StealthSonics U4 and U9 – unboxing and initial impressions


As regular readers of the blog will know, StealthSonics are a brand I had heard rumblings about in the usual HF blogs, but only came across properly for the first time at the recent Canjam London event. Some more details about the firm are available on the original blog post, but suffice to say I was impressed with their enthusiasm, the unusual approach to IEM design and most importantly, the sound they are producing from their IEM range. Without much convincing required, I decided to pick up their midrange and top end models (the U4 and U9 respectively) so I could spend some proper time getting to know the sound signature, and decide for myself if what I was hearing at Canjam was just a case of “new toy syndrome” or if these guys are on to something serious with their new range.

After a few months of use (a little longer than I usually take to write impressions, but real life has been very good at getting in the way of my audio reviewing recently), I’m pretty happy that this isn’t just another case of gearitis, and both of these models are the real deal. I will be posting a full review for both IEMs in due course, but while I work on that, I felt compelled to put some thoughts down on paper about my first few weeks. As mentioned, these were purchased outright at Canjam so the opinions given below are 100% my own, with no consideration or input given from the team at StealthSonics.

Unboxing – both models

Both IEMs  come in (almost) identical cardboard-sleeved presentation boxes, with white text on a black background and just a hint of colour with a larger than lifesize picture of the IEMs in glossy print on the front along with the model number. The sides advertise the website address and the various proprietary technologies StealthSonics are introducing (Iso-Stealth and SoniFlo Acoustics for the internal modelling of sound and airflow, Stealth Damping for the bass dampers built into the faceplates  and Stealth Kompozit for the ridiculously lightweight shell material they use). Sliding the sleeve off reveals a nice carbon-fibre effect box embossed with the StealthSonics logo, which opens in the usual bookshelf format. It also reveals the inside of the sleeve, which is covered in technical sketches used in the original design of the IEM range (apparently the chief designer has his best ideas on napkins in various coffee houses around the world), plus some more technical background for the various proprietary bits and pieces.

Popping the main box open, you are presented with the standard foam insert holding the IEMs themselves, plus two changeable faceplates for the damping technology (blue for the U4, dark grey for the U9). Below that is an oversized carry case to store the IEMs (around the same size as a Peli 1010), made of a semi-hard vinyl type material with a zip to open and a metal StealthSonics faceplate in the middle of the case. Digging into the final layer of the accessories, opening the case reveals a spare 2-pin cable (complete with microphone), a tiny Torx tool to unscrew and replace the faceplates, a variety of silicon and foam tips, a branded cleaning cloth and a nice beige cloth bag with the model number displayed on a tag.

Overall, the presentation is pretty high end, not quite matching the solid metal cases and Apple-esque feel of a high end JH/Astell & Kern IEM, but certainly getting close enough to make the purchaser feel like this is a quality product. The provision of two cables (both braided, both appearing pretty good aftermarket quality rather than the standard Plastics One effort bundled with a lot of mid-fi IEMs and CIEMs) is a welcome addition. The cable makeup isn’t specified on the packaging, but the U9 has a silver coloured “main” cable, whereas the U4 has a light blue cable, so I presume there is a slight increase in quality or different material used on the U9. StealthSonics are a Singapore based firm, and the cable designs look suspiciously like the Null Audio range of cables (down to the placement of the StealthSonics logo on a metal plate on the 3.5mm jack in place of the Null Audio logo), so I wouldn’t be surprised if they have a deal in place with the Singaporean firm to provide their cabling. Either way, it is pretty high quality and very aesthetically pleasing, so is another nice touch.

Overall, for the $499 asking price of the U4, this is a pretty comprehensive loadout, being well thought out and well executed. Even at the $999 RRP of the U9, this is still a good accessory set, with the only thing that I would personally add being a different and more robust hardcase to add that last touch of elan to proceedings.

StealthSonics U4

Initial impressions on sound

The U4 are an all-BA design, using one armature apiece for bass, mids, highs and “super-highs”. It is trumpeted on their marketing site as using a design favoured by those seeking the flattest response possible. So, if your mind works like mine I’d bet you are probably imagining a sonic landscape akin to a Dutch picture postcard, full of musical notes pedalling around sedate and distinctly flat landscapes. Possibly wearing clogs.

That’s not what these IEMs sound like.

The U4 may technically be flat in the sense that they don’t have a huge bias towards bass or treble, but I would call them more balanced that flat. Flat evokes an impression of anaemic neutrality that the U4 just don’t possess, sitting far more towards the natural/musical end of the sound spectrum.

Starting at the bottom, there is a healthy slab of midbass underpinning the sound, certainly more than you would expect from a single balanced armature setup. From discussion with their rather colourful rep at Canjam London, the U4 uses a full-size custom built BA which StealthSonics spent a fair amount of time and effort developing. It was money well spent, as this kicks out the sort of impactful bass you would be more likely to find in a dynamic driver model, giving a similar level of punch to something like the 64 Audio U8 in terms of physical slam. To be clear, we aren’t taking about DD levels of slam in the inner ear here – the physics of a balanced armature setup just don’t move that much air, but it definitely has a little physicality behind the hits.

The low end is meaty but not overblown, carrying a high level of texture and some satisfying definition around the edges of the notes. The U4 handles the grungy bass synth and frenetic drumming of “Saturate” by The Chemical Brothers very well, giving a nice sense of flutter in the inner ear as the bass vibrates away. It fares similarly well with smoother fare from Daft Punk or Sister Hazel. “Hello, It’s Me” by Sister Hazel treads a fine line between textured and liquid on the U4, capturing the velvety smoothness of the slinky bass riff without completely melting in the ear.

Sub bass runs quite deep, giving a little more than neutral quantity and a fair to middling extension before starting to lost a little emphasis as it drops into the low-Hz values. These aren’t the sort of sub bass monsters that will set the fillings of a hardcore EDM-head vibrating, but they definitely carry enough weight to round out the bottom of the sound and keep most people happy.

Mids hold a balanced position on the stage, sitting roughly in line with the bass and treble to my ears (I guess that’s what they mean by flat). Overall, the positioning of the sound is quite intimate, depending on the tips used. Using Spiral Dots pushes the sound fully forward, feeling like singers and guitarists are playing a few feet away from your face. Vocals are quite sweet in tone, with a hint of sharpness around the edges of vocal notes that stops them feeling syrupy. The tuning treats male and female singers with the same splash of sweetness, leaning towards euphoric at times. Despite the sharpness, the vocals still feel smooth, with no audible grain or distortion and a very clean feel to the sound.

The U4 have a good air of delicacy and lightness of touch in their delivery, the note weight giving enough body to instruments like piano and strings but not sounding particularly thick or beefy with chugging electric guitar or vocals. This gives a good sense of air and space on the stage, allowing for an excellent sense of placement. The relatively forward sound doesn’t leave a huge amount of depth (the big brother of the reir range the U9 is far better in this regard), but listening to “Better Man” by Leon Bridges, the U4 paint a vivid picture of where the musicians are standing in the echoing recording studio, saxophone and horns drifting in over the left shoulder, and the delicate whistling that ends the chorus clearly audible on the left hand periphery.

Despite not sounding overly thick, electric guitar based music plays well on these, the thinner edges to the notes and the excellent speed of the armature drivers providing a fast and energetic edge to tracks like “World On Fire” by Slash and “Freak On A Leash” by Korn. It is capable of handling the most frenetic music in my library without losing cohesion, good detailing and quick decay between notes combining to keep things firing along without blurring.

Resolution is very good for the price bracket (and good in absolute terms as well), the clean and clear presentation helping the StealthSonics model resolve fine detail on the background and mutual textures without coming through as overly sharpened. Scuffs on guitar strings in “Coco” by Foy Vance and the sound of chairs being moved in “Palladio” by Escala are both unobtrusive but clear in the background on the sonic canvas, without having to strain to hear them.

In terms of sibilance, with my favoured Final Audio E-type tips fitted, the U4 carry an element of rawness in the higher ranges, but nothing unpleasant. The tuning is actually a little reminiscent of Final Audio in this regard, managing to expose the raw gruffness of a vocal without sandpapering the eardrums of the listener in the process. “Whiskey And You” by Chris Stapleton is adroitly handled, the grittiness of the chorus that kicks in at the 1:46 point coming through like buttered sandpaper, with just enough grease to pass the harshness test but just enough traction to grab the listener. It’s a difficult tightrope to walk, but the U4 are tuned very well for my preferences here – I dare say that they won’t be as forgiving with anything which is inherently harsh or mixed with a lot of treble heat, but for most good quality output these will do just fine.

Speaking of treble heat, they survive “My Kind Of Love” by Emile Sande as well, but with slightly less aplomb, the rapier like delivery of the chorus sticking out sharply against the synth-laden background. Again, it isn’t harsh enough to be unpleasant, but it definitely sits on the sharp side here, the more neutral note weight of the U4 not packing enough substance to fully smooth out the scratchiness of the recording.

Treble in general is well presented, the dedicated high and super-high drivers doing a good job of laying out a crystalline but not overly fragile or crispy upper end. Violins sound incisive, cutting through the warmer tones beneath to breathe a little bit of air into the topmost echelons of the sound. There is a good sense of headroom at the top end of the frequency range, with the U4 comfortably extending up past my (admittedly limited) hearing into the super-high bands without losing emphasis. It has a stronger reach up high than it does in the sub-bass, but this isn’t an overly bright monitor, staying true to its stage and studio tuning background to provide something that feels sparkly but not overblown.

Overall, the U4 exhibit a balanced and mildly warm tuning, placing the instruments carefully on the stage and pushing the whole image forward towards the listener. It is a nicely organic and slightly sweet sound with no major hotspots or unexpected spikes in the frequency range, relying on the natural resolution of the drivers and a little hint of sharpness up top to mix a little clarity in with the warmth. It is the sort of “natural” that Campfire Audio have made a name for themselves doing, and while the U4 doesn’t sound like any particular CA model, the healthy does of musicality in the bass, silky midrange and dash of sparkle up top give this a very accomplished all-rounders tuning, capable of handling all but the most sub-bass dependent styles of music with ease. This IEM is proof that “flat” doesn’t necessarily have to mean boring, with enough nuance and substance behind the sound to steer clear of analytical and bring enjoyment in equal portions to the balance. Very nicely done.

Soundstage, separation and imaging

The staging on the U4 is reasonable in size, neither cavernous or Cavern Club in overall dimension. As mentioned, the stage is wider than it is deep, with a very good sense of spatial placement considering the relatively ovoid shape of the stage. Left/right separation is good, the U4 pulling instruments hard left and hard right as needed but not deviating too far from the main centre image. Playing something like “Trouble” by Ray Lamontagne gives a nicely spread out presentation, but doesn’t completely pull the separate strands apart, which can sound mildly unnatural on some of my IEMs.

Imaging is pretty impressive, giving each instrument a clear location in the soundscape, and just lacking a little bit of depth to be truly holographic (again, you will probably be looking at their U9 flagship if that is a major requirement).

Stealthsonics U9

Initial impressions on sound

In comparison to the U4, the U9 is a hybrid model, with a 1xDD 8xBA configuration, using the DD for the lows, a pair of BAs each for the mids and highs and a quad-BA setup for the super-highs. StealthSonics confirm this quad BA gives the U9 an unhearable 40kHz maximum extension on paper, but actually serves a more practical purpose, allowing the more audible treble ranges to be tuned in a linear fashion without spikes or rolloff all the way up into dog-bothering territory.

This is the TOTL monitor in their current range, with a more neutral overall sound than the U4. Surprisingly, despite the dynamic driver taking care of the bass, this isn’t any bassier than the U4, and in fact sounds a shade leaner in the midbass. This is their shot at a “reference” model, and there is a noticeable uptick in clarity and finer detail over the already impressive U4. The sound is balanced, with a fairly flat response through from bass to treble, keeping all three frequency ranges evenly balanced and stage-neutral in position.

Starting with the low end, the U9 has a deep and extended sub-bass, floating about neutral in terms of weight and quantity but extending down pretty deep without noticeable rolloff. It feels a little tighter and more impactful than the U4 in the lower reaches, the dynamic driver carrying a little of that authentic physical slam in the ear than the U4 lacks with its balanced armature drivers. This isn’t an imposing or physically visceral sound, and the bass blends seamlessly with the other ranges to make it very difficult to pinpoint where the crossover from dynamic to armature occurs. In the context of this sort of “reference” sound, this is a very well thought out tuning, allowing just a little extra in terms of texture and physicality in the lower end of the sound without disrupting the overall tone or cohesion.

Midbass is reasonably neutral, feeling a little less pronounced than the U4 compared to the sub-bass on a lot of tracks. I suspect in part this is due to the stronger sub-bass presence on the U9 balancing things out, but either way, it carries just enough body and weight to avoid coming across as anaemic, but definitely not enough to bother the basshead categories. It also respects the actual amount of bass in the mix, sinking into the background on bass-light masters, but firing up when required to provide a good level of bass thump. It still sits neutralish overall, but much like the NCM Bella I reviewed a few months back (another 1xDD 8xBA hybrid), the dynamic driver in use here is certainly bass capable.

Where it scores points over the U4 is in the texture and sense of physicality that it manages to convey in the lower end, with the dynamic driver adding a little weight to the excellent tone, painting the textures onto the surface of the notes with just a little more thickness. “Hello, It’s Me” by Sister Hazel still sits somewhere between liquid and chalky in texture, but the resonating bass guitar strings can be felt a little more cleanly in the ear between each note. Similarly for “Bad Rain”, the extra physical impact of the bass drum hits and more visceral growl of the bass give it the slight edge over the U4 in quality. Despite being a DD setup, the driver doesn’t feel noticeably slower than the U4 in presentation, with the StealthSonics team going for a taut and textured presentation that retains good speed rather than going for a looser, more impactful tuning. Make no doubt, this is high quality bass.

Mids are gloriously clear, integrating smoothly with the bass beneath without any hint of bleed. There is a sense of clarity and resolution that marks the U9 out as a higher bracket performer, with the instrumentation and vocals sounding crisp and airy, with plenty of black space between each note. It allows the listener to pick up small nuances and micro-detail very well, with the U9 hitting all the usual cues in my tester tracks, layering sounds on top of each other with enough space to pick out the little scuffs and other audio oddities clearly. Playing “Everybody Knows She’s Mine” by Blackberry Smoke, the overlapping acoustic guitar riff at the 20-odd second mark kicks in so cleanly it may as well just have hopped out of the shower, sitting on top of the chugging country rock riff beneath in harmony but without sinking in to the larger notes.

Similarly for “Better Man” by Leon Bridges, you are stuck by the image of the room the track was recorded in, the U9 placing sounds into a believable 3D configuration in your head. Bridges’ vocals come through with just a hint of sharpness, true to the less than stellar recording of this track. Overall, vocal tones are a little less sweet than the U4, with a less romantic tint and and a more true to life timbre, capturing the texture of Bridges’ voice well. Vocal weight is neutral-ish, coping easily with both male and female singers, moving from Gregg Allman to Leona Lewis without breaking stride. In fact, “Song For Adam” by Allman and “Run” by Lewis are both highlights for me on this IEM, the U9 shining a microscope on the inflections of each vocal phrasing and presenting a pair of absolutely beautiful and emotional sounding ballads. It isn’t quite at the Empire Ears Zeus level of dissection, with the mids being a little less forward and full than the mighty EE 14-driver flagship, but it is definitely a top-tier performer for me.

Guitars and instruments have bite (more so than the U4),  the extra detail and resolution sharpening the edges of the notes and giving things a crunchier and more crystalline tone overall. The tonality here veers somewhere between neutral and bright, although it never gets fatiguing even after extended listening. Firing up “World On Fire” by Slash, the U9 makes short work of the frenetic riffing, giving genuine fizz to the guitar work. Similarly for “Shadow Life” (the following track – I keep getting sucked into the music when I’m listening to these), the staccato chord progressions stop and start on a dime, sounding powerful and crunching as it drives the track along. StealthSonics are going after a share of the JH Audio / 64 Audio dominated stage musician market, so it makes sense that guitar is well catered for, but again, this is a high level performer.

For the sake of balance, those looking for a thicker or meatier sound with their midrange will probably have to look elsewhere, but the tuning they have gone for here works excellently with both acoustic and electric guitars, and makes rock music a genuine pleasure. Acoustic music is also very well represented,

With six drivers out of the nine being used all firing in the higher end of the frequency range, you would expect a reasonable level of prowess, and the U9 doesn’t disappoint, giving a tuning that is ultra-extended and strong all the way up out of hearing range. The treble itself is reasonably flat, without any telltale hotspots or over-emphasised spikes to add artificial detailing. In keeping with the rest of the tuning, the treble isn’t thick, but it is crystalline and ultra-detailed, adding gobs of fine texture and room sound to the background of almost any recording you care to listen to. Despite this, the U9 isn’t a harsh monitor, remaining true to the recording on more hotly mastered tracks but still managing to tame the grating roar of Chris Stapleton in the chorus of “Whiskey And You” without sacrificing the texture.

The U9 feels wide and airy in the top half of the sound, with a good sense of room acoustics and some sharp and glittering treble presentation giving the music a feel of openness and space. The fine violins of “Chi Mai” by the classical ensemble Duel sound haunting, giving a razor-thin edge to the notes as they float about the upper edge of the stage, but not sounding overly needling or prickly. The synth that floats behind them has just the right level of emphasis too, allowing the treble to shine without overpowering the bass and timpani underneath. This is a presentation for treble purists and laymen alike – my preference usually runs to a smoother and more solid treble, but I can appreciate the crystal clarity of the U9, and it fits perfectly into the overall tone of the monitor.

Soundstage, separation and imaging

As mentioned, the U9 have a very good sense of placement, with a soundstage that is more or less spherical in presentation. Holographic is an overused term, but when driven with the right gear, the U9 is one of those IEMs that can lay claim to the title, painting a precise 3D picture of the stage layout and the relative depth and width of each musical instrument in the overall picture. In terms of actual size, the stage isn’t huge, but extends just a little outside of the head in all directions, so is certainly not a between-your-ears experience. The super-tweeters help in this regard, the additional emphasis on the supersonic frequencies helping present some of the “invisible” audio cues like room noise to the listener, to allow the brain to more accurately rebuild the stage in the mind’s eye.

Separation and layering  are also top notch, with plenty of black space between each instrument. The size of the notes is average, slowing for a sense of space that stops the music feeling congested. Allied to the speed of the drivers and the excellent transient response, the U9 can handle pretty much anything you throw at them without losing their excellent cohesion and clarity. Impressive.

Final thoughts

I have listened to a LOT of good IEMs recently, and have a review queue overflowing with seriously impressive pieces of gear that I am in the process of writing up. Despite that, I keep coming back to the U4 and its sibling the U9 for “leisure” listening, which is always a good indication that something has really captured my attention. Also, my time is getting split between the two, with the two flavours they offer both being different enough to appeal for different pieces of music. There is an obvious difference in quality between the two, but importantly the U4 doesn’t feel any “lesser” because of that, holding its own as an enjoyable and very capable IEM in its own right.

To sum up, the U4 is a well executed, technically capable IEM with a balanced but musical sound signature (always a favourite of mine) and some interesting tech packed inside the ridiculously light shells. For the price, these are a very impressive entry into the mid-fi bracket from the new kids on the block, and I would happily recommend checking these out if you are looking for something balanced but technical at the same time.

The U9 take things up a notch technically, with superior imaging and a more neutral but no less engaging sound. They have a little more physical substance to the bass, and a more “precise” presentation that allows you to hear right into the music, without losing the engagement. They are aiming for a TOTL performance, and this model happily competes with the IEMs I have heard or own in the $1k+ price range, so as far as I am concerned, these hit the target there. Add in a non-fatiguing nature and excellent comfort, and the U9 are a serious contender if you are in the market to spend this sort of cash on an IEM.

As mentioned, a full review of both models will be up in due course with some comparisons, but if you have any questions in the meantime, please just drop a comment below, or check these out yourselves!



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