Intermodulation Distortion Measurement Results

[Ilkka]

Instead of showing each subwoofer’s IMD results with the other measured variables, I decided to do a comparison instead. This way one can more easily compare all the subs reviewed. I will also add future IMD results in this thread.

[Ilkka]

Only two subs could handle the 110 dB test level: DIY PA-horn with 1.8% score and Genelec 7073A with 2.4% score.

[Geoff Gunnell]

I seem to be more sensitive to IM distortion than many people, judging by my negative reation to what is to me obvious IM distortion in some flat panel speakers that doesn't bother others. So I am especially curious about your IM tests..

I assume there was no obvious audible difference between the best and worst performers on this test. IM is hard to hear on test tones and you were testing the subs one at a time.

I've read at least some of the discussion prior to the test selecting frequencies etc. Did you learn anything in these tests that would improve IM test methodology?

Do you have any clues as to the causes of IM in the worst performers? Put another way, have you learned anything yet that sub designers should do differently doing to reduce IM?

[Ilkka]

Geoff Gunnell wrote:

I seem to be more sensitive to IM distortion than many people, judging by my negative reation to what is to me obvious IM distortion in some flat panel speakers that doesn't bother others. So I am especially curious about your IM tests..

I assume there was no obvious audible difference between the best and worst performers on this test. IM is hard to hear on test tones and you were testing the subs one at a time.

Actually no. There is clear audible difference between the best and worse performers. I did record all the results as wav files, so it's easy to compare them later on.

I've read at least some of the discussion prior to the test selecting frequencies etc. Did you learn anything in these tests that would improve IM test methodology?

I don't really have extra time during measurements to explore different setting or frequencies, so I had to go with the settings and frequencies that were chosen before measurements. But I think the current frequencies are working really well.

Do you have any clues as to the causes of IM in the worst performers? Put another way, have you learned anything yet that sub designers should do differently doing to reduce IM?

I confirmed what my preliminary tests already showed; subwoofers with low THD levels will also have low IMD levels. So manufacturers should focus on getting the subwoofers to produce as high output as possible in subwoofer's working range (say 15 Hz to 100 Hz) with as low THD as possibly. That will also result in low IMD levels. The Big Five has been nailed down a long time ago: Flat, Deep, Clean, Linear, and Loud. There is no connection with high excursion drivers and high IMD levels.

[Geoff Gunnell]

... I did record all the results as wav files, so it's easy to compare them later on.

Excellent!

I don't really have extra time during measurements to explore different setting or frequencies, so I had to go with the settings and frequencies that were chosen before measurements. But I think the current frequencies are working really well.

That's good to hear. No problems standardizing on that frequency set for future tests.

I confirmed what my preliminary tests already showed; subwoofers with low THD levels will also have low IMD levels. So manufacturers should focus on getting the subwoofers to produce as high output as possible in subwoofer's working range (say 15 Hz to 100 Hz) with as low THD as possibly. That will also result in low IMD levels. The Big Five has been nailed down a long time ago: Flat, Deep, Clean, Linear, and Loud. There is no connection with high excursion drivers and high IMD levels.

Tom Nousaine and Tom Vodhanel will be pleased

Certainly no observed correlation between cone excursion and IM distortion is counter-intuitive... (edit) I do notice the low IM on your PA horn, however, which should have lower driver excursion...

Thanks for all your hard work, Ilkka.
You'd better rest up this winter; with all the new subs coming out you will have a busy year next year!

[SteveCallas]

subwoofers with low THD levels will also have low IMD levels. So manufacturers should focus on getting the subwoofers to produce as high output as possible in subwoofer's working range (say 15 Hz to 100 Hz) with as low THD as possibly. That will also result in low IMD levels. The Big Five has been nailed down a long time ago: Flat, Deep, Clean, Linear, and Loud. There is no connection with high excursion drivers and high IMD levels.

Logical thinking gets tested and proven correct, nice work. The lack of common sense prevalent in this hobby is astounding at times. Myths get started based on incorrect generalizations and then grow to become accepted stereotypes furthered by those who lack critical thinking skills. What's even worse is that few "enthusiasts" out there seem to want to learn the why or how of things, they only keep the cycle going by continuing the incorrect stereotypes.

Thanks again Ilkka, your work over the past couple of years has revealed many of these stereotypes to be nothing but myths.

[Ilkka]

Geoff Gunnell wrote:

Tom Nousaine and Tom Vodhanel will be pleased

Well yeah. Although Tom's own products aren't perfect when it comes to big five.

Certainly no observed correlation between cone excursion and IM distortion is counter-intuitive... (edit) I do notice the low IM on your PA horn, however, which should have lower driver excursion...

PA horn uses a Peerless XLS 12" driver which has a 12.5 mm Xmax.

[DrWho]

What test method was used for determining the IMD?

[DrWho]

Geoff Gunnell wrote:

Certainly no observed correlation between cone excursion and IM distortion is counter-intuitive...

I agree, but then I haven't looked at the drivers closely enough to see if there isn't actually somewhat of a trend. For example, both the porting frequency and cabinet alignment can affect the cone-excursion. The test signal chosen for determining the distortion can also have an impact on the actual cone-excursions being measured too.

[Ilkka]

DrWho wrote:

What test method was used for determining the IMD?

The test method is explained here.

"Intermodulation distortion

IMD is also one of the variables which hasn’t been used in subwoofer reviews before. By definition the intermodulation distortion occurs when the non-linearity of a device or system with multiple input frequencies causes undesired outputs at other frequencies. IMD is measured by inputting two stimulus frequencies using certain spacing between them, a few common stardards are 250 Hz / 8020 Hz (with a 4:1 amplitude ratio), 60 Hz / 7000 Hz (with a 4:1 amplitude ratio) and 19 kHz / 20 kHz. These stardards were designed for measuring electrical devices such as amplifiers. Naturally they are not suitable for subwoofers, so again I and my peer Ed Mullen had to start from the scratch with this test too.

Eventually we ended up using fundamentals (or carrier signals) at 30 Hz and at 72 Hz. 30 Hz isn’t too low for most subwoofers to produce and 72 Hz is still below the commonly used 80 Hz crossover frequency. This combination also doesn’t share any THD/IMD harmonics. The Excel spreadsheet we made calculates all IMD harmonics up to 6th order, which was more than enough because most subs didn’t have any 5th or 6th order IMD harmonics (already below background noise floor at ~30-40 dB, system's noise floor is at much lower level). There isn’t a very large database yet, but these first results suggest that subwoofers with high output capabilities and low THD levels also have low IMD levels. One should carefully examine if IMD is even worth measuring with subwoofers due this relationship.

Ideally IMD should of course be as low as possible at all levels."