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18 Sound

18 Sound 21NLW4100

18 Sound 21NLW4100: 21″ subwoofer driver with neodymium motor, 4″ voice coil and 15.2 mm Xmax – intended by the manufacturer for bass reflex and infra-bass enclosures.

Current · Last checked: 2026-11-13

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Schematic cross-section: 21″ driver with neodymium motor Side section: on the left the motor with a compact neodymium magnet, pole piece and top plate with the air gap. The voice coil sits in the gap on its former, held by the spider. To the right the cone opens out with its dust cap, with the surround at the edge of the basket. The drawing shows the construction, not the dimensions of a particular model.
Schematic cross-section of a driver with a neodymium motor. The drawing shows the construction, not the dimensions of this model; the manufacturer has a product photo.

General data

Link to section “General data”
Type
Subwoofer driver
Nominal size
21″
Nominal impedance
8 Ω Manufacturer
Minimum impedance
6.2 Ω Manufacturer
Sensitivity
97 dB 2.83 V / 1 m Manufacturer
Frequency range
32 Hz – 300 Hz Manufacturer
Voice coil Ø
100 mm Manufacturer
Winding height
36.5 mm Manufacturer
Gap depth
12.4 mm Manufacturer
Flux density
not available
Magnet
neodymium
Weight
10.5 kg Manufacturer
Baffle cut-out Ø
492 mm Manufacturer
Overall diameter
545 mm Manufacturer
Mounting depth
245 mm Manufacturer
Displacement
not available
Recommended enclosure volume
250 – 250 L Manufacturer

Sensitivity is referenced to 2.83 V at 1 m, i.e. to 1 W only at exactly 8 Ω. The manufacturer does not state Cms; the value is left empty here. The 4 Ω version has its own Thiele-Small parameters (including Qts 0.40, Bl 22.8 Tm, Mms 400 g) and must not be confused with these values.

The manufacturer gives 33 Hz as the tuning for the recommended 250 L enclosure.

What can this driver do?

Link to section “What can this driver do?”

Simulation from the datasheet values above. The suggestions are calculated, not stated by the manufacturer.

Bass reflex Sealed Calculated

The EBP is between 50 and 100 – by the rule of thumb, both a bass reflex and a sealed enclosure are possible. This is based on an EBP of 73.

Net volume
424 L
Tuning frequency
29.5 Hz
F3 (−3 dB)
26.6 Hz
F6 (−6 dB)
24.3 Hz
F10 (−10 dB)
22 Hz
Sensitivity (2.83 V / 1 m)
97 dB
Max. SPL at 26.6 Hz
124.8 dB
Max. SPL at 53.2 Hz
128.5 dB
Xmax limits below
50.5 Hz
Ports
1 × 400 mm
Port area for 17 m/s
1,244 cm²
Max. port velocity
16.8 m/s
Port length
72.5 cm
1020501002005001k708090100110120130140Frequency (Hz)Level (dB SPL)
  • at 1,500 W
  • Maximum SPL (limited by Xmax)
  • Power into 8 Ω (nominal impedance of the wiring).
  • Flattest alignment: lowest F3 without the response rising more than 0.25 dB above the passband level or dropping more than 0.5 dB at 2 × F3.
  • The 17 m/s limit (about 5 % of the speed of sound) is a common rule of thumb against port noise, not a standard. Rounded port ends allow higher values.
  • Lumped-element Thiele/Small model, Half space, 1 m, chamber losses QL 7, Le as a plain inductance, no room influence. At high levels (heating, non-linear Bl and Cms) real results are lower.

Open in the designer

Thiele-Small parameters

Link to section “Thiele-Small parameters”

Small-signal parameters as stated by the manufacturer. They change with temperature and level.

Fs
36 Hz
Re
5.1 Ω
Le
2.1 mH (at 1,000 Hz)
Qts
0.46
Qes
0.49
Qms
7.8
Vas
210 L
BL
29.4 T·m
Mms
374 g
Cms
not available
Sd
1,661 cm²
Xmax
15.2 mm
Xdamage
17 mm
Vd
not available
EBP
73 Hz
Efficiency η₀
1.8 %

Plausibility check

Link to section “Plausibility check”

We recalculate dependent values from the datasheet. We flag deviations above 3 %.

Recalculated parameters
CheckFormula DatasheetCalculatedResult
Qts from Qes and Qms Qts = Qes · Qms / (Qes + Qms) 0.46 0.461 consistent
EBP from Fs and Qes EBP = Fs / Qes 73 73.469 consistent
Xmax, geometric Xmax = (winding height − gap depth) / 2 15.2 12.05 different definition

Manufacturers define Xmax differently. A deviation from the purely geometric formula is not an error, but shows that a different definition was used.

Power handling

Link to section “Power handling”
  • Continuous power (standard not stated)1,500 W
  • Programme power3,000 W

18 Sound tests rated power handling for two hours with pink noise in the range Fs to 10 × Fs, free-air mounted, referenced to the minimum impedance. Programme power is defined as 3 dB above rated power handling – i.e. a calculated figure.

Build plans with this driver

Link to section “Build plans with this driver”

Sources

Link to section “Sources”
  1. Tier A · primary source

    18 Sound 21NLW4100 (8 Ω) – Product page with Thiele-Small parameters

    Eighteen Sound S.r.l. · accessed on 13 November 2026

    The linked datasheet PDF (…/21nlw4100--2.pdf) contains the same values.

  2. Tier A · primary source

    Application Note – Single 21'' Band-Pass Subwoofer Kit (121BP_v2.pdf)

    Eighteen Sound Srl · 23 September 2024 (Date from file metadata) · accessed on 13 September 2026