Systems ListWW Speaker CabinetsX21

Measurement Details

AmplifierPowersoft Digam K20-DSP+AESOP
Outside Temperature81 F
Date TestedSept 4, 2017
Tuning Freq. Of Test14.5 Hz
Settings & Equalization15Hz 12dB Octave Butterworth High Pass Filter


The impedance measurement for X21 cab with both vents open and loaded with the B&C 21DS115-4 driver, shows an in band minimum impedance of right at 3 ohms at 18.9Hz. This is very close to the rated vent tuning of 19Hz. The in band impedance maximum occurs at 43.5Hz where it reaches about 83 ohms. The impedance then drops as the frequency increases to another minimum of 3.4 ohms at 110Hz. With one of the vents blocked off the minimum impedance drops to a low of 3.3ohms at about 14.5Hz. This indicates that the effective vent tuning is in this range with one of the vents blocked off. The in band impedance maximum has shifted just a bit to a peak of about 77 ohms at 42Hz. The upper bandwidth impedance minimum remains at about 3.4 ohms at 110Hz. We can see a blip in the impedance curve at about 172Hz with both vents open and at about 190Hz with a single vent open. These are likely related to the vent length pipe resonance. This system would be considered a 4ohm nominal impedance. 

The raw voltage sensitivity for this system with both vents open is high, reaching about 103dB above 90Hz with a 20 volt input at 10 meter distance or with a 2 volt input at 1 meter distance. The 1 meter sensitivity is actually measured at 20 volts and normalized back to a 2 volt level (-20dB). Sensitivity gradually drops off towards the lower frequencies, but levels off in the range of 93 to 94dB from 20-35Hz. This is a high sensitivity for such low frequencies. When one of the vents is blocked off the overall mid band sensitivity drops a bit as expected. The sensitivity above 90Hz is largely unchanged but below that point the output drops off a bit quicker and to a lower average level. Sensitivity drops below 95dB at about 45Hz and very gradually drops off at lower frequencies. Sensitivity drops below about 90dB at 26 or 27Hz but remains at roughly 88dB way down at 15Hz. Due to what is commonly known as Hoffman’s iron law you simply will not see huge sensitivity numbers at 20Hz and below, like you would up at 80Hz. Not without a cabinet the size of a large shed anyway. At such low frequencies a voltage sensitivity of 85-90dB is quite good. 

In general terms the raw frequency response of the X21 cab + 21DS115-4 driver combination with both vents open, is quite smooth, with a very gradual roll off towards 20Hz and the characteristic knee followed by the sharp roll off below the vent tuning frequency exhibited by bass reflex systems. The raw voltage response from 57-173Hz fits into a 6dB total window. The response at 20Hz is about 11dB down from the peak level above 120Hz. We also see a bit of roughness in the frequency response starting at about 170Hz, which corresponds with what was seen in the same frequency range in the impedance curve. This should be the resonance related to the length of the ports rearing its head. With one vent blocked off and the lowered 14.5Hz tuning the raw voltage response fits within a 6dB window from 57-200Hz. Again we see the response get a bit rougher, this time up near 190Hz, which once again corroborates the impedance curve. The response at 15Hz is about 15dB down from the 120Hz level with a single vent working. This type of rising response shape towards 150Hz is common with high efficiency pro drivers in low tuned, vented cabinets. In fact this type of response may work better in some rooms than a truly flat ground plane response, which may result in excessive low bass energy. It is expected that some form of signal processing would be used in a finished system and would be used to tailor the response shape to the application. At minimum a high pass or “rumble” filter should be used to protect the driver from high level signals below tuning and a low pass filter to tailor the response for transition to the upper frequency speakers. The test using the Inuke 3000DSP amplifier with both vents open employed this type of simple signal processing. This processing consisted of 3 filters supplied by Mark Seaton. With this DSP applied the response shape flattens out considerably and now fits within a 6dB total window from 18-168Hz. Mark Seaton has also developed same type of DSP settings for protection and response shaping for use with the lower tuning when one of the vents is blocked. The settings are available publicly on the WW Speakercabinets site, Seaton Sound forum and here at Data-Bass. It was decided not to use them for the single vent testing with the Inuke 3000DSP amplifier, so it could be more easily directly compared against the K20 amplifier powering the system. A high pass filter was the only processing used for the high output testing with the K20 amplifier and the single vent open test with the Inuke 3000DSP. Only the measurements with 2 vents open using the Inuke 3000DSP amplifier had the more elaborate signal shaping employed. The high pass filter used for the testing with both X21 vents open was a 20Hz 3rd order Butterworth filter. The high pass filter used for the testing of the X21 with a single vent open was approximately a 2nd order 15Hz Butterworth filter. 

The measurements for decay and group delay of the X21 with 21DS115-4 driver are uneventful regardless of whether it is configured for single vent, 14.5Hz operation or dual vent 19Hz operation. The waterfall decay plots for all 4 test configurations show rapid loss of 25-30dB of energy over the full bandwidth of interest for each. This is what is wanted. The group delay plots are showing both the raw unprocessed results and the results with the high pass filters and processing engaged, in the case of the Inuke 3000DSP test with both vents open. This helps illustrates what effect the filters have on delaying the energy. With both vents open the group delay remains under 1 cycle for the entire useful bandwidth. Once the processing or high pass filtering are added it can be seen that the delay increases down towards the vent tuning but it only reaches 1 cycle below 24Hz and maxes out at about 17-18Hz where it remains below 1.5 cycle of delay. With single vent operation the group delay never reaches 1 cycle in either test. Adding the high pass filter increases the delay a small amount but it only exceeds 1 cycle below 16Hz. 

Long Term Output Testing Summary

Both Vents Open 19Hz Tuning / Inuke 3000DSP Amp Bridged with DSP engaged

Since this measurement set has signal processing engaged there is not a constant voltage being applied from the amplifier. The slow sine wave sweeps are started at the usual level that produces roughly 90dB from the system at 50Hz. We see linear behavior up to and through the 110dB sweep. At which point there is negligible response shifting and less than 1dB of output compression. The next 5dB increase starts to produce a bit of compression near the vent tuning. About 1.5dB. Another 5dB increase for a 120dB sweep finally causes significant compression with 2-3dB occurring between 16-28Hz and about 1dB near 100Hz. Of note is at this level it became apparent that the driver was starting to complain and run out of gas. An increase of 3dB was attempted for the next sweep but the Inuke amplifier clipped heavily and muted itself for a few seconds. A nominally 122dB sweep was attempted and the amplifier managed to avoid muting but it clearly limited the output between 25-40Hz during the sweep. At this point amplifier clipping, some vent air noise and a few grumbles from the driver became apparent. Output from the system during the 122dB sweep reached 110dB at 16.5Hz, 115dB at 19Hz and 120dB or more from 37-95Hz. Significant compression has set in over large parts of the bandwidth. Some of it due to the amplifier and some due to the driver. A repeat 100dB sweep after the ramp up to the 122dB sweep showed some temporary sensitivity loss due to heating of the voice coil and motor of the 21DS115-4. This was the first long term measurement set conducted.

Both Vents Open 19Hz Tuning / Powersoft K20 Amplifier Bridged
This set of long term output measurements uses a much more powerful amplifier and a high pass filter only. The initial measurement is taken with 1.93 volts applied. Everything goes smoothly up to an input of 34.4 volts. The output compression is less than 1db over the full bandwidth of interest at that input voltage. The next measurement is taken with 61.1 volts applied, which finally causes the system behavior to start to degrade. The output is now compressing by a maximum of 2dB at the vent tuning point and from 80-100Hz. Of note is that the output has actually expanded a tiny amount over the 30-60Hz octave. At this level the system still sounded relatively clean but there was some distortion or warmth noticed from the driver during the sweep. With the next 5dB increase in signal applied things get real and the system is being hit with almost 109 volts. 109 volts into a 3 ohm minimum impedance calculates out to just about 4000w. The impedance average is much higher than this so the real average power is much less, but suffice to say that the 21DS115-4 driver is being driven very hard with that much voltage applied. At this point the system becomes overdriven and produces some vent air noise, clear driver distortion as it is driven well beyond xmax and the output compresses by 3-6dB in some frequency bands. There was also a brief, narrow band, buzz from the system, that developed somewhere above near 200Hz during this sweep. It is unknown what the cause was. It could be a wire inside the cabinet, a loose glue joint in one of the ports, or the binding posts or terminal. It is hard to say. It was never heard with the burst testing or any of the lower volume sine sweeps. Regardless it isn’t a concern because this system should never be pushed that hard especially up near 200Hz. It is possible that the compression of the output seen over the 70-120Hz bandwidth may be at least partly due to current limiting of the K20 amplifier. I am uncertain. Also note the expanded output over the 35-55Hz band where it has now expanded by almost 1dB. We see much more compression at the top end of the response than with the DSP enable Inuke 3000DSP testing because the driver is being pushed much harder above 50Hz without the DSP filters. During the 108.7 volt 125dB measurement the X21 and 21DS115-4 combo produced 110dB at 17Hz, 115dB at 19Hz, 120dB at 29Hz and a loud 125+ dB above 42Hz. The repeat 100dB 6.11 volt measurement indicates that the 21DS115-4 driver had significant heat built up in it after this testing. It took about 10 or 15 min for the sensitivity to return to normal. This was the final long term measurement set conducted.

Single Vent Open 14.5Hz Tuning / Inuke 3000DSP Amp Bridged
The voltages applied for the single vent testing with the Inuke 3000DSP are the same as used with the K20 amplifier in an effort to keep it as comparable as possible. The 90dB at 50Hz nominal sweep starts out at the same 1.93 volt input. The system responds in a linear manner up to and through the 34.4v signal where output compression is less than 1dB over the full bandwidth. During the 61.1v measurement we start to see the output compress. There is a compression of output of about 1 to 2 dB occurring below 22Hz and above 80Hz. As with the testing with both vents open this is the measurement where the system started to exhibit signs of working hard. There was a bit of vent noise audible near the tuning during this measurement and some driver distortion due to high excursion. The next higher measurement was only run with a 2dB increase since the Inuke 3000DSP was close to its limits already. This would result in a signal of about 77 volts at the terminals. The Inuke 3000DSP exhibited some clipping and limiting during this sweep and protected itself towards the end of the measurement. This can be seen as a nosedive in the output above 100Hz in the measurement. A hard reset of the power was required to turn the amplifier back on this time, but the amplifier fired back up and was operational again immediately. 77 volts into a 3ohm minimum impedance calculates out to almost 2000w applied briefly into the impedance minimum. During this sweep the system managed to produce 105dB at 13Hz, 110dB at 16Hz, 115dB at 28Hz, 120dB at 43Hz and 125dB or more above 78Hz. This measurement produced some air noise from the single 6” port and audible driver excursion noise and distortion in the 20-30Hz range. A repeat 6.1v measurement indicated a moderate amount of heat built up in the 21DS115-4 driver which resulted in a temporary loss of sensitivity. This was the second long term measurement set conducted.

1 Vent Open 14.5Hz Tuning / Powersoft K20 Amplifier Bridged

The same input voltages were used for this testing, starting at 1.93 volts. Once again the system behaves very well up through the 34.4v measurement. The output compression only reaches 1dB or more way down at 12.5Hz where the vent air speed is likely at maximum. As seen in the other tests the performance starts to degrade during the 61.1v measurement. Compression of the output reaches 2dB near the maximum vent velocity range, about 1dB from 16-23Hz and above 80Hz. There was some vent noise during this measurement and some driver distortion audible in the deep bass as well. The sound wasn’t offensive per se and would still be useful at this level but it is clear that the system was working hard. The final sweep was taken with the full 5dB increase to 108.7 volts. This clearly caused significant vent noise and caused the 21DS115-4 to be driven into over excursion around 20-25Hz. The output had now compressed by a large amount, equal to 3 to 4.5dB from 10-25Hz and a substantial 4-5dB above 80Hz, when compared with the 1.93v baseline measurement. There is also a slight expansion of output near 45Hz. Some amount of the compression may have been due to long term current limiting in the K20 but it is difficult to be sure. Regardless this was a brutal test on the 21DS115-4 driver and if it was going to blow this would have been it. During the 108.7v measurement the output reached 105dB at 12.5Hz, 110dB at 14.5Hz, 115dB at 24Hz, 120dB at 35Hz and 125dB or more above 50Hz. It would be unwise to have this much continuous amplifier voltage available and used unchecked for this particular combination. The repeat 6.1 volt measurement immediately following the 108.7v signal indicated a large amount of heat buildup and sensitivity loss. It took about 15min for the driver to cool and sensitivity return to normal. To the 21DS115-4’s credit it took 4 sets of long term sweeps in a few hours and seemed to suffer no lasting effects from it a few hours later. This particular woofer is definitely broke in now though. This was the 3rd long term measurement set conducted.

Distortion Results
Distortion results for the X21 cabinet loaded with the 21DS115-4 B&C driver were captured at the same voltages as used during the long term output sweeps. The general trend seen for operation with both 6 inch vents open is that the THD stays extremely low during the 34.4v measurement. THD is 5% or less above 15Hz. The amount of distortion increases a lot during the 61.1v measurement. It remains below 15% from 14-54Hz which is good but then we see a large uptick between 55-85Hz which then drops back below 5% above 100Hz. The 108.7 volt sweep indicates that the system is being driven well beyond its comfortable envelope at that level. THD now reaches 20-50% over a large portion of the bandwidth. In comparison the results with the DSP filtering engaged in the Inuke 3000DSP look much cleaner. This is expected as the output above 40Hz is much less with the filters engaged. The K20 test without the upper end shelf filter produced 5-8dB more output. 

The distortion results with a single vent open and a 14.5Hz tuning indicated much the same trends. As expected the deeper tuning results in lower distortion down deep but increases distortion a bit in the 20-30Hz range. The 34.4v measurement indicates less than 10% THD over the entire 10-200Hz range which is very good especially considering the output being produced. The 61.1v measurement causes a large increase in THD from 18-30hz. This is where driver excursion would be highest and indicates that the 21DS115-4 is running out of excursion. Otherwise THD remains below 15% over the rest of the range. The 108.7v sweep with the K20 amplifier indicates that the system is being driven to its limit. THD has risen precipitously to 20-60% over the majority of the frequency bandwidth. 

The harmonic makeup of the distortion follows the same general trends for all of the testing. The 3rd harmonic dominates in the deep bass and where driver excursion is high. The 2nd harmonic dominates from 50-100Hz. 

Burst Output
With both vents open and using the Inuke 3000DSP amplifier the short term output from the X21 and 21DS115-4 driver combo is powerful. A passing output of about 93dB was possible at 12.5Hz but this is well below the operational range with a 19Hz tuning. At 16Hz the output comes on and just under 109dB output was recorded with CEA-2010 passing distortion levels. 112dB was recorded with the amplifier driven into clipping and higher distortion. At 20Hz the output was just shy of 118dB. The maximum output stays close to this range until 40Hz where a result of 123.4dB was recording with everything the Inuke 3000DSP had to give. The burst output is amp limited from 40-125Hz and output reaches the 130dB range up at 100 and 125Hz. The system was safe from driver mechanical damage with full output from the 3000DSP at least over the 16-125Hz range. Switching to the K20 amplifier produced roughly the same results at 50Hz and below, within about 1dB, so this indicates that the Inuke 3000DSP has plenty of power for maximum performance from this system. The K20 could push the system 2 to 3dB harder beyond the CEA-2010 distortion limits but this isn’t a comfortable area for the 21DS115-4 driver to operate in and results in greatly increased distortion. The K20 finally gets to stretch its legs at 63Hz and above and could produce bursts about 6dB higher than the 3000DSP but this extra output isn’t useful in context of a finished full bandwidth system. 

With single vent, 14.5Hz operation we see much the same story. With the 3000DSP amplifier connected the system managed to produce a commendable 94dB at 10Hz with passing distortion. At 12.5Hz a result of 105.5dB was recorded with passing distortion. That is a hefty amount of output at such a low frequency. At the 16-25Hz bands the output hits a plateau of about 110-112dB which is limited by the clean excursion of the 21DS115-4 driver. At 40Hz the output is into the 122dB range and proceeds to 125-130dB from 50-125Hz. Switching to the K20 amplifier resulted in roughly the same output within about 1dB from 10-50Hz. At 63Hz and above we see the same 6dB or so greater output as with the 2 vents test. This again verifies that the Inuke 3000DSP already has enough burst power for the system and a more powerful amplifier will not result is usefully greater performance. 

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