21-inch Cinema Subwoofer

This product has been discontinued. Please check with your Q-SYS regional sales representative for remaining stock availability.

Consider the PL Series performance loudspeakers as a replacement:

  • The PL Series is a portfolio of performance loudspeakers designed to deliver premium sound for Q-SYS systems in a broader range of applications
  • Available in multiple form factors, enabling design flexibility and optimal audio experiences for every unique space


  • 1500 W, 21-inch transducer
  • Frequency range extends to below 25 Hz without the need for B6 EQ filters
  • DCP, DCM and SF-3 signal processors optionally available

Read the SB-15121 application note:  covers the target applications of the SB-15121, as well as advantages and comparisons to other cinema subwoofer designs.

Frequency Range 1
(-6 dB, half space)
(-10 dB, half space)

23.8 Hz - 250 Hz
21.5 Hz - 250 Hz
Maximum Output 2
Calculated Peak at rated power
Calculated Continuous

137 dB SPL
131 dB SPL
Impedance 4 ohms nominal
Maximum Input Power 3
100 hours of 6 dB crest factor pink noise, 25 Hz–250 Hz
2 hours of 6 dB crest factor pink noise, 25 Hz–250 Hz, AES method

1500 W continuous

2500 W continuous

Recommended Amplifier Power 3400 W continuous maximum / 1200 W continuous minimum
Sensitivity 2 V at 1 meter, half space 99 dB, 40 Hz–100 Hz
Nominal Coverage Omnidirectional (80 Hz)
Transducers One 21-inch (533 mm) high efficiency subwoofer transducer featuring a vented 6-inch (153 mm) copper voice coil on a fiberglass former. High excursion/low distortion design, with extremely high power handling and low thermal and port compression. Neodymium magnet to provide a very high force factor and linear excursion.
Recommended Processing HPF at 20 Hz, minimum 12 dB/octave slope
Connectors Barrier strip screw terminals accept up to #10 AWG stranded wire. Four terminals; two inputs and two parrallel outputs.
Enclosure Vented enclosure with symmetrical port design, tuned to 25 Hz, constructed of medium density fibre-board and heavily braced. Features vandal resistant woofer mounting bolts.
Dimensions (HWD) inches 48.8 × 30 × 24
Dimensions (HWD) mm 1238 x 762 x 610 
Net Weight 231 lb (105 kg)
Shipping Weight 257 lb (114 kg)

1All frequency ranges specified refer to measured half-space response (2 pi).
2Calculated SPL at 1m, (half space, 2 pi), speaker operating at rated continuous power with pink noise within specified frequency range.
3Maximum input power tested in accordance with IEC 268-5 recommendations, 50 Hz–20 kHz band limiting, 6 dB signal crest factor.

Specifications are subject to change without notice.