Systems ListWW Speaker CabinetsX21

ManufactureWW Speaker Cabinets
System typeVented
DriverB&C Speakers 21DS115-4
Driver Size21 "
Number of drivers1
Internal volume11 ft3
External volume14.22 ft3
Dimentions (Width Height Depth)24 " 32 " 32 "
Weight150 lbs
Native Tuning Frequency19,14.5 Hz
Aprox. Price$1750 USD


This X21 cabinet based system is a bit different than the usual testing of either: A finished passive or active system, or a DIY type situation. In some ways it is a bit of all three, being intended to bridge the gap between true DIY and an off the shelf turn-key solution. With the X21 cabinet the cabinet construction, design and finishing are already taken care of. Simply add a driver, amplifier and DSP of your choice.

The X21 cabinet itself is a large vented cab for 21 inch drivers, that is produced by WW Speakercabinets. A cabinet for an 18 inch driver may become an option at a later date. The X21 cabinet is designed by Mark Seaton of Seaton Sound to provide good performance with a wide variety of 21 inch drivers commonly available. The cabinet comes fully assembled via freight shipment and can be ordered unfinished, for a lower cost of $671.66, or in a choice of maple veneer, in black or white for $928.20, or red oak veneer, also in black or white, for $964.11. A cloth covered grille is also available, in either white or black, to match the finish, or provide contrast if desired. The cabinet provided for testing here was maple veneer with the black finish.

Construction of the X21 cabinet is MDF material which is CNC’d and lock mitered together. The cabinet features a pair of large window braces in the center of the cabinet and an internal baffle/brace for the vents. The driver baffle is constructed of double thickness MDF with countersinking for the driver. The driver cut out also features a key notch to facilitate removal of the driver, located at the 6 o’clock position. This is certainly helpful when dealing with large flush mounted drivers. The driver mounts to the cab with large hex bolts and T-nut inserts. The driver cut-out and mounting bolt pattern are optimized for each individual driver. The vents are a pair of front mounted 6 inch diameter Precision ports, with large flares at both ends, positioned directly under the driver. The internal flares of the port are terminated into a baffle/ brace inside the cabinet. The X21 back panel is cut to accept a terminal plate with a set of heavy duty binding posts as standard. This part is available at Parts Express. Some form of cabinet lining is also recommended to help damp internal resonances in the cab. A packet of denim material was used in this case. Large rubber feet are provided on the bottom of the cabinet and the driver grille attaches via magnets imbedded into the front baffle and grille frame. WW Speakercabinets can also make modifications as needed for a specific order, on a case by case basis. The finished X21 cabinet measures 24x32x32 inches and weighs in at 115lbs empty. It certainly qualifies as a large subwoofer. Construction quality and finishing on the X21 sample I received was excellent.

The X21 design is a large bass reflex with a net internal volume of about 11cu ft or 311L. The pair of large 6 inch vents tune the air volume to 19Hz and provide significant port area to help combat vent compression and air noise. During testing the 19Hz tuning was verified as was the effectiveness of the ports at allowing large volumes of air to shift with minor compression or noise. In order to test the X21 cabinet a B&C Speakers 21DS115-4 driver was loaded into it and the pairing proved to match up well. Sensitivity and efficiency of the system is high with a rising response up towards 100Hz that is typical of a very efficient professional bass driver. (With this type of response shape a bit of DSP can be used to flatten the response shape and protect the driver below vent tuning. This type of processing was developed by Mark Seaton and loaded into the Behringer Inuke 3000DSP amplifier for the measurements with that amplifier and both vents open. The simple DSP settings, consisting of 3 filters, used for that test and those developed for other configurations, are listed below.) The 21DS115-4 driver and the large X21 vent area, allowed generous output down to the 16Hz range with both vents open.

Originally there was no intention to test the cabinet with one vent blocked to lower tuning, but based on the encouraging results with both vents open I decided to try it. As expected the system tuning drops very low to about 14.5Hz and the response shape droops and rolls off a bit faster down toward 15Hz. Also, as expected, we see a bit less output capability overall and a little bit more vent compression near tuning due to the reduced vent area. That said, even with only a single 6 inch vent open the output compression and amount of air noise through the port was not excessive. I’d consider it to be quite useful in this configuration. The burst output testing confirmed that the lower single vent tuning allows notable output down to the 12.5Hz band. In rooms imparting significant lift to the lowest bass frequencies, or those requiring slightly less than maximum output from the X21, this type of steeper response roll off down to a deeper frequency may match up quite well for HT.

The X21 cab / 21DS115-4 combo was tested with both an Inuke 3000DSP amplifier on 120v AC and also the normal Powersoft K20-DSP on 240v AC. Some observations can be gleaned from this. The 21DS115-4 driver is rated at 1700w AES power handling and has an xmax rating from B&C of 15.5mm one way. As far as drivers go it is well up the food chain and certainly no slouch. The detailed ground plane testing indicates that the Inuke 3000DSP has plenty of power for this application. In fact it may have a bit more than needed. The K20 was able to prod more output from the system as expected but only at a point where the driver is not happy to be anyway. The K20 got a little bit more from the system during the long term testing, which also produced marked thermal effects in the 21DS115-4 and also in the burst testing, but that was almost entirely above 50Hz where excursion goes way down and with extremely short duration signals. It would be unwise to operate the system wide open with that much voltage headroom available in the amp. An amplifier with the capability of the 3000DSP is a much better match. It appears that the 3000DSP does not have enough juice to mechanically damage the 21DS115-4, mostly due to the fact that B&C usually makes their woofers extremely difficult to bottom out, but judging from the testing it may be worthwhile to implement a limiter. The 3000DSP does not sound good at all when it clips. I was able to cause the 3000DSP to mute and go into protect requiring a hard restart once during testing. This was during the long duration sine wave sweeps, while being driven at maximum output. The 21DS115-4 driver has a rated DCR of 2.2 ohms which is quite low for an amplifier rated into a 4ohm bridged load. This is likely what caused the amplifier to protect when dumping full power into the impedance minimums. This behavior would likely diminish or disappear with a driver having a bit higher DCR closer to 3 or 4 ohms. An application of a light amount of limiter to keep the amplifier from clipping and keep the long term power applied down just a hair, would likely go a long way for safety and cleaning up the sound at the limit. It would probably eliminate the issues with the amp going into protect when heavily clipped into the 21DS115-4’s minimum impedances as well.

If we add up the cost of this system as tested we come up with: About $930 for the X21 cabinet, $250-300 street price for the Inuke 3000DSP and $500-600 street price for the B&C 21DS115-4, for a total as tested system cost of about $1680 to $1830. If you opt for the unfinished MDF cab this could be lowered to possibly $1425 or so depending on what kind of deals you can find on the driver and amplifier. The X21 is a large cabinet, but if the size makes sense for the application, the cost seems quite reasonable for an “Easy Button” subwoofer with this much performance.