Systems ListZOD AudioM.A.U.L.

Measurement Details

AmplifierPowersoft Digam K20-DSP+AESOP
Outside Temperature86 F
Date TestedJul 11, 2016
Settings & EqualizationNone.


Measurements for this system start off with the impedance. The DC resistance sits at about 3.5 ohms. The impedance exhibits a set of 3 peaks. The first occurs at 11Hz where the impedance reaches 22 ohms. The impedance then drops to a low of 4ohms at 15Hz, which is the minimum impedance for the system in the 10-125Hz bandwidth and denotes the effective low frequency tuning for the system is near 15Hz. The second impedance peak is at 29Hz corresponding with an area of high efficiency. The impedance then drops to about 4.5 to 5 ohms over the 50-75Hz bandwidth before rising to a third impedance peak at 87Hz. Another minor impedance peak is seen at 112Hz after which the impedance drops to 5 ohms on up to 135Hz. The minimum impedances suggest that this is a true 4 ohm system and could even be considered a 6ohm nominal system.

Sensitivity for the system was measured in an outdoor ground plane setting as per the usual procedure, at distances of 1, 2, 4 and 10 meters. A 20 volt signal, representing 100w nominal into a 4ohm load, was used for all sensitivity measurements and then normalized. The results show that the closer measurements at 1 and 2 meter distance show a bit more low frequency energy and a bit less upper bass energy. The hinge point appears to be at about 35-40Hz. The sensitivity starts out strong with 90dB way down at 14Hz, which then rises slightly to the 92-93dB range over the next octave up to 28Hz. The sensitivity continues a steady climb as the frequency is increased reaching 95dB at 31.5Hz, 100dB by 50Hz and topping out at roughly 106 to 108dB from 80-90Hz. Sensitivity then drops sharply above 100Hz to around 95dB at 120Hz. The moderate to high sensitivity combined with the relatively high average impedance should result in a relatively easy load for an amplifier. Note that this is not the same thing as saying that any amplifier will be able to get maximum performance from this system!

The overall frequency response shape, measured at 2 meters ground plane, shows a system with an extended low frequency response having an effective knee of about 14-15Hz and a gently rising response of about 8.5dB an octave from 25-90Hz. Response drops off rapidly above 100Hz and exhibits a small notch at 112.5Hz corresponding with the small peak on the impedance graph at the same frequency. Two more deep nulls in the response occur at 138Hz and 183Hz. The response above the bass bandwidth is quite ragged and unusable becoming extremely rough and ragged from 150Hz-2kHz. This is a system that is obviously intended to be used as a dedicated subwoofer below 100Hz with a steep low pass filter engaged. The overall raw response shape is +/-8dB from 14.5Hz-128Hz which is reasonable for a raw system response covering such an extended bass bandwidth prior to DSP and filtering. The addition of a simple shelf EQ near 30Hz and a low pass filter are all that are needed to produce a reasonably flat anechoic response.

Looking at the time domain response of this system, over the bandwidth of interest, 10-120Hz, indicates good results considering the size of this complicated high order system. Group delay is just at or below 1 cycle over the entire bandwidth of interest. Delay just briefly reaches 1 cycle down at 14Hz which is going to be inaudible at such a low frequency. The waterfall decay shows that the system loses 30dB of energy within 300msec which is good. Impulse response is as expected for a system with strong response down to near 10Hz and a limited upper end bandwidth above 100Hz.

The maximum long term output sweeps for the M.A.U.L. system start with a drive voltage of just over 1 volt. This produced the 90dB at 50Hz at 2 meter output that is used for the baseline start of this test. Nothing of any real note happened until, after eight 5dB increases in the voltage applied from the Powersoft K20-dsp amplifier, there became apparent a bit of air chuffing noises from the system down in the 12-16Hz range, where air speeds are highest. At this point the voltage applied was roughly 104 volts and the system was producing 115-120dB in the 12-16Hz which is higher than any other system tested to date. Otherwise the system produced no signs of danger or stress in the sound or the drivers. It must be noted that at this level the siding on the building back about 90ft or more away from the cabinet was starting to buzz and rattle. The final long sine wave sweep was taken at a level that should have produced 185 volts rms into the system but at this point the K20 amplifier started to current limit the signal so the actual voltage supplied over large parts of the bandwidth is somewhat less. The effect is most dramatic at the impedance minimums of the system which are near 15Hz, 60Hz and above 110Hz where current and applied power would be highest. During this measurement there was some increase in air chuffing noise down near 12-16Hz and there was also a bit of what might have been hatch panel resonance or buzz which developed somewhere in the 180-200Hz range. (The ascending sine wave sweep used for this testing is roughly 23 seconds in duration and begins at 2Hz and stops at 2X the ending frequency. In this case the stop frequency for the measurement is 120Hz so the test signal actually ends at 240Hz. In this case the system is being driven an octave outside of its effective bandwidth with a full power amplifier signal. Subsequent investigative testing with a low pass filter engaged, as the system would employ in use, removed the hatch panel noise as expected, since the energy being required from the system out of band up near 200Hz is reduced hugely. There will also be some efforts to further reinforce and brace the driver access hatches as this was a suspected weak point of the design from the beginning.). During the loudest 185 volt nominal measurement the M.A.U.L. cabinet was audibly rattling the building behind me and producing in excess of 120dB at 14Hz, 125dB at 22Hz, greater than 130dB by 38Hz and 135db or greater from 68-99Hz. Other than the slight bit of air noise and the hatch buzz near the tail end of the sweep it was remarkably uneventful. Typically with this type of voltage and output the sounds emitted from the DUT are not good in anyway. Typically extreme levels of: Distortion, cabinet vibration, over excursion or mechanical driver sounds, or other bad things like melted voice coils or bottomed out and blown drivers happen which causes the measurement to be stopped due to limitations with the speaker itself. In this case the output seemed relatively clean even at this level and the drivers did not make any clear distress noises or mechanical sounds. The K20 amplifier did not have any more to give during this type of sustained output testing and became the limiting factor so testing was stopped at this point. Of note is that a high pass filter was not engaged during this testing and the amplifier was unable to reach the driver excursion limits. Also of note is that the 185 volt measurement required the microphone to be moved back to 4 meters to ensure sufficient headroom in the signal chain.

The repeat measurement at the 1 volt baseline after the 185 volt measurement showed virtually no thermal effects on the drivers. An excellent result. The Output Compression Magnitude graph shows the same data but presented normalized so that it only shows the compression or expansion of output as the drive signal is increased by 5dB at a time. With the M.A.U.L. design having multiple drivers, each having very high displacement, linearity and power handling, the results here are exceptional. There is a moderate amount of compression where air speeds are highest down in the 12-16Hz range. It is a gradual compression with increased output and reaches about 1 to 2dB over 12-16Hz with an input voltage of 32.8 volts. At the 103.8 volt level output compression is 2 to 3.8dB from 12-16Hz. A moderate amount of compression has also started to set in up at the top of the bandwidth above 70Hz but it is 2dB or less. Of interest is that there is an apparent shifting of the response towards higher power levels in the 100-115Hz bandwidth. Here we see a gradual compression centered at 100Hz and a gradual expansion centered at 111Hz to offset. This is likely to be related to high air speeds in the cabinet and / or the interaction between and the loading of the drivers. It does not appear to be thermal heating of the drivers. It is difficult to say for certain since the interactions inside this system are complex. Regardless the total shift remains at or under 2dB at the highest power levels. From 18 to 95Hz there is 1dB or less compression in the output with the 103.8 volt input. There is virtually no output compression from 20-70Hz where there is 0.5dB or less. Unfortunately the 185 volt sweep caused the K20 amplifier to limit its long term power output so it is impossible to separate the amplifier signal compression from the compression of the total output so it cannot be known exactly how the M.A.U.L. would have performed during the 185 volt measurement. Even with the K20 compressing the output by a large amount the output compression was around 4dB or less from 16-100Hz so we do know the speaker system itself will perform much better than this.

The distortion results for the M.A.U.L. design indicate a very low distortion system with extremely strong output capability. The 18.5 volt measurement shows THD levels of less than 5% from about 12Hz to past 100Hz. At this input level the system is already producing output of 105dB to 120dB above 14Hz. Increasing the signal by 5dB for the 32.8 volt measurement does not do much to increase the THD. Distortion remains at or below 5% from 12Hz to 100Hz. There is a small spike in THD to 7% at 102Hz. Another 5dB increase in the drive signal results in 58.4 volts into the system and a slight increase in THD levels. Still the overall levels are very low with only 12% even down at 10Hz which is under the effective loading of the cabinet. THD is at 5% or less over the bandwidth of 12.5-95Hz. The SPL being produced is between 115-130dB over most of this range. We once again see a slight spike in THD near 100Hz where it reaches about 9% with the system producing 125dB. THD is at 9% or below from 11.5Hz to 200Hz. Yet another 5dB increase in the drive signal nets 103.8 volts into the system which the speaker turns into a powerful 120-135dB SPL over the 16-100Hz bandwidth. THD now reaches about 39% down at 10Hz which quickly drops to less than 1.5% at 14Hz while producing over 117dB! THD remains in the neighborhood of 6% or less over the bandwidth of 13Hz to 90Hz. Once again we see a slight spike in THD to 12% near 100Hz, except that it has shifted down a bit and is now centered at 98Hz. The corresponding SPL produced is 132.5dB. The final measurement should have been at 185 volts but as previously covered the K20 amplifier was not able to sustain the voltage for the full measurement at this level. Also of note is that this measurement had to be taken with the microphone at a distance of 4 meters to ensure headroom in the signal chain. It is unknown whether some of the distortion is due to the amplifier limiter circuit. At this point the THD at 10Hz has jumped up to 87% indicating that driver excursion is getting large below the system loading without a high pass filter in place. However, by this point the system is producing nearly 110dB at 10Hz. THD drops quickly from there to 10% just above 13Hz and a low of under 4% at 14Hz while outputting 120dB! The distortion then rises a bit to around 10 to 11% near 20Hz where driver excursion starts to go up again. Over the 13-90Hz bandwidth the THD is 12% or less, which is exceptional considering the amount of output being produced (120-139dB). Following the trend already established there is a small spike in THD near 100Hz where distortion reaches 16.4% at 96.5Hz while producing just under 137dB. The harmonic makeup of the distortion is dominated by the 3rd harmonic in the deep bass below 25Hz and the 2nd Harmonic from about 25-110Hz. Above that point the 3rd harmonic takes back over until almost 170Hz but this is all outside of the intended bandwidth of use for this design.

CEA-2010 style, distortion limited, short term burst output for the M.A.U.L., is very close to the estimates based on the modeling and design work. Output starts strong at over 106dB at 10Hz before jumping greatly to nearly 120dB at 12.5Hz where the system is starting to load those frequencies. Output exceeds 126dB at 16Hz and only continues upward from there before topping out at 144dB at 80Hz. The only 2 bandwidths not limited by the burst capability of the bridged K20 amplifier were the 10Hz and 12.5Hz bands. At 12.5Hz the system produced 120.7dB with the THD just barely breaking the prescribed thresholds and this may have been the amplifier driven deep into hard clipping. At 10Hz the system would burst 112dB but with greatly elevated distortion levels. However the amplifier was pushed to its limit at this bandwidth and the drivers still had a bit of excursion left so this system is truly amp limited with the K20 amplifier. 

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