Stealth Sonics U4: love or hate bass

Pros: Confusingly powerful bass in a fully balanced armature setup—how the heck you do that Stealth Sonics!?, better with faceplate off, high quality stock cable, includes microphone cable (flexibility+), lightweight, ability to alter bass tuning (removable faceplate)

Cons: bass rides everything (this may be positive for bassheads, but wasn’t for me), midbass distortion (texture loss to smoothness at its best), signature imbalance, easy to drive while never hissing, the sound signature (faceplate off) with better technical performance is less attractive, all plastic construction, no balanced cables on offer (yet)

List Price: $499

Product Website: Stealth Sonics U4

Stealth Sonics U4

Rating Disclaimer: ratings are subjective. Audio quality and value do not mean the same thing across all prices. A headphone with a 5 rating on audio at $5 does not have equivalent sound quality as a 5 rating at $500. Likewise, value at $5 is not the same as value at $5000 dollars.


Stealth Sonics caught my partner in blogging’s eye at CanJam London 2018 and he has since done initial impressions of the U9 and U4. A review of the C9 is coming, which is where I think this might get really interesting. Jackpot77 set up this review tour with Stealth Sonics with the ability of the participants to get their choice among the 3 universal units for diligently doing their review work. We had two weeks, which is always a challenge when reviewing, especially since I’ve got other stuff hot on the stove cooking in review land.

A little history on Stealth Sonics:

  • Started in Singapore, by a group of audiologists from a company called MyEar and a NASA engineer (later investment banker)
  • Customs made in Germany; universals made in Singapore

Usability: Form & Function


The unboxing experience is really nice on the Stealth Sonics universal series. As the U2 and U9 were sent only as headphones with an extra case for the tour, the only full unboxing experience I got was with the U4, whose full box and accessories were included. The box is wrapped in a paper sleeve that is unwrapped with an inserted flap. Inside the paper sleeve are diagrams that are actually from the design of the IEMs and marketing descriptions of the IEM’s features. It’s a cool presentation. With the sleeve removed there is a nice black box with Stealth Sonics imprinted and a nice carbon fibre-effect embossed pattern. Upon opening the super-duper large way too big case and the IEMs are visible along with a bit of the included cable and the extra set of faceplates for if you like plasticky chrome effects—I generally don’t. The cable is wrapped behind the foam insert in a neatly tucked away set-up on a cable winder. The cable is really nice quality and quite comfortable with my preferred heat-shrink earguides. The cable has little weights that help it stay secure over your ears.

The other accessories are inside the giant zipper case: a good quality copper microphone cable, a suede bag with the model name on it, an airline adapter, a 3.5mm to 6.3mm adapter, a branded polishing cloth, silicone ear tips (S/M/L), biflange silicone ear tips (M), and foam eartips (S/M/L). It’s a really good selection of accessories. I just wish that there was a useful travel case.

Aesthetics & Build

The Stealth Sonics universal line-up all come with two removable faceplate options. All come with a black, carbon-fibre effect painted face with a faux turbine in the middle. The second option matches the colour of chrome that tells you which model you have in your hand. The U2 comes with a red second faceplate. The U4 comes with blue. The U9 has a black faceplate. All the faceplates are secured by three Allen screws (tiny Allen key included).

Removing the faceplate functions as a bass boost, as the screw-holes function as venting to give the bass more incursion. I think they probably have some structure going on inside to make this happen, but I haven’t seen the inside, because these are loaners, and that would require destroying the IEM. That would be a shame, because these all sound good in their own way.

Stealth Sonics has a lot of marketing terms and trademarks (no patents, which is notable) inside the box of each of the universal series. I found this a bit over the top, and prone to tricking people’s perception. We hear the words we see, and the marketing is designed to pre-condition to hearing something good. Let’s talk about some marketing terms.

Marketing term Company explanation Translation from marketing to real terms
Iso-Stealth Audiology in action

We’ve harnessed extensive ear measurement data to engineer ultra-precise-fit systems that provide up to 36dB* of isolation for a consistent and safe listening experience

This means that shell is designed to fit within most ears with a good amount of surface area coverage. Tips will still provide most of the isolation. My honest belief is that they consulted with AAW another Singapore company (who also make their cables) to make a universal shell that is a modified version of the Shozy-AAW Hibiki.
SonicFLO Acoustics The Aerospace Advantage

Airflow is channelled through a perfectly-tuned drivers and delivery system that provides a pristine sonic response and high fidelity

Not sure what this means, as I am not going to break open the driver shell with a saw to see inside. In order to have bass boost happen with removing the faceplate, there needs to be something going on. I’d rather have a clear indication than market speak.
Stealth Damping Tight, Clean Bass

An advanced acoustic mechanism that manages resonance for crisp low end with none of the boominess that can overwhelm your sound

My guess, they use damping material that traps some bass and slows bass reflections inside the shell. This would reduce distortion vs plastic alone. The picture for this makes it look like the bass driver fires into a damping shell.
Stealth Kompozit Comfort and Confidence

Proprietary composite material ensures a snug, comfortable, slip-free fit—it’s soft to the touch, yet durable and water resistant, for long wear in any environment

I think this is 100% marketing speak. If it was truly proprietary, I’d wager that they’d have a patent pending. They use a hard plastic. If I had to guess, I’d wager that it is nylon or ABS coated with a soft feel coating, but I could easily be wrong. Soft feel coatings can have properties adjusted for specific applications. Maybe they have developed and manufactured a ‘proprietary mix’ but I’d guess that they’ve tailored a mixture that is produced by another manufacturer.
* a note from Balvinder Singh (co-founder) in the packaging says that 36dB is for the custom version. The specs for the universal are 26dB. Iso-stealth™ probably shouldn’t be claimed in the universal packaging.

Upon listening, I think the most pertinent features are SonicFLO Acoustics and Stealth Damping, because I’m not buying that the universal shell is any more isolating than any other or that it is made of a unique material. It might have a coating that is nearly if not unique to their product line-up, but that isn’t the same thing as a proprietary plastic.

I like that I can change the sound by taking off the faceplate, but I don’t like that this immediately down-grades the appearance. After looking at the construction, I’m pretty sure that damping plays little effect from the faceplate and that it is more likely that opening the screwholes just changes airflow and thus the sound.

If I were designing these, I would have done the faceplates differently:

  1. I would allow sound to be changed without removing the faceplate.
    1. This could be done by having the decorative turbine be elevated and used as a functional aluminum knob linked to a gear mechanism that would cover or uncover the holes.
  2. I would make the faceplate out of actual carbon fibre, not a fake printed pattern.
  3. I wouldn’t want the faceplates to be commonly user removable, so I would recess the screws (and not have them be vents) and use a small Torx screw head.


I find the shells a bit on the large size, but comfortable. I imagine that the shell is identical for the U2, U4, and U9 which means that they all have a shell designed to fit 9 drivers. The part of the shell that settles into the concha of the ear doesn’t have any contouring of the crus of helix area that would make people mistake it for pseudo-custom shapes like the those found on iBasso IEMs or Kinera units. The shells take into account the average shape of the tragus, antitragus and intertragic notch, which means that the fit is secure and comfortable at the most important contact point on the ear.

Ear structure

The soft touch coating is indeed nice to the ear. I’d guess that it is hydrophobic, but I haven’t tested them out in a really sweaty environment.

The cables are very ergonomic, and also strongly resemble the Hibiki cable, which is made by Null Sound (the cable division of AAW). They forgo memory wire in favour of preformed plastic sheathing (heat shrink, essentially). This is my favourite ear hook method on cables. The tops of my ears are sensitive—actually my whole ear is sensitive—which results in irritation when I have heavy cables on my ears or memory wire. I have no problems at all on these cables. For glasses wearers the low profile of the cable should ensure compatibility with your glasses.

Resemblance to Shozy AAW Hibiki

The Stealth Sonics universal shell is not a clone of the Shozy AAW Hibiki ($59), but it bears a striking resemblance. There is contouring in by the nozzle that is remarkably similar to that of the Shozy-AAW Hibiki. If I had to guess, I’d wager that the outer shell design was built off of the Hibiki with a slight modification of nozzle angle. The Hibiki has a more acute angle (by a few degrees, not much to it) than the Stealth Sonics universals. I find that both insertion angles work well for me and the units stay well-grounded through contact in the right places: the tragus, antitragus, and at the back of the concha along the antihelix. Given the fact that the cables on the universal series are made by Null Sound (part of AAW), and that both are based in Singapore, I’m inclined to believe that there was some collaboration there. I think that the Stealth Sonics universal shell is an upsized adaptation of the Shozy AAW Hibiki’s single dynamic driver shell designed for the 9 drivers of the U9. I have no evidence except my deceivable eyes. The pictures below are the U9, but this applies to the whole universal series. I think the Hibiki faceplate actually looks better.

Audio quality

I tested this with faceplate on and faceplate off and had a mixed experience. The bass gets better definition, but the mids get a bit warmer and sweeter. The faceplate off sound is still a little bit better, but not as big an improvement as the U2. The midbass area has quite a bit of distortion in both faceplate on and faceplate off setups. So for these comparisons, I’ll be going with the less attractive, better sounding faceplate off option.

Source Headphone Cable SE/Balanced Gain Volume ~SPL
Questyle QP2R (High Bias) Stealth Sonics U2 Stock SE Low 88 78.4
Questyle QP2R (High Bias) Stealth Sonics U4 Stock SE Low 82 78.2
Questyle QP2R (High Bias) WAVAYA Tria Cross Lambda Mikumo II SE Low 81 78.2

Stealth Sonics U4 ($499) vs. WAVAYA Tria Live ($590)

Build and feature comparison

Both headphones come with reasonable accessories. The Tria comes with a swish Florentine leather case that is on the small side but well designed for the included cable; it doesn’t accommodate after-market cables well. The U4 comes with a faux leather zip-case that is way too large and doesn’t quite do it for me. The Tria case is objectively higher quality. The case that is more likely to meet folks needs in general is the WAVAYA case, as it is a more usable size for throwing in bags. Real leather also wears better than fake leather. Fake leather like that on the Stealth Sonics case peels and cracks in unappealing ways when tossed around too much due to it basically being a thin laminate material. The Stealth Sonics line-up comes standard with a recessed 2-pin cable, while the WAVAYA Tria comes standard with a T2 terminated Linum BaX cable—I did not get this version, instead getting a Plastics One 2-pin cable that I immediately replaced with a better sounding cable. I’d personally recommend that WAVAYA buyers go with the T2 Linum BaX cable.


On Queen – Bicycle Race (DSD64) the bass is big and groovy on the U4, but the mids aren’t dirtied up by it and the bass is still pretty clear. The bass leans toward smooth over detailed, but detail is not completely lost. It sounds substantially like a dynamic driver with the faceplate open. It really is impressive how big they’ve gotten the bass and how they’ve gotten the note decay just about right. It is frequently the case that balanced armatures have decay that is a little bit too fast in the bass region. Something often not noted in reviews is that elevated treble gives bass greater definition. In the case of the U4, it also pushes cymbals and other instruments in the 5kHz to 6.5kHz spike in the range (I listened and then looked at Crinacle’s graphs, they are accurate). The mids maintain good detail and clarity and their position in the stage is good, there are no spikes in the midrange, just a good light slope upwards toward the treble which lends a touch of sweetness to female vocals and higher pitched male vocals. Comparatively, the WAVAYA Tria Live sounds more balanced with a good deal of air around the instruments and a presentation that is leaner. The U4 is spaghetti carbonara while the Tria Live is spaghetti vongole; they are both delicious, both have some salt (extra treble here) to clean up the stage and allow clarity through the mids, but U4 has a lot more richness to its signature that will appeal to some and feel all too much to others. I like both, but they are substantially different offerings. The bass is smaller on the Tria Live than what you see on the U4, but the sound is more balanced and less in your face. Freddie’s vocals are more appropriately placed in the stage with cymbals crashing behind it more frequently than in front of it, which is more appropriate. The bass isn’t as thick and groovy and doesn’t have as much amplitude, but it’s still good balanced armature bass.

So, who handles greasy fast speed and lots of impact better? On Billy Cobham – Quadrant 4 (DSD64), the drum impacts sound a touch muted and rounded off on the Tria Live, I think they would benefit from an extra mids driver to make the drum a bit cleaner and more impactful. Speedwise, they don’t quite keep up like their top tier (I’ve got the Octa too). Note transitions and boundaries have some overlap and smoothing. Performance is generally better from a speed perspective on bass and treble, fast mids have more trouble. The drum beat on the U4 sounds a bit boxy with two much emphasis. This track is just overexposed in the upper bass and lower mids for the U4. It’s very uneven sounding. Guitar placement is generally a bit too much back of the bass. These are bass led IEMs, but there is something to note here: the roundness of the bass is partly distortion. These are gaining some greater body by increasing distortion in the bass. The guitar and treble are doing better than the WAVAYA Tria Live on this with cleaner delivery, but the bass is about 10dB too loud for this track in the mid and upper bass. The answer to the initial question is that the U4 is slightly better, because of better treble and mids speed, but that the imbalance of the signature makes it a harder listen. On technical performance on this track the U4 is better, but on tone, I have a slight preference for the Tria Live due to its more balanced presentation.

The U4 bass is again dominant on Natalie Merchant – Carnival (24-96). Natalie’s voice is nicely placed and warm. The U4 is warm in general. This track is a vocal forward mastering, which means that Natalie’s vocal depth in the stage is still pretty appropriate compared to the treble presentation. The mids have nice clarity. Initial drum hits in the beginning of the track have good snap. I could use a bit more bass on the Tria Live, but some of this will be due to switching from a certified bassy beast to a more normal balanced armature presentation. The intro bass actually sounds better and more natural with less distortion on the WAVAYA Tria Live as the decay is more refined and tightly controlled, it’s not overly fast. The WAVAYA Tria Live has decay that’s just about right. The balance of the sound signature means that individual instruments have more separation, where there’s more spatial blending on the U4. Natalie’s voice is a bit airier on the Tria and warmer and thicker on the U4. The Tria Live still has warmth, but it isn’t to the heightened level of the U4.

Overall, I think I prefer the WAVAYA Tria Live for most music. For smash you in the face kind of music like metal, or bass led music like hip-hop, I’d probably go for the U4. It will come down to whether you want a more coloured signature or a more balanced signature for the music you are listening to.

Stealth Sonics U4 ($499) vs. Stealth Sonics U2 ($249)


Yosi Horikawa – Wandering (16/44, binaural) has a big synth bass drop that goes way down into the sub-bass with loads of texture. The U2 sounds incredible in the bass department here with no shortage of full extension expression. Width and height are good in the stage, but the forward signature of the U2 compresses stage depth and magnifies the scale of instruments. The magnified scale of the instruments reduces instrument separation. It’s a bit of a wall of sound effect.  Immediately, the U4 has notably more stage depth, which allows a bit more balance in the presentation. The bass presentation is smaller and less in your face, but with a bit less depth. The improved instrument separation is excellent, but I’m missing the absolute expression of the sub-bass some, as the U2 is so palpable. The U4 has this bass there, but because of how our hearing works, with more sensitivity to midbass, the midbass swallows up some of my detection of the sub-bass.

The U4 slightly thickens Tori Amos’s vocals on Silent All These Years (24-96 vinyl rip). This is probably due to the midbass/lower-mid emphasis of the U4 frequency response curve. This kind of emphasis tends to make female vocals a bit more congested sounding when they aren’t doing high notes. Bass strokes from the cello occupy a large amount of space in the stage due to this emphasis also, which is more space than they should occupy. A touch of haze goes over the piano also from this. Some would call this warmth, I don’t. Tori’s vocals are cleaner on the U2 without that added congestion. The mids are clearer in general (Crinacle’s brilliant graphs show this difference on the frequency response, U2 and U4). This track has relatively little bass, which I find means that the U2 sounds better because of how its lower mids and midbass are tuned.

Bob Dylan’s vocal on Stuck Inside of Mobile with the Memphis Blues Again (DSD64) is on level with the giant bass on the U2. On this track, the absence of a cross-over on the U2 is also noticeable as the bass note strums don’t come out completely clean and warm up the sonic soundscape when there is a lot of action happening in the mids, as there is here with guitar and bass guitar occupying similar space in the stage. With the U4, Bob Dylan’s voice gets a similar treatment to Tori Amos’s voice with added warmth that makes Bob sound a bit congested. The treble spike on the U4 rising between 5kHz and 6.5kHz lends a nice bit of clarity to the upper mids which helps the guitars get a bit more definition. The guitars still don’t escape the space of the bass guitar due to the forward tuning for the entire bass spectrum. Versus the U2, the bass expression has higher emphasis on the midbass. The U2 is giving me some sibilance on Bob Dylan’s ‘ss’ sounds and a little bit on his ‘t’ sounds. The sibilance is more pronounced on the U4. This track normally has some sibilance, but it is a little elevated with both IEMs on this.

I wanted to get an idea of how the U4 and U2 perform with speedy bass and high levels of complex instrumentation, so I put on Animals As Leaders – Ka$cade (16/44).  With the U4, bass tends to go a bit omni-bass, translation: a bass haze that kind of blankets everything. If the shape of the bass curve were the same but at a lower level of emphasis the tuning would be less of a problem because the bass wouldn’t be forward of the mids and blanketing the sound. With regard to all the other instruments, they are generally well represented with the U4 keeping up well on speed and resolution. When only one kind of bass is in the stage, these also do well, but when there is both bass guitar and synth bass, it gets hazy. The U2 has a cleaner bass presentation due to the lower quantity of midbass in the overall presentation. The U2 sounds better on this track even though the stage depth is less. The U2 does fine for speed also.

The U4 has better technical performance when it comes to instrument spacing and stage depth, but it has a tonality problem with vocals that makes it difficult for me to recommend over the U2. If you like an excessive mid-bass, the U4 will work nicely for you. If you like a basshead IEM that is more balanced with less coloured mids and omni-bass, then the U2 is better.


Specifications U4
Price $499
Driver type Four-driver (4BA)

1 low, 1 mid, 1 high, 1 super high

Frequency response 18Hz – 23kHz
Impedance 13Ω @ 1kHz
Sensitivity 114dB @ 1mW
THD ≤1% @ 1kHz
Isolation -26dB
Construction (specific) 4-way crossover

2 bore

Blue accents

Construction (shared) Polymer body with soft-touch coating (Stealth Kompozit™), removable faceplate (screw holes act as vents, allowing more driver incursion leading to greater bass presence), recessed 2-pin cable attachment
Accessories Extra large carrying case, polishing cloth, airline adaptor, Allen key for faceplate removal, 3.5mm to 6.3mm adaptor, single flange silicone tips (S/M/L), biflange silicone tips (M), foam tips (S/M/L), good quality double twist 3.5mm terminated cable with preformed heat shrink earguides (no mic), double twist 3.5mm terminated cable with preformed heat shrink earguides (with mic)
Warranty 1 year


The Stealth Sonics U2, U4 and U9 were provided as loaners as part of a review tour. In return for reviewing all three I will be given my choice of universal Stealth Sonics IEM—I’m choosing the U9. All thoughts in this review are my personal opinion.


The U4 has good details in it’s midrange and treble, and it is well-built. However, the bass is overwhelming with too much emphasis on the midbass that clouds the mids and swallows the sub-bass too frequently. The unbalanced tuning will appeal to some, but it didn’t quite work for me.

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