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The EP321 MEMS is very much a Binary Acoustics in-ear monitor (IEM). Founded in 2017, after five years of producing $1,000 IEMs, the Chinese company carved their niche with the more affordable Gizaudio x Chopin ($200) and Dynaquattro ($250) IEMs. It’s not that they sound similar – they don’t. But they share the brand’s desire to offer different, exciting sounds, whether that’s a bass-heavy, U-shaped profile or a soulful neutral warmth.
With the EP321 (currently $295 over at Amazon), Binary Acoustics is going for yet another approach. This time it’s an engaging, analytical sound profile, and it’s a treat for the price point.
Specs
6 drivers: 10mm dynamic + 6mm passive radiator + 3 balanced armature + 1 MEMS driver
Material: 3D-printed resin, stainless steel faceplates
Cable: 1.2m (3.5mm or 4.4mm termination)
Impedance: 13Ω @ 1kHz
Sensitivity: 122dB/Vrms @ 1kHz
Weight: 7.5g each
Release date: December 2025
Price: $310
It’s worth noting that I don’t have a single preferred sound profile. Sometimes I like wads of bass; other times I like a subtle, vocal-forward set. I have, however, been treble shy in the past. But having reviewed the Ziigaat Horizon and now the EP321 in close succession, I’m going to add clear-yet-balanced treble to my like list. This isn’t going to make my sub-$100 list of the best budget IEMs, but when I get around to writing the best $300 IEMs, the EP321 is going top 3 for sure.
What the hell is a MEMS driver?
Micro-Electro-Mechanical Systems (MEMS) is pretty new in the IEM driver form it takes in the EP321. And though it was first made commercially available in 2023 with the Singularity Audio Oni, that set cost $1,800, as the tech was too new, and needed an external energiser to play.
Today, the EP321 uses the same tech, but it’s refined and a lot cheaper. Ultimately, it’s a solid state speaker technology where the driver is etched onto a silicon wafer (driver membrane, not biscuit). It uses the same process used to make a computer chip, and while the traditional dynamic driver has a voice coil and magnet moving a diaphragm, its silicon structure is moved by an electrical charge.
Cool story, but what does that mean sound-wise?
Well, because it’s very, very small compared to a traditional driver, and has almost zero mass, it can start and stop vibrating almost instantly – that means it promises a noticeably fast detail in the sound, specifically in the high frequencies.
The EP321 uses a MEMS driver for the treble, and if we are to believe the marketing literature, it should provide a superior ‘snap’ to sounds like snare drums and cymbals. And, while I do find the EP321 incredibly snappy, I wouldn’t say that it’s night and day different to, say, the Ziigaat Horizon, which also has excellent treble, though driven by dynamic drivers.
It’s great when companies push the boundaries of technology, and this is clearly a technology that works really well, but I wouldn’t make the MEMS component the primary reason you buy the EP321. Yes, the high frequencies are snappy (possibly to the point of sterility for some). But I doubt anyone is listening to the EP321 and thinking, “ah yes, that sweet sweet MEMS sound!” It’s just cool new tech implemented well. Now let’s talk about the sound…
Sound profile
Overall sound: The EP321 sound is detailed and engaging, and that is largely thanks to its treble. Each time I review an IEM, I use them for as long as it takes to forget that I’m reviewing them. Then I snap back and see what I subconsciously picked up about the sound. With the EP321, it’s how technical it is – the perceived detail, the positioning of instruments (imaging), the crispiness of percussion, and clarity of voices. That’s a long-winded way of saying, ‘It’s the treble, stupid!’
Bass: With all the focus on the treble, there’s a risk of overlooking the bass. And that’s understandable. When it comes to sound perception, if you have loads of clarity and air in the treble, it can make the overall presentation of an IEM feel cleaner, even if there is a decent wad of bass on offer. And the EP321 certainly offers a decent wad of bass.
Though the bass isn’t the star of the show, as it is with something like the $240 FATfreq x HBB Deuce, it is solid and dependable. 808s (the processed sub-bass kick drum sound) of Kendrick Lamar’s HUMBLE and Outkast’s The Way You Move, have plenty of power, and my go-to bass guitar tracks – Eryka Badu’s Other Side of the Game, Donald Fagen’s Morph the Cat, and Funkadelic’s Give up the Funk – have all the texture and detail that I want.
Looking at Super* Review’s EP321 Squiglink frequency response chart, you’d expect the EP321 to be blowing chunks of sub bass your way, with a bloomy mid bass that overwhelms the lower mids. But that’s not entirely true. There’s definitely sub bass presence. The mid bass, however, is clean, clear but still impactful. It’s a few notches above neutral, and definitely isn’t lean, and it’s only when I compare it to Sennheiser HD 600’s legendary midrange that I realise it does overstep a bit into the mids. But it’s not disastrous.
Mids: There’s mids, and then there’s mids. And though there is plenty to like about the midrange of the EP321, there’s definitely more detailed, rich and natural presentations out there. But I still like EP321’s midrange (and it’s definitely helped by the treble).
As mentioned above, the bass bleeds a little into lower pitched vocals, but higher pitched vocals shine through. A good test song for this is Neil Young’s Only Love Can Break Your Heart, especially when his higher pitched vocals join the lower register backing voices. Switching to the Sennheiser HD 600 – a headphone fabled for its excellent midrange – and all vocals are given plenty of space, and the tonality is rich and natural. Back to the EP321 and the vocals are tinged both by the bass and the treble, and while I don’t think it sound bad, it does mean that the EP321 won’t be ideal if you want a super natural, tonally correct midrange.
Treble: This is the highlight of the EP321. The treble is well extended with plenty of air, so you can pick up on micro details in music. The lower treble is less elevated, well balanced and smooth – so it’s not fatiguing at all – but this is not what you would call a natural treble. It’s a very engaging, fast treble that’s quick to show you all the detail of your music.
This is the EP321’s secret sauce – being detailed but not being tiring on your ears. Because the elevated treble is around the 15kHz area, it comes across as detailed and airy, but never in your face or annoying. If the treble was a photograph, it would be a high resolution picture, but printed on slightly matt paper. There’s all the detail but no glare. It’s an attribute you’d expect in IEMs that sell for thousands of dollars, so getting it for $300 is great value – if you value this kind of engaging sound profile.
Price, value, competition
At $310, and sporting the MEMS driver, I’ve already highlighted what great value the Binary EP321 offer. When it comes to comparisons, there’s only really one that I wanted to delve into, especially if you’re looking for a similar treble-forward set – the $330 Ziigaat Horizon.
The Horizon is also a clear, clean, technical IEM, and one that I gave an almost perfect 5 star review. I like both of these IEMs, and they do similar things.
I think the midrange is where they differ the most. With the Horizon, it’s more articulated than the EP321. There’s a tad more warmth in the lower end of the EP321, so there’s less space for the lower mids to shine.
Usually I’d side with warmth, but when it comes to the Horizon, I think the whole presentation is more balanced. There’s not a lot in it, and swapping between them while listening to my test tracks, I often forgot which IEM I was listening to. But when I locked in, especially on tracks like the bass-leading Morph the Cat, I think the Horizon offered a smoother treble, and pushed vocals and other mid-dwelling instruments a touch forward compared to the EP 321s.
The Horizon’s mid bass is better presented: There’s greater distinction between bass and mids, which again works for a clearer midrange. Both have good sub bass, but I notice it more with the Horizon, thanks to that distinction between upper bass and lower mids.
When it comes to build quality, there’s one clear winner. The EP321 has a far superior build to the Horizon, and it’s $20 cheaper!
EP321’s metal and resin is more impressive, and the Horizon’s lighter 6g of black resin feels cheaper than the EP321’s 7.5g – even with / because of the sparkly glitter face place finish.
Based on materials, it feels like more of an event to pick up and listen to the EP321. Based on sound, the Horizon’s combination of balanced treble, solid sub bass and more articulated midrange wins out.
Ultimately, it’s incredibly close, so I can see the material quality of the EP321 playing a big part in someone’s buying decision.
Should you buy it?
One thing struck me during my time testing the Binary EP321 – isn’t it amazing that you can have this quality of sound and take it on your morning train commute; that you can enjoy this quality sound profile even when you walk to the shops?!
When you get to this quality in over-ear headphones, it’s an in-house listening experience only! There’s no taking the Denon AH-D5200 with you on the bus into town. You’d be laughed at. Or robbed.
I know that’s a trait of all IEMs, but that was the revelation I had while testing the EP321, because they are one of the better made, better built IEMs that I’ve tested. The Binary EP321 is a beautifully made, beautiful-sounding IEM that can be enjoyed in your favourite home music listening space, or when you get a few moments to yourself on a family holiday. It’s a class act.
