Fosi Audio IM4 Open-Back IEMs Review – Slow Musical Flow
Fosi Audio IM4 is a $129 USD pair of IEMs with a single dynamic driver at the core, multiple accessories and an interesting open-back design that we will review today.
Introduction
Fosi Audio IM4 answers the need of more entry-level Open-Back IEMs in the world, and Fosi decided to make those with the best shell, drivers and cable possible for a highly affordable price. Fosi Audio products are available for purchase from Amazon, Aliexpress, the FosiAudio Website as well as countless other shops as they are one of the most popular Chifi entry-level companies and shops want to carry their products.
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Product Link
Amazon – https://amzn.to/49q3QvO
Aliexpress – https://s.click.aliexpress.com/e/_c30OLFr1
Build Quality – Design
Fosi decided to design an open-back pair of IEMS for improved comfort and less driver flex / less void than fully closed ones, as being open-back would be more comfortable for long hours of listening. To achieve the results they were seeking, they employed high-performance dynamic drivers with a THD lower than 1%, and made IM4 available in two colors, Obsidian and Galaxy.
IM4 features a dual chamber for a hifi sound, with a 10mm beryllium driver at the center, and damping wool over the open-back shells for reduced harshness and a more relaxed sound. Combined with a smooth metallic housing and a low weight of just 7 grams for each IEM, those CNC’d aluminium IEMS bear the most recent, distinctive design of Fosi with Orange and black strong contrast.
Technically, we’re looking at a pair of IEMS with a 21 OHM impedance, a 109 dB SPL and a 2-Pin detachable cable with a 4-Core 392 Strand 5N OFC Silver-Plated Copper design. Including the cable, the full IEM weight is just 38 grams, and they come with just a single ended cable, but you can replace the entire cable if you want to go balanced.
Subjective Usage
Wearing the Fosi Audio IM4 feels comfy, they have a rather weak passive noise isolation, but which is not zero. This is because inside the shells there is the acoustic damping material that lowers maximum leakage and also isolates them a bit, leading to around 10 dB of passive noise isolation.
Although the cable doesn’t look very rigid or reliable, it is not one bit microphonic, and it aids to the rather good comfort of the IM4, as they have no driver flex, no void and feel super comfortable even if I’m wearing them for an entire day straight.
While the shells feel solid and not too heavy, everything does feel open-back. Insertion Depth is medium, while bore size is medium-large, so for ears with smaller canals, it might be problematic, but I feel no discomfort while wearing the Fosi Audio IM 4.
Sound Quality
Pairings – I have paired the Fosi Audio IM4 with all the sources I reviewed recently, including FiiO K17, XL, Argon Audio SA-2 Amplifier Driven by FiiO Warmer R2R, Acer Nitro 18 AI Laptop (with 3.5mm adapters), Hiby Digital M500, and Luxsin X9.
While testing them, I found that Fosi Audio IM4 is Very easy to drive, but also hissy, showing hiss and noise even with sources that show none with any other pair of IEMs. I don’t quite understand what causes this as they are not the most sensitive pair of IEMS and SPL is not that high either. At any rate, only sources that are known to work well with ultra sensitive IEMs will work well with IM4, as they really don’t like a strong source with lots of power and prefer low powered sources made especially for IEMs.
Overall Signature – Warm, a bit smooth, pleasing tone. Bouncy and fun, reaches 20 Hz, fun focus on 40 Hz – 80 Hz. Vocal pleasing for females. Sharp drop-off around 12 kHz, treble is generally not fatiguing, a bit absent. Good for EDM, Pop, female lead vocals. Soundstage is medium-wide. Struggles a bit at loud volumes. Textures are simpler, not analytical.
Bass – Bass is a bit slower, it has strong focus between 40 Hz and 80 Hz, but it loses below 40 Hz, as it has a roll-off below that, as expected from the open-back design. On the other hand, IM4 does have a bouncy type of bass and sound, also above 80 Hz it rolls off a bit, producing a sound that is not boomy nor veiled. As the mid and upper bass takes a step back, the whole sound has a bit less warmth and body.
Voicing And Midrange – IM4 has a sweet vocal range that works especially well for female voices, but it also has a sweet guitar tone and good emotion for pianos. They seem to have a slight upper midrange bias, so there’s less strength and presence for male voices and low-pitched instruments.
Treble – Top end is subdued and chill as IM4 has a rather low extension and rolls off above 12 kHz. You can expect a smooth, relaxed and laid-back top end that will take away the edge from most music, be it technical edge or hardness / harshness.
Dynamics And Textures – Dynamic range is on the more relaxed end of things, but IM4 is relaxed sounding and not very engaging, being the type of IEM that would rather put you to sleep than the type that keeps you awake and paying attention to details. Textures are slow, soft, and it has very little technical approach to music, being musical.
Loudness Saturation Gradient – IM4 sounds better at higher volumes, in the sense that it gets more dynamic, more engaging and more vivid, and it will sound more relaxed and softer at lower volumes. You get better bass at higher volumes, but the THD does not rise audibly. Overall, they are a top choice for someone who either wants to listen quietly and likes a gentle sound or wants still a relaxed sound, but with more bounce at a higher volume. Theoretically, this means that they don’t hold up that well to volume because they change character, but as they have good sound either way, I will let it slide.
Soundstage – Despite their open-back design, IM4 does not sound that wide and holographic and it is rather focused and natural in width and depth. Imaging is much better and so is layering, than I expected it to be for an open back pair of IEMs. They are not a wide sounding pair of IEM, and separation between instruments is a bit less analytical and less technical, but this all works really well with the musical and soft design.
Value – Conclusion
Fosi Audio IM4 has a low price, and yet they provide a rare type of sound and design, they are open-back, gentle and musical, and comfortable too. There is such a vast list of IEMS around this price available, but IM4 does stand out for their unique approach, not technical, but musical instead, while most try to be as sharp as possible around the same price.
If you’re looking for a relaxed, laid-back music experience, if you want to forget about the technical approach of other IEMs and need something that will never offend, will be open and not necessarily deep but bouncy instead, Fosi Audio IM4 will deliver really well on that sound and produce a musical, soft approach that will surely help you appreciate certain tracks that you’d otherwise forget the magic of, creating a sound that is musical, fluid and bouncy.
PROs
- Hard shells that are comfortable
- Soft and musical sound
- Bouncy bass with extension down to 40 Hz
- Soft and relaxed treble with extension up to 12 kHz
- Nice package with a good transport case
- Beautiful faceplate with a strong colorful logo
- Generally a very good deal if you are looking for this type of sound
- All music will be free of fatiguing elements
Cons
- Not very technical sounding
Product Link
Amazon – https://amzn.to/49q3QvO
Aliexpress – https://s.click.aliexpress.com/e/_c30OLFr1
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Full Playlist used for this review
We listened to more songs than those named in this playlist, but those are excellent for identifying a sonic signature. I recommend trying most of the songs from this playlist, especially if you’re searching for new music! The playlists are different for Spotify, Tidal and Youtube, and based on the songs I enjoy and are available on each!
https://www.youtube.com/playlist?list=PL_cjBXGmwSHSdGcwuc_bKbBDGHL4QvYBu
https://open.spotify.com/playlist/5J3oloz8Riy9LxEGenOjQ0?si=979ba4f082414be7
https://tidal.com/browse/playlist/330fd544-8e5b-4839-bd35-676b2edbb3d5
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