Systems List › DIY › AURA NS18-994-A SEALED
NotesThe first of the measurements that we will look at for this system is the impedance curve. When the NS-18 driver is placed in the standard 4.2 cu ft sealed test cab for testing 18” drivers the impedance maximum ends up at just under 20ohms at 36.5Hz. Down at 10Hz the impedance is 3.6ohms and above the impedance maximum the impedance drops to just over 4.1 ohms at 68Hz. This is a 4ohm nominal system. With 2 volts applied at 1 meter ground plane the sensitivity comes in at 82 dB at 20Hz which then rises to a maximum of 92.5dB at 44Hz and then drops off a bit back to 89-90dB from 90-200Hz. The response shape measured on axis at 2 meters groundplane shows that the response is smooth and relatively extended up to about 500Hz before sharply dropping off towards 1kHz. This driver could probably be used successfully crossed over as high as 450Hz. The response fits within a 6dB total window from 26-450Hz which is one of the better results from any 18 inch driver in this cab. There is a huge cone breakup at about 2kHz related to the aluminum cone which is very loud and very nasty sounding. It shouldn’t be of consequence in typical use but it was very apparent in extended upper frequency range measurements with any significant voltage. The group delay and other measurements used for looking at stored energy and ringing are clean and show nothing of concern as is usual for an unprocessed sealed system. Group delay is well below 1 cycle over the whole bandwidth inspected.
Long term output compression measurements of this system start with a voltage of 2.9v to produce the nominal output of 90dB at 50Hz at 2 meters groundplane. The NS-18 driver behaves in a predictable and linear manner up through the 28.9v or 110dB sweep where compression of the output has reached about 1dB over a large part of the upper bass bandwidth. The next 5dB increase in signal sent to the system results in 51.4v applied and produces about 2dB of compression from 50-125Hz. The signal was increased yet another 4dB for a nominally 119dB measurement resulting in 81.5v at the terminals on the cabinet and a calculated maximum applied power of roughly 1800w into the minimum impedance near 10Hz. At this point the driver made some clear excursion related distress noises during the sweep and the compression of the systems output reached between 3 to 4dB in both the deep bass and the upper bass range. During this measurement the NS-18 driver was able to muster up 106dB at 20Hz, almost 116dB at 30Hz and 115 to 113dB from 60-125Hz. The excursion maximum appears to have been at about 29Hz. The repeat 2.9 v measurement performed directly after the 81.5v measurement showed that the driver voice coil and motor had heated up quite a bit during these tests and showed about 2 to 2.5dB of thermal related short term sensitivity loss. After about 15min the sensitivity had returned to normal. Clearly putting any more power than this into the NS-18 driver would not be a good idea.
The distortion results for this system at the 16.3v or 105dB nominal drive level indicate that THD is below 5% above 21Hz and under 10% down to 10Hz. The 28.9v THD results which correspond to a 5dB increase in signal are quite well behaved as well with distortion under 5% above 26.5Hz and reaching only 13% or so below 20Hz. The next 5dB increase in signal to 51.4 volts applied causes a dramatic increase in deep bass distortion indicating that the driver excursion is now leaving its linear range. Upper bass THD is still under 5% above 32Hz but deep bass THD has now reached almost 21% at 20Hz and breaks 40% below 15Hz. The loudest measurement conducted with an input voltage of 81.5v pushes the driver well beyond its useful excursion limits with THD reaching 100% or more below 15Hz and breaking 40% at 20Hz. THD remains below 10% above 33Hz though. The harmonic makeup of the THD is nearly all 3rd harmonic in the deep bass with the 5th harmonic also appearing once the driver is pushed extremely hard. Above 30Hz where excursion is lower the distortion is primarily 2nd harmonic but the 3rd is also contributing.
Short term, distortion limited, burst output of this system is good if not world beating. It does manage a 10Hz result of just under 90dB which is always a sign of a low distortion driver. At 20Hz an output of 105.4dB was recorded and at 40Hz just over 121dB. Output eventually reaches around 125-127dB from 80-125Hz. In the deep bass below 20Hz the NS-18 driver could produce about 3 or 4dB more short term output before reaching mechanical limits if the distortion was ignored completely. Over the 25 to 50Hz octave there was not much extra output beyond the distortion limited results before clear driver limits were imminent. The amplifier was the limitation from 63-125Hz during these particular tests.