Systems ListJTR SpeakersCaptivator 2400-ULF

Measurement Details

AmplifierClosed Loop
Outside Temperature83 F
Date TestedMay 11, 2018
Settings & EqualizationGain at 2/3rds
Low Pass Filter at Maximum
Low Frequency Adjust at Half (Unless otherwise noted for individual measurements)


Testing of the JTR Captivator 2400-ULF started with 2 meter ground-plane response measurements. The flattest response of the 2400-ULF in this type of half-space environment are with the low pass filter set to its highest setting and the Low Frequency Adjust control, referred to as LFA , set to maximum, which boosts the lowest bass frequencies under 30Hz. In this configuration the frequency response was measured as being quite flat at +/-3dB from 12-167Hz. This much flat deep bass extension in a half space environment would typically result in a boosted deep bass response once the sub is placed inside of a room. With that in mind a more typical setting of the LFA control would be near its middle range, which rolls off the low bass a bit in a half space outdoor setting, but should result in a flatter response inside of a room. This would be closer to typical operation for most end users, so for most of the high output measurements: Distortion, burst output, long term sweeps, etc...The 2400-ULF was tested with the LFA control at its middle setting. In this configuration the response fits within +/-3dB from 24-167Hz and gently rolls off towards 10Hz on the bottom end. 

The low pass filter appears to correspond quite well with the indicated markings. The 2400-ULF response was measured with the control set at 5 different settings covering a broad range of settings from minimum to maximum. At minimum the effective -3dB point from the broadband response falls in the 60 to 65Hz range. With the control set at maximum the response is -3dB at about 165Hz. The roll off appears to be at 24dB octave. There appears to be a slight port resonance artifact at about 172Hz which is typical with vented alignments. Clearly the 2400-ULF extends high enough to be useful with smaller bass shy mains and low enough to effectively extend the response of even large powerful mains. 

The LFA adjustment was tested with it set at maximum, minimum and at the middle setting to get an idea of its general function and range of effect. The control appears to be a shallow shelf filter. The total range of adjustment is about 15dB as measured. The effects are greatest below 20Hz. Overall it should function well for matching the lowest frequency output of the sub to the room or environment it is placed in. 

Group delay and other time domain analysis of the 2400-ULF's behavior indicate a well behaved subwoofer without any questionable behavior. The group delay is under 1 cycle until below 20Hz where it begins to grow and reaches about 1.5 cycle at 15Hz. At such deep frequencies this is nothing to worry about and inaudible. Likewise the spectrogram and waterfall decay plots show a lengthening in decay below 20Hz as expected of a vented sub tuned extremely low, but nothing of note higher in frequency which is the much more important bandwidth of concern. 

The long term sine wave sweep measurements are conducted at the usual 2 meter distance outdoors and started with the subwoofer being driven with a signal that caused it to produce about 90dB at 50Hz as a starting point. As noted earlier the LFA adjust was set to its middle setting for all of the following measurements. The low pass filter was set to its maximum bandwidth. The 2400-ULF behaves in a linear manner through the 90,95,100 and 105dB sine sweeps where it remains at less than 1dB of output compression. Even the 110dB sweep is very well behaved with less than 1dB of overall output compression except for way down below 12Hz where it just barely reaches 2dB. The 2400-ULF is tuned very low in the 11Hz range so this is likely the vent starting to compress a bit due to high volumes of air flow, rather than the driver itself. The 115dB sweep finally starts to exhibit a bit of real output compression with about 1.5 to 2dB over the 50-75Hz bandwidth. Down near 12Hz it grows to about 3.5dB. The 120dB sweep indicates that the sub is finally reaching its limits. The response shape has started to change a bit and the output did not increase linearly to the 5dB signal increase. Compression in the output has now reached 2 to 3dB broadband and up to 5dB at the frequency extremes. The testing probably should've been stopped here but instead the 2400-ULF was asked for another 3dB on top of the last measurement for what would've been a 123dB sweep. At this point the output barely increased at all and indicated that this is truly the output limit for the 2400-ULF with this type of torturous measurement signal. The output barely increases since the 2400-ULF was nearly at its limit with the 120dB sweep so this is reflected in the output compression graph where the apparent compression balloons. During the 123dB sweep the 2400-ULF produced a hefty 99dB way down at 10Hz, over 105dB by 12.5Hz, 110dB at 18Hz and 115dB or greater above 26Hz. The peak in the output was 119.4dB at 36Hz. During the 115dB measurement a hint of vent air noise was apparent. During the 120 and 123dB sweeps there was some vent noise down near 10Hz and a bit of audible warmth or fuzz to the tone which is likely a combo of THD and amp limiting but otherwise the 2400-ULF seemed completely unfazed by being driven this hard. The repeat sweep at a much lower volume showed less than 1dB of residual compression due to driver voice coil heating. 

Distortion results captured at the same output levels as used for the long term output measurements show that the 2400-ULF has low amounts of THD until driven very hard. The 105dB sweep shows THD levels of 6% or less from 10-200Hz. The 110dB sweep increases it slightly as expected but THD remains under 10%. The 115dB sweep doesn't increase THD by that much. THD remains under 13% even way down at 10Hz which is particularly impressive. The 120 and 123dB sweeps finally get the 2400-ULF to produce about 20% THD below 16Hz but even 20% is quite low as far as 10-16Hz frequencies are concerned. The harmonic makeup of the distortion is primarily the 3rd harmonic below 20Hz where driver excursion is high with the 2nd harmonic contributing below 12Hz. From 20 to 115Hz the 2nd harmonic is the largest contributer. 

Burst output for the 2400-ULF is strong, especially below 25Hz. From the 31.5Hz band through the 125Hz band the 2400-ULF was able to burst between 118 to 120dB. Output tapers off somewhat below 30Hz as expected but remains strong with outputs of nearly 115dB at 25Hz, 112.6dB at 20Hz, over 109dB at 16Hz, 106dB at 12.5Hz and just under 102dB at 10Hz. At 20Hz and above the output is amplifier limited. At the 10,12.5 and 16Hz bands the output was limited by distortion or noise just before the amplifier limited. The maximum output without regard for distortion was less than a single dB higher at 16Hz for example. 

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