In-ear Monitors

Monitors / Monitoring > In-ear Monitors

Published January 2026

IEM pioneers ACS bring their bassist‑optimised hybrid technology to the universal‑fit market.

Regular readers may recall, last year I found a set of custom‑moulded ACS Emotion Dynamic hybrid IEMs to be the perfect solution to the challenge of playing bass in a silent‑stage setting. These just‑launched models employed a setup of twin balanced armatures for midrange and highs, plus a balanced‑armature super‑tweeter, combined with a custom‑made dynamic driver to give the impression of more ‘weight’ in the lows courtesy of the dynamic driver’s ability to ‘move more air’ than balanced armatures. Of course, given that we are talking about a dynamic driver small enough to fit inside an IEM along with all the other drivers, the vent tunnels and tuning filters, we are not talking about moving very much air... But a little goes a long way when it is coupled directly to your ear canal!

They worked very well in the specific application for which I acquired them, and having got used to them, I am not sure that I don’t now prefer them to my existing, multi‑balanced‑armature set, at least for live‑performance applications. At £895$1200 a set, though, clearly these are not within easy reach for everyone. However, with the release of the new Eclipse 500 model, ACS have brought the same technology to the universal‑fit IEM market at a lower price point.

A Winning Combination

The Eclipse 500 has the exact same combination of dynamic and balanced‑armature drivers as the Emotion Dynamic hybrids, but now mated with a choice of three sizes of foam tips. Achieving a proper fit is all‑important for getting a good low‑frequency response with IEMs, and getting the correct size of tip and, crucially, positioning it optimally is obviously the first essential in all IEM usage. But the shape of ears does vary significantly from one individual to another, resulting in some people struggling to achieve both a stable fit of the body of the IEM in the outer ear and a good coupling to their ear canal at the same time. I’m actually one of those people, having a slightly unusual ear canal size and shape, and have sometimes struggled with the positioning of universal sets.

Included with the Eclipse 500s, however, is a choice of different lengths of the coupling nozzle that mates the foam tips to the body of the IEM. The difference is barely discernible to the eye, with the largest being only 1.5mm longer than the shortest, but it will often be enough to make all the difference. I’m not aware of anyone else offering this particular mode of customisation, but it is a very good idea, and should ensure that more people are able to achieve the proper fit and positioning needed to get the full performance out of these IEMs.

So, what do they sound like? In addition to the standard foam tips, the review models were supplied with a pair of my personal custom mould tips as well so I could make a direct A/B comparison — well, as close to A/B as you can get given the time it takes to manoeuvre a pair of IEMs out your ears and insert another set! To me, they sound substantially the same when using the custom moulds and almost indistinguishable with the foam tips too, once I had found the optimum set and the ideal nozzle adaptor length. Clearly, there are more potential user variables with the latter setup than custom moulds.

Properly fitted, the Eclipse 500 hybrid set can offer much the same functional benefit as the Emotion set...

The Eclipse design differs from the Emotion model in construction, having an aluminium body versus the all‑silicone body of the custom‑mould version, and the sound‑bore arrangement is slightly different, too: the custom set has the dynamic driver and an ambient filter combined into a single bore while a second conveys just the balanced armatures, with both bores then venting directly into the ear canal. In the Eclipse version, the dynamic driver, balanced armatures and ambient filter all have separate bores, which then combine into a single outlet via both the custom and foam tips. In theory, any of that might be expected to make some sort of difference, but I’m happy enough to say that, properly fitted, the Eclipse 500 hybrid set can offer much the same functional benefit as the Emotion set for any performer needing a bit more sense of depth and weight in their IEM mix. And, crucially, the Eclipse includes the same ambient filter as the upmarket model, maintaining airflow into and out of the ear, whilst still achieving the required degree of attenuation and a good low‑frequency performance. I really don’t know how they make that work!

IEMs For All

There is no denying that IEMs have changed the landscape of live music performance in a lasting way. Some people love them, some people hate them, while others just accept them as a necessary ‘part of the job’ whenever they are required. But they are certainly no longer just the province of the fully ‘pro’ performer. With ever increasing numbers of ‘silent stages’ or ‘low‑volume venues’, if you work in any kind of ‘paid to play’ situation, sooner or later you are quite likely to really wish you had some.

One last thought occurs to me on these: custom moulds may be the pinnacle of the ‘pro’ world for optimum fit and performance, but ears change as we get older, and sooner or later, those pricy custom moulds will no longer seal properly and have to be replaced. The serious semi‑pro or amateur performer using universals, however, may be able keep a universal set in use for far longer by periodically switching to a different tip size, or, in this range, using a different nozzle length. A price tag of £699$950 is still not in everybody’s ‘affordable’ bracket, but with these at least you can look at it as a long‑term investment in both your contemporary stage‑readiness and your hearing health, too.

Summary

ACS’s Eclipse 500 hybrid model offers the same benefits as their upmarket Emotion Dynamic, at a lower price point, by incorporating essentially the same design and driver line‑up into a universal‑fit IEM.