Fosi Audio Enters the World of IEMs with the IM4, Delivering Rich Musical Detail with an Open-Back Design

• 10mm PU Beryllium-coated diaphragm
• Dual-chamber acoustic design
• Powerful N52 dual-magnet for control and speed
• Includes OFC Silver-plated cable

Fosi's First IEM

The name "IM4" reflects the phrase "I am for," a declaration of support, advocacy, and identity. It is open-ended, inviting participation inside and out in both aesthetics and sound. 

The IM4 features an open-back acoustic design and a 10mm beryllium-coated diaphragm driver, delivering a spacious soundstage and musical detail, a true reflection of Fosi's brand spirit: Hear the Unheard. Their tuning team aimed to create a balanced, relaxed, and versatile listening experience.

High-Fidelity Sound

10mm PU Beryllium-Coated Diaphragm
PU ensures smooth mids and punchy lows, while the beryllium coating adds clarity and sparkle to high frequencies.

N52 Dual-Magnet
A powerful, stable magnetic field improves control and speed, delivering cleaner sound with better dynamics.

Dual-Chamber Acoustic Design
The front chamber's nine-port design ensures balanced frequencies, while the rear chamber extends treble detail. 

OFC Silver-Plated Cable
Braided with 4 strands of 392-core 5N high-purity OFC silver-plated wire, the cable offers excellent signal transmission and shielding for richly detailed sound. 

Effortless Acoustics with Natural Flow

The open-back design of IM4 minimizes internal reflections and pressure buildup, creating a more open, natural sound with reduced fatigue. The rear chamber is filled with acoustic damping material to absorb standing waves, reducing distortion and harshness. The IM4 was tuned with listener feedback to ensure smooth, balanced sound across genres. 

Comfortable Fit

Developed with insights from millions of hearing-aid fitting samples, the ergonomic housing adapts to diverse ear shapes, delivering a secure and seamless fit. Crafted from CNC-machined aluminum alloy, IM4 offers a smooth touch that's gentle on the skin. The hollowed structure cuts the weight to just 38g, providing pressure-free comfort for long listening. 

Swappable Components

Sound Nozzles
Comes with both aluminum alloy and brass nozzles. Aluminum reduces sibilance with added brass; brass enhances mids and treble with clarity. 

Tuning Ear Tips
Includes three tuning sets: balanced, bass, and deep bass. Each available in S/M/L sizes. 

2-Pin 0.78mm Cable
The detachable 2-pin design supports 3.5mm and 4.4mm cable upgrades, allowing flexible customization. 

Specifications

• Driver: 10mm N52 Dual-Magnet Dual-Cavity Dynamic driver
• Diaphragm: PU Beryllium-coated
• Sensitivity: 109dB/mW
• Impedance: 21 ohms
• Frequency Response: 20Hz-20kHz
• Connector: 2-pin 0.78mm
• Audio Jack: 3.5mm Gold-plated plug
• Weight: 38g