Fosi Audio IM4

With the IM4, Fosi Audio enters the IEM market for the first time. The open-back in-ear is deliberately aimed at a broad yet price-conscious audience with high sonic expectations. The open construction already provides an indication of the expected sound character: an airy, spacious sound with a natural soundstage.

Materials and Build Quality of the Fosi Audio IM4

Fosi Audio machines the IM4’s housings from a single piece of aluminium using a CNC mill. The contours are very finely executed, the surfaces sandblasted and anodised. The choice of materials and the finish strongly suggest that this is a product built to last. The detachable cable also fits functionally into the overall concept and allows for easy replacement if needed.

I particularly like the orange accent on the backs, set off by the visible metal grille of the open construction. The open rear construction is inherently less prone to resonances, and the openness also imparts a naturally airy acoustic character.

Wearing Comfort of the Fosi Audio IM4

In practical use, the IM4 proves to be a comfortable and unobtrusive companion. Despite the metal housing, each earpiece weighs in at around seven grams, which is relatively light. The fit in the ear is secure without any pressure. During testing, I did not notice any limitations in wearing comfort. The in-ears remained reliably in place and allowed relaxed listening even over extended periods.

Features and Customisation

The Fosi Audio IM4 comes with differently shaped ear tip variants and an additional pair of nozzles. The ear tip pairs are supplied in three sizes and are housed in three cards labelled “Balanced”, “Bass” and “Deep Bass”.

The ear tip variants allow the sound character to be adapted to individual preferences, particularly in the bass region. Depending on the choice, the bass reproduction changes noticeably, giving the IM4 a degree of sonic flexibility.

A practical addition would be a clearly identifiable marking on the ear tips, such as a coloured element or a similarly easy-to-distinguish feature like one to three stripes.

As long as the ear tips remain in their designated slots in the cards, they are easy to tell apart. Once removed, closer inspection is required. Then again, the question arises whether on-the-go tuning adjustments are really necessary in practice.

Sound customisation is not limited to the ear tips – the different nozzles also play a role, handling the acoustic path between the rear wall of the housing and the ear tip.

The factory-installed nozzles are made of acoustically neutral aluminium. These are characterised by their balanced, even-handed delivery, while the included brass pair adds a touch more presence and, quite literally, more depth to the sound.

Sound Quality of the Fosi Audio IM4

Sonically, the IM4 presents itself as a well-balanced in-ear with a clear focus on a natural listening experience, as already mentioned. The bass is precise and controlled. By design, there is no exaggerated sub-bass, yet the frequency range extends deep enough to provide a solid foundation.

The midrange is clearly present and slightly softened in character. This tuning is most noticeable with distorted guitars or synthesisers. At the same time, this character fits cohesively into the overall picture and supports a pleasant, unobtrusive presentation.

In the treble, the IM4 comes across as airy and clear. Detail is rendered cleanly without the sound ever turning sharp or fatiguing.

Spatial presentation is one of the IM4’s most striking qualities. The soundstage feels open and wide, with precise placement of individual signals. Effects and quiet details are reproduced in a well-differentiated manner.

Dynamic range is comparatively pronounced. Quiet passages are rendered just as clearly as loud signal components. I did not perceive any compression effects even at higher volumes.

Speech intelligibility is also notably strong. Voices stand out clearly and remain consistently articulate even in complex arrangements.

Verdict

The Fosi Audio IM4 marks a successful debut for the manufacturer in the in-ear headphone segment. The high-quality choice of materials, solid build quality and functional design leave a very favourable impression that carries through to the sonic result.

The headphone impresses with an open and transparent soundstage combined with precise imaging. Fosi Audio’s IM4 consistently delivers a surprisingly spacious presentation and very high speech intelligibility. Different ear tips and tuning nozzles allow effective adjustments to individual sonic preferences.

The IM4 positions itself less as a genre specialist and more as a versatile all-rounder that is certain to attract particular interest from price-conscious and sonically discerning listeners alike.

FAQ: Frequently Asked Questions About the Fosi Audio IM4

Is the Fosi Audio IM4 suitable for use outdoors?

Due to its open-back design, the IM4 offers no passive noise isolation. It is therefore best suited to quiet environments such as home listening or studio use, rather than commuting or noisy surroundings.

Can the sound of the Fosi Audio IM4 be adjusted?

Yes. The IM4 comes with different ear tip variants (Balanced, Bass, Deep Bass) as well as two pairs of nozzles (aluminium and brass), which enable a noticeable influence on the sound character, particularly in the bass region.

What cable does the Fosi Audio IM4 use?

The IM4 is supplied with a four-core 5N OFC silver cable and a 3.5 mm jack connector. The connection at the earpiece uses 2-PIN (0.78 mm), which makes cable replacement straightforward.

Which music genres suit the Fosi Audio IM4 best?

The IM4 is tuned as a versatile all-rounder suited to a wide range of genres. Its strengths – wide soundstage, precise imaging and high speech intelligibility – are particularly well suited to acoustic music, jazz and vocal recordings.

How much does the Fosi Audio IM4 weigh?

Each earpiece weighs approximately seven grams. Despite the metal housing, this is comparatively light and contributes to comfortable extended wear.