Drivers ListB&C Speakers Drivers21DS115-4

ManufactureB&C Speakers
Motor TypeOverhung
Driver Size21 "
Driver Weight31 lbs
Coil Diameter4.5 "
Coil MaterialAL
Coil Length1.43701 "
Gap Height0.55118 "
Manufacture's Xmax15 mm
Geometric Xmax11.3 mm

TS Parameters (Measured by

Fs31.6 Hz
Res2.2 Ω
Le 1khz2.62 mH
Sd0.168 m2
Vas263.9 liters
Mms379.8 grams
Cms67 μm/N
BL28.53 Tm
BL2/Res369.9 N2/W
L/R Time Constant1.19 milliseconds
SplSens97.94 dB


B&C Speakers newest 21” woofer the 21DS115 is built on many of the same principles as their other high power bass drivers. It uses the same proprietary B&C cast aluminum frame as used on the 21SW152 and 21SW115 and shares many other design features as those other 21” drivers. Some of these include: The gasket, the heavy duty waterproofed cone and dust-cap, the triple roll accordion surround, the cone edge reinforcement on the back side of the cone, a motor based on a large neodymium slug in the pole, an aluminum demodulation ring, a large pole vent, under gap vents in the back of the motor case and under spider venting.

One way that the 21DS115 differs slightly is that its 4.5 inch diameter voice coil is a thicker, 4 layer aluminum wind. This allowed the engineers at B&C to get similar motor force to that achieved using a much larger 6” diameter voice coil due to the extra winding layers. A 4 layer coil is heavier and typically has higher inductance but if it allows a smaller diameter, this can offer some advantages. A smaller diameter former is lighter and if using an internal magnet setup can also result in a smaller and lighter motor structure, with less magnet material needed. Also the smaller diameter former takes up less real estate on the spiders so a smaller diameter spider might be used or more corrugations could be left in the spider that might provide better linearity. The spiders used on the 21DS115 driver are a smaller diameter than those used on the larger 6” voice coil 21’s in the B&C lineup but since the former is 1.5” smaller in diameter this should offset the loss in overall diameter. The downside is that a thicker coil with more winds results in more inductance in general.

In the case of the 21DS115 what we see is that the normalized motor force, which is the BL squared and divided by the voice coil dc resistance, is very high at around 370. This is quite a bit higher than the 6” coil 21SW152 in fact. The dual spiders used in the suspension are a smaller diameter than the large ones used with the 6” voice coil B&C 21’s and when combined with the smaller diameter former and voice coil, result in a moving mass that is also a little lower than the 21SW152 at a measured 380g. Both of these contribute to the drivers efficiency which is very high. The spiders used on the 21DS115 appear to be quite a bit more compliant as well and the result is an Fs that is a bit lower than the 21SW152 despite the lower moving mass. This all adds up to an extremely low Qts of about 0.200 on a 21 inch driver. The pair of 21DS115-4 drivers provided by B&C, produced a set of measured parameters that are about as close as I’ve seen to factory. Usually some variation is expected due to manufacturing, measurement and atmospheric variations. In this case the results were scary close.

The xmax rating for the 21DS115 is 15mm with an xvar of 17mm according to B&C. B&C bases their xmax and xvar figures on Klippel testing of their drivers for motor and suspension linearity, rather than old school mathematical calculations of coil and gap. B&C does provide information on the gap geometry as well. The magnetic gap depth for the 21DS115 is 14mm, which is taller than normal for a pro style woofer and the coil wind height is 36.5mm. What this means is that the mathematical coil overhang is 11.25mm. A common industry calculation used for xmax is the coil overhang with 1/3rd of the gap height added. In the case of the 21DS115 this results in 16mm. This is right in between the B&C stated figures of 15mm xmax and 17mm xvar. The listed xmech for the driver is 30mm. Adding the gap height to the coil overhang results in a spec of 25.25mm, at which point the coil would be leaving the magnetic gap completely. It is doubtful that the driver can be made to mechanically bottom. Testing of the driver confirmed this. The driver responds well to around 15 to 18mm of one way excursion, as it should. Beyond that point it becomes progressively less linear and large increases in input power result in very little extra excursion. Even in free air it took a very large amount of power to get it beyond about 20mm one way and it basically stops responding to further increases in power much beyond that point. It also begins to start distorting and complaining noticeably. It is doubtful that the driver can be truly bottomed out. This is the same type of behavior exhibited by the other B&C high power woofers. At high excursion the motor does start to make notable air noise from the motor, but it seems to be a little less audible than with the 21SW152 or 21Ipal drivers. This is normal behavior with high power woofers using extensive air cooling in the motors and shouldn’t be a factor once the driver is loaded into a normal enclosure.

The 21DS115 driver appears to offer nearly equivalent and in some ways better performance than the 21SW152, while weighing only 30lbs and with a street cost around $100 cheaper. 

B&C Speakers 21" 21DS115-4 Systems