Headphones — Fosi Audio IM4

The IM4 is an open-back wired in-ear monitor from Fosi Audio, built with strong sensitivity and dual-connector flexibility. The IM4 brings Fosi Audio's considered engineering to an open, wired in-ear format.

Key specs that matter

The transducer uses dynamic or isodynamic driver technology in an open-back, in-ear, wired configuration - an uncommon open-back approach for an in-ear design. Frequency response spans 20Hz to 20,000Hz within plus-or-minus 10dB. Connection offers both 3.5mm and 4.4mm balanced connector options. Sensitivity is a strong 109dB SPL at 1kHz/1mW, with impedance at a low 21 ohms - a combination making this earphone easy to drive from virtually any source.

Sound and use context

Expect exceptionally efficient reproduction reflecting the strong 109dB sensitivity combined with the low 21-ohm impedance, meaning this earphone reaches satisfying volumes readily from a wide range of sources, delivered through an uncommon open-back in-ear design offering some environmental awareness.

Pairing

The dual 3.5mm and 4.4mm balanced connector options let the IM4 pair directly with either portable or balanced desktop sources without needing an additional adapter, and Fosi Audio's own ZH3 DAC shares the brand's coherent engineering philosophy.

Alternatives

Fosi Audio's broader headphone catalogue offers alternatives for buyers wanting a different form factor.