Audio-Technica Sonic Fuel Earphones Review

Sonic Fuel Series 1
First, I’d like to thank the folks at Audio-Technica for providing samples of the Sonic Fuel earphones for review.

There’s a reason music montages popularized by films like those in the Rocky franchise have been so popular. Music gets us going. The right music keeps us motivated to push ourselves to our limits and past them at the gym. But a number of headphones, earbuds and in ear monitors simply aren’t made for active use, and there’s a market for sport and gym ready earphones. Enter Audio-Technica’s Sonic Fuel line of earphones, promising to be durable, to fit securely during physical activity and provide quality sound in the process. I have before me the three models in the new line, the CKX5iS, CKX7iS and CKX9iS.

So, do the three Sonic Fuel earphones have what it takes to measure up? Read on to find out.

CKX9iS 8



Included in the box is a drawstring pouch, 3 sizes of C-tip rubber ear hooks and 4 sizes of silicone single flange eartips.


A nicer, velour drawstring pouch, 3 sizes of C-tip rubber ear hooks, 4 sizes of silicone single flange eartips and a set of Comply Active Series foam tips.


A hard leatherette clamshell carrying case, 3 sizes of C-tip rubber ear hooks, 4 sizes of silicone single flange eartips and a set of Comply Active Series foam tips.

CKX5iS 1

Design and Build Quality

The entire Sonic Fuel line is designed for use in the gym or during other physical exercise and the earphones all seem to be more ruggedly built than other earphones. Though the construction is still mostly plastic, the housings feel solid and the rotating nozzles of the CKX5iS and CKX7iS are loose enough to allow for free movement but don’t seem like they’ll come apart at the seams, even during strenuous activity. I must say that they don’t exactly inspire confidence in long term usability but the
The CK9iS doesn’t have the rotating nozzle which makes it feel sturdier than its less expensive siblings but does give the impression that it is designed to focus more on the sound quality than being an “active” earphone.

The cable of the CKX5iS is a standard round cable while the 7iS and 9iS feature slightly stronger feeling flat cables. All three feature a remote and microphone as well as a built in volume limiter. I don’t call it a volume control, per se, because it doesn’t actually control the volume of your device. It can be handy if you need to change volume quickly but overall, with its rather uneven channel balance, it’s not something I used often.

CKX9iS 9

Comfort and Isolation

This is bound to be a sticking point for many people who try the Sonic Fuel earphones, as it was for me. The huge housings and rubber ear hooks (or C-tips, as Audio-Technica calls them) make them somewhat uncomfortable to wear for extended periods, even with the rotating nozzles of the two lower end earphones. That said, the sound of the Sonic Fuel earphones isn’t terribly dependent on the fit, unlike other, finicky earphones like the Sony MH1C.

Isolation on all three earphones is subpar, which I suspect is by design. In other words, don’t expect these to shut out the outside world in the same way the average vented dynamic driver IEM can, much less those with balanced armature drivers.

Microphonics were present in all three IEMs but never overbearing.

CKX7iS 6

Sound Quality

In keeping with the Audio-Technica house sound, all of the Sonic Fuel earphones share a similar presentation, being dry and edgy, with deep, punchy bass and clear, extended highs.


Sitting at the lowest end of the Sonic Fuel line, the CKX5iS sets a surprisingly high standard. The sound is clear, slightly warm, and a tad grainy. The low end boasts impressive depth and detail, still rumbling with decent authority down below 30Hz. The midrange is clear and dry but strangely, a bit hollow. It’s not a huge distraction or a big deal but it is noticeable, especially next to other earphones.

Moving into the upper midrange and treble, the CKX5iS is decently linear and well extended. It’s clear and airy and has a nice degree of treble sparkle and shimmer. It’s a bit peaky but not distracting but has a tendency to be a tad strident at times.

The presentation is slightly warm but thin, allowing the detail to shine through.


The mid-level CKX7iS is, expectedly a step up from its lower end sibling, offering a richer, fuller sound alongside the increased level of detail you’d expect. The sound is less dry and a touch less airy but it’s better for it, offering a smoother and better balanced sound signature that I really enjoyed.

Starting at the bottom, the bass is full and authoritative without ever sounding excessive. It’s more prominent than either of its Sonic Fuel siblings but remains controlled and tight with extension on par with its lower end sibling and slightly better linearity. The hollowness of the midrange I heard in the lower end model is gone and sounds warmer and more involving than the CKX5iS. The treble is still nice and clear and has enough sparkle and presence to satisfy all but the biggest of treble heads.

Presentation wise, the CKX7iS has a slightly bigger soundstage but, at the same time, sounds more intimate due to the warmer sound signature, which is a plus in my book.


At the top of the range, the CKX9iS is, expectedly, the best sounding of the three earphones, but the improvements from the CKX7iS are more subtle than one might think. On the whole, the biggest improvement is to the micro-detailing, which is definitely better than that of its lower end sibling. The sound is a tad thicker than that of the CKX7iS and the presentation is more grandiose and involving.

As a whole, all three earphones are capable of punching above their weight class. Even the lowest end CKX5iS is more impressive than many other $50 earphones I’ve heard. Detail across the range is impressive and the sound quality leaves little to be desired. The edginess inherent to many Audio-Technica earphones is a given, and the CKX5iS and CKX9iS are both prone to sibilance, particularly on two of my sibilance-heavy test tracks, One Up by Childish Gambino and Eminem’s 25 to Life while the softer CKX7iS is not as sibilance prone as its siblings.

Sonic Fuel Series 2


In summation, the Sonic Fuel line is a solid one, one that boasts a few surprises. While I take some issue with their overall fit and comfort, such is the fate of practically every earphone designed for “active” use. While I do have some reservations about the swiveling nozzles and lack of flexible strain reliefs on the Sonic Fuel earphones, I think their design is mostly solid. Having used them during my own workouts, I think they’ll be good enough for most users for active wear.

The CKX5iS is available from Amazon for around $50 or $45 for the microphone free CKX5, the CKX7iS is available for around $80 or $75 for the microphone free CKX7 and the CKX9iS is available for around $100 or $95 for the microphone free CKX9 version.

For the asking price of each earphone, I think they’re a pretty good value, especially for people who work out often. Though I have some issues with the comfort, fit and isolation, I still think they are a good choice for gym use and they have the sound quality to warrant a purchase even for folks who don’t intend to use these at the gym. If you have larger ears and ear canals, the CKX line will probably be a good fit for you, offering sound quality that’s more than worthy of their price, at each level.

About Justin McBride

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