Systems List › DIY › Rockford Fosgate T3S2-19 Sealed
NotesMeasurements for the T3S2-19 driver loaded into the standard sealed test enclosure start off with the impedance measurement. DC resistance of the system read about 1.8ohms with the impedance measurement indicating about 2ohms down at 2Hz rising to 3 ohms at 10Hz and reaching a system in band peak of 12ohms at 32.6Hz. The impedance then falls sharply back to about 2.5ohms at 90Hz before slowly climbing back to 3ohms by 150Hz. There was some induced noise in the measurement which was unusual and may have been related to vibration of the whole system due to the 1000g moving mass of the driver. Investigation is planned. The low impedance of this system classifies it as a 2ohm nominal load, so the voltage sensitivity was captured using an input voltage of 1.41v at 1m ground plane and 14.1v at 10m ground plane. Sensitivity of the system is decent but not spectacular. The average of the 10m and 1m measurements puts the sensitivity at about 77.5-78dB at 20Hz which is about average for 18-21inch drivers in this size sealed enclosure. The measured sensitivity reaches 90dB around 58Hz which is a bit higher than normal. Sensitivity continues to rise towards 80Hz where it levels off around 93-94dB from 80-200Hz which is a bit better than average. Sensitivity over the 30-50Hz range is somewhat lower than average, however down at 10Hz the sensitivity is better than normal and above 80Hz as well. This is partly due to the lack of an inductance hump in the response which would usually be in the 40-60Hz range in the test cabinet used for these measurements. The other cause is the extremely low “q” for this system due to the T3 19’s massive amount of motor force, which means that the alignment is seen as large by the driver despite only being a bit over 4 cu ft. The suggested alignment to achieve a system qtc of 0.7 is 0.8cu ft or 22.7 liters! Also it pays to remember that sensitivity is NOT efficiency. The extra motor strength means that the T3 19 will have better average conversion efficiency than many drivers with less motor force despite what the voltage sensitivity might indicate. The on axis frequency response of the T3 19 rises up until 100Hz where it levels off and remains relatively flat up to 350Hz or so before exhibiting a response resonance at 400Hz. Despite the tremendous amount of voice coil wire and the 1kg moving mass the response does not appear to exhibit any effects from inductance nor from peaking due to a high qtc alignment thanks to the powerful motor. Higher moving mass generally exacerbates top end extension and inductive resonance issues but the T3 19 design has battled these common bass driver issues quite well. It appears that this driver could be used up to the 250 to 300Hz range potentially. At 400Hz there is a peak that shows up in the impedance curve, the response and the time domain though it doesn’t appear to ring much. The group delay, spectrogram and waterfall decay measurements show a clean controlled result over the range of interest. Group delay doesn’t approach 1 cycle at any point.
The high power measurements start off with the long term output compression measurements. 3.62 volts was applied for the baseline measurement which causes the system to reach roughly 90dB at 50Hz. The T3 19 exhibits superb linearity through the next 5 sweep measurements, corresponding to a 25dB total increase in the output demands and an input voltage of 64.4 volts. At this point the compression of the output barely reaches 1dB. Another full 5dB increase in demand from the system results in about 115volts applied and caused the K20 amplifier to current limit during the sweep. That is a first with a single driver system. The effect can clearly be seen in the 114.6 volts maximum long term sweep and the long term output compression graph for the same measurement. The K20 was being operated bridged into a system with an impedance of 2 to 3 ohms over large parts of the frequency range so a ballpark calculation with 115 volts results in around 5300 watts over some parts of the bandwidth. The limiting was most pronounced over the 70-120Hz range where the impedance minimum drops quite low. The K20 is not rated into loads less than 4 ohms bridged. This being the case it is impossible to tell how much compression was the driver. The repeat 90dB 3.62 volt measurement conducted immediately after shows barely 1dB of sensitivity loss after absorbing a very large amount of energy from the amplifier. This is a great result. This and the trending of the earlier measurements tends to indicate that the driver would have completed the 114.6 volt sweep with less than 2dB of overall compression, had the amplifier not run out of long term current capability. Despite a bit of amplifier limitation the maximum output reached during this test was still commendable. Over 98dB at 10Hz, 105dB at 16Hz, 110dB at 21Hz, 115dB at 30Hz and 120dB at 49Hz.
The distortion results, at the same output and voltage inputs as the long term compression measurements, indicate a very capable and low distortion driver design. At a voltage input of 20.4 volts THD is kept very low with 5% or less from 15-200hz. The 36.2 volt measurement shows much of the same with THD below 5% at 21-200Hz. THD barely reaches 10% from 10-15Hz. The 64.4 volt measurement shows a slight increase in deep bass distortion but it still barely reaches 26% at 10Hz, is about 11% at 20hz and rapidly drops below 5% above 26Hz. The 114.6 volt measurement finally gets the driver close to its limits in the deep bass as the THD has risen sharply and has now reached 60 to 70% down below 14hz but drops sharply to 32% at 20Hz, 14% at 25Hz and drops below 10% above 27Hz. Above 28Hz the THD is under 8%. Again the amplifier limiting the 114.6 volt sweep likely has some effect on the results for that measurement but how much is difficult to ascertain. Looking at the harmonic makeup of the THD we can see that the deep bass is virtually all the 3rd harmonic and that the 2nd harmonic dominates the rest of the bandwidth above 27Hz.
CEA-2010 style, distortion limited, burst output results for the T3 19 sealed system continue much the same trend as the rest of the measurements. At 10Hz the T3 19 recorded 95.8dB with 19 % distortion. At 12.5Hz a result of 99.6dB and the rest of the results were much the same at 20Hz an output of 109dB with 17.6% THD. At 31.5Hz the output passed 120dB, 125dB was exceeded at 40Hz and from 50-125Hz the output was in the range of 128-130dB. Note that the voltages applied were quite a bit lower than normal above 40Hz. This is because the K20 amplifier would briefly mute if pushed any harder into such a low impedance load. In the deep bass at 25Hz and below the driver would produce 2 to 3dB more output but not without failing the distortion threshold for the 3rd harmonic. These results show that the T3 19 is the highest total displacement driver tested to date, by a slight amount, other than the gigantic Stereo Integrity 24”. Also of note is that the maximum voltage applied at the 10 and 12.5Hz bands was 115 volts which is the same as that used for the long term output sweeps so even if the K20 had not limited there wasn’t much more output to be had in the sub 15Hz range during the sweep measurements.