64 Audio Duo - Universal IEM - Official Thread

I wanted to share some more thoughts on the Duo because I brought one back with me from CanJam. As far as I can tell, 64A basically has three flavors of sound: their “reference” audiophile tuning (which consists of the U12t, U6t, U18s, and to some extent the Nio), their audiophile tuning (the Tia Trio and Tia Fourte), and their mainstream tuning (the U18t). The Duo straddles the line of the previously somewhat lone Nio while adding on some extra flair up top, presenting a comparatively new flavor, if not one that will be readily familiar to fans of the brand. But before I get ahead of myself, I should share some thoughts on the Duo’s physical design.

In a nutshell, I LOVE the Duo’s design. The grill doesn’t look cheap at all to me and it’s super pleasing aesthetically. Something different about the shell of the Duo is that it has been anodized what appears to be a slight shade of midnight blue (or maybe I’m just colorblind). Either way, the Duo looks fantastic. And it’s just as ergonomic too. While I didn’t get in a size comparison in the photos above, it’s the shortest of 64 Audio’s IEMs in terms of shell depth. I spent about 5+ hours jamming out to them today and not once did I feel the need to remove them outside of instinctively yanking them out when someone spoke to me. I say “instinctively” because you don’t really need to remove them; you can actually hear people talking if you just pause your music. Anyways, that’s more than enough talk about build coming from me - let’s talk about the actual sound.

As stated above, the Duo shares some similarities to the 64A Nio, and that’s primarily in one aspect: the fat bass shelf. That said, the Nio sports more of a sub-bass focus and for a sense of digging into the deepest frequencies and generating them with a sense of thickness and weight, the Nio comes out ahead in A/B. In this respect, at least intangibly, I do find the Duo’s bass to be more reminiscent of the DD used in the Tia Trio. The Duo’s bass is more nimble than the Nio’s bass response thanks to less decay, and it sounds fairly rich whilst avoiding the wooly thickness of the Nio. I also have no qualms on the fronts of bass texture. As far as I’m concerned, the quicker bass response of the Duo’s DD was the right tradeoff, as the Nio’s subwoofer generally sounds more sloppy when it comes the mid-bass. The Duo generally has pretty good bass - not the best I’ve heard for this price point, that honor goes to the Sennheiser IE900 - but certainly enough to merit comment. And remember, the Duo’s DD is tokening the entire midrange too.

Speaking of the midrange of the Duo, it’s pretty alright. The overall amplitude of the pinna compensation has not changed too much - it still leans toward the more relaxed side - but it does not peak until 3kHz unlike most of 64A’s other IEMs which tend to peak a bit earlier at around 2kHz. This lends the Duo to a slightly more upfront center image, but 64A has recessed the presence regions from 3-4kHz to avoid sibilance per usual. Intangibly, vocals consequently hang slightly higher on the stage while staying upfront for me. I don’t think this midrange presentation is quite perfect, but it is an interesting tuning decision that, again, demonstrates to me that 64A really knows how to tune. Expectedly, the timbre of this midrange is quite pleasant with none of the timbral inconsistency that characterizes most hybrids between bass and midrange. This part was really was nice to hear.

The treble on my unit seems to be somewhat different from the unit I heard on the show floor. I remember the show floor unit sporting more of a lower-treble emphasis which beget a more V-shaped sound signature. However, I will assume for now that the unit I have on-hand represents the ideal sound, as the Duo’s product page markets it as having a more laidback sound signature. This is indeed a boon with respect to coherency, as I felt that the tia treble on the show floor unit was a tad too distinct from its DD counterpart. But make no mistake: This can still be a bright IEM depending on the ear tips you’re using, your hearing, and the volume you’re listening at. In fact, I feel that the tia peaks on this Duo are higher in amplitude than on my U12t - they’re definitely higher than the Nio - and almost sharp on certain tracks with wide-bore tips. This was consistent among all three units I heard; hell, I hear what sounds like a 16kHz sheen when Taeyeon enters on “I Found You” that I didn’t know was there before. Further along the lines of frequency response, I’d say the Duo’s treble is about 40% lower-treble, 20% mid-treble, and 40% upper-treble. I don’t know if it’s quite right to market this as a relaxed IEM but, to be fair, swapping to the foam tips will definitely mitigate any unwanted shimmer as I found on Day 1 of CanJam.

Imaging on the Duo is interesting. It’s good, but it’s not mind-blowing within the context of the more open nature of the IEM. The Duo’s biggest weakness on this front would be localization of instruments. They’re somewhat undefined and clearly not as sharp as the Symphonium Helios. But to reiterate, the Duo definitely sports above-average imaging, and I think the best way to describe it would be simply “natural”. Sounds mesh into the backdrop pleasingly and there’s a good sense of ambiance to the Duo’s staging. However, there’ll be no cop-outs on the front of detail, I’m afraid. The tia treble holds its own in terms of detail, but I simply don’t find the Duo particularly detailed in the midrange which is why I called the midrange “pretty alight” earlier. It sounds slightly smoothed over in A/B with the 64A U6t which I don’t even consider a strong performer for detail, and the Sennheiser IE900 which has a disgusting upper-midrange recession. This is ostensibly a limitation of using a single dynamic driver to cover both the bass and the midrange frequencies, even a high-quality one at that.

The best part of the Duo’s intangibles would be its dynamics. They’re surprisingly good; I wouldn’t have bought one for myself if I thought they weren’t. Now, I do not think the Duo has great micro-dynamic distinction - a sense of individual instruments and vocalists popping with subtle fluctuations in volume - but it stands that the vast majority of IEMs lack this quality anyways. In fact, the only IEM I have heard with good micro-contrast at this price point would be the Symphonium Helios. That said, macro-contrast on the Duo has the good sense of weight and authority that characterizes the likes of the 64A Nio. I found myself jacking up the volume and going back to swings like Sawano Hiroyuki’s “Tranquility” at 0:50 repeatedly just to catch more of this quality while testing the Duo at CanJam. I do not know how 64A does it, and maybe you can consider it the “oh, he’s just a 64A shill” effect, but there is definitely something about the likes of the U12t, Nio, and the Duo that catches my attention and gets my head bobbing.

As for some concluding thoughts, the Duo is definitely a good IEM and I like it! But I also cannot ignore the price in good faith, and I don’t want to make the mistake of hyping something that won’t get near-universal praise. It’s crucial to recognize that the Duo sports a more niche design; it heavily emphasizes comfort and it doesn’t befit outside use like traditional IEMs unless you’re indexing more strongly for situational awareness. For me, that’s what I wanted as a compliment to my Symphonium Helios. I’d imagine this would work great for walking outside too and, as the reps at the 64A booth mentioned to me, streaming and gaming. But it still pays to recognize that not everyone else will have these use cases; solely on the basis of sound quality, I find the Duo to be solid for the price but not necessarily outside the scope of the lofty expectations I hold for 64A.

All that said, there is a lot of unique technology and innovation packed into the Duo. Friends better versed in the construction of IEMs than myself have commented on the complexity of 64A’s crossover design, and I enjoy the emphasis placed upon the user-experience with technologies like Apex Core - after all, the best sounding IEM doesn’t mean much if one can’t wear it comfortably.


Wanted to share another review on the 64Audio Duo. Seems like it’s a bit of an overlooked product in 64Audio’s lineup judging by how this thread is going :sweat_smile:. Not totally unjustified to be honest, I think.


64Audio is a brand I know little about. What I do know is that 64Audio is arguably the most successful Western IEM company with universally acclaimed products like the U12T. I also know that 64Audio is the darling of my friend Precogvision’s dreams. With qualifications like these, I decided to dive into the 64Audio IEMs and do a series covering all the ones I can get my hands on courtesy of headphones.com.

And for my first foray into the brand, I decided to go with the $1,200 64Audio Duo as it’s probably the most unique of 64Audio’s products. Unlike the other 64Audio IEMs, the Duo sports a 1 DD + 1 BA hybrid setup and an open back design. While 1 DD + 1 BA setups are common, open back shells are still relatively rare in the IEM space. I’m curious to see how this combination plays out, especially as it seems like 64Audio may have a few tricks up its sleeve with its marketed tia, Apex Core, and Wave Sync technologies.

Build and Comfort

My loaner unit didn’t come with the full accessory set, so I won’t go through all the details of that in this review. What I did get was its stock cable and carrying case. The case is a generic round-shell case that’s a little on the large side but comfortably holds the IEMs in. The 2-pin cable is… non-ideal. Not the worst cable I’ve ever come across but is one I would definitely swap out. It feels very plasticky with a dose of cable noise and memory.

The most striking feature of the Duo is a perforated grill on its faceplate that covers slit-like opening similar to the open cups of a planar headphone. Interestingly, this isn’t the only part of the Duo that’s open. There seems to be a large vent hole at the very bottom point of the Duo’s shell. Other than that, the Duo’s design is straightforward and ergonomically shaped. Its nozzle is a very reasonable 5 mm in diameter and isn’t overly short or long. While the shell is fully built out of metal, it’s surprisingly light. I find the Duo to be very comfortable. I’m not sure if it’s 64Audio’s technology or the fact that its vented but I don’t feel any pressure build up with the Duo. The downside of course is the relative lack of isolation. That said, the Duo doesn’t leak sound all that much. Think of these “open back” IEMs as being highly vented rather than truly open back like headphones are. I wouldn’t worry about it annoying nearby people unless you’re listening extremely loudly in a quiet environment.

Frequency Response and Tuning

My first impressions were negative. The Duo is a timbrally challenged IEM. There’s a significant amount of lower mids bloat, off-natural vocals, and unevenness in the treble. Colored, if you want to be polite. To be honest, its tonality reminded me of ChiFi from a few years back. Except I noticed the Duo does a surprising level of technical chops after I got over my initial shock over its frequency response. More on that later. At any rate, the Duo was unnatural enough that I put it aside after two songs and listened to a couple other IEMs for a sanity check and a palate cleanse.

Coming back to the Duo and giving it some more ear time was when I finally accommodated to its sound. While the Duo still sounds quite colored, it’s not to the point that it’s overly distracting. The bass is absolutely elevated. Midrange has the aforementioned lower mids bloat and an upper midrange recession. It’s here in this skewed midrange that gives the Duo its colored sound. Treble is hard to pin down. Its unevenness gives rise to a strong lower treble transient attack and an upper treble peak somewhere for the occasional sparkle. Yet, the Duo isn’t exactly an airy IEM.

Frequency response of the 64Audio Duo. Measurement taken with an IEC-711 clone microphone. Comparisons can only be made relative to other measurements taken by this specific microphone. A peak at about 8 – 10 kHz is likely an artifact of the measurement rig and may not exist as depicted here. Measurements above 8 kHz are not accurate. If possible, reference multiple measurements.

Looking at the measurements, it seems like it lines up pretty well with my thoughts. There’s a lot of bass bleed into the lower mids. While the upper mids and pinna gain look reasonable despite the massive 4 kHz dip, the overbearing presence of the lower mids is enough to throw off the midrange balance. Speaking of the dip, the contrasting 5 kHz peak is likely what’s giving rise to a sharp transient attack for hats and cymbals. It’s hard to interpret what’s happening in the upper treble given the lack of measurement accuracy in that region, but we can see that there isn’t much treble (excluding the artificial 8 kHz peak) until the very edge of the upper treble where it likely has negligible effect given the lack of musical energy in that region.

Instrument Notes


There’s quite a bit of midbass boominess here rather than a sense of slam. Unfortunately, the lower mids bloom is diluting the impact of these notes and definition is lost. Together, it makes the kick and low toms blur into each other. Notes have a big, blunted oomph to them. On the upside, the DD of the Duo imparts a certain grittiness, adding a much-needed subtle flavour to the sound. Snares have a thick body and while its characteristic crack does give it some definition, its energy is dulled on the Duo as a result of the large 4 kHz dip.

Hats and cymbals are the instrument that best highlights the quirks of the Duo’s sound. That lower treble transient attack I talked about above is most apparent here and sticks out like a sore thumb especially if the drummer is hitting a consistent pattern on the hats. These notes are sharp, crisp, and sometimes crass. To be frank, I found it distracting at times. To a lesser extent, a similar effect is found with the tambourine. With the Duo’s mid treble dip, the emphasis of the hats and cymbals are at the point of attack while delicate note decay is relegated to the side. I’m tempted to attribute the hats and cymbals sound to BA timbre especially when contrasted against the DD dominated drums.


With such a large helping of low end, the bass guitar is clearly heard on the Duo. It has decent definition and nicely “fills the room”, so to speak. Interestingly, the Duo is quite musical when it comes to electric guitar licks and acoustic guitar finger picking. Short passages are highlighted in ways I haven’t heard before, making familiar songs sound fresh and unique. It doesn’t happen in every tracks but when it does, it’s a very pleasant experience. It’s all the more odd, then, that electric guitars outside of these brilliant short passages have little bite and can sound like a slog due to the overabundance of lower mids and abrupt upper mids dip.


I’ll be honest here. The tonality of vocals on the Duo are not great. They aren’t “sweet”, they aren’t “lush”. They’re passable. The best thing I can say about them is that they can sometimes accentuate the creaminess in the vocals of certain male singers. Overall, voices are thick and lacking air, pulled back and relaxed. At least they aren’t sibilant. That said, vocal layering is pretty good. The Duo is able to distinguish vocalists while blending their harmonies.


Staging and imaging on the Duo are one of its standpoints. Horizontal stage is generous and the pulled back vocals gives a sense of depth. Stage height however is lacking. Imaging isn’t pinpoint accurate but there’s sufficient nuance here. More importantly, the Duo has an openness to its sound that I look for thanks to its open back nature. It helps to diminish the in-your-head, closed in feeling that a lot of IEMs have and brings a more comfortable and enjoyable listening experience.

I don’t think the Duo is a very resolving IEM in the sense that it captures the little nuances or trailing ends of tones. But at the same time, it can really highlight notes that I never noticed before. I have a theory that the perception of resolution is partly influenced by simply hearing things differently. In other words, the Duo is so colored that instruments have a uniqueness in their presentation that stands out and steals my attention. This is most apparent with midrange focused instruments like the guitars as mentioned above. It creates an illusion of resolution on the Duo. Ironically, the novelty wore off as I spent more time with the Duo and got accommodated to its tuning. Fewer and fewer tracks had moments where I would notice an intriguing passage or hear a new note. It really is a double-edged sword.

As for the rest of the Duo’s technical performance, it’s generally fine. Instrument separation is alright. Layering is good as it takes advantage of the Duo’s staging. The Duo does have a decent sense of dynamism though that may be a product of all its bass.

Should You Buy It?

No, not if its going to be your only IEM. The 64Audio Duo is best suited as a complementary IEM, one that acts as to mix up your listening and every now and then. This way, music on the Duo will have a fresh novelty to it as its contrasted against more neutral IEMs in your collection. Of course, this is easier said than done. The Duo is a $1.2k IEM, certainly not cheap by any means. Whether intentional or not, the Duo is clearly a luxury product. I guess what I’m trying to say is, you’ll know if you can afford it. If you can afford it, I don’t think it’s a bad buy at all. I think that as hobbyists, sometimes we seek out more than just “pure sound quality” but rather unique experiences to challenges our senses and broaden our perspectives. The 64Audio Duo offers a glimpse that.

One final note is that you may have noticed that I didn’t make any comparisons to other IEMs in the $1,000 range or so, such as the recently reviewed Symphonium Helios. The truth is, I didn’t feel like it was necessary. The Duo so different from its contemporaries that writing any comparisons would have been like writing two whole reviews anyway. And to be honest, this was part of the reason I chose to review the Duo first from the 64Audio lineup. It’s a unique sounding IEM that, in my eyes, stands alone from the rest of the product stack or its competition. I hope you’ll join me as I take a look at the more traditional offerings from 64Audio in my upcoming reviews.


I agree on most part Due is uncoherent mess.
Have you heard U12T? I would like to hear your impression.