Fosi Audio IM4 Open-Back IEM

FOSI AUDIO IM4 OPEN-BACK IEM

The Fosi Audio IM4 is an open-back wired in-ear monitor built around a single 10mm dynamic driver. As Fosi Audio's first IEM, it combines a PU diaphragm with beryllium coating, N52 dual magnets and a dual-cavity acoustic structure. The design focuses on a single full-range driver together with mechanical tuning options for the listener.

OPEN-BACK ACOUSTIC DESIGN

Unlike a conventional sealed IEM, the IM4 uses an open-back shell that allows greater acoustic communication with the outside environment. This design is intended to create a more open presentation and reduce the sealed-in feeling associated with many in-ear monitors. The trade-off is reduced passive isolation, making the IM4 better suited to home, office and quieter listening environments than noisy travel.

10MM DUAL-MAGNET DYNAMIC DRIVER

The IM4 uses a 10mm PU diaphragm with a beryllium coating, driven by an N52 dual-magnet, dual-cavity motor system. Its single dynamic-driver architecture covers the full audible range without requiring a multi-driver crossover. Fosi specifies a frequency response of 20Hz–20kHz and a nominal impedance of 21Ω.

MECHANICAL SOUND TUNING

Interchangeable aluminum and brass nozzles provide one level of tuning, while the included Balanced, Bass and Deep Bass ear-tip options provide another. This lets the listener adjust fit and tonal balance without software processing. As with any IEM, the final result also depends on ear shape, insertion depth and the seal created by the selected tips.

LIGHTWEIGHT ALUMINUM CONSTRUCTION

Each earpiece is CNC-machined from 6063 aluminum and weighs approximately 7g. The detachable cable uses a standard 0.78mm 2-pin connection and four cores of silver-plated 5N OFC copper, terminating in a gold-plated 3.5mm plug. The replaceable cable also makes future maintenance or compatible cable changes straightforward.