Stealth Sonics U2, U4 and U9

U4 – The warm and smooth one

  • Drivers: 4 BA (1 x Low, 1 x Mid, 1 x High, 1 x Super-High)
  • Crossover: 4-way
  • Frequency response: 18Hz – 23kHz
  • Sensitivity: 114dB at 1mW
  • Impedance: 13 Ohms at 1kHz
  • THD: <=1% at 1kHz
  • Price: US$499



The U4 are presented by Stealth Sonics as having “the flattest response possible” and I think here we might have some Babylonian confusion going on because I would personally associate a flat response with neutrality and the U4 are anything but neutral. The U4 are warm, lush and very smooth, perhaps a little too much so. The U4 have an unmistakable veil that takes a while for your ears to adjust to. Once adjusted it is a signature that is incredibly easy going and about as fatigue free as it can get. The U4 present the listener with thick notes and a woolly smoothness that is really rather pleasant. It does come at a cost of some transparency and separation is nothing special, but it does work well to produce a signature that you can listen to all day without any issues. While I was going through different types of music I found that with down-tempo EDM, such as Carbon Based Lifeforms, the listening experience was incredibly mellow and I could just slowly drift away in the music. It would not surprise me if performing artists will appreciate this signature for their stage monitors, as details are still there and there is enough air to avoid congestion, but the smoothness helps to make the U4 a practical tool. Of course audiophiles might be more critical, but they are an annoying bunch of anal-retentives with their reviews and unpicking signatures rather than just enjoy the music… Oh yeah… *embarrassed cough* Let’s move on, shall we.


The U4’s signature is dominated by its bass and that is perhaps not so surprising once you learn that Stealth Sonics used the biggest, baddest BA driver they could find to produce it. Especially with the faceplate off the bass is capable of the sort of low-end growl that I would normally associate with dynamic drivers. With the faceplate on (Stealth damping enabled) the bass is really quite impactful and it gives great weight to, say, a kick drum.

Because the bass is so dominant, it does colour the signature all the way up and I feel this is what causes the perception of ‘veil’ across the signature that will be a matter of preference whether or not it is acceptable. For bass heads I think that without the faceplate the U4 produce a really satisfying bass and I dare say I wish Stealth Sonics had tuned the U4 with that bass in mind. In fact, after the U2, this experience with the U4 leads me to question whether or not the Stealth damping technology is the right solution. It definitely tightens and cleans the bass, but also seems to physically constrain it from resonating naturally, and the U4 have an awesome natural resonance there. Faceplate off, the U4 have a formidable bass performance that can compete with much more expensive IEMs.


Where I might differ in opinion on the flatness of the response, I do fully agree with Stealth Sonics that the mid range of the U4 is silky smooth. Instruments have a full sound and the warmth passed on to them by that incredible bass results in a natural-warm tonality that is incredibly easy going. Works great for mellow blues, although it does come at a cost of some details. Details are not entirely lost, but the U4 don’t push them towards the foreground either.

Vocals are more balanced compared to the U2, as I find that with the U4 male vocals get the natural throatiness they need and there is more weight to them as well. Female vocals do get some sweetness added to them, but I personally don’t mind that and is again fully in line with the mellow character of the U4. Even the sound of the brightest soprano reaches your ears like fluffy clouds, and the U4 manage it while maintaining very decent clarity at the same time. In that sense I feel that Stealth Sonics have found a balance that works well. Smooth and fatigue free while maintaining enough detail and clarity to ensure nothing is missed.


Like every aspect of the U4, the treble is as smooth and easy going as it comes. It feels a little attenuated, but with the extension needed to add enough air to counter the warmth of the bass. But the bass reaches even to this end and the treble is convincingly on the sweeter side, lacking some sparkle and natural brightness in (for instance) cymbals or brass instruments. I do like some sweetness to the treble, but when I listen to Tchaikovsky’s Nutcracker I find that the sparkle is missing to add to the wintery/Christmassy atmosphere I love so much.

Here again I suspect that this is an intentional choice by Stealth Sonics to cater to the needs of the performance artist rather than the audiophile. The sparkle is there for the listener to pick out, but is not emphasized to avoid fatigue. I think this is a good choice. I often use IEMs to isolate myself from the environment while I work and as such can wear IEMs for very long periods of time, easily 10 hours a day, and for sessions this length of time the U4 would definitely be among my preferred IEMs regardless of price.

Page 4 – U9 detailed analysis and Conclusions.

3 thoughts on “Stealth Sonics U2, U4 and U9

    1. Thanks! I have not heard the Cupid myself yet and the Stealth Sonics review was part of a tour, so I don’t have them around anymore, otherwise I could have begged Alex (Twister6) to borrow the Cupid. Perhaps Jackpot77 (who organised the tour) over at Audio Primate knows how they compare because I know he has the Cupid as well and I sent the tour kit back to him.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s