{"id":26453,"date":"2026-04-19T00:11:51","date_gmt":"2026-04-18T23:11:51","guid":{"rendered":"https:\/\/gamerunlocked.pt\/?post_type=product&p=26453"},"modified":"2026-04-19T00:45:08","modified_gmt":"2026-04-18T23:45:08","slug":"fosi-audio-im4-open-back-iem-10mm-beryllium-driver-swappable-nozzles","status":"publish","type":"product","link":"https:\/\/gamerunlocked.pt\/en\/product\/fosi-audio-im4-open-back-iem-10mm-beryllium-driver-swappable-nozzles\/","title":{"rendered":"Fosi Audio IM4 \u2013 Open-Back IEM, 10mm Beryllium Driver, Swappable Nozzles"},"content":{"rendered":"\t\t
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Fosi Audio IM4 \u2013 Open-Back IEM, 10mm Beryllium Driver, Swappable Nozzles<\/h2>\n

The Fosi Audio IM4 is Fosi's first IEM \u2014 and an unusual one. Open-back architecture on an in-ear is not the obvious choice for a debut product, but it defines everything about how the IM4 sounds and who it's for. Single 10mm beryllium-coated PU dynamic driver, CNC-machined 6063 aluminum shell, two swappable nozzles, and nine ear tip sets in three tuning profiles. Built for desktop listening, not commuting.<\/p>\n

Open-Back Architecture<\/h3>\n

The rear chamber of each earpiece has acoustic vents that allow the driver to breathe into open air rather than into a sealed cavity. This eliminates the pressure buildup and reflections that give closed IEMs their characteristic \"in-your-head\" presentation. The trade-off is isolation \u2014 there is none. Sound leaks both ways, which makes the IM4 a poor fit for public transport or noisy offices, but an excellent choice for home listening at a desk or alongside a DAC setup.<\/p>\n

The soundstage benefit is real and consistently noted across reviews: width and depth are notably larger than what closed-back IEMs at this price deliver. The imaging is precise enough to be useful in gaming, particularly for directional cue detection in FPS titles.<\/p>\n

Driver and Acoustic Design<\/h3>\n

The 10mm driver pairs a PU (polyurethane) diaphragm with a beryllium coating. PU provides mass and compliance for natural, smooth low-frequency response; the beryllium layer adds stiffness and speed to the high frequencies, extending clarity without adding harshness. N52 dual magnets generate a strong, stable magnetic field for fast transient response and low non-linear distortion \u2014 rated at THD below 1%.<\/p>\n

The dual-chamber design separates front and rear acoustic behavior. The front chamber uses nine ports for balanced frequency output across the sound range. The rear chamber is packed with acoustic damping material to suppress resonance peaks that would otherwise color the presentation with harshness or ringing.<\/p>\n

Swappable Nozzles and Tuning Tips<\/h3>\n

Two nozzle options are included. The aluminum nozzle produces a warmer, smoother character \u2014 better for long listening sessions and genres where midrange texture matters. The brass nozzle brings forward mids and adds treble clarity \u2014 better for detail-oriented listening and gaming, where that slight 5kHz presence helps footsteps and positional cues cut through. This is a genuine functional tuning system, not cosmetic.<\/p>\n

Nine ear tips in three profiles complement the nozzle system. Balanced tips maintain the intended frequency response. Bass tips add low-end emphasis. Deep Bass tips extend sub-bass presence. Tip and nozzle combinations allow meaningful customization of the final signature without additional purchases.<\/p>\n

Build and Cable<\/h3>\n

The shell is machined from 6063 aluminum alloy using a 5-axis CNC process, sandblasted and anodized for a smooth, skin-friendly finish. At 7g per earbud, it's light enough for hours of wear without fatigue. The cable is 4-core 392-strand 5N OFC silver-plated copper, braided, terminating in a 3.5mm gold-plated plug. The 2-pin 0.78mm connector supports aftermarket cable upgrades, including 4.4mm balanced terminations.<\/p>\n

Gamer Unlocked Take<\/h3>\n

The IM4 is a genuinely interesting debut. Fosi didn't launch with a safe, neutral tuned closed-back \u2014 they went open-back with swappable tuning. For a first IEM, that's a strong signal of intent. The result is an IEM that sounds wider and more natural than most closed competitors at this price, at the cost of isolation that closed-back users may not be willing to give up.<\/p>\n

For desktop use alongside the K5 Pro, K7, or DS3 \u2014 which is exactly the context Fosi designs these products for \u2014 the IM4 makes sense. The open-back staging pairs well with a neutral DAC, and the nozzle system means you can tune the character to what you're listening to. For gaming at a desk, the wide soundstage and that mid-treble presence make positional audio notably better than most closed IEMs at this price.<\/p>\n

Honest note: start with the aluminum nozzle and deep bass tips. The brass nozzle with wide-bore tips can push the treble into fatigue territory for some listeners. And if you listen on the move or in noisy environments, this is not the right IEM \u2014 the lack of isolation is a real limitation.<\/p>\n

\u26a0\ufe0f Isolation Notice:<\/strong> Open-back design provides no passive isolation. Ambient noise enters freely and sound leaks outward. Designed for quiet, controlled listening environments.<\/p>\n

IN THE PACKAGE<\/h4>\n
    \n
  • 1\u00d7 Fosi Audio IM4 IEM pair<\/li>\n
  • 1\u00d7 5N OFC Silver-plated Braided Cable (3.5mm)<\/li>\n
  • 2\u00d7 Swappable Nozzles (Aluminum and Brass)<\/li>\n
  • 9\u00d7 Ear Tips (Balanced, Bass, Deep Bass \u2014 S\/M\/L each)<\/li>\n
  • 1\u00d7 Carrying Case<\/li>\n<\/ul>\n<\/div>\n\n\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
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    SPECIFICATION<\/th>\nDETAILS<\/th>\n<\/tr>\n<\/thead>\n
    Model<\/td>\nFosi Audio IM4<\/td>\n<\/tr>\n
    Type<\/td>\nOpen-Back In-Ear Monitor (IEM)<\/td>\n<\/tr>\n
    Driver<\/td>\n10mm N52 Dual-Magnet Dual-Cavity Dynamic Driver<\/td>\n<\/tr>\n
    Diaphragm<\/td>\nPU Beryllium-coated<\/td>\n<\/tr>\n
    Impedance<\/td>\n21\u03a9<\/td>\n<\/tr>\n
    Sensitivity<\/td>\n109dB<\/td>\n<\/tr>\n
    THD<\/td>\n<1%<\/td>\n<\/tr>\n
    Frequency Response<\/td>\n20Hz \u2013 20kHz<\/td>\n<\/tr>\n
    Acoustic Design<\/td>\nOpen-back \u00b7 Dual-chamber (9-port front, acoustic damping rear)<\/td>\n<\/tr>\n
    Swappable Nozzles<\/td>\nAluminum (warm, smooth) \u00b7 Brass (clearer mids and treble)<\/td>\n<\/tr>\n
    Ear Tips<\/td>\n9 pairs \u2014 Balanced, Bass, Deep Bass (S\/M\/L)<\/td>\n<\/tr>\n
    Cable<\/td>\n4-core 392-strand 5N OFC Silver-plated Copper \u00b7 Braided \u00b7 3.5mm Gold-plated<\/td>\n<\/tr>\n
    Connector<\/td>\n2-PIN 0.78mm (detachable \u00b7 compatible with 3.5mm and 4.4mm aftermarket cables)<\/td>\n<\/tr>\n
    Shell Material<\/td>\n6063 CNC Aluminum Alloy (5-axis) \u00b7 Sandblasted anodized finish<\/td>\n<\/tr>\n
    Weight (per earbud)<\/td>\n7g \u00b10.5g<\/td>\n<\/tr>\n
    Weight (incl. cable)<\/td>\n38g \u00b11g<\/td>\n<\/tr>\n
    Isolation<\/td>\nNone (open-back design)<\/td>\n<\/tr>\n
    Available Colors<\/td>\nObsidian (Black) \u00b7 Galaxy (Silver)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n\t\t\t\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"

    Key Features:<\/strong><\/h4>\n
      \n
    • Open-Back Design \u2014 Soundstage Beyond IEM Norms<\/strong> Rear chamber vents let the driver breathe freely, eliminating pressure buildup. Soundstage width is notably larger than closed IEMs at this price. No isolation \u2014 designed for quiet environments and desktop use.<\/li>\n
    • 10mm Beryllium-Coated PU Driver, N52 Dual Magnets<\/strong> PU provides smooth lows and mids; beryllium coating adds clarity and speed to highs. Dual N52 magnets and dual-chamber acoustic design deliver fast transients and low THD below 1%.<\/li>\n
    • Two Swappable Nozzles \u2014 Aluminum and Brass<\/strong> Aluminum nozzle for a warmer, smoother presentation. Brass nozzle for clearer mids and more forward treble. Two distinct sound signatures from a single pair of IEMs.<\/li>\n
    • 9 Ear Tips Across Three Tuning Profiles<\/strong> Balanced, Bass, and Deep Bass tips in S\/M\/L. Tip choice materially affects the bass response and staging \u2014 not just fit. Deep Bass tips with the aluminum nozzle is the recommended starting point.<\/li>\n
    • 7g Per Earbud, Detachable 2-Pin 0.78mm Cable<\/strong> 6063 CNC aluminum housing with sandblasted anodized finish weighs 7g per side. Included 5N OFC silver-plated braided cable terminates in 3.5mm; upgradeable to 4.4mm balanced.<\/li>\n<\/ul>\n","protected":false},"featured_media":26454,"comment_status":"open","ping_status":"closed","template":"","meta":[],"product_brand":[],"product_cat":[125],"product_tag":[516,1981,1984,1980,1985,1130,512,1979,1983,1982],"class_list":{"0":"post-26453","1":"product","2":"type-product","3":"status-publish","4":"has-post-thumbnail","6":"product_cat-iem","7":"product_tag-audiophile-iem","8":"product_tag-beryllium-driver-iem","9":"product_tag-budget-open-back-iem","10":"product_tag-dynamic-driver-iem","11":"product_tag-fosi-audio-iem","12":"product_tag-gaming-iem","13":"product_tag-in-ear-monitor","14":"product_tag-open-back-iem","15":"product_tag-swappable-nozzle-iem","16":"product_tag-wide-soundstage-iem","17":"pa_brand-fosi-audio","19":"first","20":"instock","21":"taxable","22":"shipping-taxable","23":"purchasable","24":"product-type-simple"},"_links":{"self":[{"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/product\/26453","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/product"}],"about":[{"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/types\/product"}],"replies":[{"embeddable":true,"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/comments?post=26453"}],"version-history":[{"count":5,"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/product\/26453\/revisions"}],"predecessor-version":[{"id":26468,"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/product\/26453\/revisions\/26468"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/media\/26454"}],"wp:attachment":[{"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/media?parent=26453"}],"wp:term":[{"taxonomy":"product_brand","embeddable":true,"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/product_brand?post=26453"},{"taxonomy":"product_cat","embeddable":true,"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/product_cat?post=26453"},{"taxonomy":"product_tag","embeddable":true,"href":"https:\/\/gamerunlocked.pt\/en\/wp-json\/wp\/v2\/product_tag?post=26453"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}