# Preliminary IMD measurements for subwoofers

Discussion in 'Speakers' started by Ilkka R, Apr 4, 2006.

1. ### Ilkka R Second Unit

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There has been a lot of discussion about intermodulation distortion (IMD) over various Internet discussion boards. Very few IMD measurements have been done on any subwoofer, most of the discussion has been basically guessing and making overly generous assumptions related to linear excursion of the woofer etc. I'm not a fan of assumptions, I'm more facts and measurements type of guy. I and Ed Mullen have been working around this IMD subject for a quite some time now and have been trying to find some common ground concerning many different questions relating its measurement and calculation. We are not sure yet whether to include this new test into the already quite wide battery of tests we take.

Theorizing can only go so far, but to get further we need some measurements and actual data so that we can see if our theories and hypothesis were correct. Since we still have a quite thick layer of snow here in Finland, I had to do my testings indoors, but that don't necessarily mean unrealiable results, especially when one is only comparing two different systems and knows the limitations of such way.

Hypothesis: Subwoofers with high excursion woofers suffer from higher levels of IMD than subwoofers with lower excursion woofers.

There has been also discussion about the relevance of the weight of the cone, weight of the amplifier and some other variables. In order to prove or disprove our hypothesis, we need two subwoofers, other equipped with a high excursion woofer, heavy cone and light amp and another equipped with not so high excursion woofer, lighter cone and heavier amp. I haven't actually measured a single one of these parameters, they are just assumptions based on some highly educated guesses made by several self-educated members of the AV community. And the test products are...Axiom EP-600 and SVS PB12-Ultra. If someone doesn't know if by now, Axiom is the one with a heavier amp.

In order to exclude the effects of room, one has to go as close to a subwoofer as possible. But not too close, because as we all know, SPL rises 6 dB everytime we half our measuring distance (if there is no reflective field, so it doesn't apply 100% when indoors), and because my microphone is only good up to 120 dB. My own hypothesis was that there is not much IMD at low sound pressure levels, so I needed to use as high levels as possible. After making some frequency response measurements with both subs, I decided to use 4" measurement distance. Using that distance FR curves were as close to each other as possible and there was still a good amount of distance for that high excursion driver to move before hitting the tip of mic. More info about my gear and how I calibrate them can be found here.

Intermodulation distortion: Nonlinear distortion characterized by the appearance, in the output of a device, of frequencies that are linear combinations of the fundamental frequencies and all harmonics present in the input signals. Note: Harmonic components themselves are not usually considered to characterize intermodulation distortion. When the harmonics are included as part of the distortion, a statement to that effect should be made.

When we measure total harmonic distortion, we use only one fundamental frequency at a time. Already then there are some amounts of IMD present, because the fundamental and the numerous harmonics interfere with each other, but that is not the correct way to produce IMD. In order to produce measurable levels of IMD, we need to use two fundamentals at different frequencies. There are some stardard frequencies for measuring for example IMD of an amplifier (for example 19 kHz and 20 kHz), but there are no standards for subwoofers. In the discussions with Ed Mullen, we decided to use fundamentals at 30 Hz and 70 Hz frequencies. At the moment we use only one pair of frequencies, but it is possible that we use more in the future. There are numerous reasons why spesifically these two were chosen, but that discussion will happen another time. The frequencies of the two-tone intermodulation products can be computed by the equation:

M f1 ± N f2, where M, N = 0, 1, 2, 3, ...

The order of the distortion product is given by the sum of M + N. The second order intermodulation products of two signals at f1 and f2 would occur at f1 + f2, f2 – f1, 2f1 and 2f2. We decided to calculate IM components up to 6th order, because that means the highest frequency of measured IM component is already as high as 380 Hz and the amplitude of components above that frequency is so small, it won't affect on total IMD value. As it was said before, harmonic components aren't usually included as a part of IMD, so we had to exclude every matching IM and harmonic component. Here is a screenshot of the IMD spreadsheet I used to calculate the results to be shown later in this thread. The needed equations can be found from the Internet, so you can make your own if you like. Although it needs some research and hard work, so no, unfortunately I won't share that spreadsheet. That is a proprietary Rissanen/Mullen spreadsheet.

In order to measure the IMD for these two subwoofers, I removed the base plate from the Ultra and placed both subs on the floor of my room (one at a time of course), so that the cones were exactly at the same place and the distance to the microphone was 4" from the level of upper surrounds (During this operation, I noticed that taking off the base plate actually changes the tuning frequency of a subwoofer just a little bit. With the base plate on, Fb of the Ultra was around 21 Hz and around 21.8 Hz sans base plate (20 Hz native tuning frequency i.e. without any port plugs). This could be one explanation for some people reporting differenced after tilting the subwoofer on its side and removing the base plate. Higher tuning frequency changes the FR profile just a bit, which can be heard as an added emphasis in the mid bass range.) I measured also the tuning frequency for the EP-600, and it was exactly the same ~21.8 Hz as for Ultra sans base plate. These measurements were done using a 1" distance. The frequency responses at 4" won't be similar, even with identical tuning frequencies, because Axiom has its vent right next to its woofer, when SVS' vents are located on another side of the subwoofer. Used 30 Hz and 70 Hz fundamentals are therefore good, because neither one of them matches with the tuning frequency of these subwoofers. That means the place of the vent doesn't cause a large difference/error here.

Here are the measured frequency responses using 4" distance. Naturally SVS has still a dip at Fb and Axiom will roll off sharply below 20 Hz and above 100 Hz due its almost brickwall filters, which can not be bypassed. But still the FRs are within +/-1 dB in 30-100 Hz range, which is really good.

I tested both subwoofers for IMD and THD (calculated for both 30 Hz and 70 Hz fundamentals) using three sound pressure levels: 110 dB, 115 dB and 120 dB. Both subwoofers could have gone even higher, but unfortunately my microphone started to clip at higher levels, so I couldn't measure them. There won't be this problem outdoors, when one can use 2 m distance. The output spectrum for each subwoofer and level was recorded and evaluated with S/T SpectraPro, and later manually calculated using the showed spreadsheet. Also the THD was calculated manually using a spreadsheet of its own. Output for each frequency pair were first calibrated at 100 dB level and then rised respectively. Axiom suffered from around 0.5-1 dB of compression at 120 dB test level, so adding input level by same amount was needed. SVS didn't exhibit from compression at used test levels. Since the maximum IMD and THD levels were around 10%, the total SPL (total power) showed by Spectra is basically only the output of two fundamentals at 30 Hz and 70 Hz. The relative levels of two fundamentals weren't adjusted, although they were naturally affected by the FR profile of each subwoofer (also by compression if there was some).

Next series of graphs and tables show the IMD and THD levels for both subwoofers at three different levels. I also calculated the sum of IMD and THD, which naturally can not be done by a simple plus operation, but using the (a^2+b^2)^0.5 formula instead.

Here are the IMD spectrums at 115 dB SPL for both subs. You can self calculate the IMD and THD levels, if you have the needed formulas/spreadsheets.

Previous spectrums overlaid (only a link due large size of the graph).

http://personal.inet.fi/private/ilkk...arison_115.png

Conclusion: High excursion woofers don't seem to suffer from excessive IMD. It can be also seen that IMD isn't a large problem with either subwoofer, combined THD levels are higher at every tested SPL. Also it seems that THD and IMD are quite strongly connected to each other. Subwoofer with a high THD levels will MOST LIKELY suffer from high IMD levels as well. Naturally much more subwoofers must be still tested, but I won't believe things would turn upside down.

The next question will naturally be, do we even have to measure IMD levels or does THD tell us the same information? Does measuring IMD add something we already don't know by measuring THD?

-Ilkka Rissanen

PS. I posted this first on AVS, but David Bott deleted it right away and then banned me. The reason was: Marketing on the site is not welcome and we believe that is why you are now here. Your account is now closed.

Yeah, thanks for nothing.

edit: More results can be found at here.

2. ### dave alan Second Unit

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WOW!! Bott is an alien.

I'm glad I checked here as I thought something like this may have happened. (Actually, I thought you retracted the posting to correct a mistake )

This is a lot of posting to digest. Good info for a start.

The conclusion that high excursion woofers don't suffer IMD more than lower excursion drivers (the Axiom's driver isn't actually a low excursion driver, is it?) should maybe have excursion plots for illustration.

IMD and THD are only slightly reated, IMO, but only a larger database will show this. IMD and multi-tone tests show more of a picture of how a sub will perform with complex audio source playback than THD alone.

Great stuff, Ilkka.

More tomorrow...hard to think when tired:b

Bott is an alien. What in the World could he have thought you were marketing??!! Still, I'm glad you moved the posting over here. It's been a while and I like it here better anyway.

Bosso

3. ### Ilkka R Second Unit

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Definitely more tests will be done. Both commercial and DIY subs will be tested.

4. ### Mark Seaton Supporting Actor

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Real Name:
Mark Seaton
Great work here Ilkka,

I too am baffled at the "excuse" for the actions taken at that other forum.

In any case, I know I've exchanged more than a few e-mails with Ed on the possibilities of such testing. This example of what you both settled on here is interesting. As Dave mentioned, it could very well take some further comparison, particularly of some poor performers before much correlation is seen. In reality, THD and IMD are intertwined as both are the result of system non-linearities, although there is more possibility of IMD rising at some faster rate than THD.

While there are certainly a variety of factors to consider, one curious comparison is that there is not a direct correlation between the ratio of THD to IMD. While I'm sure many will want to look to boil this down to a single number, the screen captures you've included will probably prove more enlightening than the chart of % distortion.

One thing is pretty well certain and clear... Once THD starts rising, you can expect to see a simultaneous rise in IMD. It will be interesting to see if testing more products there may be differences observed in how fast one rises with respect to the other.

We should also note that the Ultra and Axiom used here are interesting examples. As I understand it the Ultra driver is an underhung design with a copper sleve, which both serve to lower inductance modulation and eddy current distortion. The Axiom, while obviously not having the same excursion capability or output near tuning, is another interesting animal based on what was noted in a recent review at 6 moons. While the wording on Axiom's site is somewhat cryptic in what they are doing with their amplifier to linearize things, the observed feedback suggests that there is some form of current monitoring or similar, which *could* also help the end result while in it's operating range.

Some stuff to ponder there...

5. ### frank manrique Supporting Actor

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...of course, such mundane 'lil problems will instantly disappear (well, almost) when multiple drivers are used...like I do with my SVS 16-46 octet and B12-Plus/4s...

Nice seeing your always edifying words again, Mark!

-THTS

"...hi, my name is Frank...and am an SVS bassaholic..."

6. ### Shervin_Sabri Extra

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This is GREAT work guys!!!

outstanding Job Ilkka.....now, I would really REALLY like to hear what excuse mr. "high excursion drivers have higher IMD" comes up with!

7. ### Jack Gilvey Producer

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Nice work...funny how facts can get in the way of a nice theory. I'll dig deeper into it later, but when you and Ed get your heads together, it's kinda scary.
Did you have the DSP on?

Any change of seeing anything on the VTF3 HO? Big discussion a while back on IMD and that driver.

The more I see the type of people that you piss off, the more I respect you.

8. ### Robert SW Auditioning

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I posted a link to Ilkka's HTGuide IM measurement thread on the AVS subwoofer forum last night. A simple title of "new IM measurements". It was deleted by a AVS moderator around 8 am this morning. I am bemused by the very strange AVS forum agenda.....

Very glad to see Ilkka's work posted both here and over at HTGuide.

Bob

9. ### Ilkka R Second Unit

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Well I read that there are only around dozen HO's done yet and unfortunately I don't have a single one of them, so at this point no. But when they start spreading on this side of the Ocean, I'll definitely test one.

Although my next subwoofer tests are due to happen pretty soon and there will be some drivers equipped with Adire's XBL^2 motor, so we can see how they handle the IMD.

10. ### Ilkka R Second Unit

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Yes, I'm bemused as well. We have seen so many suspicious measurements and comparisons posted at AVS, which could be considered some sort of "marketing" way before than my more scientific study. But I guess it depends who posts them? The "mods" are blind over there, that's for sure. Trolls and people with strong agendas posting flawed data can run around freely, but people trying to compare things objectively get busted. That is sad.

11. ### Shervin_Sabri Extra

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WOW!!!

I had posted a link to this info on the "SVS owner's thread" on AVS, and it too got deleted....THE WHOLE STINKING THREAD is gone! what a joke.....

12. ### Ron Temple Stunt Coordinator

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That's alot of work Illka, nice job. I doubt I'll ever understand it . It seems AVS is making a statment about perceived marketing. Kinda interested in how it shakes out, but I'll miss your posts over there. I don't buy the allegations...keep it up.

13. ### HoraceHungry Auditioning

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You got to be kiddin!!!??

AVS have been on a steady downward slope for some time. And you can sure tell that the S in AVS no longer stands for science. How can they delete a post that is pure science, kickban the poster, and claim that it is all marketing.

This is the same forum where untrue graphs of competitors products have been posted by people who are wined and dined by said company, and they are still roaming free over there.

Fantastic! I'm lost for words

Inspired by Bender I'll say "Yeah! I'll start my own subwoofer forum, with hookers,... and blackjack! On a second tought, forget the forum and the blackjack!!"

If anyone is in any kind of doubt, this is Thomas AKA Manic Miner who is pushing the keys in the other end of wire

14. ### Peter Marcks Stunt Coordinator

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Certainly a valiant effort at quantification, Ilkka, but I am surprised that many are so quick to jump to conclusions about a topic that is more complex than it seems and that most have a very limited understanding of.

Let’s talk about the test. 120db at 4” from the woofer does not translate into very high excursion. Since both subs use 12” woofers and have the same port tuning frequency, both subs should have the about same level of excursion per a given SPL (all else equal). One would expect Inter-modulation distortion to be about equal between the two subwoofers (all else equal).

Perhaps a more relevant comparison to illustrate differences in IMD would be to compare a single driver sealed subwoofer to a multi-driver ported subwoofer.

Your results suggest that even with an excellent driver like the Ultra driver, IMD is on the same order of magnitude as THD. Harmonic distortion, being harmonically related, is usually less objectional than IMD which creates non-harmonically related tones. So I would not say that IMD is not important. In fact, I would say IMD distortion in the Ultra is the dominant source of objectionable noise (looking just at this data set)!

Please do not misinterpret our stance on IMD (and this is not directed at Ilkka, just at everyone in general). Our stance is that, rather than using one ultra high excursion woofer, we would rather use more cone area and lower excursion in order to achieve a desired SPL level. There are multiple reasons for this based on our subwoofer design philosophy, including lower IMD. Some companies will claim that IMD is BS, which is ironic and even puzzling because these same companies also use multi-driver and/or ported systems which both act to reduce elements of IMD!

The reality is that any one distortion component taken in isolation will not be enough to capture differences in performance (and I am not claiming that Ilkka said this, just trying to provide perspective). We have always believed in achieving a good balance of low dynamic, harmonic, and inter-modulation distortion instead of only focusing on one type of distortion.

Sincerely,

15. ### Robert SW Auditioning

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That is fine and I agree with that. I think the confusion arises in that I can not see a particular HSU product at the same price point that implements a different solution than its closest competition. For instance the Hsu VTF-3 MKII versus the SVS PB12-Plus. Where is the technology or specification difference in IMD versus THD?

I think a lot of us appreciate the design tradeoffs Mark Seaton has done for his new dual sealed 15" sub (temp name S^4) with moderate Xmax and a very sophisticated amp that knows how to take advantage of normal listening rooms.

I have heard the port noise that comes from plugging too many ports (in a PC-Ultra) and do like what HSU has come up with the new VTF-3 HO to get lower extension and yet keep lower port air velocity. And I like the idea behind Dan Wiggins split gap speaker motor technology that maintains a mostly flat BL without a Le increase (XBL^2). I am waiting to see real measurements that show the technical results (IMD, THD, compression SPL level, etc.). I am also planning to do some outside GP measurements of what I came up with using a 15" XBL^2 Tumult and two 18" PRs in a 200 L enclosure. I had a large basement room to fill. It sounds good now, but I really want to know the true measurement parameters.

16. ### Ilkka R Second Unit

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I'm not sure who has focused only on one type of distortion? I tried to show the correlance between THD and IMD, and found one, at least with these two subs.

Could you explain what do you mean by dynamic distortion and how one can measure it?

17. ### SteveCallas Second Unit

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This basically backs up a point that many have made when debating with you. More excursion will only result in more capability, not lower performance over the same capability level. Though Ilkka has only tested two subs so far, these results would go to show that more excursion results in more capability AND higher performance over the same capability level.

As others have stated, your argument would make sense if you wish to compare a sub with multiple, quality drivers vs a sub with a single, quality, higher excursion driver, but at a given price point with mid level subs, that can't realistically be achieved. The whole thing seems like a moot point to me.

18. ### Ilkka R Second Unit

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Kinda repeating what Steve said... How about if all else being equal except Xmax?

19. ### Ilkka R Second Unit

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Since I'm already out of subs :b, I asked a friend who has a pretty high performance subwoofer, to bring it to my place today. More results tonight for a "Sub C".

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