If there is a hole at that frequency then I would fix it with the subs EQ. But I don't understand how you can say that it is always there. I don't see how we can say that anything about the modal region is "always". Every room is different. Every room needs a specific solution. Sometimes there is a "massive hole at around 120-140 Hz", but sometimes there isn't.
Wow, am I out of sync. Sub eq for a 120 to 140 dip? That is a full octave above my subs! I use very steep sub crossovers between 60 and 70 Hz. I use eq on my subs, I use treatments for the mains. I tried three versions of computerized eq on the mains, they sound bad, worse, and terrible.
Multi subs for modal smoothing is cheap when you are DIY. It may seem strange, but I fixed more modal issues in my movie room by going to one sub, in wall behind the screen. It is equidistant from the mains so it was easier to integrate than when I had two in a non-symmetrical arrangement. By themselves, they would balance pretty well. Add the mains, big hole right above the crossover.
I am more disheartened when people go buy fantastic amps and put them on junk speakers, no room treatments, and play MP3's while bragging about their great amps.
Repeat after me:" Speakers, Treatments, Everything else"
Multi subs for modal smoothing is cheap when you are DIY. It may seem strange, but I fixed more modal issues in my movie room by going to one sub, in wall behind the screen. It is equidistant from the mains so it was easier to integrate than when I had two in a non-symmetrical arrangement. By themselves, they would balance pretty well. Add the mains, big hole right above the crossover.
I am more disheartened when people go buy fantastic amps and put them on junk speakers, no room treatments, and play MP3's while bragging about their great amps.
Repeat after me:" Speakers, Treatments, Everything else"
Sorry, I got carried away, you're absolutely right about location of maximums. My main thought was that small (up to 1-3ms) delay applied to rear subs wouldn't ruin cancellation for lowest order modes much but may have positive effect for midbass summing with mains and front subs and perhaps allow rear subs to be crossed higher (up to 200Hz).
The precedent effect would come into play. I do believe adding a small delay could help sum higher up but if this range extension is to compensate for a frontal null the sound it is compensating for will appear to emanate from behind you.
I had a room and a position where the fronts basically had no bass below 180-170 Hz. So I crossed two subs/midbasses over at 170 Hz, placed on the inside of the speakers, and that filled out the area nicely as can see below for each channel. These are midbasses and don't go very deep in frequency by the way. I'm now setting up something simliar, but with one or two extra subs crossed lower. Possibly placed in the rear of the room. We'll see what measurements show.
Attachments
No amount of signal processing would make it ok to cross over the rear subs at 200Hz - at those frequencies they will be localised as separate sources for sure. I wouldn't go above 80Hz for rear subs - they're there solely to help with the longer, lower frequency modes, particularly the front/back modes.
For the front subs the maximum frequency you can go depends on how close they are to the mains. If you were going as high as 160Hz, you'd want them to be less than a metre from the mains, symmetrically placed to their respective main speaker and driven in stereo.
Effectively if placed correctly they become an integrated extension of the main speakers rather than being a "free roaming" non-localisable subwoofer. "Helper woofer" is indeed a better description than subwoofer, however if the subwoofer is well behaved to high frequencies it can serve both purposes at once.
Or you could have a couple of small woofers that run from 80-160Hz placed close to the main speakers to optimise fill in in that frequency range, and "conventional" subwoofers that only operate up to 80Hz placed further away in a more traditional multi-sub arrangement. I'm sure that would work fine too but that's an additional two woofers when the right subwoofers could do double duty.
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The amplitude of the standing wave is unchanged, so the variation in response along the room has not been made worse. The response at this frequency is up 6dB but the same would happen if you stacked the second pair of subwoofers on top of the first pair at the front.
The point is some modes are eliminated and some are not affected at all, relative to the over all SPL increase of doubling the number of woofers. When one part of a problem is solved its easy to say "but it doesn't fix everything". Again its a case of don't let perfect be the enemy of good. 😉
This sounds very logical, and for the elimination of the mentioned drawback I'd say that for stereo sub setup 6 subs would be the new optimal number. If you don't find inter-sub delay alignment necessary (for example, if your listening point is right in the middle of the square room), and if mains go low enough we could cross all subs at the same low-enough 80Hz frequency and build them all as passive feeding from the same crossover and stereo amp if the latter can handle 2-3 Ohms undistorted (for each sub having 6-8 ohms minimum Z).
Sealed boxes would give less chance for localization due to absence of audible port artifacts. Vibration decoupling from floor and drivers facing the floor or the wall will give the same benefit. Applied Linkwitz transform would allow them to be built small enough in size. Larger number of subs also may allow usage of less max SPL and Xmax capable drivers which may give the same or lower cost compared to two subs with super capable drivers with additional benefit of more even bass all over the room.
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Really ? Since when can a sharp 10-20dB in room notch be fixed with EQ ? If it could we wouldn't need multi-subs... its a spatial problem that can only be fixed with a spatial solution. (provide fill in from an overlapping but physically displaced source)
Its the exact same problem that you try to solve with multi-subs but one octave higher, yet still below the Schroeder frequency, and still at a frequency where we hear primarily the "steady state" frequency response, so still important.
If you have an 8 foot high ceiling you almost certainly have a notch in this frequency region as it primarily comes from the 2nd order floor to ceiling mode. Maybe in the US many houses have higher or vaulted ceilings that would push it lower in frequency but in Europe unless you live in a mansion you have an 8 foot high ceiling, so the problem frequency is pretty standardised.
It is exacerbated by boundary cancellation from the front and side walls which can depending on speaker placement fall annoyingly near the same frequency causing stacking of the notches. That's when it gets really bad.
I'm not saying that every room has a problem in the 100-160Hz range, but all of the ones I've measured with 8 foot ceilings and just two main speakers have, some as deep as 15-20dB and up to 1/3 octave wide.
A main speaker with the woofer (crossed over above 200Hz) at or near the floor helps quite a bit but isn't a complete solution, to fully solve it you need another woofer that is horizontally or lengthwise displaced from the main.
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Hi guys,
no single comment, question or critics due to this idea:
http://www.diyaudio.com/forums/mult...-pattern-stereo-speakers-244.html#post3766730
i am a bit a bit disappointed i have to say ... 😉
Concerning the "peak here", "dip there", "sparse transitional range" you are mainly
talking about room idiosyncrasies IMO (around and above Schroeder frequency).
There is often a transitional range above Schroeder frequency from "pure modal" to
"statistical" behaviour.
Even above Schroeder, a distributed excitation of the room may be of advantage.
But then "helper" or "flanking subs" have to be more close to the mains:
In order not beeing localized and due to the separation for modal smoothing
has to be thought in fractions of wavelengths of the frequency range under question.
That is an aspect also Wayne is emphasizing, to keep an eye on the range
around and above Schroeder, which can be modally sparse and "non smoothly excited"
while perceptual highly relevant in also providing a transparent midrange:
Irrgularities in the "low hundreds" can account for a lot of "maskers", affecting
the midrange. I guess that is why also Earl tends to use the mains without
a highpass: They have to contribute to modal smoothing around Schroeder
and also provide a seamless passage to the "conventional stereo system" at
higher frequencies.
In most rooms pure subwoofers should not go much higher than say 80Hz IMO ...
also depending on room size (Schroeder), steepness of the lowpass, noise, harmonics,
possible FR irregularities from the subs themselves ...
Kind Regards
no single comment, question or critics due to this idea:
http://www.diyaudio.com/forums/mult...-pattern-stereo-speakers-244.html#post3766730
i am a bit a bit disappointed i have to say ... 😉
Concerning the "peak here", "dip there", "sparse transitional range" you are mainly
talking about room idiosyncrasies IMO (around and above Schroeder frequency).
There is often a transitional range above Schroeder frequency from "pure modal" to
"statistical" behaviour.
Even above Schroeder, a distributed excitation of the room may be of advantage.
But then "helper" or "flanking subs" have to be more close to the mains:
In order not beeing localized and due to the separation for modal smoothing
has to be thought in fractions of wavelengths of the frequency range under question.
That is an aspect also Wayne is emphasizing, to keep an eye on the range
around and above Schroeder, which can be modally sparse and "non smoothly excited"
while perceptual highly relevant in also providing a transparent midrange:
Irrgularities in the "low hundreds" can account for a lot of "maskers", affecting
the midrange. I guess that is why also Earl tends to use the mains without
a highpass: They have to contribute to modal smoothing around Schroeder
and also provide a seamless passage to the "conventional stereo system" at
higher frequencies.
In most rooms pure subwoofers should not go much higher than say 80Hz IMO ...
also depending on room size (Schroeder), steepness of the lowpass, noise, harmonics,
possible FR irregularities from the subs themselves ...
Kind Regards
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So many posts and not enough time in the day to reply to them all... 😉
Your link just seems to go to the top of the page, so I don't know which post you're referring to. What is the post number ?
Thanks for that, that's a very good result between 100-200Hz indeed, if it was measured at the listening position. 🙂 Did you keep a before measurement as well ?
...the link should have been corrected, it's post #2439
http://www.diyaudio.com/forums/mult...-pattern-stereo-speakers-244.html#post3766730
I see the "bookmark" in the URL but for some reason in google chrome it still just takes me to the top of the page...
I did read post 2439 but I don't really understand what you're trying to achieve, nor can I visualise in my mind what effect the odd channel layout and phasing would have...so I didn't have a comment to make either for or against... 😉
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I've tried a helper woofer beside main speakers to a relatively high frequency before - and with it more or less right next to it about 200-300Hz is as high as you can go before the separate woofer starts to draw attention to itself. So I think even a closely placed helper woofer shouldn't be used any higher than 200Hz if you don't want to spoil imaging.
I don't see how using the mains without a high pass filter contributes to "modal smoothing around the Schroeder frequency", when the Schroeder frequency would be around 200Hz and Earl doesn't use distributed subs above 80Hz. At the Schroeder frequency only the mains would be active.
For any subs except for the "helper woofers" immediately flanking the mains, I would agree - don't cross them higher than about 80Hz. Fortunately there is no need to as the higher in frequency you go, the closer to the main speakers you can (and have to) be to solve the problem. You don't need help from rear subs to solve problems in the 100-200Hz range.
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That's my experience as well. I'm always looking at the excess phase group delay in REW. If there's no peak a bit of EQ can help.
Odd sub
Concerning the "odd" "interchannel dipole" arrangement proposed ...
this was thought as a solution to put David Griesingers suggestion into
practice in a different and to my view more universal/versatile setup than
he suggests in his paper.
The setup is also more in line IMO with using subwoofers for
"modal smoothing": I will put things together once again.
As long as there is no difference at LF between L and R signal,
picture 1 represents (by mode of operation) a usual dipole setup,
while picture 2 represent a setup according to John Kreskovky's
proposal in using 2 monopole subwoofers yielding dipole-like performance:
One positive subwofer in the front and one negative in the rear.
Basically i am just using one of these antiphase setups at each side.
The fun starts, when there is going to be a difference in L vs. R signals
at LF in phase and/or intensity. I provided some explanation here to
describe what is going to happen then ...
Pictures:
Picture 1 shows a variant also allowing nearfield listening.
With both channels driven by the same signal, the setup
operates like common dipole setups ...
Picture 2 shows a variant with dipole separation equal to
the room's longest axis ...
Prerequisites:
"Loudspeaker and listener positions for optimal low-frequency
spatial reproduction in listening rooms" by David Griesinger
http://www.davidgriesinger.com/asa05.pdf
This may work as an introduction, how two monopole subwoofers
can be used acting as a dipole in a given room:
DP_woofer_room
Explanation:
- If left and right signal is identical, we get basically two dipoles: One at the
left and one at the right side of the listening seat.
modes are predominantly excited in front/back direction. Excitation of lateral
and vertical modes is reduced, like in common dipole arrangements, which are
approximately aligned with the long (front-rear) axis of the room.
- If there is an antiphase component between L and R channel, this will turn the
right and left arrangements respectively towards monopole radiation, the dipole axis
now is rotated laterally, because left and right speaker arrangements act in antiphase
now, exciting asymmetrical lateral modes by that antiphase component.
- If intensity shifts between channels, arrangements will also turn into cardioids, and
a complete intensity shift to one channel again forms monopoles in antiphase on either side
of the room - again a lateral dipole occurs, exciting predominantly asymmetric
lateral modes.
Further remarks:
- The arrangement in Picture 1 has to be equalized, to give proper
dipole operation having flat FR for L and R signal being the same.
- The ratio of separation (front-rear vs. left-right) will affect the
the relative strength of lateral pressure gradients.
It will have do be aligned for room's listening situation
- The arrangements can be seen as "direction of wavefront" rotators, which
can rotate the pressure gradient in the room by full +/-90 degrees,
depending on stereo phase and intensity differences.
- Also the introduction of weighting factors is possible:
E.g. instead of "+L and -R" for building the left dipole also "+L and (-0.5*L - 0.5*R)"
could be used, if the lateral effect is "exagerated" otherwise in a given room setup.
But in the way shown, especially the setup according to Picture 2 may be realized
using any 4 subwoofers without additional circuitry.
Concerning the "odd" "interchannel dipole" arrangement proposed ...
this was thought as a solution to put David Griesingers suggestion into
practice in a different and to my view more universal/versatile setup than
he suggests in his paper.
The setup is also more in line IMO with using subwoofers for
"modal smoothing": I will put things together once again.
As long as there is no difference at LF between L and R signal,
picture 1 represents (by mode of operation) a usual dipole setup,
while picture 2 represent a setup according to John Kreskovky's
proposal in using 2 monopole subwoofers yielding dipole-like performance:
One positive subwofer in the front and one negative in the rear.
Basically i am just using one of these antiphase setups at each side.
The fun starts, when there is going to be a difference in L vs. R signals
at LF in phase and/or intensity. I provided some explanation here to
describe what is going to happen then ...
Pictures:
Picture 1 shows a variant also allowing nearfield listening.
With both channels driven by the same signal, the setup
operates like common dipole setups ...
Picture 2 shows a variant with dipole separation equal to
the room's longest axis ...
Prerequisites:
"Loudspeaker and listener positions for optimal low-frequency
spatial reproduction in listening rooms" by David Griesinger
http://www.davidgriesinger.com/asa05.pdf
This may work as an introduction, how two monopole subwoofers
can be used acting as a dipole in a given room:
DP_woofer_room
Explanation:
- If left and right signal is identical, we get basically two dipoles: One at the
left and one at the right side of the listening seat.
modes are predominantly excited in front/back direction. Excitation of lateral
and vertical modes is reduced, like in common dipole arrangements, which are
approximately aligned with the long (front-rear) axis of the room.
- If there is an antiphase component between L and R channel, this will turn the
right and left arrangements respectively towards monopole radiation, the dipole axis
now is rotated laterally, because left and right speaker arrangements act in antiphase
now, exciting asymmetrical lateral modes by that antiphase component.
- If intensity shifts between channels, arrangements will also turn into cardioids, and
a complete intensity shift to one channel again forms monopoles in antiphase on either side
of the room - again a lateral dipole occurs, exciting predominantly asymmetric
lateral modes.
Further remarks:
- The arrangement in Picture 1 has to be equalized, to give proper
dipole operation having flat FR for L and R signal being the same.
- The ratio of separation (front-rear vs. left-right) will affect the
the relative strength of lateral pressure gradients.
It will have do be aligned for room's listening situation
- The arrangements can be seen as "direction of wavefront" rotators, which
can rotate the pressure gradient in the room by full +/-90 degrees,
depending on stereo phase and intensity differences.
- Also the introduction of weighting factors is possible:
E.g. instead of "+L and -R" for building the left dipole also "+L and (-0.5*L - 0.5*R)"
could be used, if the lateral effect is "exagerated" otherwise in a given room setup.
But in the way shown, especially the setup according to Picture 2 may be realized
using any 4 subwoofers without additional circuitry.
Attachments
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Markus your comment about looking at the excess group delay of the bass response caught my eye. 🙂
I've speculated for quite a while that looking at the excess group delay of the rooms bass response (particularly at and around the listening area) could be beneficial to see what is and isn't EQ'able, and potentially be a secondary factor to help optimise multi-sub placement, (by seeking to eliminate the large peaks in excess group delay that boundary cancellation can cause by moving the sub locations) but every time I bring it up in various threads and ask if anyone has tried it all I hear are crickets... (or sometimes people asking what the heck is excess group delay, followed by crickets 😀 )
So would you agree that looking at the excess group delay in conjunction with the normal amplitude response gives some additional insight into how best to deal with room mode / boundary cancellation problems ?
Do you see large peaks of excess group delay on any of the 120-140Hz notches you measure ? I'm sure on one of my room measurements I saw a peak of 30-40ms at that notch, and one as high as 80ms on a 65Hz room response notch. Surely such large peaks of excess group delay must be audible and harmful to the bass quality ?
At the very least I would use excess group delay to see what was EQ'able and what was fundamentally not EQ'able, but I can't see why it wouldn't help with multi-sub positioning by allowing you to see when you have achieved a near minimum-phase response that can then be successfully EQ'ed.
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Could it be null from summing with out-of phase reflections from being close to rear wall (at 120-140Hz monopoles are omnidirectional), and/or side walls? 120-140Hz equals 2.45-2.85m wavelength, its half (180 out of phase or inverse) being 1.2-1.4m. If thats the reason then moving speakers or varying baffle size must shift this dip to different frequency and level. F.e. to shift the dip to more convenient sub leveling/crossing at 80Hz you may need to move speakers almost twice the distance from walls. Please validate -am I right or missed it?
Also big and cosy sofa you're sitting on (if you happen to have one) can trap the midbass in it. Why not move it away and try to listen and measure again without it?
Crossing front (helper) subs higher than 80Hz IMO adds more complexity for alignment with main fronts, and also prevents use of simplified multisub approach with only one stereo amp. Still it seems to be quite valid, not very effective energywise, still convenient enough solution for correcting midbass anomalies. I'd rather prefer adding large low Qts driver midbass dipole for its sharp transient response. Fill the dip with corresponding dipole peak (dip boundaries are variable with baffle width).
Regarding rear subs - I agree about unnecesary localization harm possibility when crossing high. Still I want to experiment with crossing lower (80Hz) and higher (120-160Hz) with applied delay to compensate for distance error compared to distance from listener to front subs. Perhaps it can make higher-crossed rear subs to "disappear" at some point and localize lower midbass at listeners position (f.e. inside his chest). Or just benefit for smearing modal peaks and dips when crossed lower.
And last thought about 6 subs in one room in stereo - if mains go as low as 30Hz and midbass suckout ir resolvable by EQing fronts or furniture change, it may be enough to have just four subs to have in total 6 bass-radiators evenly distributed along side walls for both evening modal bumps and keeping bass in stereo mode at the same time.
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