FiR Audio M4

Good Clean Fun (Very Good Clean Fun)

PROS: Fairly uncolored 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.


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 favorable 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 recognizes 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 braided, 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 color 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.

Page 2 – Sound analysis.
Page 3 – OSLO cable pair up.
Page 4 – Comparisons, and Conclusions.

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