Drivers ListStereo Integrity DriversHT-18D2

ManufactureStereo Integrity
Motor TypeOverhung
Driver Size18 "
Driver Weight30 lbs
Coil Diameter2.5 "
Coil MaterialCU
Coil Length1.772 "
Manufacture's Xmax23.5 mm
Aprox. Price$190 USD

TS Parameters (Measured by

Fs15 Hz
Res3.5 Ω
Le 1khz3.86 mH
Sd0.1195 m2
Vas501.8 liters
Mms446.6 grams
Cms252 μm/N
BL21.14 Tm
BL2/Res127.7 N2/W
L/R Time Constant1.1 milliseconds
SplSens88.95 dB


This 18' driver was provided by Stereo Integrity and is intended for HT applications primarily. It will be offered in dual 2 ohm and dual 4 ohm voice coil configurations The first thing of note about the driver is that it is sold for $190 each which is extremely low compared to most 18' drivers. The double box packaging was actually better than that seen on many $400 drivers. This combined with the modest weight of about 30lbs should keep shipping related damage from occuring in most cases.

The SI HT driver is built on the popular black painted cast aluminum 6 spoke frame with a rubber mounting gasket supplied and large push type terminals. The cone is a heavy paper type which is coated on the outside for appearance and protection and capped off with a small plastic dust cap which is left blank. The suspension consists of a high roll profile surround that is double stitched to the cone and an 8' diameter nomex high compliance spider with the voice coil leads sewn in. The surround is thinner than usual and very compliant. Attached to the cone is a 2.5' diameter voice coil on a spiral cut CZA former to (reduce eddy currents) which is wrapped in black nomex. The voice coil is a copper wind of 45mm length. Motivating the voice coil is a moderate sized motor (for an 18') with a pair of stacked magnet slugs about 1' thick and 7' in diameter. The top plate appears to be around 9mm thick and there is what looks to be an aluminum spacer ring between the frame and the motor which provides extra under spider and coil venting. The back plate is bumped slightly in the middle and has a moderate diameter pole vent with chamfer. Additionally there is said to be an aluminum pole cap and an aluminum shorting ring inside of the inner magnet diameter both of which should aid in reducing and linearizing inductance. The xmax is rated at 23.5mm with an xmech of 36.5mm (Stereo Integrity says this was confirmed by Klippel testing). This is a rather impressive parts list for a driver which sells for $190.

After breaking in the driver in free air the small signal parameters were measured using the LIMP program. The resultant specifications line up reasonably well with the SI supplied specs. FS is extremely low due to a fairly heavy mms combined with one of the more compliant suspensions that has been encountered. SI did mention that this was their production prototype driver and as such it has been beat on and broken in thoroughly so the suspension may be just a hair looser than normal. Otherwise the motor force looks good for a driver of this class and the impedance curve looks encouraging showing the effects of the 2 shorting rings in the motor which should provide a cleanly extended top end response and better sensitivity.

Free air break in of the driver with a 16Hz sine wave near resonance to keep power levels down showed that the excursion was relatively quiet through the first 25mm of peak to peak stroke and that noise from the suspension, motor and distortion steadily increased from there. At what looked like maybe 60 or 65mm peak to peak the driver abruptly tapped the back plate. Xmech effectively seems to be closer to 30 or 35mm inward. This was confirmed by pressing in on the cone to the point where the coil touches the back plate. the driver will not suffer any damage if this happens lightly a few times but you will want to be careful with the power. Indeed Stereo Integrity confirms that the power rating for this driver is based on the mechanical overload not what the coil will handle thermally. They recommend 600w and no more for this driver. The very compliant suspension results in little power being needed to move the driver to full stroke. SI's recommendations for this woofer make sense in light of this and the driver will not make much use of more than 5 or 600w of power without getting into trouble. Depending upon the particular enclosure design more power could possibly be used but careful measurement and simulation would be needed to assess the wisdom of applying more power.

Stereo Integrity 18" HT-18D2 Systems