Disclaimer:

Good morning, MBA readers! Today, we have in our hands the Fosi Audio IM4, the first IEM produced by Fosi Audio. Can I tell you that it doesn’t feel like a first attempt at all, but something much more mature? Yes, we know this manufacturer is always improving, but this is truly a great product. There’s certainly a lot of thought behind such a result, and even though it’s a simple IEM with a single driver, that doesn’t mean it’s inferior to other single-driver or hybrid IEMs.

Thanks to Fosi Audio for this review unit.

I’m not an audiophile; I’m just a guy who likes to test out different IEMs and DACs and spends a lot of time listening to music. I fall into the audio enthusiast category.

So I’m not going to use super technical words to review it, but I will do my best to describe it.

Tech Specs:

• Shell Material: 6063 CNC Aluminum Alloy
• Driver Unit: 10 mm N52 Dual-Magnet Dual-Cavity Dynamic Driver
• Diaphragm: PU Beryllium-coated Diaphragm
• Sensitivity: 109dB
• Impedance: 21Ω
• Frequency Response: 20Hz–20kHz
• Cable: 4-core 392-strand 5N OFC Silver-plated Copper Cable
• Connector: 2-PIN 0.78mm
• Audio Jack: 3.5 mm Gold-plated Plug
• Weight (per earbud): 7g ±0.5g
• Weight (incl. cable): 38g ±1g

Packaging:

The Fosi Audio IM4 package is spacious, serious, and packed with surprises. I was surprised by the quantity and quality of accessories included with this IEM.

Inside, we find:

• Fosi Audio IM4 Earphones
• 3.5mm Cable
• Carrying Pouch
• User Manual
• Quick Guide
• 1 pair Brass Nozzle
• 3 Balanced Ear Tips
• 3 Bass Ear Tips
• 3 Deep Bass Ear Tips

If we really want to find something that could have been improved, it’s the original cable, which is very nice but not modular. I absolutely would have liked a balanced cable.

Design/Build Quality:

The Fosi Audio IM4 IEM is made from a CNC-machined aluminum block, which is more expensive than mold-cast shells. In this case, we have high-quality shells, resistant and very light (7 g per side), which, in my opinion, are truly optimal. The shell has an excellent ergonomic shape, compact dimensions, and nozzles similar to full-sized Simgot/Hidizs. The faceplates have large openings that are not only aesthetically pleasing but also make a 100% functional open-back IEM. In fact, you get great performance and soundstage capacity.

The choice of materials is top-notch, and the build quality is also excellent. Inside, we have a single 10mm driver with a PU-Beryllium membrane. The presence of a single driver may seem monotonous nowadays, but these days many of my favorite IEMs feature a single driver, but of high quality. I don’t believe that more drivers are better, but rather, a single, properly calibrated driver can offer much more. The Fosi Audio IM4 is one of this small elite of products.

Initial sound impression:

You’re not exactly sure what to expect from a first try, but in this case, I have to say that expectations were high given the products Fosi Audio is making. I can tell you that the Fosi Audio IM4 has exceeded my expectations with a very mature, rich, dense yet precise and fast sound.

Track after track, it feels like I’m listening to many tracks for the first time. A single driver delivers such a precise, natural sound with hints of warmth, with an impressive openness of soundstage, and a dense, almost tangible bass. But it’s not just that, the background of these headphones is very black, delivering an ultra-sharp image rich in nuances, but the separation is also truly impressive. The texture is unparalleled; we’re off the charts.

The open-back design makes the sound very expansive, not just an aesthetic choice, but you’ll also notice it from the external noises that manage to penetrate a little. Another notable feature is that they don’t follow the Harman curve too closely but feature universal tuning that works quite well with all genres.

Equipment used for testing:

Device:

• MacOS
• Poco M4 Pro

Software:

• Amazon Music UHD 24-bit 96kHz 

DAC:

• Fosi Audio K7 (Nice sound response & warmer)

Comfort & Fit:

The Fosi Audio IM4 has a very ergonomic and well-made design. There are no issues with comfort except for the fact that the metal is cold in the winter. Otherwise, the included accessories allow you to enjoy maximum relaxation without any particular problems. As already mentioned, the open-back design does not guarantee complete external isolation.

In my case, I used Divinus Wide Bore Eartips and the Effect Audio Ares II cable. It’s a bit of a drastic upgrade and definitely not necessary, but I wanted to test the best out of this IEM.

Final sound impression:

Fosi Audio has done a great job; for their first IEM, you can clearly see the philosophy and expertise behind the brand. This headphone embodies the brand’s commitment. The Fosi Audio IM4 is a universal IEM that’s incredibly simple, yet not banal. Listening is easy, undemanding, yet still technical, with diverse nuances and details. The soundstage and the bass’s ability to add weight and depth to the soundstage are impressive.

It’s not a particularly hard IEM to drive, but I still connected it to the Fosi Audio K7 in balanced mode.

Bass

Fosi Audio did a great job. I’m sure they also credit the driver they chose.

The bass is beautifully rich and dense, creating a wonderful atmosphere. The open-back design also gives the bass a sense of airiness, or rather, air movement. All of this also expands the soundstage. I wouldn’t call it a bass-head’s choice, but it’s certainly a very pleasant bass.

To be more precise, the internal driver can handle heavy EQ adjustments without losing refinement, so it can accommodate even the most demanding users. However, the balance, resolution, and extension achieved by Fosi Audio are truly remarkable. What you hear is that powerful yet controlled resonance, which is undoubtedly due to both the driver and the internal structure of the enclosures and the appropriate materials used to control resonances. I think I saw on the official page that they were used inside. Honestly, I wouldn’t even go so far as to say it’s a single driver sounding like it’s a single driver, given how it manages to push the sub-bass without bleeding into the mids. The driver’s sustained speed, with its precise punches, is also remarkable.

Mids

Extremely balanced, warm, engaging, and silky mids. They are very sincere, natural, and well-represented. Personally, I don’t find them to be in the background, but rather quite far ahead of the treble and more or less on par with the bass. Their presence can be increased by switching to the brass nozzle.

However, the slightly aggressive mid-bass invades the mids a bit in certain situations.

Vocals, especially female ones, when properly tracked, can almost reveal unheard-of nuances. I particularly love how the low and mid frequencies blend so meticulously without any bleed. The musical instruments also perfectly match my expectations.

Treble

The treble has a noticeable roll-off, but honestly, the decision to use a similar tuning makes them very versatile and suitable for everyone. It’s not an excessive roll-off, let’s be clear. The details are all there; in fact, honestly, for a single driver, these are well above average. What I think might be missing is that feeling of freer treble. With the brass nozzle, this sensation is immediately improved. I prefer the sound of the other nozzle, which is more relaxed.

Sometimes a quiet set is nice, and that’s my case. A set that, in a simple but not banal way, lets you enjoy hours and hours of playlists without weighing down your mind or ears.

Soundstage and Imaging:

As you can see from the photos, the Fosi Audio IM4 is truly open. In that photo, the light enters through the connector and exits through the front grille. It really works, and its soundstage is truly expansive and full of air. The imaging is 90% of the time clear, except on some tracks that aren’t very clean and a bit confusing. For a single-driver IEM, that’s really good. The layering is also really well done, obviously not as good as with multi-driver IEMs, but it does the job well. Are there more technical IEMs? Absolutely.

Comparison:

My direct comparisons take as an example two highly competitive IEMs in the single driver category, namely the latest two Hidizs but the Simgot EA500 and EA1000 could also be taken into consideration, but they are now models with a few years on their shoulders (still very valid models).

vs Hidizs MK10

These are two single-driver IEMs, but the Fosi Audio is more refined. Starting with the shells, the Fosi Audio IM4 is of superior quality in terms of workmanship. Closed-back vs. open-back design. Both are very good drivers. So which one should I choose? It’s a tough choice, but in my opinion, the Fosi Audio sits between the MK10 and the MK12, so it’s a step above the MK10s. More refined, a little richer, and more exciting.

vs Hidizs MK12

In this case, the Hidizs Mk12 is a very unique higher-end headphone with a very good driver capable of absurd resolution. Needless to say, it costs more and is indeed more refined, but the Fosi Audio IM4 is equally surprising in its price range.

Conclusion:

These IEMs have shown me that Fosi Audio is truly committed to bringing high fidelity into everyone’s home. Their constant commitment is leading to the production of ever-improving products that meet consumer needs. The Fosi Audio IM4 has officially become one of my favorites. 

Very easy to listen to, the sound, at least with the black nozzle, is very relaxing and smooth. As we’ve seen from the photos, it’s truly open at the back, and this feature gives spectacular resonance to the bass, which has great presence while also expanding the soundstage wonderfully.

Its sound is rich, exciting, and never boring, and it adapts perfectly to any genre, although perhaps modern music is best suited to it. For the price of 119 USD (100 Euro), I think it’s an excellent IEM that manages to compete even with brands that have dominated the market for longer.

9.2/10

Pros: 

• Super Good Build Quality
• Really nice stock accessories
• Extremely precise & detailed
• Well-tuned, super detailed, and separation
• Bass definition and control 
• Resonance masterpiece
• Open-Back isn’t just for design
• 1 Single high-quality driver
• Wide soundstage
• Price is quite impressive

Cons: 

• To be honest, this is perfect for my taste
• Little bleeding of mid-bass in the mids
• Treble could be too safe for some and perfect for someone
• No Modular Cable

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