THD Analyzer Test of the Subwoofers

Mark (Basspig) Weiss

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Very interesting test.. 20 Hertz signal, driven to Bassmaxx ZR18s, SPL of 110dB as measured on SPL meter. Behringer ECM8000 omni capsule mic measuring the sound, fed to Behringer MX1804X mixer, output of which drives Tektronix AA501 Distortion Analyzer and HP 3580A Audio Spectrum Analyzer.

Not taking into account hum and noise, the distortion reading (unweighted) is 0.4%. On the spectrum analyzer, with the test signal stopped, there is 60Hz hum at -60dB, accounting for at least 0.1% of the total distortion reading. Of course there are many other factors, including room boundaries vibrating and re-radiating their own distortion, so this test is not completely accurate and probably contains much noise which adds to the THD % figure. I suspect as such, because distortion seems to go down as I increase level, up to a point.

The measurement mic has a distortion rating of 1% at full input at mid frequencies, so it's probably higher at very low frequencies and only kept down because the level at which the test is conducted is below the mic's max SPL. 110dB seemed to be the sweet spot between noise floor and room/mic distortion. It is safe to say that any distortion produced by the subwoofer is way below 1%.

If I could test in an anechoic chamber with a mic that has zero distortion, I could get a better picture of what the ZR18s are doing, but even with the distortion introduced by the environment and test setup, the figures look pretty darned good for a subwoofer.

Even with the sound system shut off, there is still a big spike at 60Hz on the spectrum coming into the distortion analyzer. No doubt that is contributing to the reading as it shows up as a third harmonic. Ground loops place a lower limit on THD tests because the noise is counted as distortion.
 

DonH50

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0.4% THD+N at 20 Hz and 110 dB is incredible.

Since you have the spectrum analyzer, you could just RSS the harmonics to calculate THD alone. You will need to use something slightly different than 20 Hz if you don't want hum and 3HD on top of each other.
 

Mark (Basspig) Weiss

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0.4% THD+N at 20 Hz and 110 dB is incredible.

Since you have the spectrum analyzer, you could just RSS the harmonics to calculate THD alone. You will need to use something slightly different than 20 Hz if you don't want hum and 3HD on top of each other.

I found that the limiting factor is room ambient noise. There is an absolute noise floor of 30dB due to the computer fans in the room. There is an upper limit on the room generating rattles and sympathetic harmonics on the upper side. And perhaps worst of all, there is a ground loop between the dist analyzer and the mixer I used to preamplify the mic. I'll have to explore different wiring methods to minimize that. Running the mixer right up to just below clipping seemed to lower the distortion reading, meaning that much of the figure is from the hum. If I engage A-weighting on the AA501, the distortion figure goes to 0.04%, but I don't see how that could be measuring 20Hz under those circumstances. If I could put a 60Hz notch filter in there, I could possibly read with more accuracy.
 

DonH50

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You could build a little 60 Hz notch filter... The spectrum analyzer should allow you to average out the noise, and if you used 19 or 21 Hz and averaging you should be able to pull the fundamental and distortion spurs out of the nose (though 30 dB is pretty high, especially relative to 0.4%!)

If you can shove the mic right near the speaker you might suppress some of the room noise...

A-weighting won't read the sub, it's designed for vocal applications... http://en.wikipedia.org/wiki/A-weighting
 

Mark (Basspig) Weiss

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Yes, though that's a kludge, and I think I can solve the ground loop problem. Then the bigger problem is room noise, but yeah, I can shove the mic in front of the drivers.. but then the mic would overload and I'd be seeing mic distortion.
A-weighting is a spectral shaping that simulates Fletcher-Munsen curve on human hearing, so definately useless for 20Hz measurement.
 

Mark (Basspig) Weiss

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I've been analyzing my test setup and have found some alternative wiring methods to get the 60Hz hum down another 20dB. In the process, I took distortion measurements on the AA501 analyzer and found that the mixer, preamp I'm using contributes .03% distortion itself. I am now taking the pre-EQ/Fader output to get rid of some noise and distortion. Another issue is the blocking capacitor downstream of the phantom 48VDC.. it's dropping the fundamental output because the response rolls off somewhat by 20Hz.




At least the setup is much cleaner now, and I measured the noise with the mic plugged in, and it is -84dB below preamp clipping, which represents 0dB at the top of the HP 3580A display. The noise floor is showing about -90dB at frequencies other than 60Hz.

As soon as I can get everyone out of the house, I'll conduct a followup test. Also try some different mics to see which mic has lower distortion.
 

DonH50

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I have in the past modified the phantom power dc blocking capacitor in various components to extend the LF response. Rarely, as except for my measurement mic and a few others, the mics roll off before that anyway...
 

Mark (Basspig) Weiss

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Interesting contrast.. just measured my Dynaudio Acoustics speakers.. 9-1/4" long-throw woofers.
20Hz test signal, 3 watts power level, 90dB SPL attained on Cel 201/1 meter. 29.9% THD on Tektronics AA501.
 

Mark (Basspig) Weiss

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I switched out the ext proc loop and the Sonic Hologram, swapped the Behringer ECM8000 mic for the B-1 mic, and with 20Hz @ 110dB, the total harmonic distortion reading of the Bassmaxx woofers in operation shows 0.27% on the Tektronix AA501.
 

treitz3

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Don, you should see his "magic carpet ride" video posted many moons ago.......
 

Mark (Basspig) Weiss

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I tested all six mic preamps on the Behringer MX-1804 and found varying amounts of distortion. Channel one has .035% THD. Channel 3 is lowest at .0128%. I will use channel 3 for future measurements.
The blind aspect of this test is that I don't know how much distortion the mics are contributing to the overall signal. Also, some of these mics were tested at a NIST-traceable lab by another owner and found to be 3dB down @20Hz (specifically the ECM-8000) and 5dB down @ 16Hz, so the mic response would need to be known so that a compensating network could be applied to ensure that the fundamental was not being attenuated, relative to the harmonics, skewing the distortion readings.
The other thing I forgot to mention was that I reversed the AC line plug on the mixer, reducing the 60Hz blip on the spectrum analyzer by 8dB. It really DOES pay to minimize the signal chain, remove unneeded processor loops and have the shortest path from signal generator to speakers.
Relative to to full signal before clipping (+19.75dBm), the noise floor is about -86.4dBm on this mic preamp, with input connected to the SG505 generator with the output muted.
Tomorrow, I will do some testing at mid frequencies, to see how the mids and tweets do. I'm also planning to rotate through a bunch of mics and compare results.
 

DonH50

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I have barely used an 8000 but noticed it was not as flat nor went as low as my earthworks mic.

Mic distortion can be all over the map and of course depends upon loudness at the mic as well as its design. Normally THD is (by far) dominated by the speakers and I have rarely seen THD specs published for mics. Your monsters are the exception and mic nonlinearity is probably in the picture, especially at those volumes...

You could move the mic back and see if the distortion tracks distance to estimate how much the mic's THD is in the picture.
 

fas42

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Mic distortion can be all over the map and of course depends upon loudness at the mic as well as its design. Normally THD is (by far) dominated by the speakers and I have rarely seen THD specs published for mics. Your monsters are the exception and mic nonlinearity is probably in the picture, especially at those volumes...
A fascinating quandry: how do you determine a microphone's distortion? By using a "perfect" speaker, and playback system of course, I'm sure I saw one of those in a catalogue recently, just a moment, I'll try and track it down ... :D:D

Frank
 

Mark (Basspig) Weiss

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Anyone got an Earthworks M-50 they want to loan me for a week?
I'm doing some more tests at 20Hz with the mic input #3 that has the lower distortion.. now I'm down to around 0.25% THD @ 20Hz @ 110dB SPL at the mic location (Cel 201/1 meter reading).
I did a brief test at 1,000Hz, but only at 100dB (because my African Grey parrot is with me at the moment and my daughter is in the room drawing pictures) and THD reading was 0.25%. I think some of that is room noise, as the figure was fluctuating. I plan to test at 110dB when I put my bird upstairs and the kid goes upstairs and see if I get a lower THD % figure.
A better measurement mic would be a great help.
 

Mark (Basspig) Weiss

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I did some testing today on the midrange speakers. Got my test signal up to 110dB (ouch, that was painful to listen to) at 1KHz threw a mic 3' in front of the array and adjusted the gain on my mic preamp so it was below clipping. AA501 said 0.175% THD. Spectrum analyzer showed 2nd harmonic almost 70dB down from the fundamental.

 

c1ferrari

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Hi Mark,

The pics aren't translating.
 

Mark (Basspig) Weiss

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Aug 3, 2010
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New Milford, CT
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Hi Mark,

The pics aren't translating.

There all in English :)
I posted them on Facebook, with viewability set to Public. Not sure why they're not showing up here..
 

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