Fit and Comfort

Seen above is the right channel of the 64 Audio Solo placed into an artificial ear mold with the included wider bore size M ear tip installed. I have average-sized ears, and the ear mold above represents my own experiences well enough as a proxy. The ergonomic design of the shells shaped for the ear concha should work well for most ear types, especially given the more petite size of the shells compared to most IEMs on the market. In general, the gentle curves all around the shells make for at least two to three points of contact with the ear to help support and keep the IEMs in place. Note that the nozzle length + angle, as well as the connector housing on the shells, makes for the cable to potentially hit your ear on the top, as opposed to neatly tucking away behind it. I found it being a constant reminder of the IEMs in my ear, and the coarser shells did that too. It's not a dealbreaker, but something to be aware of as I found myself tucking away the cable every 15 min or so. Perhaps a longer extension similar to the U4s/Volür would have helped here. You can also try to angle the shells straighter in the ear, but this can risk breaking the seal in the ear canals. Having a deep and secure fit here is quite important given the lower extent of isolation offered by the Solo—it is rated for -12 dB isolation compared to the -20+ dB you can get from fully sealed IEMs, and how deep the fit is can also impact the treble response as we will soon see. The more open feeling prevents any built-up air pressure in the ear canals and reduces the potential for listening fatigue. But keep in mind that this means you can hear the external ambient when nothing is playing—a good thing if you want to be aware of what's going on around you, but less so if you listen at low volumes. Likewise, those nearby may be able to hear what is being played through the Solo. The shells weigh ~7 g each, so it's not going to weigh your ears down over time. Overall, I'd say the Solo can be extremely comfortable provided you get a good fit and the cable potentially hitting your ear doesn't bother you.

Audio Performance

Audio Hardware

The 64 Audio Solo uses a single full-range 14.2 mm planar magnetic driver, but there are a few tricks up its sleeve to help distinguish it from other such single planar magnetic IEMs on the market. For one, and this is where the Tia branding on the box comes in if you were confused about it, the Solo contains two of the three Tia system elements in the form of the single bore nozzle and tia acoustic chambers. Combined with the tuning done, while it does not have the actual tia tweeter, you still can expect to see a similar airy upper treble response that has become a hallmark of 64 Audio IEMs. I also mentioned the integration of Apex core here, allowing for the planar magnetic driver's acoustic chamber to be integrated with the same patented tech we saw in the 64 Audio Duo before. It makes for a feeling of openness and a more realistic staging, which can add to the overall comfort and sound experience, but also means the IEMs are less isolating than average. The biggest distinction comes in the form of two Helmholtz resonators used here, which I believe is the first such implementation with planar magnetic drivers. These are acoustic devices 3D printed into the acoustic chamber, similar to what's being used by Dan Clark Audio with its AMTS tech, forcing air in and out in a controlled manner to resonate at specific frequencies which can then be amplified or attenuated.

The reasoning behind this is simple. Go look at all the planar magnetic IEMs reviewed here so far, and you will notice a general lack of control in the treble. This can make for a feeling of shouty vocals, sibilance, and fatigue depending on the tuning and how sensitive you are. Helmholtz resonators are not new to audio, or even IEMs, with some having even used it recently to help cut down the lower mids to make for a more pronounced bass by comparison. With the Solo, 64 Audio is using the two resonators to cut down ~5 kHz and 8.5 kHz in addition to a slight yet wideband reduction of the higher frequencies to make for a controlled treble, that can help put out a more balanced tuning too. The Solo also includes LID (linear impedance design), helping correct the uneven impedance from the electronic shelf filter used for the treble reduction. This also helps ensure the tonal response of the Solo is unaffected by the output impedance of the source being used. The end result is a slightly less sensitive set of IEMs with a rated impedance of 16 Ω and sensitivity of 100 dB/mW—still easy enough to be driven from a decent portable DAC/amp though!

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 that 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 a capable and transparent DAC/amp—I used the Questyle M15 here, but also used the provided dongle adapter which performed identically. 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 Solo. 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 basically identical across the entire 20 Hz to 20 kHz range! Keep in mind that this is a review unit, although one that is part of the first batch of finished production units as evident by the retail packaging. So, while I can't confirm this will be representative of a randomly chosen retail unit, 64 Audio has certainly upped its QC game to where randomly measured retail IEMs this year have shown similarly excellent channel matching. Measurements taken after 25 hours of testing, which included these playing a mix of various songs as well as white or pink noise and sine sweeps, showed no difference under the same testing conditions. There was no perceived burn-in effect thus, and none was measurable, either. Keep in mind that the two Helmholtz resonators and the electronic shelf filter make it impossible to find the coupler resonance peak, let alone match it at 8 kHz as I usually do for consistency. This means the effect of insertion depth is not as easily seen that way, but there are still easy telltale signs. Look at the response with the anthropomorphic pinna in place, for example, wherein the response exhibits deviation just after the two resonator frequencies at 5 kHz and 8.5 kHz. The shallower the insertion, the more the deviation will be from the ideal deep fit scenario in the coupler itself, and this is why I mentioned ensuring a deep fit is important here as the sound signature can change appreciably if you don't get a good seal.



Here is the average frequency response for both channels of the 64 Audio Solo 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. This is in addition to a second graph using a newer target that's based on a more scientific methodology involving a -10 dB tilt (-1 dB/octave) applied to the diffuse field target for the newer, more reliable B&K 5128 but then compensated for my exact 711 coupler instead. Do scroll down to the targets in my database linked above and see the new 5128 section to the left where you can click on the yellow question mark for a brief primer. The bottom line is this target is closer to what many people are likely to prefer out of IEMs and headphones alike. I have chosen to plot this graph with the frequency response normalized/compensated to the target to make it easier to see how the Solo is tuned—balanced with bass boost, forward mids, and controlled yet articulate treble.

Your experience with the 64 Audio Solo will depend a lot on the fit and seal achieved, as well as how deep you can get it in your ear canals. I mentioned above how the lower/mid-treble is being targeted by the two Helmholtz resonators in addition to the electronic shelf filter, and the extent of the 5 kHz and 8.5 kHz dip is impacted by the insertion depth and seal. Using the Spinfit ear tips with a shallower insertion made the lower treble more prevalent to my ears, which can be nice if you want some bite to vocals and certain instruments, but can also be a touch too hot for female vocals in particular given the generally forward mids here. The shallower fit also cut down the mid-treble too much for my liking to where piano keys and strings felt somewhat lifeless. Despite the measurements above indicating a flat mesa here, it's more like a smoother series of quick dips and peaks that make for easily the most controlled planar magnetic treble I have ever listened to in IEMs. There's no sibilance or the more prevalent timbre issues associated with planar magnetic drivers here, which alone makes the Solo worthy of consideration. This then gives way to a characteristic planar magnetic driver peak in the upper treble that I have seen in other such drivers, although coincidentally it also happens to line up with where the typical 64 Audio tia tweeter upper treble peak is. If you are sensitive to this region, please note that this peak can be irksome and a potential dealbreaker—especially since it comes off more pronounced as a result of the tamer mid-treble before. Here again having a deeper fit helped me a lot, preventing ringing from cymbal hits and instead adding to a more realistic soundstage.

The 64 Audio Solo is not an open-back set, and yet you feel everything is open sounding. This adds to a fatigue-free listening experience, allowing you to get more immersed in the music as long as you are in a quiet environment. In fact, my first listening session with the Solo had me thinking this was very similarly tuned to the U4s with the m15 Apex module, which is good since that combination remains one of my favorites in all audio. But the more I listened, the more I felt the Solo came off more detailed and less dynamic at the same time. In particular, trailing ends of tones felt less emphasized compared to what the U4s offered. Some of this has to do with the planar magnetic driver being extremely responsive with minimal transient time. This reduces any potential of music "smearing," allowing for excellent layering and detail retrieval too. At the same time, leading notes can feel diffused, which is confusing to the brain since the actual soundstage only just goes past your ears—it's realistic, not artificially wide—and there is a chance you may feel it to be less impactful too. The overall presentation can be jarring if you are not used to full-range planar magnetic drivers, especially with timbre still coming off slightly metallic with kick drums and guitars. On the other hand, you get very precise and deep-reaching bass which I personally love a lot. Sub-bass afficionados should not expect a "boom boom" party in your head as the Solo is more about presenting detail and accuracy with ultra-low distortion over pure impact and slam. The bass amount in general was a touch too much for my liking, although I can see why this is a safe choice for the target audience. It helps balance out the forward mids and works well to provide a certain tactile feeling that you won't get with more typical dynamic or BA drivers, at least not easily. I am left wondering what would happen if the Solo was more isolating though; I suspect it would sound closer to the other planar magnetic IEMs in the market, which can be a double-edged sword for reasons mentioned before.

The bass to mids transition is executed well to where I don't find the bass to be bloated or boomy. The Solo comes off slightly warm sounding, but you already know my preference for it and I appreciated the minimal bass bleed into the mids regardless. I will again give a shoutout to the overall clarity and the ease with which the Solo puts out individual notes, with precise imaging and layering a strong point here no matter what kind of music you listen to. Some of this can be attributed to the driver itself, although the treble not just taking over the tonality as with pretty much any other planar magnetic driver also helps a lot. But a part of me is also wondering whether the Helmholtz resonators were a solution created to solve a problem that didn't have to be sought after the first place. Planar magnetic drivers can be great to get around any front volume leakage issues, yet the Solo being so open sounding somewhat negates that. The more open presentation works for my orchestral and jazz library quite nicely, although I wouldn't have minded some more ear gain. In fact, you will notice the typical 64 Audio coloration here in the 1-4 kHz region that allows for female vocals to be forward facing, helping fans of certain pop genres to adore the Solo, while some others may feel the mids could sound thicker.


I wish I had the likes of the Campfire Audio Supermoon, Moon Rover, and Astrolith on hand here to properly compare against the 64 Audio Solo, given Campfire Audio also happens to be a US-based brand which has recently released multiple premium planar magnetic IEMs. I have tried out the first two multiple times recently though, so I can tell you the Supermoon goes for a more aggressive take on a similar tuning with more bass, more upper mids, and more treble in general. It's the closest sounding to the first generation of the new planar magnetic IEMs from China and ultimately was too aggressive sounding for me. The Moon Rover goes for a newer 12 mm driver and sounded more linear to me, albeit the lower mids were less clean at the same time. The treble felt more controlled than the Supermoon, so this is a set that could be potentially worth considering too. I should mention that the Moon Rover wasn't the easiest to get a good fit and seal for me though, even with multiple ear tips tried at the shows I demoed it at. I have not heard the Astrolith yet, it uses two planar magnetic drivers and seemingly goes for a more V-shaped tuning with a lot of bass—it has already generated a lot of fans online.

What I do have here is a bunch of the Chinese brands IEMs that use a version of the 14.x mm planar magnetic driver platform which was all the rage a few years ago, and continues to be the case today. If anything, there are now more brands joining this party as evident by 64 Audio, Campfire Audio, FiiO, and others soon. I can't tell if whether the 14.2 mm driver used in the Solo is made by the same factory that makes the other drivers, but ultimately it sounds different enough to where it might as well be a completely different setup. Shown above is the frequency response of the brand new LETSHUOER S12 2024, an update to the original S12 which was a big hit when it launched over two years ago. The older S12 had the trademark hot treble response that pretty much all those IEMs exhibited, although LETSHUOER seems to have figured out how to dampen the driver, with the new S12 being a lot tamer. In fact, it measures very well and is likely to be another hit for the brand. The Solo still showcases 64 Audio's R&D expertise with the Helmholtz resonators and the electronic filter though, you can see where the S12 has a more relaxed pinna and upper treble, thus making that already more pronounced mid treble sound notably bright and fatiguing in some cases. There is a big price differential for sure, and I'd even say the new S12 is more isolating and arguably sounds punchier. From a pure value basis, the Solo can't really compete against the new S12. But then again, you could say the same about pretty much any expensive set of IEMs compared to more affordable ones. I want to clarify here that the Solo, while still using a full-range planar magnetic driver, sounds remarkably different from the others to where it's not as direct a comparison as you might think. Ultimately, go for what sounds better to you and, if you can afford it, the Solo is the best planar magnetic set I've ever tried. This includes the equally pricey, but now discontinued, Audeze Euclid that came from a time when companies seemingly threw darts at a board to see what worked. I haven't heard any of the open-back Audeze planar magnetic earphones though—also discontinued, for what it's worth!

Stepping away from planar magnetic IEMs, I've noticed there's a general dearth of releases lately in the $1000-2000 price range. This makes it harder to compare the Solo to other sets, especially since I don't have some sets on hand such as the Symphonium Audio Crimson/Europa and ThieAudio Prestige LTD, all of which have impressed me each time I tried them at an audio show. As it turns out, 64 Audio itself provides options with the Duo and U4s. The Duo was recently discontinued, and we now know the Solo effectively takes its slot in the product line, but can still be purchased from the brand's retail partners. It is a proper open-back set and uses a 1 DD/1 tia hybrid driver configuration to put out a more warm and airy sound with rich bass, albeit sounds even more open and less isolating than the Solo. The Duo was designed for those who want even more awareness of their surroundings, and in return I found it to having slight imaging and detail retrieval issues in addition to a brighter treble response than many will find pleasant. I think it's fair to say the Duo was a cool set to try out but ultimately never comes up in the conversation now to where the Solo is way more likely to do the job for 64 Audio. But then there's the U4s—a 1 DD/2 BA/1 tia hybrid set that costs less than the Solo, can be more comfortable in the ears with an easier-to-achieve deep fit, offers more customization with its Apex modules changing the bass response significantly, and can be even more isolating than the Solo making for a more dynamic bass. The Solo handily bests the U4s in resolution and accuracy though, so those who want to feel they are paying for that extra detail will no doubt find the Solo more up their alley. It's also the set to go with complex tracks, be it synth or orchestral, allowing for greater instrument separation and more precise imaging. If I may be so bold as to use this analogy, the U4s is more like a Swiss army knife whereas the Solo is a sharpened chef's knife. I should also mention the Solo coming with two cables—both of which are nicer to use than the U4s cable—is a plus point here too.