In-ear Monitors (IEMs) have transformed the on‑stage experience for many performers, but they aren’t always ideal for bass players. Could a new hybrid design be the answer?
Balanced‑armature drivers are widely used for in‑ear monitors (IEMs), being smaller and lighter than other types of drivers whilst offering a detailed, accurate sound, as well as a degree of efficiency — something that is particularly important in the battery‑powered world of hearing aids for which this type of sub‑miniature balanced armature was first developed. But balanced armatures work best when they are only asked to reproduce a specific range of frequencies, which means that for high‑quality, full‑range audio applications they generally have to be used in multiples, This introduces issues around crossovers, filters, integration and tuning. And, small as they are, there is a limit to how many you can get inside something designed to fit in your ear!
The dynamic drivers also used in some IEMs, by contrast, are effectively just miniature loudspeakers. With the weight of a wire ‘voice coil’, albeit a small one, on the back of the diaphragm, they can’t be as ‘fast’ as a balanced armature, but given its larger surface area, a dynamic driver is able to ‘move more air’, which is something you generally want at low frequencies. The latter, of course, is something of a generalisation, as you can build a balanced armature specifically to ‘move more air’ by giving it a more powerful motor and a bigger diaphragm, but then it is no longer as small and efficient... Logically enough, balanced armatures and dynamic drivers have been used together, utilising the strengths of each technology, in conventional headphones for some time now, before inevitably spreading into the world of IEMs.
It may sound simple enough, but combining two very different technologies within the tight confines of an IEM is actually anything but simple, and the fact that many IEMs use the same drivers and yet end up sounding significantly different perhaps tells us that there is more than a degree of expertise and artistry involved in their design and tuning.
IEMs & Bass Players
Which brings us to the point of this review. I’ve been a custom‑moulded‑IEM user since 1997, with my first and all subsequent sets being made by one of the pioneers in this field, the British company ACS. I’ve had different sets over the years, as the technology has advanced but also because our ears don’t stay the same shape as we age, so it is necessary to have new scans taken from time to time to make sure that your custom moulds do actually achieve a proper seal against outside sound.
...our ears don’t stay the same shape as we age, so it is necessary to have new scans taken from time to time to make sure that your custom moulds do actually achieve a proper seal against outside sound.
My current pair is a multiple balanced armature set that I have been very happy with — at least until I was faced with the prospect of playing bass at a silent‑stage venue. A few experiments soon revealed that whilst I felt I could hear the bass perfectly clearly in a mix, it didn’t ‘feel’ particularly comfortable or satisfying. More compression seemed to make it worse, and whilst EQ on individual monitor feeds is readily available in most digital mixers, the sort of EQ that might correct this issue is going to eat up headroom and potentially run into distortion. What I felt was missing, even when I was using high‑quality amp and speaker modelling, was the familiar sense of ‘weight’ and resonance in the bass sound. IEM‑equipped bass players have sought out a number of innovative solutions to this problem, including vibrating platforms or seats, body‑worn vibration packs and, more recently... hybrid IEMs. The latter combine dynamic drivers and balanced armatures, with the cone driver’s ability to move air making the difference in terms of the perceived weight or impact in the low frequencies. Ultimate Ears, one of the leading USA‑based IEM suppliers, have hybrid sets, and as I learned at the recent Sound On Sound GearFest UK show at Tileyard, London, so too do ACS. After verifying that my current models were still a perfect fit, ACS were able to use my existing ear scans to send me a custom‑mould set of their new Emotion Dynamic hybrid in‑ear monitors.
But there’s a further complication here, in that I’ve been using IEMs equipped with ACS’s passive ambient filters. These allow a controlled amount of outside sound to leak in to facilitate communication, without my having to use ambient mics or hastily remove custom moulds whenever anyone tries to talk to me in anything other than sign language. Having now had that facility, I would never want to be without it, but good low‑frequency IEM performance is usually said to rely on achieving a good seal in the ear. Keeping this passive ambient filter working without potentially undermining the extra low‑frequency punch gained by incorporating the dynamic drivers looked like it might be a real design challenge.
Depth & Resonance
My new custom moulds arrived in clear, medical‑grade silicone, allowing me to see the full intricacy of the internal construction. The dynamic driver, made to a custom specification for ACS, is rear‑vented with a tuning filter and has its own bore, being joined along the way by the bore from the ambient filter. A second bore acts as the outlet for the two balanced armatures that take care of the mids and highs, augmented by an extra balanced‑armature ‘super‑tweeter’.
All very impressive looking, but does it work as intended? Yes, I think it does. The bass has the same basic tonality as it did before, but now there’s more of what I can only describe as resonance — the kind of cabinet‑formant‑tone resonance that is the difference between playing with a really clean DI and an actual speaker. It just feels a bit more like I’m playing bass through an amp behind me, even without any of the very specific physical sensation that brings. It is tempting to imagine that there might be some small amount of physical coupling from this design, but if there is, then it is nothing that I can discern without scientific vibration instrumentation. On balance, I think this extra depth is just being done with sound.
The very low end does seem like it is sensibly curated without being too conservative — the low ‘B’ on my Stingray 5 doesn’t threaten to detach my fillings, but there is still some heft to it all the way down.
So, job done on that count, but, of course, you don’t just listen to yourself in an in‑ears mix: the rest of the balance has to sound right, too. And by ‘right’, I mean the sound that gives you what you need in a live‑performance situation. I’m sure it is different in the world of IEMs for hi‑fi listening, but I find it is no great advantage for IEMs for live performance to be ‘flat’ in their frequency response. Having sound squirted into your ear canals at close range is an inherently unnatural situation anyway, and what really matters in my experience is that you can clearly hear transients rather than texture. The leading edges of things are where the timing is, and whilst it is common to get your own mix in your IEMs in the era of digital mixers, and probably your own overall EQ as well, you are not going to get individual sources EQ’ed just for you, so a bit of extra clarity in the presence region of your IEMs never goes amiss, and there’s plenty of that here.
Sensitivity in the Emotion Dynamics is a little lower than my previous all‑balanced‑armature set, but the nominal 28Ω impedance puts them very much in the easy‑to‑drive zone. I had no problem getting more than enough level from a variety of headphone amps — an important consideration when sources are likely to be battery powered, as wireless belt packs need to be.
The standard ambient filter supplied gives 17dB of reduction, which I find to be just right, but you can opt for 10dB or indeed 26dB to better match the environment in which you will be working. I’m not sure how much difference that would make to the sound, but clearly these IEMs are a finely tuned overall system: I experimented with blocking the ambient filters whilst listening to playback, resulting in a much soggier bass, intruding on the lower midrange, and also a collapse of the nicely ‘out of the head’ soundfield into something much more just between the ears.
Supplied, as usual, with both carrying case and portable electronic dryer and sanitiser, these are cutting‑edge custom moulds and by no means cheap, but it seems the ACS hybrid technology itself will very soon be available in their universal‑fit range as well at a more affordable price point. Universals can work well for some people, depending on physical variables like the shape of your ears, and also on the amount of trouble you take to find exactly the right size and shape of tip to get a good fit.
These Emotion Dynamic hybrids are the most effective live‑sound monitoring IEM set I have so far experienced.
Final Thoughts
In real‑world use, IEMs are always part of a whole system, every part of which must work reasonably well to give you a positive experience. You must have enough isolation so you are actually hearing your IEMs; the mix in your ears must be the one you actually need to perform properly; if you are using wireless, that part of the system must be of decent quality and reliable; and finally, you have to have learned how to be comfortable with the very different experience of performing with IEMs. The actual sound quality of the units themselves shouldn’t really be the limiting factor in the outcome, but variations in the subjective quality of the sound do very much exist, sometimes even between units from different manufacturers using the same complement of drive units.
I recognise here that I have no exact equivalent point of comparison for the subjective aspects of this review, as I’ve never had hybrid custom moulds from another manufacturer — there is, of course, little incentive to go to the trouble of getting new scans from someone else unless you are unhappy with what you already have. But I do have a lot of experience with other IEM models in universal form that I would contend lends validity to my conclusions. Leaving aside the initial goal of adding some apparent ‘weight’ for bass players, and surely drummers, too, these Emotion Dynamic hybrids are the most effective live‑sound monitoring IEM set I have so far experienced. ACS reference their patent‑pending IRIS technology (Intelligent Resonance In Silicone), as a construction method that allows very precise acoustic tuning, with further fine‑tuning in the form of their SSI Sound Stage Imaging. The latter, I think, may be using the ambient filters to achieve a wider soundfield extending outside the head. Whatever they’ve done, it results in not just great‑sounding IEMs, but also, thanks to the openness of the ambient filters, something far less challenging for healthy long‑term wearing and listening. Maybe silent‑stage gigs aren’t going to be so bad after all!
Specifications
Frequency range: 10Hz‑22kHz.
Input sensitivity: 108dB SPL/mW @ 1kHz.
Impedance: 28Ω @ 1kHz.
Noise isolation: tuned vented port with 10, 17 or 26 dB reduction of ambient noise.
Drivers: Custom dynamic transducer, three Knowles balanced armatures (mid, high and super‑high).
Casing: Lacquered 40‑Shore medical‑grade silicone.
Cable: detachable 1.3m, with two‑pin connection and memory ear hook.
Connector: 3.5mm jack with included 1/4‑inch adaptor.
What Is A 'Balanced Armature'?
The ‘armature’ bit is just a small metal strip or ‘reed’ that is pivoted in the centre to hold it in the middle of a coil of wire that sits within the field of a permanent magnet. We use the term ‘balanced’ because the magnetic force on the reed is precisely equal from both poles of the magnet. So, it just sits there, until you apply an alternating current to the coil, inducing a magnetic force in the armature that pushes and pulls it between the opposite poles of the permanent magnet, corresponding with the polarity of the electrical signal. So far, still no actual sound, of course: the sound we hear comes from a small, lightweight diaphragm, mounted within the same casing, driven into vibration by a physical connection from one end of the armature and vented to the outside world through a small hole.
Why bother making sound this way? Well, it can all be extremely small, as the coil only has to be big enough to induce movement in the tiny, lightweight armature, as opposed to the voice coil in a conventional speaker, where the coil has to move itself in the magnetic gap, in order to move the cone it is attached to.
Who Needs Ambient IEMs?
There can be no doubt that the development and widespread use of in‑ear monitors has revolutionised the live‑performance experience of many musicians and sound engineers. For the latter, the elimination or significant reduction of wedges and side‑fill speakers can clean up the front‑of‑house mix like nothing else, and for many performers, especially singers, moving to in‑ears was the first time they had ever been able to really hear themselves properly on stage!
Using accurately fitted IEMs in both ears, so they achieve a good seal against outside noise, makes your working volume level entirely independent of your environment, whereas earbuds or ill‑fitting universal IEMs will often exclude so little of the backline and ambient sound that you then need to have your in‑ear signal uncomfortably loud to be able to hear it — a worse outcome than not using IEMs at all.
But there is no doubt that using a fully ‘isolating’ set of in‑ear monitors can make you feel slightly disconnected from your audience and indeed your fellow performers. The latter is especially a problem in rehearsal, where what would once have been a few quick words can turn into a bit of a pantomime of lip‑reading, gesticulation and reluctant IEM removal and re‑fitting. That is, unless everyone maintains the discipline of only speaking one at a time, and into a microphone that they know is in everyone else’s IEM mix... and that never happens.
There are other drawbacks to fully sealed IEMs, moreover, especially for singers, which you can experience for yourself just by speaking or singing with your fingers in your ears — you’ll hear a kind of droning head resonance know as ‘occlusion effect’. Also, sealing off your ears from normal air flow for long periods of time is also not particularly healthy, as they will get both hot and moist.
The precise workings of ACS’s patented ‘tuned vented port’ designs are somewhat beyond my grasp of the physics of miniaturised acoustic systems, but I can say that they work spectacularly well in my experience. Instinctively, the expected trade‑off is some kind of compromised bass response, but the incorporation of the venting in the new ambient hybrids with their impressive bass response seems to prove that doesn’t have to be the case.
Pros
- A fundamentally more familiar and satisfying IEM sound for bass players.
- Isolation level options.
- Ambient venting without LF loss.
- Acoustic imaging that seems to extend outside the head.
Cons
- Pro‑level price bracket.
Summary
A set of IEMs designed to make bass players happier may actually turn out to be just as helpful to players of other instruments. Not cheap, especially in custom‑moulded form, but the tech may soon be available in affordable universal models too.