Acoustune HS1551 CU

General Information

Transducer: Φ10mm Single Dynamic Driver
Impedance: 32Ω
Sound Pressure Level: 110dB/mW
Frequency Range: 10Hz~25KHz
Cable: MMCX Re-cable, L=1.2m (4-core)
Plug: Φ3.5mm plug (gold plated), L-type
Shell: Chamber – 100% Brass CNC milling, Housing – 100% Aluminum CNC milling
Color: Grand Blue & Gold / Burgundy & Gold
Accessories: Earphone, Cable, Ear-tips (4 types)*, Earphone Case, Cable Clip & Tie, Warranty, Safety Instructions
* Ear-tips: AET02 (F), AET06 (M+), AET07 (S/M/L), AET08 (S/M/L)
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Pros: Smooth, Natural, Great Build Quality
Cons: Treble Extension Can Be Better

Acoustune is a Japanese audio company founded in 2013 and they specialize in making in-ear monitors. Their iems are of a single dynamic driver configuration. I would like to thank Acoustune for the review unit of the HS1551 CU. More information can be found on Acoustune official website, Acoustune .


  • Driver Configuration: 10mm Dynamic Driver
  • Frequency Response: 10 Hz – 25 kHz
  • Sensitivity: 110 dB/mW
  • Impedance: 32 ohm
Unboxing & Accessories

The HS1551 CU comes in a black package with a black protective cover that sports the brand logo, description and image of the iem at the front. At the back of the protective cover, there are specifications and description of the model. There is metal hard case with the brand logo on top in the black package. After unlocking the case, there are the iem, cable, instruction manual, warranty card, a variety of tips [ AET02 (F), AET06 (S+/M+), AET07 (S/M/L), AET08 (S/M/L) ] and a brown leather carrying case that contains a leather cable wrap and a velcro cable tie. There is a great amount of accessories provided and there is a premium feel to them.

IEM Build & Design

The HS1551 CU has red housing made of aluminium CNC milling and gold chamber made of brass CNC milling. There is solid construction. It boasts an unorthodox industrial design which helps to create a nice unique appearance. On outside of the iem, there is a circular shaped metal sticking out. There are 3 metal screws on the faceplate. On the inside of the iem, there are L & R markings on the left and right side respectively. The nozzle is slightly angled with metal mesh and there is a vent. The brand and model name are printed on the rims. The iem utilizes MMCX connectors and on the MMCX sockets, there are blue and red ring on the outer edge to differentiate between left and right. I am able to fit the iem comfortably in my ears.

Cable Build & Design

The cable has a braided design and it is black in color. It uses MMCX connectors with a opaque black housing. On the MMCX connectors, there are L & R markings to differentiate between left and right respectively. The chin slider and y splitter are black in color. The jack is 3.5mm gold plated right angled and the housing is black in color. There is strain relief. The cable is rather flexible.

Sound Analysis


The HS1551 CU has moderate sub-bass quantity and it is extended fairly. The depth is good. The sub-bass reproduction is full and it takes on a smooth manner. The rumble is expressed in a natural manner. Musicality is brought out well. The bass decay is moderate and it is not the fastest. There is still punch but it does not exert in a hard-hitting manner. The bass texture is rendered smoothly and it provides a soothing listen. The mid-bass has great quantity and the slam is presented with a weighted feeling. In addition, the warmth helps to improve the musicality.


The midrange is lush and smooth. The thickness helps to aid in the vocals reproduction. There is a good amount of body and vocals sound sweet. The lower mids has a nice amount of quantity and male vocals benefit greatly from it. Emotions are conveyed effectively and there is no dry or hollow feeling. The upper mids has slight forwardness and along with the control, female vocals are expressed in an intimate manner. There is finesse and musicality is showcased well.


The treble on the HS1551 CU is extended in a moderate manner and it has body. It is able to present the smoothness which provides a soothing listen. There is no sibilance or harshness. There is a relaxing treble performance but at times, it lacks the excitement to liven up the sound. The amount of air rendered is moderate which gives space at the top end. The crisp is little with slight sparkle. The details retrieval is good.


The soundstage has moderate naturalness and the width has good magnitude. The depth is slightly closed in with sufficient space rendered. Positioning of vocals and instruments is fairly accurate.


Acoustune HS1551 CU vs Acoustune HS1503 AL

The HS1503 AL has less sub-bass quantity than the HS1551 CU. The HS1503 AL commands greater extension than the HS1551 CU. HS1551 CU takes on a smooth approach and its bass texture is rendered more smoothly. There is a nice tinge of warmth. Decay on HS1503 AL is slightly quicker with more pace. Rumble on the HS1551 CU is presented with naturalness. The mid-bass on the HS1551 CU has more body and it gives a weighted feeling to the slam. Each bass note on the HS1503 AL is articulated with a cleaner hit and it has more attack. The midrange on the HS1551 CU has extra body than the HS1503 AL. The lower mids on the HS1551 CU is slightly thicker which benefits male vocals. The upper mids on the HS1503 AL is boosted and the forwardness contributes to the intimacy of female vocals. In the treble section, the HS1503 AL has slightly better extension while the HS1551 CU has additional body. There is slightly more air on the HS1503 AL and there is no sibilance and harshness on both. The HS1551 CU is much smoother in its treble. There is additional sparkle on the HS1503 AL to inject excitement to the overall sound. The HS1551 CU showcases a more matured presentation. Lastly, the soundstage on the HS1551 CU expands naturally. The width magnitude on the HS1503 AL is slightly larger and the HS1551 CU has the better depth.

Acoustune HS1551 CU vs Beyerdynamic Xelento

The HS1551 CU has more sub-bass quantity and Xelento extends more. The sub-bass reproduction on the Xelento is impactful and it is able to bring out the necessary punch. The HS1551 CU is more conservative and it has additional warmth. The bass texture on the HS1551 CU is rendered with smoothness. The bass decay is significantly quicker on the Xelento with agility. Each bass note on the Xelento is expressed with tightness and it showcases the technicality aspect well. The mid-bass on the HS1551 Cu has more quantity than the Xelento and the slam feels more complete. The Xelento executes the slam in an agile manner and it hits harder. The midrange on the Xelento has a higher transparency level and boasts better details retrieval. Emotions are conveyed effectively on the HS1551 CU. The lower mids on the HS1551 has more body than the Xelento and it does male vocals justice. The upper mids on the Xelento is more forward which benefits female vocals reproduction. Next, for the treble, the Xelento has the edge in extension with greater amount of air rendered. It has sparkle to inject excitement into the sound. The HS1551 CU has body in the treble and takes on a smooth presentation. Musicality is showcased well on the HS1551 CU. Lastly, for the soundstage, the HS1551 CU expands more naturally. The Xelento has greater width magnitude while the depth on the HS1551 CU is more closed in. Positioning of vocals and instruments has better accuracy on the Xelento.

Acoustune HS1551 CU vs HiFiman RE800

The HS1551 CU has more sub-bass quantity than the RE800 but the RE800 is able to command greater extension. The RE800 has the edge in having an agile sub-bass reproduction. Its speed helps to increase the engagement level. The bass decay on the RE800 is quicker while the bass texture on the HS1551 CU is rendered with more finesse and smoothness. The mid-bass on the Prism has extra body and the slam has more control. The RE800 presents its slam in a quicker manner. Each bass note on the HS1551 CU is articulated well and the rumble is natural. The midrange on the HS1551 CU is fuller and satisfying to listen to. The HS1551 CU has slightly more body in the lower mids department and male vocals are expressed better. For the upper mids, the RE800 has additional boost and it creates forwardness for an intimate experience. Female vocals are livelier. Next, in the treble section, the RE800 is being expressed with better articulation and extension. There is no sibilance and harshness on both. The amount of air rendered on the RE800 is more and the HS1551 CU has additional body. Treble presentation on the HS1551 CU is smoother and controlled. In terms of soundstage, the width on the RE800 is slightly better but the depth on the HS1551 CU is less closed in.


The HS1551 CU is an organic and smooth sounding iem that is able to provide natural bass rumble, warm midrange and smooth treble. Emotions are conveyed effectively and it is sweet to listen to. In addition, its unique industrial design gives it a striking look. The packaging has a premium feel and it is very complete with an abundance of quality accessories. The Acoustune HS1551 CU is able to give a musical listen.

For more reviews, visit .
Pros: Awesome Cyberpunk concept design
Clear and detailed mid
Tight and heavy bass
Smooth treble
Good for 4.4mm balanced terminal
Cons: Imperfect fitting
Depending on the device, a lot of difference in bass detail.
Very tricky upgrade-cable matching

To be a specialist in one field, must be unique and different than others. 'Acoustune' is an earphone brand of Japanese company 'DICS' which produces 4.4mm balanced Pentagon terminal which is internationally accredited.

This company developed a special driver called 'Myrinx' designed as a human artificial tissue close to the eardrum, producing a variety of earphone products ranging from the HS1004 to the current lineup. Let me introduce HS1551cu, a representative product of this company.

HS1551cu, its package is quite impressive. 8 pairs of ear-tips in a separate plastic case, an exclusive leather case for earphone and a leather cable tie, and a hard-carrying-case for them all. The accessories are all set for practical use so that you will not have to buy additional.

The gold-colored chambers of HS1551cu's earphone bodies are made of brass and have each one ‘Mirinx’ drivers. The backs of the chambers act as a handle, so you can hold HS1551cu tight when you wear or when you disconnect its cable. The MMCX connector terminal can be matched with various upgrade-cables and not loosened easily due to its high-quality finishing.

The cable which I have used is old-version, is difficult to wear due to their thick thickness and stiff insulators. But now, have been improved to give more flexibility and thinner thicknesses so that can wear HS1551cu easier. Even with these improvements, its bores are short and the red shells surrounding its brass chambers are large, may not be worn easily for some users.

HS1551cu tends to be warmer sound-signature than colder. The impression of dense and deep reverberation from the bass and the expression of sharply smooth mid and treble make this tendency stronger. The middle and lower range of the mid-range is emphasized, and the vocals and the instrument played in the main are pulled forward, and other sessions show around, so it has a good performance of listening to music as well as other media such as video contents.

Because of its bass which has a dense reverberation like small particles jammed together with strongly emphasized on lower-bass, it delivers a tightly packed ambiance. It will give a nice appreciation by the exquisite harmonious combination of the brass instrument and the bass drum reverberation details in a song Moanin’ from ‘Art Blakey and The Jazz Messenger’. However, due to the strong reverberation emphasis and slow response characteristics of bass, there is a limit to catch up the bass details of fast bass.

There is a strong roll-off in the upper-treble. Because of this feature, the overall sound-signature is slightly suppressed and gives a gentle propensity, but the dynamic range is wide enough so that doesn’t blunt the clarity or sound darker. Even though with a dynamic driver type diaphragm, the detail of the midrange is good. It has less reverberation and faster response-characteristic than the low frequency so that it has a moderate temperature and gives a clear sound expression. So, you don’t have to be tricky about choosing music genres regardless of vocal’s voice-tone.

Depending on the quality of the audio source, the variation of bass detail is very large. In a low-quality performance or less power device environment, the change in middle and treble is not that severe, but the bass response changes unstably, making the impression that the bass is cluttered and distorted. If you want to hear more clear bass detail when using the 1551cu, should consider improving the audio system.

As a company that designs 4.4mm balanced terminals, it performs well on devices with 4.4mm balanced terminals. Its dynamic range of the bass gets extended so that make overall sound-stage more natural and wide. If want to do that, I recommend matching it with Acoustune’s upgrade-cable called ‘ARC03’ (MMCX to 4.4mm BAL).

Usually, it suits some upgrade-cables which shows strongly emphasized mid and smooth treble. But, most of the cables show bad results, Since the midrange’s phase is unstably changed when using balanced terminals, it sounds awkward to hear, so upgrade-cable matching is quite tricky.


I am Bigheadfiler. Yes. I am Big Head guy reviewer from South-Korea. My first language is Korean and I am still learning how to write in English. Please understand if see any missing sentences or words. All of the contents are written and taken by me. Thank you.

The in-ear headphone is offered by ‘Acoustune’. The content of the review has been written without any restriction because the authors' freedom is respected.
Thank you for supporting and pointing out. I had been really concerned to decide about making sub-title. Acoutune HS1551cu, its diaphragms are made of artificial ear drum materials. Because of its engineering concept, I did make decisions for this title.
Will Chiu
Will Chiu
Thanks for the review. I just recently got one for myself and they are fantastic. But I am having a hard time to find a 2.5mm cable for the iem. Do you have any recommendations? Thanks
Their 2.5mm stock cable is the best than any other brand for HS1551cu. I am not sure where you can get it. Please ask Acoustune directly about it.
Pros: Rich bass and mids without being too aggressive (not basshead IEM, but can easily satisfy bass lovers). Rich, smooth mids and extended highs that are far from being sibilant. Superb build quality.
Cons: Bass could be too boomy for some listeners, the cable is a little microphonic, not an analytical sound signature.

I’ve been anticipating the release of the Acoustune HS1551 for ages, and now that I’ve finally gotten my hands on one, I’m happy to say that this is a very refined single dynamic driver design, not only in terms of sounds but also looks and build quality.

I first encountered Acoustune in the Hong Kong High End AudioVisual Expo 2016 in the Soundwave Booth, when the Acoustune HS1551 was still a prototype known as the HS1301 (as far as I know). I had never heard of Acoustune before, but I was open to trying new IEMs, and although I only had a brief moment with them, they sounded really good. Very satisfying bass coupled with non sibilant and extended highs, all coming out of a single dynamic driver. The metallic form also appealed to my tastes, and so ever since then I’ve been waiting for their final release, and I finally got a chance to try it at this year’s Expo.

Acoustune’s HS1501/1551 prototype at the 2016 Hong Kong High End Audio Visual Expo.
I would like to thank Soundwave Hong Kong for lending us this demo unit of the Acoustune HS1551 CU for our review. We are not commercially affiliated in any way. The Acoustune HS1551 goes for $3980 HKD – around 500USD.

As far as I know, Acoustune is some sort of joint Hong Kong-Japanese partnership, and they’ve been around for at least a few years, developing several single dynamic driver designs. They pride themselves on using Myrinx driver membranes in their drivers which they claim can provide the highest resolution audio (at least in earphones!). I’ve tried their HS1004 at HMV as well, and was also pleasantly surprised by it – quite a tight and spacious midbass sound with strong highs, but it was a little sibilant and unrefined at certain moments. However, I followed their posts on Twitter about the development of the new HS1551 and HS1501 series closely, and seeing how much attention to detail they put into it as well as the length of time taken, I knew it was going to be something good.

Note: The HA1551 CU is different to the HS1501 AL; don’t get confused by the confusing naming system.


For people who don’t want to read too much, here’s the verdict: The HS1551 CU is just so warm, liquid, and comfortable sounding. But it’s not all about the bass here – there’s a smooth transition from bass to mids so vocals never disappear, and while the highs don’t take centre stage at all, they’re still extended and noticeable in the background. It’s got a fantastic design and build quality too.


  • Luscious, warm, bass and mid frequencies give a comfortable fullness to bass and instrumental/vocal range
  • Bass is solid but not overly aggressive; liquid sounding at times combined with the smooth midrange (but not so much that it’s loose and flabby)
  • Very refined, extended highs twinkle in the background – not even a slither of sibilance here folks.
  • Just so natural sounding, it really lets you focus on the emotional qualities of the music rather than the minor details.

  • Bass could be slightly loose sounding for some discerning listeners
  • If you want something brighter or more analytical, this probably won’t be for you. Go check out the Acoustune HS1551 AL or balanced armature options from other companies.
  • Not the widest soundstage but it is very decent.
  • I know some people hate memory wire, but since the cable did not have memory wire it did not always settle over my ears and I had to readjust the cable around them sometimes. At least the cable is replaceable!

Transducer: Φ10mm Single Dynamic Driver
Impedance: 32Ω
Sound Pressure Level: 110dB/mW
Frequency Range: 10Hz~25KHz
Cable: MMCX Re-cable, L=1.2m (4-core)
Plug: Φ3.5mm plug (gold plated), L-type
Shell: Chamber – 100% Brass CNC milling, Housing – 100% Aluminum CNC milling
Color: Grand Blue & Gold / Burgundy & Gold
Accessories: Earphone, Cable, Ear-tips (4 types)*, Earphone Case, Cable Clip & Tie, Warranty, Safety Instructions
* Ear-tips: AET02 (F), AET06 (M+), AET07 (S/M/L), AET08 (S/M/L)

Packaging & Accessories

The Acoustune HS1551 CU comes in a pretty hefty box. Removing the cardboard packaging and you’ll come face to face with a tiny metallic briefcase complete with sturdy set of hinges and a briefcase style latch. If you’re laughing at the slightly ridiculous size of the carry case and are wondering how anyone could carry this around, don’t worry because they’ve actually included a beautiful and much more portable leather carry case.

Regardless of the size though, it’s clear that they’ve paid lots of attention to giving the best first impression possible to the user, and it does give me that little bit of audiophile geek unboxing giddiness getting to open this elaborate packaging up to see the two shiny red and gold earphones nestled inside the foam container, just enticing me to take them out. Wrapped around the circular foam block is a 3.5mm braided, plastic sealed MMCX cable ready to be attached to the beautiful earphones.

Hidden underneath all of this is a large set of tips for your choosing with a variety of sizes and types including silicon and foam. I did not notice any huge differences between the yellow and blue centred tips, but you may want to try tip-rolling them.


The Acoustune HS1551 CU is in my opinion one of the most attractive and unique IEM designs I’ve seen. The beautifully milled smooth aluminium shells combined with the more industrial looking brass “damper rod” (that’s what they called it on the packaging) that protrudes from the centre exudes a bold, futuristic appeal that you wouldn’t be surprised if it was one of Iron Man’s gadgets. Construction is rock solid and flawlessly put together – no rough edges to be seen anywhere, the parts are seamlessly combined and secured with 3 tiny hex screws.

While the form may seem odd and angled strangely at first, it actually is pretty straightforward once you take a closer look at it. Ignoring the more angular, detailed exterior, the HS1551 CU is actually very smoothly rounded on the ear facing side. In fact, there are no sharp edges to be found at all, with every corner finely chamfered and rounded for comfort. A clever design choice made by Acoustune is that unlike most dynamic driver designs which have the vent facing outwards, they hid the driver vent right next to the sound nozzle, significantly decreasing the chance of external noise from affecting it, and it does make for good amount of isolation.

If at this point you are still wondering how this odd looking IEM is worn, it’s actually a cable over ear design that will make all the sense once you try it on. Which brings me to my next point. The rather heavy stock cable has many things I love such as having a hefty L-plug, a thick, dual braided, plastic sealed cable which splits into two individual cables, all very sturdily built. I’m pretty sure that it’s going to be a long time before any of the strain points fail. The big, heavy set aesthetic of the cable does really match the bold, industrial design of the earphones but it does have some problems. It’s a little too sturdily built in my opinion, as the stiffness of the cable sometimes causes a slight amount of microphonics when I’m moving around. Another point is that because there is no memory wire, the cables sometimes rebound from the position behind my ears and I’ve found myself occasionally having to adjust them back in place. Finally, the stiffness does lead to a bit of microphonic noise when using it on the go whenever the cable swings and comes into contact with anything. Maybe they should have used something slightly more delicate looking and softer. At least there is an option to replace it!


There’s just something about a good single dynamic driver design that sounds so good. I started my audiophile journey as a diehard dynamic driver lover, discovered the possibilities of multidriver balanced armature designs, fell in love at the obvious strength of combining both types of drivers, but to this day I still have a huge fondness for single dynamic driver designs and in fact it has been growing with better and better dynamic drivers being implemented. Compared to multi driver balanced armature designs, single driver designs typically have the advantage of better, natural bass response and an unmatched sense of cohesion, but often suffer by the bass being too boomy and loose, less detailed mids and even sibilant, weak high frequencies but a high quality, well implemented dynamic driver can overcome all of those obstacles. Veterans like Sennheiser have never given up on dynamic drivers, while newer ones such as Campfire Audio are pushing the possibilities of driver design, and it seems like Acoustune is also keen on dedicating their craft into the perfection of single dynamic driver design, hitting a new sweet spot with the HS1551 CU.

Listening to the Acoustune HS1551 CU is just like that warm, comforting feeling when enjoying a nice cup of hot chocolate at home when it’s cold out. Like receiving a warm hug, the HS1551 CU gently but firmly pumps out an extremely smooth, natural bass response, the kind that only a dynamic driver can produce – full of air and with natural, robust impact. Every low frequency detail, from the lowest subbass rumble to the quicker strikes of the tom toms is reproduced with a warm clarity. Bass impact is very controlled and actually quite gentle, with nary a sign of bass bleed, and just the slightest boost in quantity in the lower bass frequencies so that it’s still “musical” and “fun” sounding when it needs to bring out the big bass. There’s definitely enough bass quantity to enjoy heavier genres like rock and trance here, but it doesn’t have a very aggressive majorly boosted bass attack unlike something like the Campfire Audio Vega, which hits very hard.

The liquid warmth of the low frequencies persists all the way up to the midrange, producing rich, full bodied vocals and instruments. While the low and mid end of things is full of luscious warmth, the HS1551 CU is also superbly refined up top as well. Carrying just a tad bit of a bright edge and flavour to the vocals and highest details, the HS1551 CU adeptly adds these smooth final touches to its presentation making the music just that much more convincing. Highs are extended enough for those details to be obvious, but far, far away from any sense of sibilance. Listening to this IEM is an engaging but relaxed affair; a slight boost in the mid and high regions to pique your interest in the minutae of the singer’s voice, or the finely resolved pitter patter of the live audience’s clapping in the distance is what distinguishes it from weaker IEM designs. Apart from my own preference for warmer sound signatures, it’s this refined sound tuning that seals the deal for me.

The HS1551 CU is neither a reference nor a extremely “fun” hard hitting IEM, but it manages to straddle both of these extremes in its own way. The bass might be a little too warm or boomy for some, the midrange not analytical enough, or the highs not bright enough, but for those looking for an IEM that can just hit that sweet spot in the middle with magnificent mids, resolved highs, with a bit of oomph from the bass end, and let you listen to it for hours on end, the Acoustune HS1551 CU is the perfect choice.


Notice the relatively flat 20-500Hz low end with a very gentle boost; followed by a small peak at 1-2kHz, this makes for a very lovely and rich low-mid frequency listening experience!


  • Sennheiser IE80
    As they are in a similar price range, I thought to compare it to the legendary IE80. Personally, I think the IE80 sounds a little flatter and more neutral, with less sparkly highs, and the low end also sounds a little darker as well. I prefer the Acoustune over the IE80, but if you want more aggressive bass impact and don’t really care for vocals or highs, the IE80 would be preferable. Otherwise, the Acoustune wins in all other aspects.
  • Campfire Audio Lyra 2
    I would say the Acoustune is a better value for money deal than the Lyra 2, but sonically, they are quite different. The Lyra 2 has a slightly more aggressive bass, and the rendition of details is more precise and fine, with slightly more high frequency detail. In contrast, the Acoustune has fuller vocals and is richer, but also more laidback. I really like them both!


I really like the Acoustune HS1551 CU. Sure, I have some complaints about the cable, but it’s got an amazing design, build quality, and most importantly, it’s just got a great sound signature. It ensures that none of the details are missing, but has that gentle boost on the low end to keep things fun. There are not many single dynamic driver IEMs out there that sound so refined. Definitely a recommended buy!

Reposted this on HeadFi for the lovely community!


I got HS1551CU as well. I tried to replace stock copper cable with cheap silver plated copper cable FiiO RC-SE1. The difference is very apparent - much less bass, much more upper mids and treble, and also much less warm tone (pretty neutral tone), and more clarity. I miss the warm tone of stock cable (though i would prefer a little less of it than with stock cable), and somehow the amount of bass (I would also prefer a little less of it than with stock cable as well). But it shows, that with right cable, the optimal synergy between clarity (+detail retrieval) and amount of bass, and also right tone may be achieved. Therefore I ordered pure Copper Toxic cable BW, which I believe will be the right one for me. It should have the clarity while keeping the warm tone, and making the bass tighter. No.5. or any other fine pure copper cable may be the right cable for you based on my findings, but also silver plated copper cable may sound good to you, if you prefer neutral tone.