64 Audio A4s Custom In-Ear Earphones Review

If you’re a fan of high-performance In-Ear earphones, there’s a good chance that you’ve come across at least one of 64 Audio’s vast range of multi-driver earphones during your research into the hobby. From their two-driver Duo (which impressed me greatly when I reviewed it earlier this year) right up to their range-topping Fourte, the US company (based in Vancouver, Washington) has carved-out a hard-won reputation for creating a vast range of universal IEM models for different budgets and listening preferences. 64 Audio has also developed a number of patented innovations for their IEM range, including “tia” (Tubeless In-Ear Audio) and “apex” (Air Pressure Exchange) which has helped them to stay at the forefront of portable audio and develop a legion of audiophile devotees. 

What you may not know, however, is that 64 Audio is also a leading producer of Customer In-Ear Monitors (CIEMs) that are favoured by some of the world’s biggest names in pro audio - from on-stage performers and recording artists, to producers and mixing engineers. 

In late 2021, 64 Audio announced the launch of their second “hybrid” CIEM, the $1999 64 Audio A4s Custom In-Ear Earphones, which are available for order now at Addicted To Audio. The A4s boasts a four driver design that also includes a dynamic driver to deliver strong bass performance that 64 Audio suggests will be particularly useful for bass players and drummers - or, audiophiles who are partial to enjoying a rich, dynamic bass performance in their music. 

The A4s presents a relatively affordable entry-point into 64 Audio’s “tia” driver technology that graces their higher-ended models, and is a compelling all-round package in terms of design, sound, and performance that deserves a look-in by professionals and audiophiles alike. Before we go into the A4s’ technical breakdown, let’s take a quick overview of what the “c” in CIEM is all about if you’re new to this particular branch of in-ear audio. 

What are Custom In-Ear Monitors (CIEMs)?

Custom In-Ear Monitors are chiefly developed to help musicians monitor the sound of their own (or their bandmates’) performance accurately during the demanding environment of a live performance. If you have thousands of screaming fans screaming and singing in unison in front of you (or, in the case of my own band “dozens” of fans), then it can be a little tricky to hear all the musical and timing cues from the rest of your band, and you might not even be able to hear what you’re playing or singing yourself! 

Traditionally, this was solved on-stage with the use of foldback amplifiers, but one of the side-effects of having to crank these up to overcome the ambient sound of a live performance space is that you can end up with pretty bad hearing damage, pretty quickly. Not only do CIEMs allow musicians to monitor their own live performance, but they also provide strong levels of passive noise isolation, making it easier to concentrate on the sound of their own music while preventing hearing damage over time. 

As their name implies, Custom In-Ear Monitors are, well, customised for the individual wearer. If you’ve used traditional IEMs in the past, you’ve probably noticed that their performance (particularly when it comes to bass) as well as their passive noise-attenuating abilities are highly dependent on the seal that you’re able to form using either silicone or foam tips. CIEMs are individually moulded to the shape of the user’s inner and outer ear, with each unique design being created from impressions that require an audiologist’s assistance to create. The result is a seamless IEM shell that sits completely flush against the contours of your outer ear, with a protruding nozzle that inserts neatly into your inner ear to form a completely snug, flush fit. 

Due to their superb fit and isolation, as well as being tuned with audio professionals in mind, it’s no wonder that CIEMs are also often sought as the ultimate in-ear solution by audiophiles and portable hifi connoisseurs. 

How to order and customise your own 64 Audio A4s

Choosing a pair of CIEMs isn’t as simple as choosing a pair of “regular” universal-fit IEMs off the shelf - there’s a bit of a process involved to make your one-of-a-kind pair. If you’re wondering which particular 64 Audio model will be best for your needs and listening preferences, it’s worth taking a look at this video they’ve produced which may help give you a steer towards the 64 Audio CIEM that’s going to be best for you:


1) Getting impressions made

The first thing you’ll need to do is to have some impressions made with your local audiologist. Most audiologists will be familiar with the process, as they usually create all kinds of hearing protection, hearing aids, or bespoke listening devices for their customers. It’s a fairly quick and affordable procedure, and usually costs anywhere between $50-$100 depending on where you go. 

After checking that your ears are clean, they’ll inject a semi-solid plastic-like goop into your ears, which takes around a minute or so to set. When it’s ready, they’ll pull them out for you to then take home and send through to Addicted To Audio after you’ve chosen the model and customisation options that you’ll be going ahead with. Here’s what my impressions looked like: 

2) Choose your model and customisation options

The A4s isn’t only “customised” due to its personalised moulded shape, but you can also tweak its aesthetics according to your own individual tastes. Take a look at the A4s product page on the 64 Audio website to choose what colour and faceplate options you’d like to choose for your own A4s - you can even adorn it with your own initials, pattern, or logo if you like! Next, get in touch with your nearest Addicted To Audio store to let them know about your final customisation options, and make a payment to get your order underway. 


3) 3D scanning of your moulds

Unless you already have 3D scans of your impressions made from previous CIEMs, you’ll need to send your impressions back to Addicted To Audio so that they can make a detailed scan of your impressions using their state-of-the-art 3D scanning tool. Once they’ve made the scans, they’ll then send these on to Ultimate Ears to start manufacturing your one-of-a-kind A4s, which usually takes around six weeks to produce and ship back to you from the US (depending on the volume of orders they have at any given point in time - check with your nearest Addicted To Audio store to understand wait times). 

64 Audio A4s technical overview

The A4s is a “hybrid” multi-driver CIEM, in that it uses a complement of four different drivers in order to achieve its unique sound signature. Two balanced armatures are responsible for reproducing low-mid and high-mid frequencies, while 64 Audio’s signature “tia” driver acts like a “tweeter” for higher frequencies. The key point of difference in the A4s was 64 Audio’s decision to also employ a dynamic driver in order to create an impactful bass shelf, which gives the A4s a powerful and yet organic sound that helps it stand apart within the 64 Audio custom line-up. If you choose one of the translucent shell colour options when building your own A4s you’ll be able to look inside at the complex series of drivers, crossovers and wiring that sit inside each shell - it’s a pretty cool piece of engineering and I’d highly recommend you tick the “translucent” box over the opaque version simply to behold 64 Audio’s handiwork.

The four drivers are linked together via a passive four-way crossover network and together create a claimed frequency response of 10Hz-20kHz, with an impedance of 11-ohms and sensitivity rating of 107dB that makes the A4s exceptionally easy to drive with any number of on-stage, in-studio or portable source gear. 64’s patented Linear Impedance Design (LID) circuit has been used in the A4s to “correct the non-linear electrical resistance of the multiple driver sets”. What does this mean? Seeing as the A4s is designed to work with source devices of varying output impedances, the LID system helps ensure a consistently flat impedance to avoid any unwanted changes in frequency response. 

64 Audio explains that their “tia” technology is designed to help reduce resonance and distortion, and they’ve used two components of this in-house system in their newest hybrid IEM: the use of the “tia” driver for high frequencies, plus a single-bore design that terminates in a large-ish single mesh-covered nozzle at the business end of each A4s. 

The A4s is not a fully sealed design - there appear to be two tiny vent holes on the upper edge of each shell, and there are also the two prominent silver “plugs” on the outside faceplate of each shell which contain 64 Audio’s “Apex” modules. Their Apex system is designed to help release the discomfort of air pressure from the ear canal while still retaining a high degree of passive isolation. The A4s ships with the “M20” module as standard, which is rated as providing -20dB of passive isolation as well as providing the strongest bass performance. Stepping down to the optional “M15” or  “mX” modules will lessen isolation to either -15dB or -10dB respectively, while proportionally reducing overall level of bass but also opening up the A4s’ sense of air and clarity. 

64 Audio A4s key specifications:

Transducer Type

Dynamic & Balanced Armature

Transducer Configuration

1 dynamic low

1 BA low-mid

1 BA high-mid

1 tia high

Bore Configuration


Frequency Response

10Hz - 20kHz


107 dB/mW (94mV) @ 1kHz


11Ω @ 1kHz


Integrated 4-way passive crossover


20dB w/ m20 module

-15dB w/ m15 module

Ships with M20 Module standard

64 Audio A4s packaging and accessories 

Waiting for your new CIEMs to arrive after sending away your impressions can often feel like an eternity, but when the A4s finally arrived back from 64 Audio it was a first-rate experience from the get-go. The A4s is presented inside a premium black and red box bearing a message from 64 Audio founder Vitaliy Belonozhko plus a hand quick start guide with directions around fitting them in your ear, and tips on avoiding hearing damage. 

Inside the box, the A4s is safely stored within one of 64 Audio’s excellent metal screw-top storage cases (it looks a little like an ice hockey puck) which feels strong enough to withstand being run over by a tour bus. Each custom A4s will come with the owner's name laser-etched onto the lid of the case, which is a nice personalised touch and helps make them truly feel “yours”. An IEM cleaning tool is also provided inside the A4s’ case to help clear the nozzles of earwax and other debris, which you should try and do often, as being a deep-fit CIEM this will become noticeable if you’re listening to them a lot. 

Two different cable options can be chosen when designing your own A4s - either the more common two-pin 0.78mm system, or “IPX” variety, which is a more compact and robust version of a traditional MMCX connection. My review pair came with IPX cables, and while it may be harder to track down a pair of aftermarket cables if you’ve chosen to go down this route, it really is a more durable and reliable connection and has become my preferred style of detachable IEM cable. The 48-inch braided cable that comes with the A4s is terminated in a standard 3.5mm single-ended jack for connecting your CIEMs to all kinds of source gear. I really dig the 64 Audio cable on the A4s - it’s pliable, lightweight, and doesn’t kink or tangle easily. Thankfully, it also avoids annoying microphonic noises from bumps or from rubbing against your clothing. 

64 Audio A4s fit, comfort, and isolation 

You always hold your breath at first when trying on your new CIEMs for the first time, hoping that the impressions you had made translate to a comfortable and flush fit. 64 Audio uses a 3D-printing process to create the A4s that they call “3D-Fit”, which was developed to help preserve the details of your individual impressions while retaining the integrity of the acoustic tuning features inside each shell. 

64 Audio nailed the mould on my review A4s shells - they’re an absolute Goldilocks fit, sitting completely flush and snug inside each of my ears. They’re also terrifically comfortable, and don’t feel at all like they’re creating pressure or poking against any part of my outer ears. I’ve worn them for 8-hour+ work days without taking them out once, they’re that comfortable. They’re also relatively easy to insert and remove, and don’t take too much cajoling to get in and out of place. 

Passive isolation on the A4s is excellent - I have no way of measuring if it’s actually the -20dB that’s claimed when the M20 modules are used, but I reckon this rating is probably on the money. With each CIEM inserted, the level of noise reduction is close to that of a decent pair of active noise cancelling headphones. And when music is playing, the outside world essentially disappears. If you’re planning on getting your own A4s then I’d suggest that you can probably forget about ever needing a pair of ANC cans. Even when you find yourself in a noisy environment, the details and dynamics of whatever you’re listening to are still clear and prominent - if you need stage monitors that’ll tell you exactly what the rest of your bandmates are doing, the A4s will make a terrific choice. Passive isolation does take a noticeable deduction when swapping in a pair of M15 modules, so these are probably better suited for quieter environments or when you actually need a little background noise to creep in. 

Listening to the 64 Audio A4s

From the moment you first listen to the A4s, you know that you’re in for a fun time. They’re an extraordinarily rich and dynamic-sounding IEM thanks to the addition of their dynamic driver whose hand is clearly evident when listening to bass-rich music. The low-end on the A4s is extremely pronounced, but tastefully so - it’s substantially elevated all the way down to the deepest sub-bass regions, and yet retains a strong degree of speed and composure. I’ll go so far to say that the A4s’ bass implementation is among the most enjoyable I’ve heard in any IEM, or headphone for that matter. Balanced Armature bass can be articulate and precise, but there really is no substitute for a dynamic driver when it comes to properly dynamic bass that you can feel

The excellent bass guitar work in Steely Dan’s Peg sounds punchy, prominent (but not overcooked) and easily followable in the mix on the A4s, but doesn’t overshadow the instrumental or vocal tracks. The A4s make well-recorded drums and percussion sound absolutely killer - I kept going back to The Noose by A Perfect Circle over and over simply to revel in the tasty thump of the kick drum and the sizzle and snaps of the cymbal and snare work at the 1:50 mark. 

The A4s’ overall tone is somewhat on the warmer and more relaxed side, mostly due to its elevated bass experience, but also because it has a velvety and somewhat restrained upper treble. There’s still plenty of information and shimmer on display up top in Hiatus Kaiyote’s Fingerprints, but it’s not as extended nor abundant with micro-detail as some of the higher-specced 64 Audio multi-driver IEMs such as the u18s. This tuning and driver implementation does favour tone and texture over out-and-out technicalities, and while it’s definitely a resolving IEM, the A4s won’t trade resolution blows with the best of them. But, you wouldn't buy yourself a pair of these to get super analytical - I think the A4s is best suited for makers and lovers of music. Whether you just want to groove along with some tasty house music like D.A.N.C.E by Justice and bliss-out on the gnarly bass, or enjoy the stripped-back musicianship of Keith Jarrett’s piano and Charlie Haden’s double bass on their Jasmine record, the A4s offers an easy-to-love tuning that both natural-sounding and decently linear from top-to-bottom. The A4s’ upper mids/lower treble aren’t the most forward, with Natalie Merchant and her backing instrumentalists taking a figurative half-step back from the microphone in her track Carnival (but damn does that bass sound good…), but there are a couple a gentle peaks in the upper treble that seem to land right at the point where snares and cymbals hit - which is perfect for stage monitoring. 

I listened to the A4s with a range of source devices, ranging from the Sony NW-A100TPS 40th Anniversary Digital Audio Player to the FiiO M17 Portable High-Resolution Audio Player, and I found that it sounds frankly excellent on whatever you throw at it source-wise. CIEMs like the A4s are designed to perform reliably and sound faithful using wireless monitoring gear, and the casual listener can also happily plug them into the headphone jack on a Macbook or iPhone dongle to great effect. My go-to source device for IEMs is the Astell&Kern A&future SE200 Digital Audio Player, with the volume set at 55 (out of 150 volume increments) being the sweet spot for most music out of its single-ended jack. With a decent discrete source such as the SE200, the A4s has a decent ability to layer separate tracks within a decently-wide “head-stage” that is noticeably less cramped than the more intimate Ultimate Ears UE Live, but it’s not the most laser-focused 3D image.

Final thoughts

The A4s is perhaps the easiest-to-recommend custom in-ear on the market at the moment, not only for its relatively affordable asking price, but because it also has a decadent bass-rich and refined sound signature that’s exceptionally easy to love - even if you’re a hardened lover of a flat “reference” signature. 

Combine the A4s’ rich and inviting sound signature with a design, build and customisation package that’s simply first-rate, and you have all the ingredients you need to either jump on stage, or jump into your favourite albums and simply enjoy music for what it is, rather than analysing it to pieces. If “fun > flat” sounds like your music philosophy and you’re looking to get into the world of custom in-ears, look no further.  


64 audioCustom in ear monitorsIn-ear earphones