Systems List › DIY › FocusWorks Audio GUJ18V1 Sealed
NotesMeasurements of the FocusWorks Audio GUJ18v1 driver loaded into the 4.2 cubic foot or 119L sealed cabinet start off with an impedance curve that is about 5ohms below 5Hz rising to about 8ohms at 10Hz. The system resonance comes in at 32Hz where the impedance reaches a maximum of 51 ohms before dropping back down to a low of 10.1ohms at 54Hz which gently rises to 13ohms up at 125Hz. This is a high impedance system which is easily an 8ohm nominal load. The voltage sensitivity is measured with a 2.83 volt input at a distance of 1 meter ground plane. The maximum sensitivity occurs between 36-45Hz where it reaches a maximum of 93dB. At 20Hz the sensitivity is about 81.5dB and about 88dB at 125Hz. The overall response shape measured at a 2 meter distance ground plane exhibits a moderate hump at 40Hz and a slight roll off above 45Hz towards 100Hz where the response is about 5dB down from the maximum. Overall the raw response shape fits within a 6dB total window from 27Hz to beyond 200Hz. The time domain measurements indicate no ringing or issues over the primary bandwidth of interest for subwoofers which is 10-120Hz.
The high output measurements start with the long term output compression sweeps. These start with a drive voltage of about 4.6 volts to produce an output of about 90dB at 50Hz at 2 meters. The GUJ18v1 driver performs well and does not show any significant compression in the output until a drive voltage of 46 volts at which point just 0.5dB of compression is observed. A 5dB increase in the voltage results in 81.6 volts applied and a slight increase in compression of the output to just about 1dB at maximum. Another full 5dB increase in the signal results in 145 volts being applied and the output compression now reaches 1.5 to 2dB broadband over the 35-120Hz bandwidth and 3dB down near 10Hz indicating the driver voice coil is now heating significantly and the driver is running out of excursion. This was also indicated by some audible distortion and distress from the driver in the deep bass. During the 145 volt measurement the GUJ18v1 driver was able to muster up 95dB at 10Hz, 108dB at 20Hz and a peak of 119.5dB at 37Hz before leveling off to about 115dB from 85-120Hz. The repeat 4.6 volt measurement indicated that there was some thermal buildup in the voice coil and motor as evidenced by a temporary drop in sensitivity of about 1dB above 40Hz.
The distortion measurements captured with the same drive voltages as used for the long term output measurements indicate that the GUJ18v1 has quite low levels of distortion above the system resonance. The 25.8 volts measurement shows under 16% THD at 10Hz, just 6.3% at 20Hz and 3% or less above 30Hz. The 46 volt measurement shows a slight increase as expected by driving the system 5dB harder. THD is now 21.5% at 10Hz as excursion is getting large, 11% at 20Hz and still well under 5% above 30Hz. The 81.6 volt measurement indicates that the system is now being driven hard and starting to reach the limits of its performance envelope in the deep bass. THD is now over 50% at 10Hz but still only 17% at 20Hz. THD remains well below 5% above 35Hz. The final 145 volts sweep the GUJ18v1 driver is taking a lot of input power and is driven well past its linear xmax below 20Hz. THD is 100% or more below 15Hz and 43% at 20Hz. The THD has also increased dramatically to 20% near 30Hz as well. THD remains at 6% or less above 45Hz even when driven this hard. It should be noted that while the driver clearly was overdriven in the deep bass during this sweep it did not mechanically bottom or suffer any damage. It is unclear whether the GUJ18v1 can be hard bottomed as it was not pushed further but it makes clear warning signs that it is being driven too hard that would be difficult to ignore. One other thing that came up during the distortion testing was a slight peak in the THD in the 140-160Hz region. The level is low and never reaches 5% even during the 145volt sweep but this is seen on the other measurements for the GUJ18v1 and the GUJ21v1. It appears to be related to a resonance formed by the dust cap, former and pole vent. It was audible during the loudest sweep measurements but not the lower volume ones. However it should be noted that driver has to be driven very hard at about 150Hz to excite this and with a typical low pass filter as used for a subwoofer system around 80-100Hz this is a very unlikely scenario. This is not something specific to the GUJ drivers either. Most drivers with long formers, large coils and a large pole vent will exhibit something similar in the lower midrange.
The short term burst output of the GUJ18v1 driver is indicative of a driver with strong dynamic capabilities. Deep bass output is strong and reaches just under 109dB at 20Hz with passing distortion. At 40Hz the GUJ18v1 is able to muster up just over 120dB and the output reaches a plateau from thereof about 123 to 124dB. With the bridged K20 amplifier the output at 50hz and below was limited by distortion and driver excursion noise. If driven hard enough the driver would make an abrupt change in sound that appeared to be related to perhaps the surround stretching tight. Above 50Hz the output was amplifier voltage limited. Of note is that the maximum output from 20-40Hz was barely more than that with passing distortion indicating that the driver is using almost its full xmech excursion during this test in those bandwidths.