18 Sound 8M400F – LF Driver

18 Sound 8M400F – High-Sensitivity 8-Inch Ferrite Midrange with ISV Voice Coil and 9 kHz Bandwidth

The 18 Sound 8M400F is a dedicated 8-inch ferrite midrange driver that produces 100.5 dB sensitivity and a 200 Hz – 9000 Hz bandwidth from a 51 mm ISV voice coil with a copper shorting ring on the pole piece. Where the MB-series 8-inch drivers are mid-bass transducers designed for displacement and bass extension, the 18 Sound 8M400F is a midrange transducer designed for maximum output per watt in the vocal and presence frequency bands — the open-back ferrite complement to the sealed-back neodymium 8NM610.

The 9000 Hz bandwidth is the widest of any conventional 8-inch driver in 18 Sound’s range. It extends well beyond the 8NM610’s 5000 Hz, the 8NW650’s 6300 Hz, and the 8MB400’s 5200 Hz. The 18 Sound 8M400F reaches into the lower treble — covering vocal fundamentals, presence, sibilance, and harmonic detail from a single transducer. In many system architectures, the driver can function as both the midrange and the lower portion of the HF section, reducing the number of drivers in the signal chain.

The 100.5 dB sensitivity from an 8-inch midrange is exceptional. It sits between the 8NM610’s 104 dB sealed-back figure and the highest mid-bass sensitivity (8MB720/8MB710 at 99.5 dB). The copper shorting ring on the pole piece is the feature that separates the 18 Sound 8M400F from the mid-bass MB series — reducing voice coil inductance, linearising the impedance curve, and improving transient response and phase control in the mid frequencies.

18 Sound designed the 18 Sound 8M400F for use as a midrange in horn-loaded, direct-radiating, sealed, or reflex configurations — the widest enclosure versatility of any 8-inch driver in the range. 18 Sound recommends a 6-litre reflex enclosure tuned to 100 Hz, with sealed enclosures working from as small as 3 litres.

At 4.9 kg with a ferrite magnet, finned basket with direct thermal contact to the magnet structure, and humidity-repellent cone treatment.

Design and Construction of the 18 Sound 8M400F

The 18 Sound 8M400F uses a 51 mm (2-inch) aluminium ISV (Interleaved Sandwich Voice Coil) wound on a high-strength glass fibre former. The ISV places conductor on both surfaces of the former for mass-balanced, linear motor behaviour. The coil is bonded using unique high-temperature resin adhesives that maintain structural integrity at the elevated temperatures the 250 W AES power rating generates.

The 13.1 Tm force factor driving 16.9 g of moving mass produces 100.5 dB sensitivity and a Qts of 0.26 — the most motor-dominant open-back 8-inch midrange alignment in the range. The 317 Hz EBP confirms extreme electromagnetic damping suited for horn-loaded and vented applications.

A copper shorting ring on the pole piece is the technology that separates the 18 Sound 8M400F from the MB mid-bass series. The ring reduces voice coil inductance, linearises the impedance curve through the upper operating range, and improves transient response and phase control in the mid frequencies. For a driver operating to 9000 Hz, impedance control through the copper ring is critical for maintaining smooth, predictable response across the full bandwidth.

The curvilinear cone uses a high-damping wood pulp composition. 18 Sound’s in-house developed cone treatment provides humidity repellency and also dampens bell-mode resonance significantly — addressing both environmental durability and acoustic performance.

The triple roll surround provides controlled compliance through the 3 mm Xmax. The short excursion is by design — the 18 Sound 8M400F operates in the midrange band where sensitivity and bandwidth matter more than displacement.

The finned basket maintains direct contact with the magnetic structure, significantly improving thermal connection and heat dissipation. This direct thermal path reduces voice coil temperature during sustained operation, lowering power compression beyond what the coil’s thermal mass alone supports.

At 4.9 kg, 210 mm overall diameter, 186 mm baffle cutout, and 106 mm depth, the 18 Sound 8M400F is heavier than the mid-bass drivers (8MB400 at 3.6 kg, 8MB500 at 3.4 kg) due to the stronger motor assembly and copper ring.

Performance and Applications for the 18 Sound 8M400F

The 18 Sound 8M400F delivers 100.5 dB sensitivity across a 200 Hz – 9000 Hz bandwidth. The 100.5 dB is 5.5 dB above the 8MB400 and 8MB500’s 95 dB — a massive sensitivity advantage that reflects the completely different design philosophy. The MB series trades sensitivity for displacement and bass extension. The 18 Sound 8M400F trades displacement for maximum mid-band output.

The 9000 Hz upper bandwidth covers the entire vocal range, the presence band, and the lower brilliance band from a single 8-inch cone. In a three-way system, the 18 Sound 8M400F typically requires only a tweeter or small compression driver above it to complete the spectrum. The high-damping wood pulp cone maintains controlled output through the upper mid-range without the breakup modes that stiffer cone materials can exhibit.

The four enclosure options — horn, direct radiating, sealed, or reflex — give system designers maximum flexibility. The 0.26 Qts and 317 Hz EBP produce the best results in horn-loaded and vented configurations where the motor’s control delivers the tightest, fastest midrange output. Sealed enclosures from 3 litres work for the most compact midrange chambers. The 6-litre reflex recommendation tuned to 100 Hz provides extended mid-bass support below the midrange band.

The comparison to the 8NM610 positions the 18 Sound 8M400F within the 8-inch midrange category. The 8NM610 delivers 104 dB from a sealed-back neodymium platform at 4.5 kg — the highest output with built-in rear enclosure. The 18 Sound 8M400F delivers 100.5 dB from an open-back ferrite platform at 4.9 kg — lower sensitivity but wider bandwidth (9000 vs 5000 Hz), more enclosure options (horn/direct/sealed/reflex vs sealed-back only), and ferrite’s lower cost. The 8NM610 is the sealed-back output champion. The 18 Sound 8M400F is the versatile open-back bandwidth champion.

Within the full 8-inch range, the 18 Sound 8M400F and 8NM610 are the two dedicated midranges — both optimised for mid-frequency output with short Xmax and high sensitivity. Everything else at 8 inches (8NMB420, 8MB500, 8MB400, 8NW650, 8NTLW2000, 8NMB750, 8MB720, 8MB710, 8NTLS2000) is a woofer or mid-bass. The midrange tier is a different performance class.

Conclusion – 18 Sound 8M400F Midrange Driver

The 18 Sound 8M400F is the widest-bandwidth 8-inch midrange in 18 Sound’s range. The 9000 Hz upper frequency reaches further into the treble than any other conventional 8-inch driver — including the sealed-back 8NM610’s 5000 Hz. The copper shorting ring provides the impedance control that makes 9 kHz output possible from a cone midrange. The 100.5 dB sensitivity delivers the mid-band output that professional systems demand.

The ISV coil provides linear motor behaviour. The copper ring provides impedance linearisation. The high-damping cone provides smooth, natural response. The finned basket provides thermal management. And the four-way enclosure versatility — horn, direct radiating, sealed, or reflex — gives system designers the widest application freedom of any 8-inch driver in the catalogue.

For dedicated midrange sections, horn-loaded applications, compact multi-way systems, and any application where an 8-inch midrange needs to deliver 100.5 dB sensitivity across the widest possible bandwidth from a proven ferrite platform, the 18 Sound 8M400F is the 8-inch midrange that covers more of the audible spectrum at higher output than any other open-back 8-inch in 18 Sound’s range. ISV, copper ring, 9 kHz, 100.5 dB — the bandwidth benchmark.