The Fosi IM4 is the first IEM from Fosi Audio, an open-back single dynamic driver in-ear monitor featuring a 10mm beryllium-coated diaphragm. It launched in 2026 and is currently priced at 99 USD.
The Simgot EA500LM is a single dynamic driver earphone that hit the market in early 2024, featuring a 10mm lithium-magnesium dome diaphragm. It’s currently priced at 89 USD.
FOSI IM4 SPECIFICATIONS
- Type: Open back IEM
- Driver: 10 mm dynamic driver
- Diaphragm: PU Beryllium-coated Diaphragm
- Sensitivity: 109dB
- Impedance: 21 Ohms
- Frequency Response: 20Hz-20kHz
- Cable: 1.2m Silver-plated OFC Copper
- Cable connector: 2-PIN 0.78mm
- Cable termination: 3.5 mm gold-plated plug
- Shell Material: Aluminum Alloy
- Weight (per earbud): 7g
- Weight (incl. cable): 38g
Current price and availability:
SIMGOT EA500LM SPECIFICATIONS
- Driver: single full range “dual-magnetic, dual-cavity” dynamic driver
- Diaphragm: lithium-magnesium dome
- Impedance 21Ω±15%(@1kHz)
- Sensitivity 123 dB/Vrms (@1kHz Gold Copper nozzle & Red Silicone Ring)
- 123dB/Vrms (@1kHz Silver Steel nozzle & Red Silicone Ring)
- 124dB/Vrms (@1kHz Silver Steel nozzle & Black Silicone Ring)
- Frequency response range: 10 Hz–50 kHz
- Effective frequency response: 20 Hz–20 kHz
- Connector: 0.78mm 2-pin
- Cable material: high-purity silver-plated OFC copper cable
Check the current price here:
LISTENING COMPARISONS
Associated equipment:
- Amplifier: Topping A90
- DAC: RME ADI-2 DAC FS
- Tips: SpinFit CP145 on the EA500LM, Tangzu Tang Sancai Wide on the Fosi
Graceful Touch by Tord Gustavsen Trio
The EA500LM is organic and delicate, with warmth and clean piano notes that carry attack and dynamics. The IM4 does nothing wrong but feels a little less dynamic and alive.
The Tunnel by Tord Gustavsen Trio
The IM4 sounds very nice, spacious and well balanced, detailed and with good imaging. The EA500LM is equally good, yet there is something about it that makes it feel even more alive and organic.
In the Mountains by Espen Eriksen Trio
The EA500LM delivers a powerful and punchy yet detailed bass, while the piano reproduction is gentle but clear. The IM4 is similar, but with better bass detail; the piano lacks a little presence in the treble by direct comparison, which leads me to prefer the EA500LM.
Summer 3 Vivaldi Recomposed by Max Richter
The EA500LM is crisper and generally clearer, but struggles more than the IM4 when things get very busy. The IM4 feels darker and thicker by comparison, but not at all in a bad way.
Vicarious by Tool
The IM4 handles busy passages better than the EA500LM, which can get a bit congested in more complex sections, while the IM4 stays considerably cleaner. That said, both have a satisfying amount of bass and a pleasant tonality.
Jambi by Tool
Here the EA500LM handles things better. The IM4 is similar, but sounds a little flatter and less energetic, though perhaps more neutral.
Sunrise by Norah Jones
The EA500LM sounds excellent: spacious, detailed, warm, dynamic and clear, with a warm-bright character. The IM4 is similar but less spacious, and with a more warm-dry timbre.
WRAPPING IT UP
Sound Signature
Generally speaking, the sound signatures are not too dissimilar. Both have nicely present bass that does not overpower the rest, a well-balanced midrange, and clean treble that is not overly sharp, though the IM4 is darker in direct comparison.
Treble
The treble is sharper and thinner with the EA500LM, though not in an unpleasant way. The IM4 has a less intense, slightly drier and more laid-back treble.
Midrange
The midrange is quite similar between the two; which of them sounds more upfront or intimate depends on the track, owing to frequency response variations. Both have nicely present midrange and neither are recessed in any way.
Bass
The amount of bass is quite similar between the two, though there is a sense of more boldness with the EA500LM and more texture with the IM4.
Soundstage and Imaging
While there is nothing wrong with the soundstage size and imaging capabilities of the IM4, the EA500LM mostly sounds more spacious and images more convincingly.
Detail, Dynamics, and Timbre
The general detail level is quite comparable. On tracks where there is not too much going on, the EA500LM can feel a little more detailed in certain areas, while when things get busy the IM4 often handles complexity better on certain tracks.
When it comes to dynamics, I tend to find the EA500LM consistently more dynamic, except on very busy passages where it gets a little crowded and the IM4 maintains its composure much better. That said, this is very track-dependent and it is difficult to generalise too much.
The timbre is different. The IM4 has what I’d call a “dry warmth” while the EA500LM has a “bright warmth” . The IM4 is more neutral and flatter sounding, while the EA500LM is more vibrant and organic.
CONCLUSION
Both the EA500LM and the IM4 are very good IEMs around 100 USD. They present music differently and each has its own strengths. The IM4 may be the better all-rounder, but the EA500LM edges ahead on less complex acoustic tracks in particular. Both come recommended.
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