FiR Audio M4


Headphoneus Supremus
Pros: Fairly uncoloured sound with outstanding detail and texture, very high quality bass, very high overall technical performance, excellent build quality, stock cable, great leather case
Cons: Lower treble can be a bit much for some people
FiR Audio M4 - Good Clean Fun (Very Good Clean Fun)

I would like to thank FiR Audio and Project Perfection for providing me with the M4 in exchange for my honest opinion. No incentive was given for a favourable review.

  • Drivers: 1 x dynamic driver (low), 3 x balanced armatures (mid, high)
  • Frequency response: 10hz - 20Khz
  • Impedance: 6.4ohms
  • Proprietary tubeless design
  • 3rd gen Atom pressure release system
  • Price: US$1,899


FiR Audio is a relatively new name in the industry, as it was founded only in 2018, but the person behind the brand is considerably better known: Bogdan Belonozhko. I was going to say here that Bogdan is one of the "famous Belonozhko brothers", but that sounded too much like it could be a trapeze act and would not have been entirely accurate either because I believe there are some Belonozhko sisters out there too. Then I thought about saying Bogdan is part of the "Belonozhko family", but that made it sound more like a New York crime family. So Bogdan is just Bogdan, part of a family that has become very well known and respected within the audio industry. Formerly a CEO at 64 Audio, he left that company to his brother Vitaliy and went his own way with FiR Audio. I think with FiR Audio Bogdan built up a very interesting company that sets itself apart from many others by offering not just IEMs, but also the tools to keep those in tip top shape.

One of the first FiR products I came across was The Headphone Vac, an IEM vacuum cleaner that comes in two sizes and features a bunny holding corn as a logo. Why a bunny holding corn? Because the Vac was supposed to be produced in Iowa where there are (of course) more members of the family, cousins this time I believe, and the "ears" of corn is what a bunny lives on during winter. Quite clever actually... Bunny ears, corn ears, IEMs you put in your ears and a family that breeds like rabbits. (Just kidding on that last one.) Especially for performing artists such tools are a really sensible investment in order to keep their monitors in optimal condition, which should provide them with a longer life as well. For audiophiles it can also help avoid the agony of having to send their precious IEMs to the manufacturer for cleaning, or worse, repairs that could have been avoided. To me this suggests that Bogdan and his team at FiR Audio are not just trying to push their IEMs, but aim to offer a complete package that recognises the investment their customers make.

The M4 come in a satin black box with golden FiR bunny logo and the model of your IEMs. The box is the same for the other FiR IEMs (the M2, M3 and M5), but with subtle differences in the graphics. Opening up the box reveals a personal note from the FiR Audio team and behind it the first layer with the M4 themselves and the cable, which is covered by a small black card with FiR Audio sticker. Removing the first layer reveals the second one with a leather case and warranty card. The case is great and I think has the most useful design of the various cases I have come across. It is a round case that works perfectly for storing the IEMs with any cable attached, as there is enough space for even chunky aftermarket cables (save for something ridiculously oversized as a flat braded, 12-wire SilverFi R5). Because it is round it allows the cable to curl up more naturally so it does not tend to develop awkward bends over time, which is something I sometimes have when storing in (for instance) a rectangular Peli 1010 case. The case is also made of leather and so is soft enough to protect the IEMs when moving around, where some harder cases also seem very hard on the inside. The design of the case is very nice with the FiR Audio logo embossed on it, a beautiful colour that coincidentally matched the case of the Cowon Plenue 2 I used for this review and then a really clever way of storing the tips at the bottom. FiR Audio include a basic selection of tips with foam, silicone and a set of double flange tips, plus (of course) a cleaning tool.






Build quality and fit
The build quality of the IEMs is superb. It is an all-metal design and I am a huge fan of that because to me it always inspires confidence that I can use them without too many worries. I would happily stuff these in a pocket and not worry about it, while with resin shells I am often too concerned about possible cracks developing when I do that. The big thing with how FiR Audio designs its IEMs is that they have a completely tubeless design. The aim with this is to get the IEMs to produce a more natural sound than you usually get with IEMs by increasing the air volume available within the monitors. Usually IEMs are designed with sound tubes running from the drivers and this means only the volume inside of those tubes is available for the sound. By removing these tubes the whole inside of the shell becomes available, which means a lot more air volume for the sound to develop. FiR Audio combine this with their third generation pressure release system called Atom, which is like Adel and Apex, and it counters the occlusion effect you normally get with IEMs very effectively. This makes the way you perceive the sound feel a lot more natural, like it is coming from beyond the IEMs. The shells have a great ergonomic shape that is very comfortable and because wearing them does not build up any pressure within the ears, it does not really feel like wearing IEMs at all.



The included cable is a good quality and very supple one built up with 8 thin wires. There is no memory wire in them, just pre-bent heat shrinks that are a little longer than I usually see, but it did not affect comfort. One thing of note is that the connectors for the universal models are MMCX instead of FiR's own RCX connectors that come as standard on their CIEMs. I understand that the reason for this is to cater to those audiophiles who like to use aftermarket cables. On the one hand I like that they went with a more common connector for the universals, on the other I do find it a shame not to have the advantage of the more reliable RCX connector.

Listening was mainly done using the Cowon Plenue 2 and some with the Lotoo PAW6000. While the Lotoo is my main DAP for reviewing these days, I switched to the slightly sweeter sounding Cowon because it resulted in a better treble response for my personal sensitivity (discussed in more detail below). I also switched tips from the included stock tips to (I believe) Sony tips that also slightly helped reduce the treble issues I was having. This combination worked perfectly for me and avoided all earlier issues.

The signature of the M4 is what I would call fairly uncoloured, with a very well done lift in the bass to add pace and impact, and a subtle bit of excitement coming from the lower treble. So perhaps you could say the M4 have a slight U-shape, but really well balanced and I am not sure if "U-shape" is really the best description here. I don't have them around anymore or I would have certainly have included a comparison, but the M4 remind me (from memory) in many ways of the 64 Audio U12t. In case of the M4 though it comes with added excitement from the bass and a little from the treble. Where the U12t never quite did it for me, the M4 have gone from strength to strength the more I used them. I like a bit (or often "a lot") of colour to IEMs and the M4 add it in a very subtle, but extremely well balanced way. In my first impressions I wrote the following:

" My first impression is that these might well be perfect for people who do not like too much colour to their sound, but are looking for some fun and excitement with a (very) high level of technical performance."

This feeling has only been strengthened over time. The M4 are exceptionally good technical performers that find a near perfect balance between accuracy and fun. High praise, I know, but it is something that stems from many hours of listening and a continued sense of surprise by just how good the M4 are with every type of music I used them for. They are highly versatile and impress with everything from Beethoven to Pearl Jam. In that sense they remind me of the 64 Audio Tia Trio, which also kept surprising me with their versatility.

The tubeless design in combination with the Atom pressure relief system helps to generate a spacious stage that is very wide and deep with good height, giving a very natural feel to the presentation. Coherency is great and I personally do not find that the hybrid design is causing the sort of separation between the bass and the rest of the signature that sometimes seems to happen when different driver types are used. Everything works together harmoniously. Separation is great with lots of air around instruments, as well as providing accurate and stable positional information. To be honest, apart from a slight issue I had with the lower treble lift hitting my sensitivity on occasion, I can't really fault the M4. Contenders in the category "desert island IEMs" (if you can only bring one set)? I think so.


Oh my... Oh my, oh my... The M4 really impress with their bass. This is not an Empire Ears Legend X-type of 'I have my head stuck in the subwoofer' bass (which, let's be clear here, I adore and has left two Weapon IX sized holes in my life ever since I heard them), but this is without a doubt one of the very best quality bass I have heard. I have been analysing this bass from every angle, using every type of music I have and it always sounds "right" in every single aspect. The bass is very well controlled so it does not muddy the mids and its placement always seems to be exactly right. This might well have to do with the placement of the dynamic driver within the tubeless design, where I believe the dynamic driver sits at the back of the shell.

Listening to the Rolling Stone's 'Hoo Doo Blues' is great. The track has a very prominent kick drum, which the M4 reproduce with an incredible amount of body, texture and detail, yet it does not in any way interfere with rest of the instruments or Jagger's voice. Everything remains super crisp and clear. The physicality of the bass is much the same. Combined with that outstanding level of detail and texture, it gives a very clear representation of the instrument. Bass instruments have a very realistic sense of size and weight to them, but they never overwhelm. Similarly the Tuba in Tuba Skinny's 'Jazz Battle' sounds big and bold and gives you a sense of the size of the instrument, yet it is so impressive to hear how well it is controlled. Classical music? Yes, there too. Bass instruments sit in the back where you would expect to find them and still you get an unmistakable sense of their size and the physicality of the instrument to an extend that I have not yet encountered.

The bass in general extends very well and is capable of a proper rumble and the physicality I mentioned already, which together with a very tight nature is also great for popular music and EDM. It is quite an articulate bass that does not feel particularly slow or sluggish, despite being produced by a dynamic driver. Imagine Dragon's 'Thunder' has a really nice heartbeat to its bass, not super quick, but very nice and precise with just enough resonance to give that heartbeat body. It makes the track wonderfully engaging and fun.

The mids are not as full as I would normally like them to be and initially I was worried that the leaner mids would come at the expense of a natural timbre. I don't think I could have been more wrong here. If anything the mids are accurate, very accurate. I often use Beethoven's 5th symphony conducted by John Eliot Gardiner to get a good sense of timbre and I was very impressed by what I heard. Out of nowhere came some of the most accurate sounding woodwinds I have heard so far. Not full sounding, but accurate and so easy to distinguish different instruments, even those very close in tonality were distinguishable with ease. The piano sounded very good too, not quite up there with the Rhapsodio Eden (which were addicting for piano), but a joy to listen to nonetheless. When listening to Chopin Nocturnes it is amazing how much detail there was in every single note. Because the Nocturnes are solo piano pieces, you get a genuine sense of every note and the weight with which it is being played. The M4 reveal this in great detail and give an accurate sense of the dynamics between the notes.

Vocals are very clear and balanced. They do feel somewhat recessed and I don't think the M4 distinguish themselves with particularly dense vocals. There are definitely a few IEMs I would prefer for vocals due to a more forward position and greater density. Still, the M4 are very good for vocal music nonetheless. Female vocals are clean even at the highest octave, while male vocals have the chestiness to distinguish them clearly. So in choral pieces this means that neither male nor female vocals are favoured and placement of the different voices comes across very clear and precise.

The treble of the M4 is very well extended and offers an exciting, yet well-balanced sparkle. There is a lower treble lift that, while using stock tips and the PAW6000, was on occasion pushing it a little too far for my treble sensitivity. For instance the track 'Flight 420' by Astronaut Ape had treble details and sparkles that were fatiguing for me to listen to. However, this depended on specific album and even individual tracks. The album 'Interloper' by Carbon Based Lifeforms, which is a similar type of down tempo EDM with lots of details and sparkles in the highs, did not give me any issues. Of course by switching tips and source I avoided the issue altogether and 'Flight 420' became perfectly safe for my treble sensitivity.

The sparkle the M4 produce is very clean and extremely detailed. Cymbals sound great although they can, depending on the recording, have a slight brightness that I don't feel is entirely accurate. Then again, with decent quality recordings it does sound very detailed and accurate, so perhaps there is your explanation. Not so much the M4, but rather their revealing nature also revealing poor quality in recordings.

DITA Oslo cable
Usually I will include a few interesting cable pairings in my reviews, but with the MMCX of the M4 I am unfortunately not able to do that. All the cables I have available have 2-pin connectors and I don't have those nifty little adapters in order to use them with MMCX IEMs. However, Project Perfection did send along one cable for me to try out: the DITA OSLO cable. @twister6 has reviewed it separately (link), but this time Project Perfection specifically wanted us to have a look at the synergy with the FiR M4.

The OSLO cable is a very high purity copper cable that has been specially treated and as such the name stands for Oil-Soaked Long-crystal Oxygen-free (OSLO). The build quality of the cable is great. It is a fairly thin and very supple cable that comes (in this case) with high quality MMCX connectors, the signature round DITA Y-split and of course the Awesome Plug, which I am a very big fan of. Since the DITA Dream XLS came with its own OSLO-XLS cable, I have been using this cable a lot and found it very comfortable. The main difference here with this OSLO cable and the M4 is that the connectors sit on top of the M4's protruded sockets and that makes it a little high compared to the M4's stock cable where the connectors slide over and sit nice and flush. Not a big deal, but if the OSLO's MMCX connector would sit the same way, I think it would improve comfort even further. Of course that would be an issue for other IEMs where the MMCX sockets are recessed, so perhaps it is unavoidable.


The OSLO cable also comes with a little bottle of the oil solution that is used to treat the cables, which can be used on the connection of both the Awesome plug (i.e. the four connectors you see when changing the plug) and the MMCX (or 2-pin) connectors. An aftermarket cable supplied with a bottle of oil, you say? Cue the snake oil comments! Given the hostility of some to anything to do with aftermarket cables, I think that is quite a gutsy move. Even I was sceptical about it, but I gave it a fair try anyway.

And to be fair, if you call it a "contact enhancer" it all becomes a bit less controversial. I used it on two IEMs that have seen a lot of use, just to see if I could notice a difference. It was not possible to A/B because once it's on, it's on and I wasn't going to clean the connectors and reapply it several times. I am dedicated to doing thorough reviews, but I do have my limits. I first tried it with my DITA Fealty and felt that perhaps there was indeed a slight improvement in clarity, or perhaps just a cleaner and a little more stable sound. The difference was not huge and to be honest, I doubt I will ever use it purely for sound improvement. However, there was one point where I did find it incredibly useful and that was when I applied it to the Final E4000. Those have seen a lot of abuse as part of my boot camp review, where I went running in the pouring rain, the freezing cold, the blistering heat and had sweat pouring over the MMCX connectors during indoor bike trainings (talk about a thorough review...). By now the connectors have started suffering from frequent cut outs and I was about to accept their sad demise. Applying the contact enhancer however completely solved the issue and it gave the E4000 a new lease of life. So in that sense it was unambiguous that it indeed helped improve the quality of the connection.


The pairing of the M4 with the OSLO cable is a really good one where I feel the synergy between the two is bringing a few notable changes. The overall brightness of the M4 is toned down a little to the point where I can use the PAW6000 as a source again without having any problems with the treble (still using the Sony tips though, but purely because I get the most comfortable fit with those). The stage seems to open up even more than it already was and creates one of the biggest stages I have heard so far that stretches very wide with great depths, although height still seems to be similar. The bass of the M4 gets even more texture and a bit more body without loosing the tight control or impact. The little bit of extra warmth this generates helps to make the mids a little fuller and I feel this gives the mids a bit more naturalness. That is of course because it is more in line with my own personal preferences, as I tend to prefer a slightly warmer signature. Because of this the mid range also gets a little bit of the liquidity I mentioned in my review of the DITA Dream XLS, where in a very similar way to the Dream XLS, the M4 and OSLO pairing gives this sense of the notes flowing from one instrument to another. This is something that I felt very clearly with choral music, where for instances in Bach's Cantata #140 the voices flowed almost like water (hence the term "liquidity") without negatively affecting separation. Treble I feel is quite significantly affected -perhaps due to my sensitivity- and becomes sweater. I personally think this gave cymbals a more natural sound, less bright than with the stock cable, but still very well extended and incredibly detailed while having a great sparkle. That worked brilliantly for my treble sensitivity. [insert: big grin happy face]

I think the synergy between the OSLO cable and the M4 is very good and that combined with the comfort of the cable and convenience of the Awesome Plug, it makes for a very good upgrade from the stock cable. It is a little warmer and so it will depend on personal preferences if that is the direction you would wish to go.

-64 Audio Tia Trio-
These are very similar IEMs in terms of build quality and driver configuration (4-driver hybrid vs 3-driver hybrid) and so I initially expected the M4 to be similar to the Trio -I know, I am not always the most logical person- but they differ more than I had expected. The Trio are darker sounding, giving them a more laid-back feel compared to the M4. The M4 push details forward more clearly and yet present it in a way that feels more open and natural. Not that the Trio have the occlusion effect, their own tubeless design and built in Apex pressure release system is also very effective, but it seems like FiR Audio have managed to improve on it with the M4. The bass of the Trio feels a bit looser and woollier compared to the bass of the M4 and the M4 seem to dig a little deeper, keeping it tighter and retrieving more texture and detail at the same time. The mids of the Trio are perhaps a little warmer, but here I find that they share a similar natural tonality. The treble on the Trio feels a bit more polite and I personally never had any issues with it.

Overall I think these are both excellent technical performers and the main difference is in their tuning. The Trio are more easy-going, as well as more fun and engaging in my opinion, whereas the M4 are a little more restrained (i.e. good clean fun) and push details more clearly forward. I feel the M4 have the upper hand in pure technical performance, but it can be a bit misleading because of the Trio's darker signature. With both I have this same sense that the more I listen, the more impressed I am by what they are capable of. If it is between these two, then I think it mostly comes down to personal preferences. I personally prefer a slightly darker signature, so in that case the Trio has the edge, but at the same time I can't ignore the bass of the M4, it is just too good.

-DITA Dream XLS-
I recently reviewed the Dream XLS and already did a comparison there, but I wanted to include it again based on the longer time I have now spent with the M4. These two IEMs are very different and the M4 are definitely the more energetic ones that bristle with energy. If I compare the two with, for instance, Wolf Alice's 'Yuk Foo', the difference in energy is really noticeable. The Dream XLS are much more subdued in the bass and the treble, although the detail and texture is at a very similar level, the M4 simply push it forward a lot more. The treble of the M4 also has a bit more brightness and there are sparks flying with this track, which is not something that can be said with the Dream XLS. Mids of the Dream XLS are a little fuller and warmer and they are comparatively speaking, more mid-centric than the M4. The level of detail is really close, although as I mentioned it is presented in a different way. Both the IEMs excel at it and you will be hard pressed to find others that can do it quite like this: lots of detail and yet non-fatiguing. Be it with the caveat that, depending on your own treble tolerance, it might be good to roll tips with the M4 and use a relatively smooth source.

Build quality with both is excellent and you really get IEMs that will no doubt last a long time. In terms of fit I think the M4 work a little better for me personally, but I have always liked the fit of Bogdan's IEMs (I count previous and current models of 64 Audio's IEMs here too because he was involved in their development). The Dream XLS do have the edge in giving a more luxurious feel and those come with a better stock cable, but they are a bit more expensive as well.


With the M4 FiR Audio have produced a highly versatile set of IEMs that have a fairly uncoloured sound with a well-balanced bit of excitement coming from the bass and treble, while performing at a very high technical level. I always enjoy exploring IEMs and will generally find things I really like about them, but it is rare that I am so consistently impressed by IEMs as with the M4. They work for every type of music I have tried and always perform at the highest level regardless of price. With only a minor caveat concerning the lower treble lift, I can highly recommend a demo.