The21 SUB PRO PASSIVE SEALED is a jewel in the pool of subwoofers designed for the highly educated customer that is on the quest for the highest performance and understands how to differentiate sound quality. When this customer experiences the The21 SUB PRO PS unparalleled output and uncolored sound, the measured test results we publish translate perfectly to realized performance. ASCENDO’s policy is to be completely transparent and publish measurements done by independently certified third-party labs to provide factual data, thereby enabling informed, educated decisions. The The21 SUB PRO subwoofer is so far above and beyond the competition in every aspect; refined musicality, tightness, and output levels suitable for a 21"" driver. It is also engineered to operate perfectly in sync (in phase) with our Infrasonic subwoofers; The24 PRO, The32 PRO, The50 PRO, The80 PRO when there is a desire to reproduce movie content below 20Hz with the same authority, precision, and musicality.

Compared to conventional subwoofer motor concepts, the ultra-modern double-gap motor of this custom-made 21" driver with two outer and two inner voice-coils enables more linear travel and much faster directional changes with higher sound power and significantly lower power compression.

Specifications (selection)

Speaker type

21-inch forward-firing, dual gap motor structure with 2 inner and 2 outer voice coiles, neodymium motor.


18 mm excursion each direction (36 mm linear), voice coil remaining in homogeneous magnetic field, linear and lowest distortion

Passive | Active


Recommended amplifiers 

1Ch DSP + Amplifier are required. Min. recommended amplifier: 109 Vrms, 154 Vpk, 27,3 A; equivalent to 3000W@4Ω with correct RMS & Peak limiting & compression settings.

Recommended ASCENDO amplifiers  

(With additional external 1CH DSP: AIA2-6004), DSP4-10K2  



SPL (peak / half space 2 pi)

Measurement data
139 dB @ 63 Hz peak 

Frequency range

Measurement data
55 Hz - 500 Hz / -3 dB
28 Hz - 500 Hz / -15 dB


97 dB  / 1W/m

Power handling 

 4000 W


4 Ω 

Air volume displacement 

2,88 L / in comparison 21" standard cinema subwoofer 1,76 L

Directivity horizontal x vertical in Deg°


Input Terminal



Internally braced birch ply and MDF


23,62 " | 600 mm


43,31 " | 1100 mm

Hight with feet 

44,29 " | 1125 mm

Depth without grille

14,57 " | 370 mm

Depth with plug

14,57 " | 370 mm


112,44 lbs | 51kg


Structural paint black

Custom colour options 

Piano lacquer, colour of choice, matt or other finishes upon request

Shape customization 

Upon request 


Floor standing with feet


The21 SUB PRO ACTIVE SEALED is supplied with a front cover as standard

Technical modifications subject to change without further notice