Drivers ListAurasound DriversNS18-994-A

ManufactureAurasound
Motor TypeUnderhung
Driver Size18 "
Driver Weight43.5 lbs
Coil Diameter4 "
Coil MaterialAL
Coil Length1 "
Gap Height2 "
Manufacture's Xmax19 mm
Geometric Xmax12.7 mm
MSRP$750 USD
Year2015

TS Parameters (Measured by Data-Bass.com)

Qts0.472
Qes0.533
Qms4.11
Fs21 Hz
Res3.1 Ω
Le 1khz0.41 mH
Sd0.1164 m2
Vas276 liters
Mms392.5 grams
Cms146 μm/N
BL17.37 Tm
BL2/Res97.3 N2/W
L/R Time Constant0.13 milliseconds
SplSens88.66 dB

Notes

The Aurasound NS18-994-A driver has been around for a long time in its current form. Over 10 years. That is a long time for a design to stay around unchanged. Despite being a design that is over a decade old, it is in many ways much more advanced than most bass drivers on the market today.

The NS18 is built on Aura’s Neo-Radial motor design which is an arrangement of NDFeb rare earth magnets arrayed around the voice coil in close proximity to it. The motor system is a shaped outer case in which the cluster of magnets are glued at the top in a position that would normally be where the top plate forms the outside of the gap. The pole piece has a very large, shaped vent to help ventilate the motor and an integral screen to prevent debris from entering the motor. The motor also bolts directly to the frame with 8 bolts, to allow very quick and simple swapping of the entire top assembly. The design of the motor and high attention to the shape focus all of the magnetic energy through the steel so that there are no stray fields and the maximum potential of the magnets used can be harnessed. The voice coil is a single layer, aluminum ribbon wind of 4” in diameter, with a wind height of 25.4mm or 1 inch. The gap depth, or height of the magnets in this case is 50.8mm, or 2 inches. The coil is arranged in an underhung arrangement. A simple mathematical calculation of xmax gives a value of 12.7mm one way. The rating from Aura is 19mm so this appears to be a calculation including 50 percent of the coil height as well.

The frame is a proprietary Aura piece and is a heavy gauge, 4 spoke, cast aluminum design. The Aura logo is on 2 of the spokes. The frame has plenty of clearance for large excursions and would easily handle a very heavy motor. The pair of spiders are about 10 inch diameter and they are mirrored to reduce distortion. The surround is injection molded rubber. The frame also has a heavy gauge aluminum cosmetic trim ring which covers the surround edge and cleans up the appearance of the front of the driver. The frame also has large openings underneath of the spiders to vent the gap and motor and screens to keep out any dirt or debris. The cone is a flat black aluminum with a steep profile. The dust cap is also aluminum and is a pointy bullet type of profile. The terminals are block type terminals with a set screw. High quality but the Philips head screws feel like they could strip out over time. Despite the driver being a single voice coil there are 2 sets of terminals. Total weight of the driver is about 43.5 lbs which is middle of the road or even considered light by today’s standards.

Aura’s basic specs for this driver are a 4ohm nominal load, claim an Xmax of 19mm one way and an Xmech of 38mm. Power handling is rated at 800w rms and the sensitivity rating is a rather low 86dB. That’s not much for an 18” driver. The factory motor force rating comes in at about 93.2 which is rather low as well. Aura also claims that this design is exceptionally low distortion though. After a bit of break in time the parameters were pulled from this driver using LIMP. The results match up very well with the factory data. The Fs is rated at 20Hz factory and a result of 21Hz was measured. DCR of 3.1 ohms was very close. The measured piston area using 1/3rd of the surround was extremely close to the factory 1164cm too. Measured MMS was within 7.5 grams of the 400g factory rating. The measured impedance curve matched up very well with the factory provided one as well. BL was measured at 17.37 versus the factory 17 rating. Overall there is excellent agreement with the factory specs. This indicates that there still seems to be tight quality control on this drivers production after a decade or so. The impedance curve indicates that this is likely to be a very low inductance driver with an excellent top end which should be useful well into the mid-bass. This is indicated by the extreme flatness and extension of the impedance out past 1kHz with very little of the rise you would see with typical subwoofers. In fact the inductance at 1Khz is a minuscule 0.41mH. Considering that this is a 4ohm driver that is very low indeed. The voice coil is a 4 inch diameter unit but it is also only a single layer and a relatively short 25mm wind height so it actually has very little mass compared to most of the overhung sub drivers tested for Data-Bass. This helps the inductance immensely as does the motor arrangement which has much less steel directly next to the coil compared to other designs. This is also part of the reason that the motor force is not that great. Underhung motors inherently have less coil in the gap and that will drop the overall efficiency greatly.

Free air break in and testing of this drivers behavior indicated that it is very quiet in operation and moves a lot of air through the motor. The fact that the pole vent and under spider vents are very large in area help to prevent air noise. Mechanically the driver is also remarkably quiet through the first 1 to 1.5 inches of peak to peak stroke. It starts to produce a bit of clatter and distortion when driven around 2 inches peak to peak. It will do the 2.5 inches peak to peak that Aura claims but it is obviously becoming highly non-linear at this point and eventually makes a noise that is very unpleasant. It does not appear to be bottoming since the design actually seems immune or at least very difficult to bottom. The coil has to leave the gap entirely to make physical contact with the back plate. It may be possible but the driver clearly warns you well before that point. The effective excursion for this device seems to be in the 1.5 to 2 inch peak to peak range and beyond that point should be considered a safety cushion. One issue that did rear its head while impedance testing this driver is that it has one of the nastiest break up resonances I have encountered right at 2kHz. You would not typically run this driver up much past 400-500Hz in any case but there is no chance of using a shallow slope even crossing over that low. The resonance at 2kHz is REALLY loud if excited.

Cosmetically this driver is one of the best finished, sexiest drivers I have personally seen. It looks great from every angle and the whole thing just screams quality and precision engineering. The specs appear to be dead on with the factory parameters published a hundred years ago and every indication is that this will be a very low distortion device as long as it is kept within its excursion envelope. It will be interesting to see how this driver handles high power because the coil is very light and the driver efficiency is not that good either. The long term testing may need to be cut a bit shorter than usual to avoid smoking the coil. We will see. This driver has a lot of positives and I would expect it to do best in systems where it is allowed to produce dynamic and varied content and not subjected to punishing amounts of power for extended periods. In other words this driver is more suited to home theater or moderate volume music listening in a home setting rather than car audio competitions or sound reinforcement.

Aurasound 18" NS18-994-A Systems