Delta 21" Birch Plywood Passive Subwoofer 1500w RMS 4 ohm


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This 21-inch Neodymium Subwoofer is encased in my 18mm Birch Marine Plywood cabinet. This passive neodymium subwoofer delivers over 139 dB SPL. My driver in the box is 1500w RMS and 4-ohm impedance.

Use these to deliver the Sub Bass frequencies that an 18" cannot deliver. Use these at low volume under the 18" subwoofer to truly experience low frequencies.

Key Features:

  • 8mm, 13-ply birch marine plywood wooden enclosure
  • Countersunk Extra Tick "ding proof" grille to prevent grill damage when on tour
  • Acoustic foam behind the grille to prevent moisture and dust ingression
  • Point Source
  • Frequency Range = 29~1000Hz
  • Impedance = 4 ohms
  • Power = 1500w RMS - Program 3000w - Peak 6000w
  • Max SPL (dB) - 139
  • Cross Over (Hz) -  80- 150 dependent on your system
  • Connection - 2 x Speakon in/out

To get even lower sub low bass plug the ports and seal the cabinet.

Marine Birch Plywood 21" cabinet finished in Heavy Duty black splatter paint it will withstand all the knocks and scrapes gathered through touring.

Fitted with my purpose made BWN21 12kg driver with its 5-inch voice coil which goes down to 27Hz.

The neodymium 21" driver is very efficient and takes modest amplification to drive it. Study our BP4k amplifier.

If you are presently using an 18" speaker you will notice a huge difference by adding  this single or double 21"

Speakon in and Speakon out enables you to link two subs together giving a 2-ohm load.

Important information about subwoofers

Subwoofers are designed to reproduce low bass audio signals between <30Hz and <150Hz. Driving them with full frequencies will cause permanent damage, so they should only be operated with a passive low-pass filter, a crossover network, or an active electronic crossover or speaker management system.

Other Features

  • British Designed 21" NEO Driver with British Paper Cones
  • Point Source Ported Design
  • Five Inch voice coil
  • SpeakON NL4 in/out
  • Easy Lift Handles
  • 2mm metal grille backed with black acoustic foam
  • Rugged birch marine plywood 18mm enclosure
  • Program: 3000W
  • Peak: 6000W
  • Carton Gross Weight = 50kg
  • Speaker Net Weight = 46kg
  • Product Dimensions = 80cm x 65cm x 77cm
  • Carton Dimensions  = 84cm x 69.5cm x 82.5cm


  • 1500W (RMS) / 6000W (peak) power handling
  • British Designed NEO Driver
  • Point Source
  • 1 x BWN21 1500w NEO Woofer with Pinch Terminals
  • 5" voice coil
  • Rugged birch marine plywood 18mm enclosure
  • RMS: 1500W
  • Program: 3000W
  • Peak: 6000W
  • Nominal Impedance: 4 Ohms
  • Frequency Response: 27Hz - 150Hz
  • External Crossover to be used
  • Connections: 2 x SpeakON NL4 in and out
  • Easy Lift Handles: 2 x Steel Acrylic enamel, black
  • Grille: 2mm metal grille backed with black acoustic foam
  • Grille: Finish: Acrylic enamel, black
  • Max SPL 130dB peak
  • Ported Subwoofer
  • Countersunk Extra Tick "ding proof" grille to prevent grill damage when on tour
  • Acoustic foam behind the grille to prevent moisture and dust ingression

Speaker Driver Data

  • Overall driver diameter 548 mm
  • Net weight 12 kg
  • Magnet NEO
  • Flux Density 1.1 T


  • Nominal Impedance 4 Ω
  • Musical Power 3000 W
  • RMS Rated Power 1500 W
  • Sensitivity@1W/1m 96 dB


  • Voice Coil DC Resistance Re 3.6 Ω
  • Resonance Frequency Fs 35.0 Hz
  • Voice Coil 125mm
  • Mechanical Q Factor Qms 10.5
  • Electrical Q Factor Qes 0.45
  • Total Q Factor Qts 0.43
  • Mechanical Moving Mass Mms 335 g
  • Mechanical Compliance Cms 252 um/N
  • Force Factor BL 24.3 Wb/m
  • Equivalent Acoustic Volume Vas 260.0 It.
  • Maximum Linear Displacement Xmax ±14 mm
  • Reference Efficiency ηo 2.250 %
  • Diaphragm Area Sd 1680 Cm2
  • Losses Electrical Resistance Res Ω
  • Voice Coil Inductance@ 1K Hz LE m H
  • Magnet NEO
  • Voice Coil Winding Copper
  • Voice Coil Former Glass Fibre
  • Cone Paper
  • Surround Treated Cloth
  • Dust Cap Paper
  • Basket Aluminium
  • Depth 252 mm
  • Bolt circle diameter 525 mm
  • Baffle cut out diameter 505 mm
  • Flange and gasket thickness 15 mm


Your Guarantee is for 12 months and starts from delivery date but please read on. The warranty is recorded with the BishopSound Order number which begins with a B. Please download and keep a copy of your invoice and keep it safe.

We know problems can occur from time to time so even after 1 year I am here to help. We carry parts and we treat everyone as we expect to be treated ourselves so we will work hard to get a win win.

We still help people who bought from us at Carlsbro 30 years ago with parts (if they are available) repairs and solutions. BishopSound prides itself on treating every customer as a family friend.

In short if the problem was caused by me its up to me to fix it. If you had an accident then I will try my hardest to fix it at little or no cost to you. This guarantee is in addition to, and does not affect your statutory rights or the other right of consumer purchases.

For more information, please see our Terms & Conditions

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