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.

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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.