Modal ringing and positioning

[goneten]

Hi there,

I recently bought some bass traps from GIK, 4 tri-traps, stacked floor to ceiling in my front two corners. Overall they have made a noticeable difference to the bass.

However the bass still doesn't stop and start on a dime, as some say, it still isn't as tight sounding as I would like it to be. I know, the remedy to this situation is to add more bass traps. Now I've been told before that modal ringing, the phenomenon that gives us this nasty boomy bass can't be fixed with positioning. That positioning affects the frequency response but not the time response.

So I tried something today. My room is rectangular in shape but I've always sat the short way within the room. So I rearranged my system to sit the long way and the difference was astounding.

I have a single MFW-15 subwoofer and just until recently I thought that the design itself might be boomy, because even with the tri-traps installed (without EQ) the bass doesn't sound anywhere close to the way I would imagine it to sound, so I assumed that perhaps the sub was the culprit. Well, I was wrong. Now that I've positioned my system to face the long way within my room I'm now getting that 'stop and start on a dime' bass that not even 4 tri-traps could provide. It's like as if I've added like 20 bass traps or more to my room.

So I would like to know why this is the case. Why is it that sitting the long way in my rectangular room made my bass sound so much clearer and articulate with far less boom. I can imagine my frequency response would have changed but that alone can't describe why bass notes decay so quickly now compared to before. I thought modal ringing could only be reduced with bass traps.

Unfortunately, I can't keep my set up positioned this way so tomorrow I'll have to rearrange things again. But I'm really amazed at the differences.

Regards,

[Glenn Kuras]

It is a few things, but the biggy is the back wall is further from you now. You are not getting as much of the CRAP (very tech term ) coming from behind you.

BTW we really try to tell customers this all the time about setting up properly. It really does make a heck of a difference. As you have pointed out.

Glad to hear the room is rocking now.

[goneten]

Hi Glenn,

Yeah, well unfortunately it's only temporary. Now if only I could get the same improvement but sitting the short way in my room. I suppose I could achieve the same result by adding more bass traps but I wonder how much more would be required to make the same subjective improvement.

I've only got one rear corner behind me and the other one is wayy off to the left; it's on the kitchen side (room opens up to a small kitchen area) so it's really lopsided. If I treat the back wall behind me I'm sure that will make a difference but the sound just was so open when playing down the long path.

I don't even know if that could be replicated using acoustic treatment. But the bass issues I'm sure can be sorted out it's just that it would mean a lot more work needs to be done. Some say that the first few traps you add make the biggest subjective improvement and that additional traps bring about minimal gains over and above that. So I'm guessing I would need some drastic coverage to make the same subjective improvement.

Regards,

[goneten]

So any explanations as to why the decay times seem much lower ?

Regards,

[goneten]

Just an update : My earlier impressions might have been over-exaggerated especially with the bass 'starting and stopping on a dime'.

That isn't really the case. I guess I was just shocked at the differences. Bass notes definitely seem to die out quicker in this new configuration.

The bass sounds, I don't know, lighter ? It's difficult to describe. The ringing hasn't been eliminated but it seems to have been reduced. I've listened to a bunch of songs with a strong punchy bass line and it definitely sounds more articulate.

In order for me to get the 'start and stop' bass that is so elusive I would still need more bass trapping. How much more I have no idea.

Regards,

[tINY]

The biggest effect of moving subs around is not exciting certain modes.

If you have 2 subs, you can position them to excite even fewer modes. For an experiment, but that sub in a tri-corner and walk around the room - this is going to excite all the modes.

Then, you can play with the position of your sub and your mellon to see if there is a combination that is clearly the best....




-tINY

[goneten]

I didn't move the sub around but I did move my seated position so that it was facing the long way in the room.

I just don't understand why the bass sounded so much more responsive and immediate. That, to me, tells me that ringing has gone down. So then it's true that sub positioning or listener positioning can reduce long decay times within a room ?

Regards,

[bpape]

Most likely because you were out of the modal area in question and you didn't have the buildup from the rear as close to you.

You can impact perceived frequency response via positioning but it will do nothing in terms of decay time.

Tri Traps in the corners in front of you and 244's centered on the wall behind you is a common way to 'fix' short way orientation. It gives you bass control at both ends of the room, at the sides and the middle of the width, and deals specifically with nulls off the back of the room which can give the impression of too much bass at surrounding frequencies.

Bryan

[goneten]

Originally Posted by bpape

You can impact perceived frequency response via positioning but it will do nothing in terms of decay time.

Sorry if I'm misinterpreting anything but are you saying that what I experienced was more a case of a frequency response change than a decay time change ? Because I used to have the SMS-1 and even with a flat response, it never sounded this good.

Regards,

[Weasel9992]

Originally Posted by goneten

Sorry if I'm misinterpreting anything but are you saying that what I experienced was more a case of a frequency response change than a decay time change ? Because I used to have the SMS-1 and even with a flat response, it never sounded this good.

Regards,

That is pretty much what Bryan is saying. Decay times are not affected by moves within the room, but frequency response at the listening position is. What you did when you reoriented the room was changed your primary mode to a frequency that was lower than it was along the other orientation. So maybe you went from a primary mode at 70Hz down to 35Hz, or less...so by the time you have modal energy that could potentially affect your mixing you're at your third or fourth octave..or higher..where there's much less energy. That's why we always advise people to set up firing down the longest dimension...because that takes best advantage of a room's raw modal responses. It only gets better from there with treatment.

Frank

[goneten]

Hey Frank,

So due to change in position I 'pushed' those room modes further down where they wouldn't likely affect music frequencies as much.

All modes have a ringing associated with it, correct ? So from my point-of-view it sounded like the ringing had been reduced because the modes were no longer interfering with the musical spectrum of frequencies but if measured, the ringing would still remain the same.

Is that correct ?

Regards,

[tINY]

If you don't move your sub, the modes behave the same way. If you move your head, you may not be in the right part of that standing wave to hear it well.

If you position your sub in the right place (say the exact center of the room) it won't excite some of the modes (the primary modes in the case of the room center).




-tINY

[Weasel9992]

Originally Posted by goneten

Hey Frank,

So due to change in position I 'pushed' those room modes further down where they wouldn't likely affect music frequencies as much.

All modes have a ringing associated with it, correct ? So from my point-of-view it sounded like the ringing had been reduced because the modes were no longer interfering with the musical spectrum of frequencies but if measured, the ringing would still remain the same.

Is that correct ?

Now you're on the right track. The modes never change position...they are where they are as a function of the size and shape of the room...but they become less relevant as the each dimension becomes larger. When you turn to shoot lengthwise down the room the modes are there, but less relevant because they lose a bunch of energy before they become high enough to become problematic.

Frank

[goneten]

Thanks for the explanation Frank. Thanks Tiny and Bryan for your advice too ! Much appreciated.

I think besides adding more bass traps what I also need is some EQ since I have none in my system at present. I think the boom I am experiencing is a combination of poor decay times and large peaks within the response. If the peaks are cut I'm sure that will help reduce the boominess I've been experiencing in some music.

Regards,

[Weasel9992]

Originally Posted by goneten

I think besides adding more bass traps what I also need is some EQ since I have none in my system at present. I think the boom I am experiencing is a combination of poor decay times and large peaks within the response. If the peaks are cut I'm sure that will help reduce the boominess I've been experiencing in some music.

As long as you resolve all the issues you can with treatment first, then a **VERY** conservative application of room EQ can be very helpful. Typically you'd need something more sophisticated than a parametric or (worse yet) graphic EQ though.

Frank

[goneten]

Frank, I was thinking of buying the Anti-mode which works in the frequency and time domains. Testing of this unit over at AVforums does, indeed, confirm that ringing was reduced.

You should see some of the graphs, verified with REW. Frequency response was much flatter but more importantly the ringing had been reduced.

Regards,

[Weasel9992]

Originally Posted by goneten

Frank, I was thinking of buying the Anti-mode which works in the frequency and time domains. Testing of this unit over at AVforums does, indeed, confirm that ringing was reduced.

You should see some of the graphs, verified with REW. Frequency response was much flatter but more importantly the ringing had been reduced.

Regards,

I took a look, and no matter what they say it's just not possible. You can't reduce modal energy by employing what amounts to EQ, no matter how sophisticated it may be. That's not my opinion...that's physics.

Caveat emptor.

Frank

[goneten]

I took a look, and no matter what they say it's just not possible. You can't reduce modal energy by employing what amounts to EQ, no matter how sophisticated it may be. That's not my opinion...that's physics.

I don't understand. Did you take a look at the measurements ? When tested, at AVforums (by Russels) the ringing had been measured to be reduced.

Doesn't measurements trump everything else ? If their measurements were just wrong then how can any measurements of any kind be considered valid or trustworthy ? I'm just trying to understand your position after seeing the evidence for myself. If ringing had not been affected then surely the measurements would have demonstrated no change, right ?

Thanks.

Regards,

[Weasel9992]

Originally Posted by goneten

I don't understand. Did you take a look at the measurements ? When tested, at AVforums (by Russels) the ringing had been measured to be reduced.

Doesn't measurements trump everything else ? If their measurements were just wrong then how can any measurements of any kind be considered valid or trustworthy ? I'm just trying to understand your position after seeing the evidence for myself. If ringing had not been affected then surely the measurements would have demonstrated no change, right ?

It doesn't trump physics. I hear you...the plots and waterfalls say what they say and I'll leave it at that. From the standpoint of physics I'm not sure how one affects a time-domain problem in the frequency domain, which is essentially all it does. I don't know what else to say.

Frank

[Ethan Winer]

Originally Posted by goneten

at AVforums (by Russels) the ringing had been measured to be reduced. Doesn't measurements trump everything else?

Yes, but I didn't see any evidence. All of the waterfall graphs in that thread were so coarse as to be impossible to read. Even if it looks like the ringing was reduced, it probably was not:

1) If you lower a peak with EQ the ringing looks lower too, even though the decay time itself stays the same.

2) It is sometimes possible to reduce ringing via EQ at low frequencies with very careful adjustment of the EQ parameters. But as soon as you move the measuring microphone two inches the critical balance is lost and the ringing comes back. So by extension, it's not possible to eliminate ringing at both of your ears at the same time.

I've tested this extensively:

EQ Versus Bass Traps
Audyssey Report

In the second article above I tested the Audyssey system which makes all sorts of claims that were easily disproven. It couldn't reduce ringing even at the same location the microphone was placed.

--Ethan

[goneten]

All of the waterfall graphs in that thread were so coarse as to be impossible to read. Even if it looks like the ringing was reduced, it probably was not

What do you mean when you say the graphs were 'so coarse' ? Don't understand.

The reviewer had some good comments I felt. I used to own the SMS-1 EQ unit and no matter how flat I made my response, it always sounded boomy. No matter how flat the response 'looked'. But here :

Before and after Anti-Mode :



Regards,

[goneten]

Even the reviewer mentioned that an EQ device like the SMS-1 requires some effort on behalf of the listener to set up the filters with appropriate Q otherwise you add ringing.

That's exactly what happened to me. I couldn't measure the ringing aspect since the SMS-1 only shows a frequency response plot. But these measurements taken in the time domain makes me think that it will provide me with a worthwhile benefit, especially down low. The Anti-mode looks like it's a step in the right direction, at least looking at the graphs. User impressions have been extremely positive.

I mean, Ethan I understand what you're saying and I've read some of the articles you've mentioned, especially the AVS thread since I predominately post there. And I don't want to sound like I'm arguing, it's just that there always seems to be two sides to a coin. Even if you look at the 63-75 Hz range, the ringing cleared up and there were no drastic cuts applied. I mean, there were no peaks at all yet the ringing has clearly been reduced.

Doesn't that count for something ? By looking at these graphs do you not think that there would be any benefit ? I am fully aware that EQ'ing can't reduce ringing everywhere. But there is only major position within my room.

Regards,

[JohnPM]

Originally Posted by Weasel9992

From the standpoint of physics I'm not sure how one affects a time-domain problem in the frequency domain, which is essentially all it does.

All filtering operates in the time domain, the FR plots we like to use are just a convenient way to see the effect of the filter's time domain changes on frequency response. Each modal resonance has an effect on the response of the room/speaker/EQ system. The mode's effect is not altered by EQ, the EQ is cascaded with it. Correct choice of EQ parameters (given sufficient flexibility in the parameter settings) allows the resonant effect of a mode (the poles of its response, to use system theory jargon) to be exactly countered by zeroes in the EQ response. The EQ filter has poles of its own, but for a filter which cuts rather than boosts those poles have a lower decay time than those of the mode it is countering. The net effect is to replace the long decay of the mode (at the mode's frequency) by the shorter decay of the filter. The limitation of EQ is that the amplitude of the mode changes with position, but the amplitude of the filter's cut does not, so although EQ can reduce the decay, the level can only be controlled to the desired level at those points in the room where the mode has the same amplitude as at the position for which the EQ was adjusted.

[Ethan Winer]

Originally Posted by goneten

Before and after Anti-Mode :

I didn't see those graphs there. All of the graphs I saw were scaled so you couldn't really see where the ringing "went through the floor" so to speak.

As John said, EQ can "work" at one very small spot in the room only. Elsewhere the improvement is less, or even made worse. Versus bass traps that reduce peaks, raise nulls, lower ringing, and make all of those better everywhere.

--Ethan

[Weasel9992]

Originally Posted by JohnPM

All filtering operates in the time domain, the FR plots we like to use are just a convenient way to see the effect of the filter's time domain changes on frequency response. Each modal resonance has an effect on the response of the room/speaker/EQ system. The mode's effect is not altered by EQ, the EQ is cascaded with it. Correct choice of EQ parameters (given sufficient flexibility in the parameter settings) allows the resonant effect of a mode (the poles of its response, to use system theory jargon) to be exactly countered by zeroes in the EQ response. The EQ filter has poles of its own, but for a filter which cuts rather than boosts those poles have a lower decay time than those of the mode it is countering. The net effect is to replace the long decay of the mode (at the mode's frequency) by the shorter decay of the filter. The limitation of EQ is that the amplitude of the mode changes with position, but the amplitude of the filter's cut does not, so although EQ can reduce the decay, the level can only be controlled to the desired level at those points in the room where the mode has the same amplitude as at the position for which the EQ was adjusted.

Thanks John!

Frank

[goneten]

So Frank, have you changed your position a little ?

Regards,

[Weasel9992]

Originally Posted by goneten

So Frank, have you changed your position a little ?

Regards,

Nope. I still wouldn't rely on any filter of any sophistication to fix modal or ringing problems in my room. I was grateful to be corrected by John though.

Frank

[goneten]

Originally Posted by Weasel9992

Nope. I still wouldn't rely on any filter of any sophistication to fix modal or ringing problems in my room.

How would you tackle problems at very low frequencies, like 50 Hz and below specifically ?

Regards,

[Weasel9992]

Originally Posted by goneten

How would you tackle problems at very low frequencies, like 50 Hz and below specifically ?

Regards,

Below 50Hz I'd look for (or build) a tuned product...if you can't do that then some very conservative application of an EQ-type product can be helpful. Don't get me wrong...it's not that I think that they're useless, just that they're not a substitute for a well-treated room.

Frank

[terry j]

Originally Posted by Ethan Winer

But as soon as you move the measuring microphone two inches the critical balance is lost and the ringing comes back.
--Ethan

I hear this all the time, but in the bass region is it true? (due to the wavelengths involved I guess)

For sure, the higher frequencies are very much affected by the slightest movement of the mic, which makes it extremely problematic with the idea of drc for full range use...(which IS the correct mic position?? left ear, centre of forehead, right ear type stuff. The answer is of course that it is fraught with 'danger' at the higher frequencies)

Here is some measurements I took last night, granted they are not waterfalls so maybe they have no bearing on this discussion, but it is a series of five measurements across my LP, a span of at least two feet (as opposed to two inches)



fully smoothed, would be a total mess with five measurements otherwise. Even so, the bass region (which we are talking about here) is pretty close, and the upper frequencies are wildly different.

This may be a bit more accurate for our porpoises (we are dolphins??), unsmoothed and 200 hz cut off



Yep, for sure minor variations (my speakers drop like a stone under 35 hz, so not sure if stuff below it is important??) And of course by 'sods law' last night happened to be very windy and rattling windows etc during the measurements, I'd hazard a great deal of any variation in these graphs are due to that. Still, they happened to be sitting here on the computer so may as well use them for this illustration.

Maybe the associated waterfalls reaaallly change by comparison with the minor FR change, but the 'fear' that the slightest change in position produces profound changes in the bass response seems to me to be unfounded.

unless I am completely missing some point??