Home » Reviews » Custom IEMs » Bellos Audio X4: An Engineer’s Perspective – A Custom In-Ear Monitor Review

Bellos Audio X4: An Engineer’s Perspective – A Custom In-Ear Monitor Review

DISCLAIMER: Bellos Audio provided me with the X4 in return for my honest opinion. I am not personally affiliated with the company in any way, nor do I receive any monetary rewards for a positive evaluation. I’d like to thank Bellos Audio for their kindness and support. The review is as follows.

Bellos Audio is an up-and-coming in-ear manufacturer from Nashville, Tennessee. What their youth doesn’t tell you is the 20 years of experience they have in the form of Vlad and Bogdan Belonozhko. Formerly of 64 Audio, the pair founded Bellos to address some of pro audio’s most prevalent obstacles: Cost, lengthy turnarounds, strenuous repairs, on-stage comfort and fragile components. With their expertise in 3D-printing and a wealth of proprietary tech, Bellos want to deliver comfy, affordable and long-lasting IEMs quickly to you. Did we mention they sound really good too? Today, we cover their flagship X4: A warm reference that makes $999 a near-bargain.

  • Driver count: Three balanced-armature drivers and 1 dynamic driver
  • Impedance: 16Ω @ 1kHz
  • Sensitivity: 116dB @ 1mW
  • Key feature(s) (if any): In-Air Canals, ATOM Venting
  • Available form factor(s): Custom acrylic IEMs
  • Price: $999
  • Website:

Packaging and Accessories

The X4 arrives in modest packaging; a sleeved cardboard box with high-res images and text all around. There isn’t fancy embossing, textures or foils. It’s functional, no-frills, and the type of packaging you’d associate with artist-oriented products. Inside, you’ll find the X4’s puck case in bubble wrap. Personally, I’d have preferred a foam-lined interior with the case sitting in a circular cut-out. Dressing up some of the bare cardboard would’ve helped the unboxing experience too. But, for the working pro, whose main concern is the monitor reaching them in one piece, Bellos’ effort here should very well suffice.

Thankfully, the X4’s case is the very opposite of modest. It’s a metal, puck-shaped chassis with a screw-on top, engraved with the company logo and client’s name. The case is compact, and the lid has smooth action. And, inside are the X4’s with its cable tied with a twist tie. Again, as a consumer, a leather strap would’ve been nice. But, speaking as a drummer and engineer who’s likely to lose such a strap in the midst of a busy set, I’d instead suggest a Velcro strap attached to the cable. I’d love to see something like that in Bellos’ next product.

The case’s internals are very similar to their partner FiR Audio’s cases. It has a foam insert with cut-outs for the included wax pick and In-Air Canal tips, which we’ll get into later. The tips come in 3 sizes; small, medium and large. And, the case has a velvet lining throughout to ensure max safety for the IEMs in storage.

The X4 comes with an 8-wire SPC cable with FiR Audio’s RCX connectors and a right-angled, single-ended 3.5mm plug; both housed in a translucent plastic. Its Y-split and cinch are finished in metallic silver, which nicely match the X4’s logos. Being a pro-focused product, I’m not expecting heavy, metal connectors or overt branding. No-frills, it is. But, where it matters – the lack of fussy memory wire, weight, flexibility, craftsmanship and finishing – I really can’t fault it. It’s a well-made, all-ends-met cable that’s been a joy to use whether behind the console, the drums or my desktop. Plus, with FiR’s RCX, you’re also guaranteed secure connections at least 5000 times over.

Aesthetics, Build and Wearing Comfort

When ordering your Bellos Audio CIEM, you can choose between 3 colors – matte black, matte green and matte gray. I went with black for my X4. And, they share a common faceplate design; the Bellos Audio emblem inlaid in metallic silver. Despite the relative lack of options compared to their competitors, I think Bellos’ clean design looks superb. The matte shells are unique to the industry (apart from Bellos’ partner FiR Audio), and the silver art contrasts crisply against them. I think the alloy-steel faceplate screws add an industrial aesthetic as well. So unless multi-hued swirls, exotic faceplates or custom artwork are an absolute must for you, I think the Bellos look should perfectly suffice for most.

As far as build goes, I cannot fault my X4’s at all. The matte texture is consistent all throughout, the metallic inlay and alloy-steel screws are sunk-in perfectly, and the faceplates join both shells seamlessly. Additional structures like the In-Air Canal nozzles, the RCX sockets, the tiny ATOM vents and the internal nameplate, which contain the user’s initials and serial number, are made just as well. Like FiR’s, Bellos’ shells are printed with above-average thickness, so they feel reassuringly hefty in the hand. This is the quality you can expect with builders as seasoned in 3D-printing as the Bellos brothers, and any musician who gets their hands on these, I’m sure, will instantly feel like they’ve gotten their money’s worth.

Through their proprietary In-Air Canal technology, I believe Bellos Audio have crafted the most comfortable pair of customs I’ve ever worn. I own somewhere between 20-30, and the X4’s feel (at least in the inner ear) unlike any of them. In place of a hard, acrylic nozzle is an ultra-pliable, soft silicone; even softer than the 2 silicone customs in my collection. And, it genuinely feels like nothing is in my inner ear. I feel acrylic hanging onto my outer ear, then nada beyond that. The closest comparison I can make is wearing a UIEM with tips too small, so you aren’t achieving a solid seal. Except here, you’re achieving full isolation and bass. It’s, again, a truly unique feeling, and it’s genuinely one of the most notable innovations I’ve seen in CIEMs in a long time. Bravo.

As far as the outer ear goes, I’d say the X4’s are closer to medium-sized customs. They aren’t as filling as 64 Audio’s, nor as rounded and inner-ear-dominant as VE’s or Custom Art’s. They more so grip the top of the ear without filling the concha bowl too much. And, the inner ear, as said, basically feels nothing. Most importantly, Bellos’ nanometer-precise 3D-printing means its weight and pressure are distributed as well as possible. It’s a very comfy custom that I’ve literally used to mix for 5-6 hours straight before experiencing any discomfort. So, any engineer mixing long shows or working long stretches on the band tour bus should find great comfort with these.

The Pro Pitch

Bellos Audio prides itself as a brand that puts musicians first, and they’ve developed a pipeline with this very principle in mind. From their 3D workflow, to their performer-friendly technologies, it’s clear where their priorities lie.

Turnaround Time and Serviceability

Bellos Audio guarantees a turnaround time of 5 working days after they receive a usable pair of ear impressions; a far, far cry from the 6-to-8 weeks (at least) most brands will put you through. Taking into account the 2 weeks shipping to and from Bellos, you’ll get your IEM within 3 weeks of ordering, which is astonishing. You can even shave off an extra week if you already have ear molds in digital, STL form.

And, if your earphones ever run into any issues, Bellos’ customs all have removable, screw-on faceplates. It’s unlike the rest of the industry, where faceplates are glued on with resin adhesive, then sealed with the shell in lacquer. Customs like that require breaking open, almost, to repair the in-ears, then rebuffing and repolishing after the fact. Bellos’ design makes repair far easier, far less invasive and far quicker. This means, if your in-ears break just days before a big show, you’ll likely have them back in time for dress rehearsals. Heck, if you’re local, you can probably send them in in the morning and have them back by lunch.

In-Air Canals

This is a Bellos Audio original that seeks to revolutionize custom IEM comfort. Unlike traditional CIEMs that have full solid nozzles, Bellos’ are cut short, and in their place are stems for silicone tips. This means your tougher outer ear will be covered fully by precisely-printed acrylic, and the only material in your sensitive inner ear is soft, pliable silicone. This not only increases comfort to, again, a vanishing degree, but these soft, malleable tips will also maintain a seal when your ear canal inevitably changes shape whilst talking or singing, which hard acrylic nozzles can’t do.

It’s also worth noting that the tips included with my X4’s are a new revision, which Bellos started sending out late last year. They’re now a matte-black versus the clear originals, and what Bellos have changed is their ability to hold onto the nozzle better. So, the in-ears won’t fall out with the tips stuck in your ear when you’re moving on stage. And, they should also provide a better seal, which’ll lead to stronger isolation and bass response.

Again, as someone who’s spent a decade with customs and seen nothing but incremental shifts, I can confidently say this is one of the biggest technological leaps I’ve seen in a long, long time; it’s certainly the most practical. I’ve been able to pull literal all-nighters mixing and mastering on the X4 with no qualms in comfort. I’d argue this feature alone is worth considering a Bellos IEM as your next, and it’s something I look forward to seeing their competitors attempt to replicate.

ATOM Venting and Isolation

Courtesy of their partners FiR Audio, Bellos’ customs come equipped with ATOM venting in the form of a tiny hole along the seam of the faceplate. This allows a small amount of air to travel to and from the ear canal, releasing any pressure that may build up there. Because of this, fatigue will set in slower, allowing longer, more comfortable listening. And, it results in airier, more open staging as well.

Now, the drawback to ATOM venting is the amount of isolation the in-ear provides. Rather than the -26dB traditional customs give, the X4 only has -17dB. In practice, though, I haven’t found it to negatively impact my performance behind the console or drum kit. Mixing front-of-house in a venue with a fairly-loud PA system, it only takes low-to-moderate volume to drown the speakers out and focus solely on the in-ear mix. And, on stage, the tiny hint of ambience – and I do mean tiny – actually helps stoke the feeling of a live performance. So, unless your venue is obscenely loud, or you need the utmost isolation from external noise, the X4’s -17dB should easily suffice.

RCX Connectors

All of Bellos’ customs also come with FiR Audio’s durable RCX (Rigid Coaxial) connectors. They look similar to the more common MMCX, but they’re rated at a much higher 5000 mating cycles vs. the MMCX’s 500 (per Amphenol’s datasheet). So, they’re less likely to come loose over time – they won’t come flying off when you’re performing on stage – and you’re less likely to have to send them back in for repair(s). Connections with RCX are also firm and tactile, which, again, just adds to how solid these X4’s feel to use.



Picture of Deezel


Church-boy by day and audio-obsessee by night, Daniel Lesmana’s world revolves around the rhythms and melodies we lovingly call: Music. When he’s not behind a console mixing live for a congregation of thousands, engineering records in a studio environment, or making noise behind a drum set, you’ll find him on his laptop analysing audio gear with fervor and glee. Now a specialist in custom IEMs, cables and full-sized headphones, he’s looking to bring his unique sensibilities - as both an enthusiast and a professional - into the reviewer’s space; a place where no man has gone before.


Leave a Reply

Your email address will not be published. Required fields are marked *