Stealth Sonics U4 – Boom, Boom, Clap!



The U4 is thick, pillowy and L-shaped; emphasised on the low frequencies. However, this emphasis spans even higher up the range; short of the low-mids. So, what it contributes are strongly full-bodied instruments, as well as a rich, warm and syrupy timbre. That richness is undercut slightly by the sparkle of the top-end, even if the general treble region has been kept relatively laid-back. The mids have a great linearity, bridging the lows and highs seamlessly. Despite being neutrally positioned, the high-mids shine through all that warmth, giving vocals authority; toe-to-toe with that big-sounding bass.

Spatially, the U4 is an admirable performer. The stage – although not necessarily out-of-head – is never stuffy, congested or claustrophobic; a wonderful achievement, especially given the presence of the bass. The U4 also manages impressive composure for a signature with this much body. A black background is constantly visible behind all the warmth, and you can follow along where everything is in the track fairly well. Obviously, considering how full-bodied everything is, clinical separation and micro-detail retrieval aren’t at the top of the U4’s priorities. Nevertheless, by virtue of top-end extension, control and midrange resolution, the U4 is a notable technical performer in spite of its rich, warm and fat tonal balance.


The U4’s star player, its low frequencies are full-bodied, rich and energetic. Though, unlike most other L-shaped sigs, the bass here is more melodic and silky in timbre; emphasising the sound and presence of a bass note, rather than focusing all its energy into impact. The result is a bass that’s very musical and listenable too. It doesn’t steal the spotlight from the lead instrument by rattling the listener’s skull with mounds of sub-bass. Rather, it complements those mids to form rich, warm and well-rounded instruments. Finally, the region sports excellent extension, coherence and linearity, so it never ever rolls off whenever it alternates between mid-bass hits, sub-bass rumbles and upper-bass melodies, and vice versa.

Given these technical achievements, the U4’s low-end is quite resolving as well. Layering perhaps isn’t as taut as it could possibly be given how much body they’ve given the lows and given the more laid-back nature of the top-end. However, Stealth Sonics have certainly given the bass more authority than I would’ve expected – allowing it to have a nice balance between presence and control – most likely by virtue of their proprietary technologies. The tone of the bass is delectably warm and smooth, making it – again – more ideal for easy listening than analysis and taking apart. Instruments like bass guitars and kick drums bloom and intermingle, for example. So, if that’s the timbre you’re looking for, the U4 will deliver.


Past the low-end lift, the U4 dips throughout the lower-mids. What this does is add headroom, clarity and definition to instruments so they don’t become too full or too harmonic. That role is already performed by the bass, so the low-mids take a step back. As a result, instruments are clear-sounding, clean and refined with just the right amount of wetness to sound natural, without becoming congested or mushy. There’s then a steady lift towards 2-3kHz to ensure vocals have the presence and authority required to stand out from the lows. This is capped off with wonderfully clear upper-mids.

The 4-5kHz range is my favourite bit of the U4’s tuning. A lift there gives vocals this vibrant, glowing quality that allows them to strongly pop as the commanding lead instrument. The best part is that the lift sounds effortless and perfectly in line with the rest of the U4’s signature. It’s not a lift that attempts to forcefully pull the upper-mids forward and away from the lows and highs. Rather, that clarity blends seamlessly with the rest of the ensemble and completes those full-bodied notes with a zingy, vibrant glow; like a squeeze of lemon juice in a rich, hearty stew. Tori Kelly’s First Heartbreak is as resonant as ever on the U4. Pair that with doses of air, and you have well-rounded, organic, effortlessly clear mids.


That 5kHz lift seems to have bled over a tiny bit into 6kHz, as the U4’s low-treble can have a touch of brightness to it at times. It’s largely track-dependant. For example, it’s non-existent on Tori Kelly’s First Heartbreak, but then it rears its head through the guitars on the very next track: I Was Made For Loving You. And, it’s rife on Nick Jonas and the Administration’s brightly-mastered and energetic State of Emergency. So, it’s a bit of a hue that may or may not appear, but in any case, it shouldn’t be of any concern, since it never crosses over towards brittle or sibilant territory; just a bit of colouration is all.

In order to ensure smoothness, the U4 dips its middle-treble moderately. With cymbals and hi-hats, there’s a very slight smoothening or feathering effect that comes from a 7-8kHz dip. This is ideal if you like your percussion a touch gentler-sounding, though it may not be ideal if ultra-clarity is what you’re looking for. Nevertheless, enough energy is injected in to the upper-treble for a clear and resolving response. The U4 is not mushy or muddy in any way, which I continue to be impressed by given its tonal balance of choice. And, wonderful treble extension allows those transients to come through against a black, stable background with admirable spatial qualities, commendable stereo spread and well-paced decay.

General Recommendations

The U4’s L-shaped signature makes it ideal for those who enjoy smoother, thicker-sounding in-ear monitors. Its bass is a specific highlight; for better or worse. Shown below are the U4’s strongest traits, so see whether or not it’s ideal for you:

A smooth, warm and forgiving signature: The U4’s risen low-end gives it a rich, pillowy signature that softens and fattens instruments. At the same time, its top-end is kept smooth and forgiving, whilst being decently extended as well. So, if you’re sensitive to top-end glare and you’re looking for a sub-$1000 in-ear to fit the bill, the U4 is a very likely candidate.

Full-bodied, rich and bountiful lows: If you enjoy a warm, full-sounding bass and you prefer your lows more melodic than skull-rattling or punchy, the U4’s bass response will please. It possesses a linear lift up until the low-mids, which gives its bass a sing-song-y quality, rather than a rumbly one. If this is your ideal sort of presentation, the U4 is a desirable option.

Upper-mids that are both clear and well-founded: Despite the U4’s low-end emphasis, a cleverly tuned upper-midrange and low-treble gives higher-pitched vocals a vibrant, crystalline quality. Female vocals sing with sweetness and shimmer, which is rare in these sorts of signatures. This is ideal if you want both rich, hefty lows and radiant, airy instruments.

Obviously, the U4’s tilted signature will be less appealing to those who prefer the opposite. This certainly isn’t a bright or sparkly-sounding IEM. If the attributes below are ones you prioritise most, the U4 may not be the ideal monitor for you: 

A crisp, clean, treble-emphasised presentation: The U4’s emphasis on fullness and harmonic richness naturally prevents it from sounding compact, tight and ultra-fast. It’s a mellower, more pillowy in-ear more ideal for big picture listening than analysis of the minutia. So, if you’re a critical listener yearning for utmost detail, the U4 isn’t the ideal monitor for you.

Vastness, expansion and clinical separation: Again, the U4’s thicker, richer instruments take up quite a bit of space. While this makes it easy to get immersed in the U4’s euphonic presentation, it’s not ideal if your preference is large amounts of space and tighter notes. If you want a spacious, atmospheric presentation, there are more suitable options out there.

A lean or neutral low-end: At this point in the review, it’s become awfully clear that the U4’s bass is quite strongly shaped. It’s a plentiful bass won’t hesitate to make its presence known. Again, it’s a well-executed colouration that plays well with the rest of the frequency response. Nevertheless, it still isn’t for those who prefer neutral, thinner or brighter tunings. 

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About Author

Church-boy by day and audio-obsessee by night, Daniel Lesmana’s world revolves around the rhythms and melodies we lovingly call: Music. When he’s not behind a console mixing live for a congregation of thousands, engineering records in a studio environment, or making noise behind a drum set, you’ll find him on his laptop analysing audio gear with fervor and glee. Now a specialist in custom IEMs, cables and full-sized headphones, he’s looking to bring his unique sensibilities - as both an enthusiast and a professional - into the reviewer’s space; a place where no man has gone before.

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