level, otherwise it might be achievable with personal calibration of headphones. However, companies like GRAS make measurement systems that mimic the human ear, they are called ear simulators. In theory, if you combine an ear simulator measurement system with a headphone that measures flat on it, you would be able to make a headphone that has a flat frequency response at your ear. It’s not just about that, flat frequency response is something that you want from a loudspeaker in an anechoic chamber, but is it something that you want at your eardrum? I’m not sure I can answer that. It all goes back to the question above - if the ear resonances do considerably vary from person to person, in that case it would mean that even the ear simulator measurement system would not be accurate and consistent. It’s an endless loop, you can quite literally start questioning everything - do you even want a flat frequency response at the eardrum? We know you want a flat frequency response in an anechoic chamber for loudspeakers, but can you just assume that you want the same for a headphone? I don’t know.

In other words, you might bet able to make a headphone “reference-grade” on its own - which means that the headphone is technically-wise “uncolored” and has a flat frequency response - but we have yet to confirm how much ear resonances vary. If they vary a lot, this would mean that you wouldn’t be able to achieve a headphone that sounds flat to the listener without individual calibration/tuning - but you can probably get somewhat close with ear simulator measurement systems.

We can conclude two things from this:

  1. A technically flat frequency response could be achievable with the help of an ear simulator measurement system. This would only be the case if the ear resonances don’t vary significantly from person to person.
  2. You cannot achieve a “true” flat frequency response without tuning the headphone individually for the listener. In other words, to achieve a flat frequency response for your ears, the headphone would have to be calibrated and tuned to your ears. This is something that is subjective, because this frequency response would sound flat only to your ears.

If all of this sounds a bit complicated, it’s because it actually is. Our ears and perception of sound are quite complex. To achieve a flat frequency response that a person actually perceives as a flat frequency response you would have to do various tests, make casts, measurements, etc. You can get a very good understanding of this by watching the following video:

This is a very important video, you have to watch it to get a basic idea of how much our perception of sound alters the original sound… all because of things such as our ear canals, pinna, and concha. All of the subjects use the exact same headphone (Sennheiser HD600) and listened to the exact same sound (pink noise), but the way they heard this pink sound vastly differed. Hopefully you can translate this to a larger scale and imagine how a frequency response would be altered and could also vastly differ from person to person.

Perhaps it’s easier to picture this if you think of our ears as filters. If a headphone is flat on its own (on a measurement system that is not an ear simulator), it is technically-wise correct, it is indeed flat according to the graph and the microphones, but since our ears are not the measurement microphones, we all hear a different and altered frequency response - our very own frequency response. If different shapes of the ear result to similar ear resonances, then it is crucial to simulate these ear resonances when tuning a headphone. If there is any sort of repeating pattern in our ear resonances, then it is essential to use an ear simulator to get accurate results - tuning a headphone on a measurement system without taking our ears into consideration is useless. The headphones will be listened to by humans, not microphones.

The term “flat” needs to be well defined in order to understand what “flat” means in the headphone world, what it means to have a flat or neutral sounding headphone - this is something that we haven’t really done yet. Applying the same principals that are present for loudspeakers may not be the correct approach to tuning of headphones. Once we define this term, we can then go on to try and achieve it in practice.

Does Ollo Audio S4X have a flat frequency response?

Yes. That’s the simple answer.

To me, this is the most important aspect of this headphone, it’s the marketing claim, so it should live up to it. The only thing I can test the S4X for is the flat frequency response, otherwise I would be basing it on my subjective hearing or on other subjective factors. Since this is a headphone that is marketed as a reference-grade headphone, there would be no point in telling you how this headphone sounds to me in terms of music listening, it would hold absolutely no value.

One thing is very clear, and it’s that S4X is aimed to be a tool, not a music listening headphone. Most users use headphones to listen to music, which means that the headphone needs to sound pleasurable to their ears, however, sound engineers are looking for accuracy. They are looking for specific characteristics to help them with making music, hence why I consider a “reference” headphone a tool. To me, a “tool” should be something that you can rely on, and fulfilling that in the headphone space is a highly difficult task. This is not to say that you cannot use the S4X as a music listening headphone.

At first you might think that no professional sound engineer will depend on a set of headphones to mix or master a track (or any other type of sound engineering), and you would be correct for the most part. However, there are exceptions. Not everybody has a studio of their own, not everybody has the budget to use a studio, not everybody has a studio that is close to their home, and not everybody has the perfect environment to make their bedroom or living space a “studio”. This is why Mr. Rok started his company, he was working late at night and his wife wasn’t really happy with that… it was at this moment that Ollo Audio was born. There are many people that face the same problem, especially younger people. You should also know that there are no standards when it comes to sound engineer’s preferences. Some prefer to use one studio monitor, others prefer another, some prefer headphones, some don’t - this just tells you that it all has to do what the engineer is comfortable with, it’s not as though you have the absolute standard studio monitor that is “the one”. Just take a look at 10 highly successful sound engineers and what equipment they use, you will find that it highly differs.

I also want to state something that I think is very important. I cannot put Ollo Audio’s statement of “neutral” to the test, and it’s simply because we don’t have a clear definition of what “neutral” is. If there were to be a definition, it would need to be free of listener’s preference bias, because preference is something else. Preference is what you personally like to hear from a headphone, it’s not what determines if something sounds like it should sound in its “true form”. However, I do know that Ollo Audio used the IEC 60318-1 ear simulator, and I know they tuned it according to the IEC 60318-1 standard. They definitely took into consideration how our ears alter sound (ear resonances), and they tuned the S4X with that in mind.

The Challenge of making a reference-grade headphone

At this point you should be able to see that it is a great challenge to create a reference-grade headphone, or just a headphone on its own. What I have said above is a present problem for all headphones because it is a problem that is caused by our complex ear structure. Currently, all of the headphones on the market are universal - meaning they are not custom-made for your ears. Since our ears are vastly different, without custom-tuning the manufacturer cannot know the frequency response at your eardrum. I like the quote from Ollo Audio - “A true flat is different for every individual”, and this is because a flat in the free field (in this case it’s the space between the headphones and your eardrum) is not flat by the time it reaches your eardrum.
The second largest challenge that is completely impossible to avoid is preference. Each individual has a different personal preference, that is why it is crucial to listen to audio product in-person before buying them. Again, in theory, a reference-grade headphone shouldn’t be a headphone that depends on listener’s preference.

But how do you even create a reference-grade headphone? What is a reference-grade headphone anyway? This is a hard question to answer, mainly due to the fact that companies have used this term as a marketing strategy to influence studio users to buy their products. If you go on and read the details to these so claimed “reference-grade” headphones, you will find nothing but a bunch of words that hold no value, words that do not explain the technical details as to why this headphone is marketed that way. I like the fact that Ollo Audio put in the effort to actually explain what their headphone is trying to achieve and what they mean by “neutral”. Reference, neutral, uncolored, all these terms are often mixed up… leaving us to have a lot of different ways of wording something without a clear definition of either of the terms.

After carefully researching the existing “reference” headphones, I came to the conclusion that Ollo Audio S4X is among the few that have a clear aim with a clear (scientific) explanation. I think Ollo has a very good approach to make an “uncolored” headphone that is aimed to be mass produced, aka commercial product. I have my own personal reasoning behind this opinion. I theorized that the ideal concept is to have a flat frequency at the eardrum, this would be the most uncolored headphone - however, as stated by Ollo Audio, this is impossible to achieve because it would require personal calibration and tuning… which you simply cannot do by selling a commercial (universal) product. Basically, if a headphone measured flat, it would not be flat by the time it reached your eardrum, this is the challenge… it brings me back to the ear resonances, because if they do not vastly differ from person to person, then the headphone will sound very similar to everybody, and my statement above would be false. You cannot avoid the ear structure that changes the sound by the time it reaches to your eardrum, but perhaps you can study how our ear structure alters sound and implement those changes to the tuning of a headphone, this would ideally let you at least mimic how it would sound at the eardrum - this is already being done with ear simulators.

~After giving it some thought, to have a product that is the closest to being technically flat, it would have to have a flat frequency response before the sound (from the headphones) is distorted and altered by our ears. The reasoning behind this is that each person would get an altered version of this very flat frequency response, the level to which this flat frequency response would get altered would vary from person to person. However, it should be the only way to unbiasedly create an uncolored headphone without individually calibrating it to the users ears - basically every user would hear a “slightly” altered version of the original flat frequency response.
If you were to tune the headphones to any other frequency response, the frequency response that the user would hear could greatly differ, which means that the level of coloration would differ from person to person - this would mean that it wouldn’t be as close to an uncolored frequency response as if it was originally tuned to be flat~ Or so I thought. The above statement (~) is completely wrong, and it is something that I assumed at the beginning. I should note that in the statement above I was talking about a measurement system that does not mimic our ears. While our ear structure definitely varies from person to person, it (in theory) alters the sound in a similar way. This means that even with different ear anatomy in humans, we can draw an average of how each part of our ear changes sound - it can be observed and averaged, as can be seen in an ear resonances graph. A flat frequency on measurement microphones would be useless because nobody would hear it like that, it would be “uncolored” and flat only for the measurement setup… and we are humans, not the microphones. That is not how we hear through our ears. What you want to do is implement into the frequency response itself is how our ear changes sound (ear resonances), I believe this is what ear simulators and software do. They mimic the way ear, meaning they try to alter sound in the same way our ears do (e.g. boost in the 3kHz region).

Uncolored is a term that I would associate with a flat frequency response. In theory the aim of a flat frequency response should imply that no frequency response stands out, but the terms like “neutral” and “reference” are much more complex. These two terms are very subjective and weakly defined, it’s not as though you can easily define a truly “neutral” or “reference” headphone. It is much more complicated than a frequency response… which is problematic enough on its own. You would need a very strong and detailed scientific explanation behind these two terms. Since neutrality consists of factors & elements outside of the frequency response (such as soundstage, imaging, probably separation, etc.), factors that you either cannot measure or they are things that are complex to measure, you wouldn’t be able to accurately define these terms on paper.

The Solution and how Ollo Audio approached problem

Now that you are aware of the large challenge of designing a commercial headphone that will be listened to by people with different ear structures, I want to cover Ollo Audio’s approach to “overcoming” this challenge.

Knowing that they cannot design a true flat headphone without personal calibration, they took advantage of the challenge itself. How so? Well, the first step is to acknowledge the problem and study it. By knowing the problem, you can attack it strategically and take advantage of it. Because it affects and alters the sound by the time it reaches our eardrum, the problem in this case is our ear structure.

With a firm base and lots of existing research on this matter, Ollo Audio was able to also use it to develop their headphone around it. Instead of designing a headphone that measures flat on a measurement system, they altered the frequency response of the S4X in a way that a flat response would be altered by our ears.

Here are some of the factors that they looked at:

  1. They used the Sennheiser HD600 as a base, which is a very good starting point. It’s a headphone that has been used as in the studio space for quite some time, so at least they could study what the consumers consider as “neutral”.
  2. The Harman Curve was also something that the research department from Ollo Audio was studying and taking into consideration.
  3. HRTF and how our ear anatomy affects sound

Beyond this, there isn’t much that can be done, perhaps lots of scientific research and studying the existing studies. The best you can do when making a commercial product is to mimic the ear, look at the averages, and implement these factors into the tuning of the headphone itself. That’s exactly what Ollo did.

The hard truth

… is that there is no solution that has been found, at least for now. Not only do our ears vary on their on their own, but measurements have their own limitations. Each manufacturer has their own method of measuring, and even if they have the same equipment they will not have the same results. There is no “holy standard” and for this reason sound will continue to be subjective.

This is not something that is specific to this pair of headphones, but all headphones. Each frequency response chart is different, was taken in a different lab, by different people & manufacturer’s. Comparing these charts from different companies is not accurate (especially when you do not know all the details about how they were made, and with what equipment they were made) but at least it might give you a general idea about a product.

The only tool you can trust is your ears, anything beyond that at this point is something that you cannot fully rely on. If it sounds right to you, you will perceive it as right. As life goes on, you will form different sound perceptions, but they will be exclusive to you - it’s not something that will sound like that to other people. The anatomy of the ear is complex enough on its own, it alters the sound on its own, however, the neurological side is something that is not often brought into conversation… but if you can imagine how much stuff happens with sound perception on a neurological level, it is something that we have yet to explore and study. The only right and wrong is in your mind, so trust your ears, don’t depend on other people’s opinion.

With speakers the question of how the “neutral” response should look like has been answered (if a speaker measures flat in an anechoic chamber), but the same question remains unanswered for headphones.


After spending weeks researching and reading several studies, I believe Ollo Audio not only lived up to its marketing, but also successfully made a tool for the sound engineers themselves. With limitations from both our ears and measurement equipment, the manufacturer had to be very mindful of how they will make the S4X, this required lots of observing and studying.

This is a statement directly from Mr. Rok:
“As there are so many different standards and approaches in the field of equal loudness contours, we decided to base our product development firmly on the feedback from our customers. We listen and do what we can, to adjust sound, comfort and even the purchase process, to accommodate as many pro audio needs as possible.”

To my ears this proved to be true, as though I found the S4X balanced across all frequencies. From Mr. Rok’s interview, he explained that the S4X is tuning is somewhere between the IEC 60318-1 standard (used by G.R.A.S.) and the Harman Target curves. Since Ollo’s target audience are sound engineers and professional audio industry, they made sure to test their product with them and took their feedback. I think the main reason why they were successful with this headphone is because they neither relied too much on neither measurements nor consumer feedback - they found the perfect balance between the two. They basically combined their own research with feedback from the target consumer market (engineers).

Mr. Rok further explained that the S4X is tuned closer to the IEC 60318-1 than to the Harman Target. This should give you a more direct explanation of tuning nature of this headphone.

All of this being said, I have to say that I believe that this headphone lives up to its asking price. There are a lot of things Ollo Audio provides besides a good headphone - 5-year limited warranty, 1-year of full warranty, widely accessible and affordable spare parts, and the easily replaceable nature of the headphone. These things matter quite a lot to somebody who needs a product that they can rely on, especially for professionals - being able to order spare parts and replace them at home is something that definitely holds high value. And most important of all, they provide a special trust factor. If you are skeptical or just want to try the S4X for yourself, you have a 30 days trial period - on top of that you also have the option to pay in installments (€20 a month for 12 months).

At the end of the day, you have to remember that Ollo Audio is a small company. What makes it a special small company is that it is primarily a direct-to-consumer company. This is important because it lets Ollo Audio use the majority of their budget towards the research department and future product development. It is the factor that is letting Ollo Audio do what they are doing now.

And I want to say that I am highly disappointed in people who continued to compare the S4X to the older models without even trying or measuring the new S4X. While the older models were not perfect, it’s clear that serious improvements have been made with the new X model, making it completely independent of its predecessors. Another thing has greatly disappointed me, is the fact that people who have reviewed this headphone have reviewed it from an audiophile perspective… which virtually holds no value to the target audience that will use this headphone in the professional audio field. I really hope somebody like @Resolve can form a more technical or scientific-based review, because I think that is a crucial approach for a headphone like this (whereas it is not crucial for music oriented headphones that aren’t of such strict technical nature). If there is anybody who should be approaching a review by listening, it’s the sound engineers - the people who these headphones were aimed at.

P.S. Ollo Audio has announced that they are expecting to release a closed back model in 2021, so keep a lookout for that. Mr. Rok said in an interview that he is expecting further improvements in performance.

Sources and research material

For anybody who is interested to do the research themselves, these are some useful links that are definitely worth a read:
1. David Griesinger touches on several subjects:
Must-read studies by David Griesinger (these are cached webpages of his work, some of these are presentations, so they are missing images):

Frequency response adaptation in binaural hearing
Binaural Hearing, Ear Canals, and Headphone Equalization David Griesinger
The necessity of headphone equalization
Recent concert hall research findings and a method to equalize headphones to an individual at the eardrum

You will notice that David Griesinger’s studies mainly focused on actually getting a speaker experience with headphones (hence why many of his studies revolve around binaural hearing). The majority of my assumptions have been based off of my understanding of Griesinger’s work.

2. Must-watch explanations from @Resolve who did a great job at explaining everything in a very understandable fashion. These videos are great if everything seems very confusing:



3. Ollo Audio’s brief explanation on of the S4X


Or if it fits your taste, a reality-show-like series where Mr. Rok and his team touch on the S4X:


To get a basic understanding of what HRTF is:

4. Useful explanations of graphs and measurements:
https://www.reddit.com/r/headphones...on/fbxbp9j/?utm_source=share&utm_medium=web2x - Oratory1990’s brief explanation as to why we haven’t solved the question of the ideal frequency response for headphones.

5. Controversial
https://nwavguy.blogspot.com/2012/04/what-we-hear.html - a very well written article explaining the neurological side of things.

To further add on to the link above, you also have a lot of things going on a psychological level. This is where things get way too complex to take everything into consideration. The power of our mind is very strong, more so than people think. I am not sure about the name of this psychological test, but it has been done many times: if you present two identical objects to a person and ask them to find the difference, their brain will make up the difference even though it doesn’t exist. If you can think of it the same with the audio equipment, then you can understand that a human mind can be convinced that there is a difference between two cables, two sources, two formats, etc. And this is not even taking into consideration the post-purchase rationalization… which on its own can quite significant.

Here are two important definitions from psychology (hopefully you can apply them in the audio space):
Illusion is the distortion or misinterpretation of a real perception.
Delusional perception is when a correctly sensed and interpreted stimulus is given some additional significance.

Having done research and reading serious studies on hallucinations has significantly helped me understand the power of our mind. I’ll leave this here: “How is a real itch to be distinguished from a hallucinated one if in neither case the presence of an object in public space is required?”

I want to give special thanks to the people from HiFi Guides and The Headphones Community for helping me out with my research. I want to thank everybody who presented a valid argument, because without those people I would not have come to the realizations that I came to

Special thanks to @MaynardGK (HiFi Guides) and Resolve (The Headphones Community) for leading me to major progress with my research.


Congratulations sir for a very interesting and engaging insight/review :+1: it’ll be interesting to see what the closed back version’s like.

as for…“because I am pretty sure manufacturers do not tune their headphones differently in different parts of the world” Unique Melody deserves a mention here for doing 2 tunings for their MEST, a JP and Universal version for what it’s worth :man_shrugging:


Damn. Well deserved praise!

I just don’t think business-wise it would be convenient for the manufacturers to put out different tunings for different parts of the world, but hey… we will see how this industry changes over time

Watched the vid, left me questioning. I did see a comment under the video claiming that he bought them after they were hyped up by a reviewer, AFAIK, I am the only one who has put out a review recently. I want to clarify that in no way did I hype the headphone up. I don’t do the “hyping” stuff. I based the review on my findings and my research.

It is a question whether DMS spoke directly to the founder, Mr. Rok, because it is possible that the customer service has no clue about tuning and technical details. When communicating, I had to state that I want to speak with Mr. Rok directly, then I was forwarded to speak with him.

Either way, here is the founders response under DMS’ video:

Comments under the video are very mixed, people are questioning both sides

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LOL. DMS posts a YT vid that is far from favourable. Rok, starts with “like your style, nice channel”, attempts to address DMS’s concerns without ever going into attack mode, then ends by putting a positive spin on the situation. Seems like the guy would rather have friends than enemies.

NTR reviewed it too and hyped it to gods end

Also it seems like there is a bit of a rivalry between the Ollo s4x and the 560s

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I understand the rivalry, there is the 560s, Avantone Planar, Austrian Audio HI-x55, many reference headphones out there. And there are hypers too - some people just find everything good. I truly did my best to understand their approach, I did so by researching on my own… spending hours on research daily.

I think that the founder needs to come out with a statement.

On reflection, I think DMS has misunderstood what Ollo is doing with the S4X. DMS apparently thinks the FR graphs on the Ollo site are compensated using the same target that Ollo is using to actually tune the S4X. Instead, the graphs are compensated with the default reference curve that comes with their measuring rig.

Ollo/Rok states on their web site that what they actually tuned the S4X to is user feedback from members of their target audience, presumably through multiple iterations. IOW, no theoretical standard is involved as DMS assumes.

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OLLO Graphs…like a London Underground map…but


And Average line would have been great…

I really love this part of Ollo’s website. It was fun individually comparing it to other headphones, seeing how it differs.

I’m still waiting for their new headphones! I believe others are waiting as well :wink:

Mixed impressions here. Pungent odor coming from the bag and very microphonic cable (upgraded cable, not stock) in conjunction with the metal headband.
Most notable attribute is mid forwardness. Mids sound better than 6XX and 660S (which is no small feat ofc). Bass and treble take a backseat, bit of lack of punch and slam with drums (vs R70X). Subbass is weak. It appears that my pair has a sound that is tilted to the right with no clear center image, something weird going on there so my impressions are subject to change. Will compare it to a 2nd pair to see if mine is faulty. If they are, I’ll probably send them to Ollo for a checkup since the shipping costs are moderate.
Edit: I compared them to a 2nd pair and mine seems to have a channel imbalance. Until that gets fixed, it wouldn’t make sense to post more impressions (though the impressions above about bass, esp. sub bass (no surprise) and treble will probably stay. If you want moderately priced sub bass you gotta go with closed backs or Fostex/Denon biodyna. As for treble, it’s not like as if there’s many treble spikes and troughs or as if it’s lacking extension.