Fit and Comfort

Seen above is the right channel of the 64 Audio Duo installed into an artificial ear mold, and I am using the size M black silicone ear tips included with the IEMs. This is my typical combination for personal use and illustrates the nature of the achievable fit with these IEMs. I do have average-sized ears, and the ear mold above represents my own experiences well enough as a proxy. The semi-custom design of the shells shaped for the ear concha with ergonomics in mind works out well for most ear types, especially given the smaller size of the shells compared to most IEMs on the market. In general, the soft curves and design make for at least two to three points of contact with the ear, which adds support and keeps the IEMs in place. The longer nozzle also has the the ear tips go in far enough to make for a better fit and seal, although the company's Apex Core tech with the semi-open back and fully open front means isolation is not a strong suit in a loud environment, with 64 Audio rating 12 dB isolation from the Apex Core implementation. Audio being played will still overcome most ambient noise though, so it's similar to open-back headphones in that regard. You would want to be more courteous of those around you than the other way round, which makes for an extremely comfortable set of IEMs you can keep in your ears for hours on end, so much so that I even had them in when I didn't necessarily have anything playing if I was, say, crossing a road and just temporarily paused whatever was playing. The cable extends further past the IEMs than I'd like, but the angled housing cuts down on the angle of the pre-formed ear hooks that still naturally go over and behind the ears as seen above. Pair this with the cable clip and cinch, ideally route the cable behind your body or use the cable clip on the front, and you now have a securely installed set of IEMs that allows you to be in touch with your external environment while also being anything but physically fatiguing, especially given these weigh ~6.5 g each.

Audio Performance

Audio Hardware

Here we see the so-called exploded wireframe of the 64 Audio Duo, which effectively shows the various components that go into making it. It also shows the complex 3-piece face plate assembly which results in the open front, semi open-back design. At the top of this image is the daughter PCB for the cable connectors, but it also contributes to what 64 Audio calls Wave Sync. You see, as the name suggests, the Duo has two drivers—a 9 mm titanium film dynamic driver (DD) with neodymium magnets and its own dedicated manifold acoustic chamber and the excellent tubeless in-ear tia audio driver that exclusively caters to the higher frequencies. This means the crossover from the DD to the tia driver is at ~6 kHz, and the tia driver is also placed by the nozzle itself. So despite the DD being placed closer to the ear canal than in most IEMs, the increased distance from the DD coupled with the high frequency crossover point would otherwise be quite hard to execute, and Wave Sync is an all-pass crossover filter that compensates for time or phase effects while working around the physical distance between the drivers to still allow for a cohesive frequency response transfer without sound waves interfering or even canceling out. This also has the added benefit of any dynamic driver effects being more substantial given its physical proximity to your ears, which we'll discuss more soon.

The other piece of technology aside from the previously discussed Apex Core is LID, or Linear Impedance Design, which was missing in the U18t and affected the sub-bass response when powered off a source with a higher output impedance factor there. There's no reason to worry about it with a reasonable source these days, but LID accounts for it further by utilizing a proprietary circuit to "correct the non-linear impedances of the drivers, restoring proper interaction with the source and preserving the desired sound signature." Effectively, this means you can run the Duo off a basic dongle or laptop 3.5 mm output without worrying about it compromising the frequency response. This certainly helps too, given the Duo is a weird IEM that is planar-like in its power requirements. It has an average rated impedance of just 9 Ω, but paired with a low sensitivity of just 98 dB/mW to where it will benefit from a source that can put out more current for the same power. Overall, it's still easily driven, requiring all of 16 mW to hit 110 dB transient spikes. A good portable DAC/amp will suit your needs just fine when without an available 3.5 mm audio jack on the go, and I'd certainly pick one over a standard dongle adapter to make sure there's no source-based bottleneck.

Frequency Measurement and Listening

I will mention that I have a general preference for a warm-neutral signature with a slightly elevated bass, smooth treble range, detailed mids, and good tonal separation. I also generally prefer instrumental music over vocals, with favored genres including jazz and classical music.


Our reproducible testing methodology begins with a calibrated IEC711 audio coupler/artificial ear IEMs can feed into enough for decent isolation. The audio coupler feeds into a USB sound card, which in turn goes to a laptop that has ARTA and REW running and the earphones connected to the laptop through the sound card. I begin with an impulse measurement to test for signal fidelity, calibrate the sound card and channel output, account for floor noise, and finally test the frequency response of each channel separately. Octave smoothing is at the 1/12th setting, which nets a good balance of detail and noise not being identified as useful data. Also, the default tuning was used for testing, and no app-based settings were chosen unless specifically mentioned. Each sample of interest is tested thrice with separate mounts to account for any fit issues, and an average is taken of the three individual measurements for statistical accuracy. For IEMs, I am also using the appropriate ear mold fitted to the audio coupler for a separate test to compare how the IEMs fare when installed in a pinna geometry instead of just the audio coupler. The raw data is then exported from REW and plotted in OriginPro for easier comparison.


The IEC711 is such that you can't really compare these results with most other test setups, especially those using a head and torso simulator (HATS). The raw dB numbers are also quite contingent on the set volume, gain levels, and sensitivity of the system. What is more useful information is how the left and right channels work across the rated frequency response in the 64 Audio Duo. The left channel was separately tested from the right one, and colored differently for contrast. I did my best to ensure an identical fit for both inside the IEC711 orifice, so note how the two channels are nearly identical. In fact, even after seeing the minor offset for the right channel from 3–8 kHz, my ears and brain could not tell the difference, including when I was deliberately trying to hear it. Anything past the coupler resonance has to be taken with a grain of salt, and even here I had no channel mismatch in practice to where this randomly picked demo sample turned out to be one of the better-matched IEMs I have tested. Measurements taken after 50 hours of testing, which included having these play a mix of various songs and white or pink noise and sine sweeps, showed no difference. There was no perceived burn-in effect thus, and none was measurable, either. The response with the anthropomorphic pinna in place matches the ideal scenario in the coupler quite well overall, which is an indicator of how good the seal was when installed in the artificial pinna.


This is the average frequency response for both channels of the 64 Audio Duo plotted against my personal target taken from VSG.squig.link, which also gives you an idea of my personal preferences to better correlate any possible biases. The tuning of a set of headphones or earphones does not have to match my target as long as it is tuned with some direction, makes sense, and is executed well. After all, no one set will appeal to everyone, and having different options is what makes this hobby so interesting and hard to quantify. With the Duo, 64 Audio is clearly taking a different approach when it comes to sensory integration with the ambient environment, and the aim here is to have a relaxed tonality with a "non-analytical top-end while retaining air and sizzle" along with promises galore when it comes to the soundstage, imaging, and even sub-bass response paired with "deep and rich low frequencies."

A quick look at the frequency response above shows the Duo adopt a warmer sound signature, and the tonality itself is in fact very similar to what we saw before on the U18t. However, the Duo executes this far better by not only going with a dynamic driver rather than the somewhat-weaker woofers in the U18t, but the dynamic driver handling both the low and mid frequencies, which has the warmer tonality play out very well in practice. I will even say that this is the best implementation of a warmer-tuned IEM I have heard to date. The bass is rich with 9 dB of impact compared to the lowest SPL measurement at ~800 Hz and maxing out in the mid-bass proper at 50–60 Hz. Sub-bass extension is quite good too, with a minimal, appreciated decrease all the way down. But note that the 12 dB net isolation from Apex Core means this is not an isolating set, and I would not use the Duo for the likes of electronic music. The mid-bass has good detail and impact though, especially considering I went in expecting little in this region, and there is no sense of mid-bass bloat, either.

The transition from the lows to the mids is well executed given the same driver handles both, as there is no sudden change in how the leading and trailing edges are perceived from one driver type to the other, and timbre felt natural throughout. However, this is also where I noticing the main weakness of the design, as even forward-facing male vocals seemed to lack the same detail as snare drums and bass guitars. It's still more than just good when it comes to general media consumption—podcasts, movies, voice calls, etc.—but I expected more here. In fact, I get what a few other reviews meant by calling the Duo's technical performance a letdown compared to the technical powerhouses that the Trió and Fourté are purported to be. Central imaging is tricky since there is a tuning artifact that affects it courtesy the pinna gain shift to 3 kHz, and I found the mids to be smooth but not resolving enough. That said, the soundstage is impressive, which is where the open front comes in. It creates a sense of space that is far beyond the average set of IEMs, and 64 Audio came close to achieving a balance of both the spaciousness we associate with headphones and the impactful intimacy of IEMs.

Based on general results from AES works on HRTF preferences, the pinna gain compensation will be more of a miss than a hit, and it certainly isn't to my own tastes, either. I would have liked more volume from 1–3 kHz, and those who prefer acoustic music will likely agree with me. This affects the balance of male and female vocals too, with the latter coming off quieter by default. This and the recessed 3–4 kHz region combined, as well as first having used the Spinfit tips, is why I initially thought there was something off with the crossover. The wider bore ear tips resulted in a tuning more in line with what I want to see, but the recessed region was obviously done on purpose to make it non-fatiguing and avoid sibilance. Anything past this area comes with a warning from me—if you attempt to listen to orchestral music with lots of instruments that have fundamental and harmonic tones in the so-called presence region, do so in a quieter environment. The open nature of the Duo means the bass response will be affected the most in a louder environment, which may by extension make the treble response come off overly bright and somewhat annoying to many. The 8 kHz coupler resonance also comes off at closer to 7 kHz in my ears, and piano keys in particular ring more than I'd like. This is the weakness of the dynamic driver also catering to higher frequencies before the tia driver in the mid treble does its job by adding presence, and air past 10 kHz. Depending on how sensitive your ears are, this makes the Duo brighter in some cases and darker in others. Ultimately, my hunt for the perfect DD + tia implementation continues!


I mentioned before how the Duo is my at the moment favorite implementation of a warmer signature, including over the likes of the $1500 Effect Audio Axiom, $2000 Lime Ears Pneuma, and even DUNU's take with the $220 FALCON PRO. I also clearly prefer the bass of the Duo over the $3000 64 Audio U18t, although the latter is significantly more resolving and handles the mids and treble better. My personal preference is towards a more impactful bass that is rich in quality over quantity paired with a well-executed mid and treble response, so the $900 DUNU ZEN PRO is still my preference in the price range of the Duo. The bass response is quite close between the two, and the Duo certainly has an edge in the treble response too, but the ZEN PRO bests it with a more balanced upper mids execution and arguably better accessories. Lastly, I will again mention that the Duo is the only one of these IEMs that goes with the open front and semi-open back design, which makes comparing it directly to just about anything else on the market hard owing to its specific feature set that affects the use case and experience based on the environment.