Home » Reviews » Earphones » Fir Audio M4 Review – Detail Freak

Fir Audio M4 Review – Detail Freak

Pros –

Compact and comfortable metal shells, Excellent bass power and control, Superb resolution and detail retrieval, Holographic imaging, Easy to drive relative to competitors, Minimal wearing pressure

Cons –

Sharp note presentation combined with bright signature can wear on the ear over time

Verdict –

The M4 is one of the best in-ear monitors on the market for those that enjoy a clean, tactile sound and want enormous detail retrieval presented in a holographic manner.

Introduction –

Fir Audio are a relatively new company founded in 2018 by Bogdan Belonozhko. Usually, most would rave here about his past experience as the CEO of 64Audio, about how he split off from his brother Vitaliy to form a new company with a new ethos. While similarities in approach can be drawn, Fir’s designs stand on their own and with this, the company has ossified a distinct and unique persona. The M-series jettisoned Fir to become a household name in the high-end IEM space. This prosumer line up spanned from the M2 to the flagship M5 with the VxV targeting the audiophile crowd more directly. The M4 sits just below the flagship M5, yet perhaps this outlook doesn’t do it justice. For the M4’s clean, vibrant tuning serves as an antithesis to the warm, coherent M5, meaning a case can be made that it is a valid co-flagship option. As usual, Fir’s legendary technologies are present here albeit without the Sonion E-stat tweeter system seen on the M5. It remains a technically astounding design that stands strong in a price class with no shortage of impressive competition.

The M4 retails for $1899 USD. You can read more about it and treat yourself to a unit on Fir Audio.

Disclaimer –

I would like to thank Asher from Project Perfection and the team at Fir very much for their quick communication and for their generosity in accommodating a discounted M4 for honest review. All words are my own and there is no monetary incentive for a positive review. Despite receiving the earphones at a reduced cost, I will attempt to be as objective as possible in my evaluation.

Contents –

Specifications –

  • Drivers: 10mm Dynamic driver woofer, 1x BA mid, 1x BA upper-mid, 1x BA high
  • Frequency Response: 10 Hz – 20 kHz
  • Nominal Impedance: 6.4 Ohms

Behind the Design –


Image provided by Fir and Project Perfection

The M4 is fit with Fir’s air transferring open model (ATOM), a pressure release system similar in form and function to ADEL and APEX seen on many other high-end IEMs. ATOM differs in structure from competitors, consisting of foam materials and a nano-thin membrane filter system. It was designed to reduce ear fatigue by minimising pressure on the ear drum but in the same sense I do find these technologies to greatly enhance wearing comfort for enthusiasts too, greatly reducing the sensation of wearing pressure generated by the in-ear form factor. In addition, there are sonic benefits in the form of a wider soundstage and more open wearing sensation and Fir reason this comes with minimal impact to passive noise isolation.

Direct Bore Drivers

Image provided by Fir and Project Perfection

We’ve seen tubeless drivers on a few models, think 64Audio’s TIA drivers and Campfire Audio’s T.A.E.C. By bypassing the usual tube and damper system used by most IEMs (especially hybrids due to the sensitivity mismatch between drivers), manufacturers are able to extract better extension and less distortion from traditional BA drivers. Fir’s monitors sport entirely tubeless drivers and the direct-bore aperture refers to the tweeter driver which has been placed within the nozzle of the IEM itself. By widening the aperture, Fir reason that a greater air volume can pass through them, permitting a more natural and organic sound with a faster transient response.

Tactile Bass Technology

Image provided by Fir and Project Perfection

Fir have outdone themselves with their tubeless design, not only applying it to all 3 BA drivers but the DD woofer as well. The dynamic driver fires into the entire chassis rather than having its own chamber. This maximises the air volume it can work with and enables some mass loading with the metal chassis, helping to improve detail and optimise decay. This results in an especially tactile and controlled bass performance.

Unboxing –

Where the VxV offered a very minimalist unboxing, this is not the case with M4 that comes in a larger hard box with eye-catching gold-foil inlays. Sliding off the outer shell reveals the earphones protective within a foam inlet and cable snaking their way down channels in the foam. The cable is then coiled below which is a great addition as some cables come kinked out of the box; here it uncoils nicely with no tangles or preformation. Below is the leather carrying case with a lovely red stain. It has a suede interior that is soft to the touch and prevents scratches. In addition to housing the IEMs, the case has a foam inlet containing the ear tips and cleaning too. It holds them snugly which means they can be accessed during commute and travel where the listener may want to switch to more isolating foam tips for example. Inside are 3 sizes of silicone tips in addition to 2 pairs of foam tips. A warranty card, user manual and round sticker are also included.

Design –

If you’re familiar with any of Fir’s designs, you’ll have very much the same experience here. What we observe are clean, minimalist shells that achieve beauty in their simplicity. The M4 is an especially captivating model to me with its champagne faceplates and, as opposed to the VxV, employs sand-blasted aluminium all around. In fact, Fir tells me each shell has been CNC milled form a single block of aluminium, albeit the faceplates have been separated so the interior can be assembled. It does mean the faceplate matching is nigh perfect which contributes to the premium look and feel. In addition, it means there’s less glue and no risk or snapping the nozzles and stronger connectors.

Speaking of which, up top are Fir’s RCX connectors. They are similar to MMCX but slightly outset, locking into an inlet in the Fir cable for additional integrity. This does mean third party cables will require either custom RCX connectors or adaptors. That said, the stock cable impresses with its 8-wire braid and silver-plated copper conductors. It’s light, supple and tangle-resistant with minimal memory and microphonic noise conduction. I experienced no connectivity issues during my testing and found the ear guides were well-shaped and comfortable. The M4 has a 3.5mm straight plug with good strain relief and a case friendly protrusion. The whiskey jacket is a good match for the champagne shells so I felt no need to change the cable.

Fit & Isolation –

I think it’s admirable that, despite the slew of technologies and innovations that went into this earphone, Fir were still able to achieve such a compact design. The M4 isn’t tiny, but much smaller than your average IEM, especially amongst its high-end competitors. Its simplistic styling also means it doesn’t have the superbly locked-in faux custom fit of some competitors, but guarantees better comfort for a wider range of ear shapes. For me, I found its rounded edges and slender proportions to offer perfect wearing comfort over longer listening sessions with no hotspot formation. Its nozzles are long and well-angled but not excessively wide. This permits a slightly deeper than average fit. Meanwhile, the ATOM modules mean wearing pressure is minimised, yet the over-ear fit and well-designed nozzle mean zero fit stability has been sacrificed. In turn, there is zero driver flex and, due to the very small vents, wind noise isn’t exacerbated when outdoors either. Similarly, isolation is strong if shy of a fully-sealed IEM – I’m sure the custom M4 would also do much better in this regard. Still, this is easily suitable for regular commuters and even air travel when paired with more isolating foam tips. Altogether, a smart design that achieves excellent wearing comfort and zero wearing pressure.

Next Page: Sound & Source Pairings



Ryan Soo

Ryan Soo

Avid writer, passionate photographer and sleep-deprived medical student, Ryan has an ongoing desire to bring quality products to the regular reader.


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