Context
The only two IEMs I own — the Tin T2s and the FiiO FH5s — also happen to be the only two IEMs I’ve heard.
It wasn’t long after getting into the audiophile hobby (about six years ago) that I wanted a way to listen to music in the go that I would look forward to. That quest — in which I’ve lived with a few pairs of over-ear, Bluetooth, noise-cancelling headphones, a few pairs of wired, over-ear headphones, and the aforementioned two pairs of IEMs — has ended with the 64 Audio U4s.
It wasn’t long after getting into the audiophile hobby (about six years ago) that I wanted a way to listen to music in the go that I would look forward to. That quest — in which I’ve lived with a few pairs of over-ear, Bluetooth, noise-cancelling headphones, a few pairs of wired, over-ear headphones, and the aforementioned two pairs of IEMs — has ended with the 64 Audio U4s.
Frequency Response
If I had to give an elevator pitch for the 64 Audio U4s, I would describe them as a four-member band crammed into an IEM housing.
The four band members — in order of priority — are the bass, treble, lower-mids, and upper-mids.
1. Bass: The bass of the U4s can best be characterized as *amorphous*; it's able to tighten up when a song calls for it and present itself in a more diffused way when it's time to get out of the way of the other parts of the frequency response. Despite the bass being the part of the frequency response that typically stands out the most to me, I wouldn't go so far as to classify the U4s as bass-head IEMs. A more apt description would be *bass-driven*, just as a band's rhythm section is often its driving force. This likening is because of the leadership role that the bass plays in the U4s, I found doing anything to reduce the U4s' bass to do them a disservice.
2. Treble: Cuts through the bass, providing a nice sense of contrast.
3. Lower Mids: This part of the frequency response has a subtle vibrato at times, kind of like a bow being pulled across the string of a cello. I also suspect that this part of the frequency response not being adjacent to the bass in terms of priority allows the lower mids to meaningfully contrast with the bass.
4. Upper Mids: The most neutral part of the frequency response. I suspect this region will be the most controversial one. However, I believe this was done to serve as a point of reference for the other three more iconic parts of the frequency response. It's also worth noting that I wouldn't consider this part of the frequency response to be recessed, but rather that it doesn't seem to have been elevated relative to the other parts of the frequency response. It could also be argued that the relative recession of this region invites the listener to "lean in" to hear what's happening, which I consider a cool, alternative way to draw attention to a part of the frequency response.
I suspect that each of the four parts of the frequency response I heard could be mapped to one of the four drivers in the U4s, with the dynamic driver handling the bass, the Tia driver handling the treble, one regular BA handling the lower mids, and the other regular BA handling the upper mids. I've never heard an IEM that gave me the ability to distinguish all of its drivers from each other, so this was a pleasant surprise.
Speaking of firsts, I was surprised to find that the U4s trained my ears to notice subtle details in my music. I realized this when I returned to my ZMF Aeolus to find that the repeated two-hit symbol sequence at the beginning of "Total Invasion" by Killing Joke as clearly audible; I was not able to hear these notes on my Aeolus before, so the only logical explanation is that I was listening to them differently than before.
The four band members — in order of priority — are the bass, treble, lower-mids, and upper-mids.
1. Bass: The bass of the U4s can best be characterized as *amorphous*; it's able to tighten up when a song calls for it and present itself in a more diffused way when it's time to get out of the way of the other parts of the frequency response. Despite the bass being the part of the frequency response that typically stands out the most to me, I wouldn't go so far as to classify the U4s as bass-head IEMs. A more apt description would be *bass-driven*, just as a band's rhythm section is often its driving force. This likening is because of the leadership role that the bass plays in the U4s, I found doing anything to reduce the U4s' bass to do them a disservice.
2. Treble: Cuts through the bass, providing a nice sense of contrast.
3. Lower Mids: This part of the frequency response has a subtle vibrato at times, kind of like a bow being pulled across the string of a cello. I also suspect that this part of the frequency response not being adjacent to the bass in terms of priority allows the lower mids to meaningfully contrast with the bass.
4. Upper Mids: The most neutral part of the frequency response. I suspect this region will be the most controversial one. However, I believe this was done to serve as a point of reference for the other three more iconic parts of the frequency response. It's also worth noting that I wouldn't consider this part of the frequency response to be recessed, but rather that it doesn't seem to have been elevated relative to the other parts of the frequency response. It could also be argued that the relative recession of this region invites the listener to "lean in" to hear what's happening, which I consider a cool, alternative way to draw attention to a part of the frequency response.
I suspect that each of the four parts of the frequency response I heard could be mapped to one of the four drivers in the U4s, with the dynamic driver handling the bass, the Tia driver handling the treble, one regular BA handling the lower mids, and the other regular BA handling the upper mids. I've never heard an IEM that gave me the ability to distinguish all of its drivers from each other, so this was a pleasant surprise.
Speaking of firsts, I was surprised to find that the U4s trained my ears to notice subtle details in my music. I realized this when I returned to my ZMF Aeolus to find that the repeated two-hit symbol sequence at the beginning of "Total Invasion" by Killing Joke as clearly audible; I was not able to hear these notes on my Aeolus before, so the only logical explanation is that I was listening to them differently than before.
Soundstage and Imaging
The soundstage of the U4s has a bit of hight, but not much depth. However, the real standout of the U4s is its imaging. Its imaging stands out because of the subtle changes in vocal positioning within the middle of the stage. Whereas most audio output devices make a sustained note sound like it's coming from one location, the U4s highlight slight fluctuations in vocal positioning within the same note. This prompted me to pay more attention to parts of songs that I normally wouldn't focus on that much. I don't have to try that hard to find songs that expose this property, but if you'd like a song to start with, I'd recommend "Issues / Hold On" by Teyana Taylor.
Listening Impressions
If you only listen to one song on these IEMs, I'd recommend considering making that song "Haunted Jukebox" by Saint Etienne. This is because I believe this song effectively showcases all four "band members" living in this IEM, demonstrating the merits of this slightly unorthodox way of tuning an IEM.
If you're jonesing for an IEM that will likely give you a new, fairly consistently enjoyable perspective on your music, I'd recommend giving the 64 Audio U4s a shot.
"On Fire" by Sebadoh features a certain bass note that sounds like it's coming from the lower third of the U4s in a way that no other parts of the song did, resulting in an a novel, enjoyable, and engaging listening experience.
If you have a bit more time, I'd recommend checking out "Close to Me" by Juls to get a different perspective on the U4s' bass by hearing how it handles more modern, electronic bass.
If you're jonesing for an IEM that will likely give you a new, fairly consistently enjoyable perspective on your music, I'd recommend giving the 64 Audio U4s a shot.
"On Fire" by Sebadoh features a certain bass note that sounds like it's coming from the lower third of the U4s in a way that no other parts of the song did, resulting in an a novel, enjoyable, and engaging listening experience.
If you have a bit more time, I'd recommend checking out "Close to Me" by Juls to get a different perspective on the U4s' bass by hearing how it handles more modern, electronic bass.
Timbre
The U4s are just shy of sounding natural. This is probably on account of their emphasis on the sub-bass and treble, as opposed to the mid-bass and upper-mids. I consider the latter pair of frequencies to be the more natural-sounding one, which would explain why the ZMF Aeolus (my main over-ear headphones), sound so natural/organic to me
Sound Isolation
Sound isolation is satisfactory to me, but certain sounds — typically certain higher-pitched ones — can cut through the sound isolation of these IEMs.
Power Requirements
I had no trouble driving the U4s off of my 3.5mm-to-Lightning Apple dongle DAC. While they were a bit harder to drive them my FiiO FH5's, it was not by much. I would even go so far as to say that I like that the U4s are a little harder to drive than the FH5's; the FH5's were a bit *too* sensitive for me, often leaving me with little room to raise the volume before reaching uncomfortably loud levels. I noticed a bit of background hiss while using the dongle, but only if no audio was playing.
My 16" M1 Pro MacBook Pro's built-in 3.5mm headphone jack was the only source of mine that negatively affected the U4s' sound. As I mentioned earlier, the bass is the most important part of this IEM's frequency response; therefore, messing with the bass in any way could easily mess up the delicate balance between the four main components of the frequency response. To that end, the MacBook seems to reduce the bass of whatever's plugged into it. In the U4s' case, the bass reduction causes its treble to stand out too much, causing it to come off as slightly *piercing*.
My 16" M1 Pro MacBook Pro's built-in 3.5mm headphone jack was the only source of mine that negatively affected the U4s' sound. As I mentioned earlier, the bass is the most important part of this IEM's frequency response; therefore, messing with the bass in any way could easily mess up the delicate balance between the four main components of the frequency response. To that end, the MacBook seems to reduce the bass of whatever's plugged into it. In the U4s' case, the bass reduction causes its treble to stand out too much, causing it to come off as slightly *piercing*.
Fit
My ears are pretty small, which is one of the reasons why I haven't gotten too invested in the IEM space. However, I took a chance on the FiiO FH5's and — with the help of the stock double-flange tips (or Simbiio W tips), I was able to achieve a pretty good fit. Given that, it's little wonder that the U4s — whose dimensions are pretty similar to that of the FH5's — also fit my ears pretty well. Nevertheless, the U4s' lack of pseudo-custom shells makes achieving a good fit a bit finicky. To that end, I've found that holding my head upright — as opposed to angling my head slightly downward (the usual posture I adopt while inserting IEMs) — significantly improves the fit. Additionally,
Accessories
Few accessories come with the U4s, but everything you need is included. Unfortunately, the quality and/or utility of these accessories ranges from... fine to downright poorly designed.
The brush does its job and has a little magnet in the top, the purpose of which I'm uncertain.
The stock ear tips sound the best and the foam ear tips sound okay. However, none of the three sizes of stock ear tips fit my left ear. I think that an ear tip in between the small and medium sizes would fit my left ear best. In the meantime, I've been trying to make do with the small ear tips. The Symbio W tips improved the fit of the U4s, but slightly messed with the sound. With all that being said, I'd still take either of those tips over the Spinfit ones that came with the U4s. Those tips — to my ear, at least — paired so badly with the U4s that I was left questioning why they bothered to include them; the treble sounded smothered, which isn't great for an IEM whose treble performance is one of its standout features.
The cable is a bit kinky and has a bit of memory, but isn't microphonic.
The case is nice, although I could see the felt-lined interior clinging a bit to residual ear wax on the U4s' ear tips if they contact each other. One caveat to this case, however, is that the lid isn't a screw-on one. Instead, it just relies on the friction between it and the case. The lid manages to stay on well enough for the most part, but struggles to stay on if the contents of the case peak out from the base of the case. The leather material doesn't make a lot of noise, as compared to, say, an aluminum case (which I think 64 Audio includes with some of its IEMs).
The shirt clip is by far the worst part of this package. It seems like they just 3D-printed it, although I'm not sure how they actually manufactured this item. The part that's designed to clip onto a shirt is adequate, but the part that's designed to attach to the cable is awful. This part of the clip has little room to insert the cable into, which led to me accidentally breaking this part of the clip the second time I used it to try to attach the clip to my shirt. I could have left it on after the first time, but it pinches the cable so tightly that I was worried leaving it on would damage the cable. Lastly, it feels a bit cheap, which is what led me to think that it was 3D-printed. This wouldn't bother me all that much on a significantly cheaper IEM, but given that the U4s cost over $1000, I believe that everything that comes in the box is subject to scrutiny.
The brush does its job and has a little magnet in the top, the purpose of which I'm uncertain.
The stock ear tips sound the best and the foam ear tips sound okay. However, none of the three sizes of stock ear tips fit my left ear. I think that an ear tip in between the small and medium sizes would fit my left ear best. In the meantime, I've been trying to make do with the small ear tips. The Symbio W tips improved the fit of the U4s, but slightly messed with the sound. With all that being said, I'd still take either of those tips over the Spinfit ones that came with the U4s. Those tips — to my ear, at least — paired so badly with the U4s that I was left questioning why they bothered to include them; the treble sounded smothered, which isn't great for an IEM whose treble performance is one of its standout features.
The cable is a bit kinky and has a bit of memory, but isn't microphonic.
The case is nice, although I could see the felt-lined interior clinging a bit to residual ear wax on the U4s' ear tips if they contact each other. One caveat to this case, however, is that the lid isn't a screw-on one. Instead, it just relies on the friction between it and the case. The lid manages to stay on well enough for the most part, but struggles to stay on if the contents of the case peak out from the base of the case. The leather material doesn't make a lot of noise, as compared to, say, an aluminum case (which I think 64 Audio includes with some of its IEMs).
The shirt clip is by far the worst part of this package. It seems like they just 3D-printed it, although I'm not sure how they actually manufactured this item. The part that's designed to clip onto a shirt is adequate, but the part that's designed to attach to the cable is awful. This part of the clip has little room to insert the cable into, which led to me accidentally breaking this part of the clip the second time I used it to try to attach the clip to my shirt. I could have left it on after the first time, but it pinches the cable so tightly that I was worried leaving it on would damage the cable. Lastly, it feels a bit cheap, which is what led me to think that it was 3D-printed. This wouldn't bother me all that much on a significantly cheaper IEM, but given that the U4s cost over $1000, I believe that everything that comes in the box is subject to scrutiny.
Closing Thoughts
The 64 U4s is a fully realized vision that's worth the asking price; everything I hear sounds intentional, meaning that the main reason to take issue with its sound is that it doesn't align with your preferences. Nevertheless, given that I decided to keep them, it should go without saying that that caveat doesn't apply to me. If you're looking for a different perspective on your music that's still enjoyable and are able to stomach the cost, I'd definitely recommend giving these a try.