BP-SD21, 21″ Band Pass Subwoofer – The Sub Engine



Legacy Listing of DISCONTINUED PRODUCT. BP-SD21 Product No Longer available in ceramic magnet version. See the BP-SD21-S3

Our BP-SD21, The Sub Engine has the low frequency output to match its eye-catching appearance. It boasts folded-horn output from a box one half the size of one of our WH folded horns. The 21″ low frequency driver has the lowest power compression we have ever measured thanks to its massive magnet structure and huge 6″ diameter voice coil. Each BP-SD21 is capable of handling 2,000 Watts program power. This speaker has been turning heads with the output equal to a typical double 18″ box, but it is nearly as small as a usual single 18″ box.

Also available, the BP-SD18 Sub Engine Jr., an impressive 18″ version of the same concept.



System Type4th Order Vented Band Pass
Active or PassivePassive
Program Power ( Watts )2000W
Bi-ampable Electronic Crossover Recommended 4th Order at 35 Hz to 80 Hz
Nominal Impedance (Ohms)8 Ohms
Sensitivity (dB @ 1Watt/1m)106 dB*

  • Measurement Made Using 2.83 Volt, 4th Order, Band-limited Pink Noise from 38 – 80 Hz
  • One Cabinet Measured 106 dB and Two Cabinets In Parallel at four Ohms were measured to be 109 dB
Max SPL (dB)138
Frequency Response (Hz +/- 3dB)38 Hz – 80 Hz
Crossover Frequency (Hz)80 Hz Low Pass
Driver ConfigurationSingle 21″ Woofer
LF Driver(s)Single 21″ Woofer with 6″ Voice Coil
LF Program Power ( Watts )1000 W Continuous, 2000 W Program
LF Impedance (Ohms)8
Inputs – Speakon Neutrik 4-pinDual NL4 , Paralleled
Flying HardwareOptional
Bar Handles4
Enclosure Materials3/4″ to 1/2″ Multi-Ply Birch
16 Ga. Perforated Steel
Finish / CoveringBlack Catalyzed Paint
Optional Finish / CoveringDuraBlast polyurea coating
Dimensions (DWH xback W, inches)30″ x 24.5″ x 30″
Dimensions (DWH xback W, cm)76 x 62 x 76
Weight (lbs/kg)168 lbs / 76 kg

Additional information

Weight168 lbs
Dimensions30 × 24.5 × 30 in