64 Audio Duo Universal In-Ear Earphones review

The newest release from renowned IEM manufacturer 64 Audio, the $2149 64 Audio Duo Universal In-Ear Earphones, is their most affordable universal IEM model to date. The Duo, as its name implies, is a dual-driver IEM with a single dynamic driver plus a balanced armature driver tucked away inside each shell in a hybrid configuration. 

The Duo features many of the signature proprietary technologies pioneered in their more expensive models such as “Tia” (tubeless in-ear audio) and “LID” (linear impedance design), whilst also bringing some innovative new design features to the table along with a unique tuning that makes it much more than simply the “entry-level” model in the 64 Audio lineup - the Duo has a different sort of listening philosophy baked into it. 

“In-ear”, or “out-of-head”? 

Listening with a pair of IEMs is generally a far more intimate experience than the one that a pair of full-sized headphones provides - especially those of the open-back variety. IEMs are far more compact devices (by design, obviously), and the sensation that those tiny drivers create when they channel music directly into your inner ear can feel far more personal and small compared to the more open-air, spacious effect that comes with the added space between your ear and the driver of a full-sized pair of cans. Sometimes, this can be a great thing - IEMs can create an extremely focused sound that reveals hyper-levels of detail, all the while creating a layer between the outside world and whatever album you happening to be shuffling by virtue of the way they seal your inner-ear.

This presents somewhat of a conundrum if you’re on the fence between buying yourself a nice pair of IEMs, or a set of over-ear headphones. Need to keep things compact and plan on doing most of your listening while you’re out-and-about? An in-ear solution is probably your best bet. Or are you planning on doing most of your serious listening at home in a quiet environment while you’re firmly planted in the one spot? If that sounds like you, then you might be putting most of your research into a pair of full-sized headphones. 

As well as the obvious benefits of both earphone formats, this generally requires you to  trade-off between portability and the added “stage” that a full-size headphone creates. And this brings us to the subject of this article, the new 64 Audio “Duo”. When designing their newest Universal IEM, 64 Audio sought to recreate the spacious “open-air” feeling that wearing an open-back headphone can create and so went about designing it with some interesting and innovative new features to help create a rather unique offering that sets it apart from their other Universal IEM offerings. 

Apex Core

The most striking feature of the 64 Audio Duo is without a doubt its unique black mesh grille on the outer face of the IEM shells. Offset by a striking polished aluminium, the grilles are a little reminiscent of the sort of open-air mesh you’d find on a pair of open-back headphones. And it’s not just an aesthetic choice - 64 Audio has employed a new technology here that they’ve called “Apex Core”. 

The perforated grille has been designed to create a less-obstructed airflow between the IEM and the outside air to release pressure outward for a wider, less fatiguing sound. Often, IEMs can create an uncomfortable sensation due to pressure that builds up inside the body of the IEM and your ear canal that gets tiring after listening for a while, and also reminds you that you have two small nozzles stuck inside your ear - not exactly conducive to a fun listening experience. 

Wave Sync 

If you’ve come across 64 Audio IEMs before then you’re probably used to seeing multiple balanced armature drivers arrayed in a complex cross-over network - there are no fewer than eighteen in their 18s (check out Stephen’s 64 Audio 18s Universal-Fit Earphones review for a closer look at this model). However, the new Duo (as you can probably guess by the name) only uses two drivers - a dynamic driver for low/mid-range frequencies and a “Tia” balanced armature for highs - their lowest driver count in a universal-fit IEM to date.

The two different-style drivers have been paired together in a hybrid configuration with a relatively high crossover point (crossover = the point at which one driver “hands-over” to the other in the frequency range) using a new technology that 64 Audio has called “Wave Sync” - an “all-pass filter” that ensures the phase of the two drivers is always correct without having to change the physical distance between the two drivers. The benefit to you, the listener? 64 Audio explains that Wave Sync creates an entirely coherent sound whereby there are no perceptible timing issues as audio signals move between both drivers, making for a more linear, immersive listening experience. 

64 Audio Duo specifications

Driver Type/Count

2 Drivers - One dynamic driver and one balanced armature driver

Driver Configuration

1 tia high, 1 dynamic low

Frequency Response

20Hz – 20kHz


98dB/mW (98dB @ 1kHz @ 1mW (95mV)



Integrated 2-way passive crossover


-12dB internal apex technology

64 Audio Duo design and build

The Duo’s unique silver and black mesh outer fascias make it an attractive and differentiated-looking pair of IEMs without looking overly “blingy” or gaudy. The rest of the Duo’s shell is made from a single piece of smooth, nice-to-the-touch dark-finished aluminium with no visible seams, with only a single round vent hole on the bottom of each shell to relieve sound pressure. 

The Duo has the smallest profile of any 64 Audio Universal-fit IEM, no doubt due to the fact that only two drivers have had to be taken into consideration when designing and sculpting their overall outer shape. They are relatively small as multi-driver IEMs go, and fit easily into my outer ear without too much outward protrusion at all - the Duo’s shape and materials make them an extremely comfortable pair of in-ears and I can wholeheartedly vouch that they’re great for all-day listening without the slightest hint of pressure or discomfort. 

The Duo features 64 Audio’s “Tia” or “tubeless in-ear audio” inner architecture which it shares with its older brethren. Rather than having a tube channel audio from each driver to the outer nozzle, both the dynamic and balanced armature drivers are allowed to radiate freely 

You wear the Duo in the “over-ear” cable fashion, with a detachable cable connected to the top of each shell via the common “two-pin” system which makes swapping cables a simple affair. With the cable wrapped over the top of your ears, the Duo stays firmly and comfortably in place. The cable itself is an all-new affair from 64 Audio - it’s made from four strands of silver-plated, 26 AWG OCC copper and coated in a smooth, rubberised black plastic. It feels nice, avoids kinking, and is generally great to live with - it just gets out of the way and lets you carry on with enjoying your music without any annoying tangles or unwanted microphonic noises. The cable is terminated in a single-ended 3.5mm jack for connection to most standard device outputs, so if you plan on hooking up your new Duos to a balanced output you’ll need to BYO cable. 

64 Audio Duo packaging and accessories

As you’d expect from a premium audio product at this price, the Duo arrives in a suitably appropriate, premium hinged black cardboard protective box which opens up to reveal the IEMs nestled inside cutout foam along with a generous assortment of accessories. 

64 Audio provides one of their excellent round storage/carrying cases with the Duo, which is finished with the kind of soft leather-like material that would be entirely welcome with a very expensive watch (if you dabble in that hobby as well, you’ll know what I mean). 

As well as the aforementioned cable, 64 Audio has generously equipped the Duo with three sets of silicone ear tips, three sets of SpinFit tips, and three sets of TrueFidelity tips for a total of nine tips all up including S/M/L sizes for each variety). Not only is this helpful in terms of helping create the most comfortable fit and seal, but they each lend a slightly different quality to the sound as well as offering different levels of external noise isolation. The stock tips have a wider opening at the nozzle and have a shallower insertion depth than the SpinFit silicone tips, which creates a more snug fit in my ears. This makes the SpinFit tips better at creating a seal against outside sound, as well as a firmer low-end response. The supplied foam tips create the best seal against outside noise, and further darken the sound another notch with a greater sense of mid-bass weight. 

Isolation is an interesting point on the Duo - while they aren’t technically an open-back design, they don’t offer quite so match passive noise reduction compared to your run-of-the-mill IEM. Whereas most IEMs reduce outside noise by 20-25dB (give or take), the Apex Core system in the Duo means that the outer shell attenuates less outside noise - by design. 64 Audio rates the passive isolation of the Duo at -12dB, and while this may seem substantially less on paper, they actually block more of the outside world than I was anticipating. If you’ve hit “pause” on your music and someone comes up to you to have a conversation, you can generally leave them in and chat almost like normal. With music playing and the foam tips installed, you can still hear your fingers click (my standard “scientific” isolation test) but traffic noise from outside on the street is almost inaudible. I wouldn’t recommend the Duo as your first pick if you’re looking for something to listen to on flights on a noisy bus, but in normal real-world conditions, you ought to be fine. 

Going back to my original point about the trade-offs of open-back headphones, I actually find their open-air design somewhat advantageous when it comes to feeling not entirely disconnected to your surroundings. Like it would if you were listening to a pair of speakers, you’re still able to hear colleagues around you in an office setting (just don’t sit too close to others) or hear the doorbell if you’re expecting a delivery at home. Listening to the Duo neatly splits the difference between IEM immersion and open-back spatial-awareness - at once you feel entirely connected to your music while still feeling like you’re connected to the world around you. They’re great for ocean-side jogs, listening sessions in the park, or for when you simply can’t afford to hear that “boarding now” announcement at the airport. My advice is that the Duo is best auditioned in real life to get a proper feel for how they’ll feel and sound (like all audio equipment, really). 

Listening to the 64 Audio Duo 

Thanks to its light and comfortable design the Duo tends to “disappear” on your head after you pop the tips in your ears, wrap the cable around the ear and plug them into your musical source of choice. 64 Audio claims that the simplified approach to tuning with their two-way “Wave Sync” crossover can also make you “forget that you’re wearing IEMs” - one of their stated goals with this design. A couple of tracks into The Strokes’ “The New Abnormal”, I found that I actually agreed with this assessment. The 64 Audio Duo has a warm, natural sound that feels entirely linear and devoid of any irksome peaks or troughs that spoil your listening party. Because the Duo sounds tremendously pleasant and immersive, it has the effect of pulling you in to your music rather than simply piping it down your earholes. 

In terms of voicing, the Duo has a gently “V-shaped” tuning with generous lashings of rich bass energy; a mildly recessed and yet nicely textured mid-range; and a treble region that somehow manages to be both gentle and forgiving whilst still managing to provide a level of upper sparkle and shimmer that creates a nice sense of “bite” and clarity. While the Duo has a very even-handed and relaxed character that never gets grating nor sibilant, it manages to add just enough of a sprinkling of pixie dust to add a Goldilocks-like layer of shimmer to proceedings. The Duo is definitely a fun-sounding IEM, but it never gets overly fun to the point where someone at your party breaks your Nana’s vase and it’s time for everyone to pack up and go home. 

Australian neo-soul band Hiatus Kaiyote’s 2015 record “Choose Your Weapon” is both extremely well-produced, and extremely fun - the mix of singer Nai Palm’s vocals, great instrumentation and expertly-recorded drums also makes it an enjoyable album to put a pair of IEMs like the Duo through its paces. The Duo’s excites with plenty of dynamic thump from the bass and kick-drum parts, and yet never gets out-of-hand when it comes to cymbal hits and fast, treble-heavy passages - it has a real rounded, forgiving gentleness to it. Instrument and vocals have a lovely organic texture courtesy of the Duo’s dynamic driver, but the gentle recession of the lower mid-range gives the Duo a more subdued, less “in your face” character that also adds to its sense of space. Ryan Adam’s reverbed vocals in his track “To Be Without You” actually sound like they’re decaying away somewhere into the room that you’re sitting in, rather than being contained within the two small IEMs that are connected to your ears. The plucks of his acoustic guitar strings sound as good as any headphone I’ve heard in terms of tonal believability, and the hit of the toms do feel like they’re “visibly” coming from two metres behind the band - the Duos do have a nice sense of depth as well as space. 

You’re probably wondering at this point whether 64 Audio has managed to make the Duo sound like an open-back pair of headphones - yes, and no. Their relaxed tuning and warmer sensibilities make them feel more like a Sennheiser HD650, say, than the sheer width and clarity of a Sennheiser HD800s, to borrow an open-back headphone analogy. The Duo does have a nice sense of left-right staging that certainly extends beyond your ears, but its smoothed-over nature and honeyed top-end tend to smear individual tracks into each other a fraction rather than pulling them apart and separating them with a clear sense of “air” between them. This isn’t a criticism by any means - the Duo simply has a lovely smooth, warm tone that feels comforting and close rather than force-feeding detail in the presence and treble regions. This last point may give you the immediate impression that they’re not an overly detailed pair of IEMs, but that’s not an entirely fair assessment. The Duo is able to provide plenty of contrast and information in terms of the micro-dynamic nuances in The Claypool Lennon Delerium’s excellent retro-psychedelic track “Boriska”, but its the pleasant tuning and macro-dynamic abilities that are more likely to win fans here. 

Powering and pairing the 64 Audio Duo 

Like all IEMs that come across my desk, I like to start off by listening with pretty humble devices. Apple’s $14 USB-c headphone adapter is about as humble as they come, and the 64 Audio Duo sounded surprisingly excellent. The Duo’s 9-ohm impedance is definitely on the lower side, but its 98dB sensitivity means that require a fraction more juice than most other multi-driver IEMs to get things going. With the volume set at 50% and plugged into my Pixel 6, the slow jam “Arizona” that’s the final track on Kings of Leon’s “Because Of The Times” album sounded pretty bloody convincing. This track gives you the impression that it’s been recorded in a vast hall with the band members strewn around it, along a soaring synth track that’s been played in a Dolby Atmos theatre. 

Content that the Duo was forgiving and content enough to perform well with even the most modest of sources, I then fired that same track up with the eminently more capable Astell&Kern ACRO CA1000 Desktop Amplifier & DAC. Those same band members each took a step further out into the hall, and that 7.1 surround system turned into an 11.1 system - the Duo really did step up to the occasion, and does reward the listener when paired with a high-quality, discrete device. Being a little lower in sensitivity does open up the possibility of using the Duo with desktop amplifiers - while the Simaudio MOON 430HA Headphone Amplifier is maybe a little overkill for a pair of IEMs, there was only the barest background noise evident - usually, the 430 HA creates an out-of-control HISSSS with IEMs. 

Final thoughts

I think it’d be unfair to think of the Duo as the “entry-level” model in the 64 Audio Universal-fit line-up. Rather, I think it’s appropriate to think of it more as an excellent way to get a taste of the 64 Audio’s technology and multi-driver know-how in an IEM that has definitely forged its own character. The Duo will appeal to lovers of a smooth, silky and natural tone who also like the added playfulness of a generous low-end plus a splash of added upper treble sparkle. 

I can even recommend that if you’re in-store and trying out a few pairs of open-back headphones that you change gears a little and take the 64 Audio Duo for a spin. You might just find that these little in-ears sound far bigger than they ought to, and I can see more than a couple of you realising that a brilliantly-realised IEM like the 64 Audio Duo can take the place of your full-sized headphones - this review pair has become my go-to lean-back-and-chill-out couch listening companion of choice at home, and I haven’t felt like putting on an open-back pair of headphones for a good couple of weeks (I’ve even been using the Duo for all my gaming and video-watching!). 

A less-isolating IEM may seem a little incongruous at first, but the spacious character of the 64 Audio Duo, to me, seems to open them up to more listening occasions than your standard IEM. The fact that they’re pocketable and you can take this kind of sound with you is simply the cherry on top - they’re terrific. 

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