Radioactive Bunny.

PROS: durable metal shell with a beautiful faceplate design, interchangeable atom modules, kinetic bass you can feel, more detailed natural sound tuning (vs XE6), new premium cable and other accessories.

CONS: price, uni only (for now), small size of atom xs module (uni).

The product was provided to me free of charge for the review purpose in exchange for my honest opinion.

Manufacturer website:  Fir Audio.  Available for sale directly or official retailers like Bloom Audio.

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Intro.

Last year when Fir Audio introduced their new Frontier Series, I jokingly asked Bogdan if this is the “Final Frontier” or should we expect more of the “Next Frontier” surprises.  After the release of Xenon 6 (XE6), Krypton 5 (KR5), and Neon 4 (NE4), Belonozhko brothers had a very busy year keeping up with a demand from basshead audiophiles while continuing to collect the feedback which paved the way for the Next Frontier release.  As I mentioned in my original Frontier Series review, the term “basshead audiophile” would seem like an oxymoron because basshead sound tuning is usually geared toward consumers who don’t care as much about the quality of the bass or the resolution of upper frequencies.  But audiophiles are a different breed with a basshead fetish for both, the quality and the quantity.  That was the attraction of Fir Audio Kinetic bass driver under the mesh grill cover of the shell, delivering a physical analog quality bass you can feel.

The actual quantity of the Kinetic bass varied between their different models and depending on atom module selection.  If you wanted more bass impact and fuller body warm organic tonality, XE6 was the way to go, and if you preferred a scaled down bass slam and improved retrieval of details, KR5 was a better choice.  But what if you want something in between?  Apparently, that was common feedback from a number of Fir Audio customers who didn’t want to compromise the bass quantity in favor of upper frequencies clarity.  And that is how a new family member of Frontier Series, Radon 6 (RN6), was born, to meet the demand of both requirements.  So, is RN6 a new IEM or a retuned version of either XE6 or KR5?  I received RN6 review sample just before CanJam NYC ’23 and spent over a month testing and talking about it on Head-fi.  Finally, I’m ready to share and to summarize my findings in this detailed review of Fir Audio latest Radon 6 IEMs.

Unboxing and Accessories.

The packaging of RN6 is similar to other Frontier Series IEMs, a compact box with a customized sleeve artwork to reflect the color theme of these IEMs and to give you a summary of key features.  Inside, there is a secure foam tray for IEMs and the cable, a genuine leather protective round case, Fir Audio “space force” sticker, and a number of other accessories including a set of foam eartips (S/M/L), a set of Symbio eartips (S/M/L), a cleaning tool, 0.9mm hex key bit for atom XS modules, and a small holder with additional atom XS modules.  You get a total of 3 pairs: black, silver, and gold.

RN6 is only available as universal, though I have seen a few exceptions with people who received a custom version.  Perhaps, if the demand is there, Fir Audio will offer CIEMs in the future.  As a side note, a custom version is typically geared more toward musicians, but they are becoming very popular with audiophiles as well who want a perfect fit with a maximum sound isolation.  Other Frontier series iems do offer a custom version which obviously comes without eartips, and if you choose a replaceable atom module you get X version instead of XS.  XS is tiny and requires tools, while X version is much bigger and can be handled without a tool as long as your finger nails can grab it.  Fir Audio even has the option for their other Frontier Series CIEMs to be configured with internal atom module.  But for now, RN6 is universal only.

At first, I received the original pure copper 8-wire cable that comes standard with Frontier Series iems, but later I learned that RN6 will have a new pure silver cable with OFC shielding.  The new cable didn’t even make it to CanJam NYC, became available only after the show when I received it for testing.

The cable features 2 thicker conductors going to each side, 4.4mm termination with a comfortable grip metal housing connector and Fir Audio logo, a matching finish metal y-split and a sliding chin-slider with “fir bunny ears” logo, pre-shaped earhook heatshrink, and 2pin connector with a compact straight metal housing and corresponding L/R marking on inside and “ears” on outside.  The cable has a soft pliable pvc insulation in dark brown finish.  It is very comfortable to wear on the go and has zero microphonics.

Design.

Last year Fir Audio introduced three models under their Frontier Series, each with a unique name and number corresponding to a driver count under the hood.  Xenon 6 (XE6, $3,899) uses 6 drivers: 10mm kinetic DD bass, 2x open BA mids, 1x open BA high-mid, 1x open BA high w/sound reflector, 1x EST ultra-high, and 28ohm impedance.  Krypton 5 (KR5, $2,999) uses 5 drivers: 10mm kinetic DD bass, 2x open BA mids, 1x open BA high-mid, 1x open BA high w/sound reflector, and 22ohm impedance.  Neon 4 (NE4, $2,299) uses 4 drivers: 10mm kinetic DD bass, 1x open BA mid, 1x open BA high-mid, 1x open BA high w/sound reflector, and 22ohm impedance.  The new Radon 6 (RN6, $3,299) has the same 6 driver config as XE6: 10mm kinetic DD bass, 2x open BA mids, 1x open BA high-mid, 1x open BA high w/sound reflector, 1x EST ultra-high, and 28ohm impedance, but its price is cheaper than XE6.

Regardless of the driver config, the exterior dimensions of these IEMs are the same, only varying in shell material/finish and faceplate design.  Top of the line XE6 features a bit heavier machined stainless-steel shell with gold plating exterior and sapphire glass faceplate over carbon fiber with infused gold flakes.  The next one, KR5, has an aluminum shell with a shiny chrome-like plating and a sapphire glass faceplate over carbon fiber with a granite-like pattern.  NE4 also has aluminum shell with a similar chrome looking plating, sapphire glass faceplate, and unique granite-looking faceplate pattern.  The new RN6, which also shares top of the line crown, has a machined aluminum shell with all black plating exterior and sapphire crystal glass faceplate over carbon fiber with infused gold flakes, similar to XE6.  Obviously, SS shell will be the heaviest, RN6 and KR5 weights are same, and NE4 is a bit lighter probably due to having less drivers.  Each universal pair has infamous “fir bunny ears” artwork on the right side and either RN6, XE6, KR5, or NE4 unique font print on the left side.

But one thing will remain common between all Universal and Custom Frontier series models, the main feature of the design, the Kinetic bass.  It refers to 10mm Dynamic Driver that is open and ported under a mesh cover while facing outside through the back of the shell.  By design, it offers a hybrid conduction system, allowing you to hear the low frequencies in two ways.  One is through air-conduction of the sound hitting your eardrums via the nozzle, and the other one is by means of bone-conduction from the back of the driver transmitting directly to the ear cartilage through the bones in your head, converting to kinetic energy as it reaches the inner ear (cochlea).  That is what this Kinetic Bass is about, a hybrid conduction where you can both hear and feel the bass.

One thing I did notice, while the driver size is the same, the actual Kinetic driver opening of XE6 is bigger than RN6, KR5, and NE4.

Also, every IEM will have Atom (air transferring open module) pressure relief system which reduces ear fatigue during extended listening and at higher volume by venting pressures that build up in a sealed ear canal.  As it was already mentioned, UIEMs use small screw-size Atom XS modules, while CIEMs with removable module option use a regular size Atom X.  XS does require a tool to remove it.  Not going to deny it, it’s a finicky process where you have to be very careful not loose XS module, so make sure to use a well-lit area and on a flat surface.  Furthermore, as you screw the module in, there is no hard stop, it will continue turning once inside, which is OK and expected.  Per Fir Audio, “The thread on the module is only to help back the module out when removing, there is no thread in the receiving pipe.  You can simply press the module in on insertion, gently until it stops, no spinning required.  This feature uses what we call a soft thread.  It can’t strip so no worries on that.”

Frontier Series IEMs come with a set of 3 Atom XS/X module pairs (black, silver, gold), and you can buy optional red module pair:

• + Isolation 17dB: gold
• N Isolation 15dB: silver
• – Isolation 13dB: black
• O Isolation 10dB: red

Frontier Series also features Fir Audio Open Acoustic system which is part of the tubeless driver design.  BA drivers are open with no sound tubes, radiating directly into the sound chamber, referred to as Sound Reactor which uses natural acoustics instead of dampeners and filters for the shaping of the sound before it reaches your ear canal.  The high BA driver, also open, is located inside the nozzle (canal stem) and uses a Sound Reflector instead of Reactor, to direct high frequencies into the ear canal.  Basically, everything is tubeless and all sounds get mixed together naturally while directed to your earcanal.

Another unique feature of Frontier Series is Rigid Technology which improves the reliability and durability of the design.  In theory, it could even allow a user to “service” IEM thanks to its modular components which can be easily replaced instead of sending it back to manufacturer.  The universal 2pin shell socket is part of this rigid modular design which can be replaced by removing small hex screw holding the connector in the shell.  Also, you will find a snap screen at the tip of the nozzle which can be replaced by removing a spring ring that holds it securely in place.  And when it comes to CIEM models, the 3D printed shells are 20% thicker than average for a better protection of the internal components.

The fit.

Frontier series universal shells sit comfortably inside my ears, sticking out just a little bit.  The nozzle has an average length, not too long or too short.

Page 2 – Sound analysis and Eartips selection.
Page 3 – Cable pair up and Source Pair up.
Page 4 – Comparison and Conclusion.