[John Casler]

I'm doing something similar, but with one sub at the front and one at the seating position.  I'll deal with my room mode at 28hz with the XO because the subs will have 28hz as the upper cutoff.  My primary emphasis is to keep output in room because I watch late night movies, so I'm pretty sure that changing things in the time domain will mess me up.  If I start equal but opposite phase waves at the same time, my net in room is 0, which should drastically reduce what tries to escape.  If I set the waves off  ...

WHoa   That is interesting stuff, but I'm not sure I understand what your aiming for.

I hear you saying that you think running two subs (out of phase) will somehow "reduce" what tries to escape????

I need more info on how you see this as working.  Is this loosely based on placing two speakers "face to face", and playing them out of phase to cancel one another?

[JohninCR]

John,

I'm just creating a dipole sub with an on axis distance differential between the front and rear waves of about 4 meters.  It won't be true dipole in room because one will be at the front wall and 2/3rds the way back, so it will probably be some kind of cardoid radiation pattern, but off axis and between the 2 units there will be a very substantial dipole-like null.

Because of the huge separation distance, my 6db/oct dipole bass rolloff won't start until down at about 15hz.  From my upper XO point of 28hz I will have a decreasing response due to the dipole influence (above 15hz the out of phase wave from the front box reinforces the primary wave up to 43hz where it peaks at +6db).  I'll probably just use a 6db/oct low pass filter starting at 6-10hz to offset the dipole sloped response which should flatten things out nicely.

Unfortunately, I do give up room gain and I do still have to worry about sound transmission through the front and rear walls, but at the other surfaces output will be significantly reduced.  With the door almost centered in the null, the output from the 2 subs will be almost zero at the door.  

The seating area will be directly behind the coffee table sub, so the highest output will be there and right at the front wall.  The rear wall is concrete, so I probably won't have to do anything there.  The front wall adjoins the house, so I plan to use 8-10" of multiple layers of panels and absorbant to prevent bass from going into the house and also eliminate much of the boundary gain resulting from the near wall placement.

The net result should be extreme output only in the seating area, so if I watch to watch Saving Private Ryan late at night I can listen at realistic levels and I'm the only one who feels the bombs going off.  In addition, room rattles and the possibility of structural damage from LF waves are greatly reduced.

I've been thinking about this for a while and the only compromise I've been able to come up with is that I can't use 2 drivers in the coffee table unit to cancel out their mechanical vibration because the enclosure would be too big.  I can still use 2 ports at the sides of the coffee table to widen the prime on axis listening area though.

[klh]

I'm excited to get the IB up and running. The hole has been cut in the ceiling, and the inner layer of the box is built. I've taken a break for a little while because it's been well over 100'F for a the last couple weeks and I'm not looking to forward to spending a number of hours up in the attic! That and I'm waiting for a few wires to come in the mail. The Ave 18"s are huge!

[youngho]

The graphs presented in that German paper still don't make sense to me. Look at the average SPL of sub-23Hz bass relative to supra-23Hz bass in graph 1, then again for graph 2. As we all know, modes represent standing waves, which essentially represents a phenomena of constructive and destructive interference related to wavelength and room boundaries. If measurements are taken in or near an antinode, then cancellation of the mode should result in lower SPL (i.e. the peak goes away). The average SPL of supra-23Hz bass in graph 2, then, should be lower than in graph 1, unless the entire system were driven with more power, in which case the average SPL of sub-23Hz bass in graph 2 should have been higher.

In addition, the location of the listening position would place the listener relatively close to a node for the second-order length axial mode, and the measured peak at this frequency in graph 1 would seem a little unusual.

Consequently, the graphs simply don't make logical sense to me. It would be nice to see these results validated. Are there any other links regarding DBA?

[ekovalsky]

The graphs presented in that German paper still don't make sense to me. Look at the average SPL of sub-23Hz bass relative to supra-23Hz bass in graph 1, then again for graph 2. As we all know, modes represent standing waves, which essentially represents a phenomena of constructive and destructive interference related to wavelength and room boundaries. If measurements are taken in or near an antinode, then cancellation of the mode should result in lower SPL (i.e. the peak goes away). The average SPL of supra ...

They way I interpret those graphs, in #1 there is extensive cancellation bass frequencies in the 28-56hz range secondary to boundary interference and maybe anti-modes.  This cannot be fixed with DSP room correction devices.  In #2, these cancellation phenomena are largely negated by the output of the rear subs.  Rather than the SPL being increased in this area by the output of the rear subs, it is just not being diminished by the reflected output of the front subs which is the case in #1.  

Based on the obvious 28hz and 56hz peaks seen in #1, this room is probably about 20' long (and maybe only 10' wide as the 56hz peak is quite strong).  In #2 the amplitude of both peaks is diminished, particularly at 56hz where it is dropped at least 12dB.  This suggests disruption of the resonances at the frequencies corresponding to the room dimensions, i.e. true modes.  Of course, this particular problem can be readily handled by DSP room correction or parametric EQ.

Below 23hz I don't think there is much information to be discerned from the graphs.  The subs used are clearly vented (look at the pics at the end of the .pdf I previously linked to) so below their cutoff ~ 24hz there is not much usable output.  What you see measured below 20hz probably reflects 'room gain' potentiating the rapidly dropping SPL from the vents.   I would like to see a similar comparison using subs with usable output down to 16hz.  Since there are no modes this low, and at these frequencies the subs just pressurize the room, there may well be some increase in SPL from the addition of the extra two subs.  In fact if you look closely at #1 and #2 there may be a slight boost occuring from 20-23hz with the DBA where there is probably still some usable output from the vents.  In any case, I would not expect to see the full 3dB gain that normally would occur by doubling the number of subs, since they are being run out of phase.

Here are a few other DBA links:

MAINS loudspeaker of Ahrensberg, Germany See the bottom sections of their subwoofer page.  Now this a speaker manufacturer's propaganda, so yes I realize maybe their goal here is to just sell more subwoofers.  As an aside, this looks to be a very nice speaker line... anyone from Germany here familiar with them ?  Florian ?

Info: What is Subwoofer (DBA) Array? The walls of the listening room and their reflections are responsible for the standing waves are formed between parallel walls.  Particulary when these resonant waves develop increased amplitude, the rendition quality and the subjective impression of the bass are substantially worsened and also depend very strongly on the listening position. These resonances develop with the frequency (and their second, third, etc. multiples), with which the wavelength is twice as large like the distance between two walls. In a 20ft room these disturbances arise to long area therefore with approximately 28 cycles per second and 56 cycles per second. With rising frequency (84 cycles per second, 112 cycles per second etc.), the interference levels become progressively smaller. Insufficient, imprecise low bass is therefore often not a result of the Subwoofers themselves but rather attributable to the interference from listening room. With bass absorbers and traps this cannot be resolved, as they would be much too large and would possess insufficient absorption.  

The solution for optimal low bass is the so-called DBA = double bass array. The DBA is composed of at least two but preferably four four, as identical as possible subwoofers. Two subwoofers are placed as usual in the proximity of the main loudspeaker/satellites or in the front corners. The other two woofer are positioned accordingly at the rear wall of the listening room. These two rear subwoofers are adjusted in such a way (with the inverted polarity and a propagation delay) that their signal practically "swallows" the radiated sound from the front subwoofers.  Thus much of the standing waves disappear, and there is a substantially more linear low bass response at the listening position. The acoustic impression is completely substantially improved. And also the volume distribution over the entire area is much more even. For the acoustic success of a DBA the correct combination of phase, propagation delay and the positioning of the subwoofers is crucial. Correctly implemented the low bass performance is outstanding, impossible to obtain without DBA ... we measure DBA configurations locally, off quadruple configurations.  This service is included in the price when desired!  Are you are looking for outstanding low bass characteristics?  We will gladly give you further information!

Klein-Hummel paper.  See particularly pages 15-16.  

... what still remain are the natural frequencies resonanting in the longitudinal dimension, which usually have the strongest and most harmful influence on bass rendition. As a counter measure, generous acoustic absorbers can be used and/or and a second identical bass array (DBA = double bass array) can be added at the back of the room.  These extra subwoofers are driven with a delay corresponding to the travel time in the longitudinal dimension and with reversed polarity. The rear array thereby works as a kind active acoustic sump and eliminates the influence of the natural resonances in the longitudinal dimension....

Here's something else I read on a German audio forum, which may interest a lot of apartment dwelling audiophiles, or homeowning audiophiles like me who listen late at night and have families that don't like to be woken up!  JohninCR this is for you!  Until I try it I won't know if it is true, but it seems plausible.

Ein Double Bass Array hat auch wunderbare Eigenschaften, den Bass für die Welt ausserhalb des Hörraums kaum/nicht hörbar zu machen.

In other words, the double bass array also has the marvelous characteristic to make the bass less audible outside the listening room.  

[John Casler]

 ... what still remain are the natural frequencies resonanting in the longitudinal dimension, which usually have the strongest and most harmful influence on bass rendition. As a counter measure, generous acoustic absorbers can be used and/or and a second identical bass array (DBA = double bass array) can be added at the back of the room. These extra subwoofers are driven with a delay corresponding to the travel time in the longitudinal dimension and with reversed polarity. The rear array thereby works as a kind active acoustic sump and eliminates the influence of the natural resonances in the longitudinal dimension...

So it would seem that the three basic variants are:

Quad Sub System #1 = 4 subs activated to have the front pair have dominance and the rear pair activated to in some way "actively trap" or attempt to "neutralize" the energy of the front pair.

(I have my doubts that a "pair" of rear subs can offer much neutralization, except to attempt to set up a "mirror image" like I suggest in #2, which is an entirely different approach)

Quad Sub System (my Push/Pull system) #2 = 4 Subs activated to act in mirror imaged synergy to create the greatest air movement in the center of the room, and possibly the mirror image will set up the exact opposite mode/node configuration to reduce the depth or height of response problems.

Quad Sub System #3 = 4 Subs activated to act as a "modified" dipolar sub system

This is an interesting area and many know I have been experimenting with it for quite some time (about 7 years).  Ever since I saw Brian's suggestion for VMPS HT, with the "push/pull".

Over the last two years, I have moved more seriously into a quad push/pull sub configuration and even only yesterday, made another improvement that seemed to acheive another level of Bass Detail (will report in the VMPS circle)

Bass is complicated!!!  There are so many variables to deal with that it makes it impossible to assess via a computer equation.

It is also very hard to get accurate "in room" measurments that are meaningful.  That is, test tone bass cannot duplicate the dynamic variations of "musical" bass, since the attack and decay times of real music are not "static".  They are transient dynamics of various frequencies, attack and decay rates.  Specific frequencies have harmonic relationships with other frequencies making the measurment, adjustment exercise a good starting point.

I would think that "ultimately" a wise DSP group will figure a method of measuring bass (and other) information based on reading what is being played in the "digital stream", and then measuring what is produced in the room.  I'm talking about music, not test tones of various types.

I wouldn't think that too difficult to measure the musical "bits" and have a corresponding "in room" correlation system that would "correct" for real music, in real time.

I think then, we will have a nice "doorway" to step through.

[JohninCR]

This topic is extremely interesting and now it has me thinking that I may need a 3rd sub at the back of the room to cancel at least a substantial portion of the on axis energy.  Have only it on the time delay, but instead of a boxed speaker, make it a true dipole with an absorber centered close behind it to soak up it's on axis rear wave energy.  I still would have zero room pressurization.  Off axis of the 3rd sub would be 0.  Rear wave on axis is dissipated and the front wave on axis is cancelled.  It might work.

The alternative would be to build a false wall in front of my rear concrete wall with 3 sections tuned to resonant frequencies of 20, 50 & 80hz.  Put absorbant in the airspace.

Which do you think would be more effective ?

[ekovalsky]

John,

In your Push/Pull setup where exactly do you have the four subs positioned ?  Are they all operating in the same polarity or do some have inverted polarity ?

And where would you position the four subs in the "modified" dipole arrangement ?  Would this be two stacks of two, with the subs of each stack facing the opposite direction and with inverted polarity ?  


I agree this is an interesting topic.  Unfortunately until I build out my room I don't have space for rear subs and won't be able to try these arrangements out myself.  

Measuring a music signal is not going to be easy.  And obviously a Radio Shack or similar SPL meter is worthless because of frequency averaging -- it will measure flat response if there +12 peaks and -12dB nulls at nearby frequencies.  For the forseeable future I see no way around test tones and FFT.

[John Casler]

Here is a Q&D (quick and dirty) drawing of my set up, which is not to scale.

I haven't taken it to the exact "mirror image" stage yet, for practical reasons (I wanted to keep the front subs in the same plane as the RM30s)

However, the front and rear subs are "exactly" (as of yesterday) equidistant from the listeners ear.

The improvement was a gradient increase in "detail and texture" to the bass.  This was by no means missing before, but it did offer a noticable improvement.

I would suspect even more, if I was able to produce an "exact" mirror image of placement and postition of all 4 subs with respect to room boundaries, and the listening position.

Eric, the rear subs are 180 degrees out of phase with the front with a simple reversal of the speaker cable.

The Dipole arrangement, was not mine, but the one described by JohninCR.

[ekovalsky]

Here is a Q&D (quick and dirty) drawing of my set up, which is not to scale.

I haven't taken it to the exact "mirror image" stage yet, for practical reasons (I wanted to keep the front subs in the same plane as the RM30s)

However, the front and rear subs are "exactly" (as of yesterday) equidistant from the listeners ear.

The improvement was a gradient increase in "detail and texture" to the bass.  This was by no means missing before, but it did offer a noticable improvement.

I would suspect even ...

Thanks for the diagram, a picture is worth 1,000 words!

It seems the main effect of your current setup is cancellation of the primary waves (coming from the fronts of the subs towards the listening position).  This will occur most efficiently at the equistant line between the front and rear subs, i.e. your listening position.  It should be effective in keeping the sub primary wave from hitting the walls and bouncing back into the room.  Most of the bass you are hearing at the listening position is likely from secondary reflected waves off the walls, floor, and ceiling which will sum in an unpredictable pattern -- this occurs because the frequencies produced by the subwoofers are omnidirectional no matter which way the drivers are oriented.  I would expect moving the listening position forward or backward from your current location would significantly change bass quality/quantity, and of course separate the temporal arrivals from the front and rear subs.

Adding progressive delay to the rear subs would shift the line of maximum cancellation behind the listening position towards the rear wall.  If pushed all the way to the rear wall this should prevent reflections off the rear wall from returning into the room to create interference peaks and nulls.  It should also result in consistent, temporally correct bass throughout the room, as what will heard by the listener is mainly the undisturbed primary wave arriving from the front subwoofers.  

It is interesting that by adding or not adding delay to the rear subwoofers may completely change what is heard at the sweet spot -- secondary reflections (no delay) or primary wave (with delay) .  I wish I had four subs now!  John since you already have the four subs, if you ever see a good deal on a device which could add delay to your rears, maybe a pro item, it would be fun to play around with it.  Certainly I'd be interested in knowing how these two setups compare with music signal.

I'm not sure how the pseudo-dipole arrangement will work out.  Hopefully JohninCR will keep us informed of his progress.  In the future when I have four subs and a room large enough to implement a DBA I will write a detailed paper including measurements and subjective impressions.

[John Casler]

Thanks for the diagram, a picture is worth 1,000 words!

It seems the main effect of your current setup is cancellation of the primary waves (coming from the fronts of the subs towards the listening position).  This will occur most efficiently at the equistant line between the front and rear subs, i.e. your listening position.  It should be effective in keeping the sub primary wave from hitting the walls and bouncing back into the room.  .

Hi Eric,

I think you missed the fact that the subs are 180 degrees out of phase, so it is just the opposite.

If the subs were "in phase" the waves would "oppose" at the equidistant position and nullify each other.  When they are run "out of phase" they offer flat, clean response.

And I might mention that the "in phase" situation would not keep the primary waves from hitting the wall since when the waves collide, they don't cancel each other totally, but only at the point of collision (node).  They then still continue on in opposite directions to other room boundaries.

Most of the bass you are hearing at the listening position is likely from secondary reflected waves off the walls, floor, and ceiling which will sum in an unpredictable pattern -- this occurs because the frequencies produced by the subwoofers are omnidirectional no matter which way the drivers are oriented. I would expect moving the listening position forward or backward from your current location would significantly change bass quality/quantity, and of course separate the temporal arrivals from the front and rear subs.

Again, since you missed the "out of phase" part, it is just the opposite.  If the Subs are run Out of phase and placed in and exact mirror image position they provide one of the "smoothest"room responses since the damage done by one is (to some degree) mirrored by the other, which undoes some of the damage (in a perfect world).

While this is totally impossible except under extreme (again perfect world circumstances) circumstances, it would work that way.

In my system, the response in the center line of the room forward and backward (to the ear) is very smooth until you get to the very front almost between the speakers, where it "rises" quite a bit, due to the fact that I have the RM30s running full range with woofers pointed in.

I ran through my "reference disc" about an hour ago and just the little changes to the rear Subs, moving then to be closer to the "equidistant" position, has uncovered bass detail and texture to an even greater degree.

Adding progressive delay to the rear subs would shift the line of maximum cancellation behind the listening position towards the rear wall. If pushed all the way to the rear wall this should prevent reflections off the rear wall from returning into the room to create interference peaks and nulls. It should also result in consistent, temporally correct bass throughout the room, as what will heard by the listener is mainly the undisturbed primary wave arriving from the front subwoofers.

I fail to see how this would happen, sound energy can either oppose, or sum with other sound energy, but it cannot "neutralize" or dissipate the energy unless it turns it to heat or light.  Other sub energies will either "oppose" (dip) or "sum" with (peak) direct energies, but will not cease being active energy, until they are are absorbed, dissipated, or leave the room.

As an illustration just drop a ball in each end of a swimming pool and watch how the ripples spead out to meet each other, collide and then carry throught the collision to continue on till they hit the walls of the pool and then bounce back into the soup of waves and ripples.  

The point is that the opposing waves didn't  cancel each other totally.  They continued on their way going through each other.  They only cancelled each other, at the point where they collided.  That is the only place they have an effect.

I have a feeling that the German article is either leaving something out or "leading" us along.  I have been thinking about the active trap idea and don't think it can work, unless they do the mirror image sound energy release like I suggest in my system.

Ethan might have some ideas, but the only way that any of the bass energy released by the speakers is "stopped" is if it leaves the room by escaping, or is trapped (absorbed and turned to heat)

It is interesting that by adding or not adding delay to the rear subwoofers may completely change what is heard at the sweet spot -- secondary reflections (no delay) or primary wave (with delay) . I wish I had four subs now! John since you already have the four subs, if you ever see a good deal on a device which could add delay to your rears, maybe a pro item, it would be fun to play around with it. Certainly I'd be interested in knowing how these two setups compare with music signal.

As I see it the "only reason" to add delay is if you don't sit equidistant from the rear subs.

I, early on, had 2 crossovers, each with "adjustable phase", the NHT-2 and the Dahlquist DLP-1.

This allowed me to essentially add delay by phase controlling each pair (front or rear)

To be honest, there were just too many controls and distances to try and compute.  I played with it for several weeks with small increments, but you didn't see me posting about any big differences.  I do know that it isn't as good "bass wise", as what I have now.

Now that said, if we have the Behringer (Oh yeah I looked into it) that allows simple adjustment, (I think it was less than $100 used and not much more new)) just what adjustment would we use?

I think in my set up, that the rear subs would only be delayed, if they were placed closer than equal distance from the listener, but the signal would be "advanced" if they were then place further away.

The whole game will change depending on your goals.  Mine are different thant the German example you gave.

It seems the German (concept #1) system is to try and neutralize or as you said "sump" the bass from the frontal sub array as it reaches the rear wall.

My system (concept #2) is that you fire two pairs of subs with the rears out of phase, which causes them to "help" each other move the air in the middle of the room, and then since they are all equal they have the mirror image of room interaction in all the same places except "JUST THE OPPOSITE"  which makes overall room response smoother and more accurate.

I have done a lot of serious (amatuer) thinking on the subject as well as much experimentation.  From what I know right now, (but that could change) the German system won't do what has been stated, unless there is information left out.

Simply running the rear subs delayed and out of phase, won't "soak up" the bad bass energy/reflections.  If it does, I'll be the first in line (well maybe amoung the first 1000 or so)

And I can only say that the bass I heard 1 hour ago was beyond the quality of any "musical/two channel" bass I have ever heard anywhere anytime.  And I am a happy camper.

So it would have to be better than that.

I may have an AudioCircle member visiting me in about two weeks.  We'll see what he thinks

[ekovalsky]

John,

I am confused on the polarity/phase issue. Maybe you can educate me...  

If you have two subs at the same distance from a particularly point in space (such as the listening position) and the output of one sub is inverted in phase relative to the other, why would their outputs not cancel at that point ?  It shouldn't matter whether the two subs are right next to each other or on opposite ends of the room.  You seem to imply that in your current arrangement the outputs of the front and rear subs sum, rather than cancel, at the equidistant listening position.  This is what I would expect with the front and rear subs wired with the same polarity, not with inverted polarity.

For instance, if you sit 9' from the front sub (~125hz wavelength) a waveform of amplitude x will reach the listening position at time=9 msec for any given frequency.  If the rear sub is not delayed, and is also 9' away even if in a different direction, a waveform of intensity -x will arrive at the listening position at time=9msec at the same frequency.  Doesn't the vector sum of x and -x yield a combined amplitude of zero ?

Obviously if the distances are not equal between the front and rear subs, cancellation or summation will occur at varying degrees at different frequencies, depending on location.

I'm not trying to be confrontational in the least, and I'm sure your system sound fantastic.  Rather I find real world room bass acoustics fascinating and challenging, and am trying to learn as much about it as possible.

Ethan, Big B., other experts... feel free to jump in!

[jgubman]

I have 2 subs (both VMPS Largers) and tested John's "push/pull" theory using the room correction software on my Meridian (provides a graphical interface w/ frequency sweep, waterfall and impulse response graphs).

I have on sub on the front wall, about 3 feet in from the corner, and the other sub on the rear wall, about 3 feet in from the other corner:

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W/ both of the subs wired in phase, average SPL at the chair was higher (~5 dbs), but the peaks and nulls were much more exaggerated.

I tested a lot w/ music and movies and ended up sticking w/ the front in-phase w/ the main speakers and the rears 180 degrees out of phase (swapped the wires, like John).

Anyway, that's my exerience in my room.

[ekovalsky]

I have 2 subs (both VMPS Largers) and tested John's "push/pull" theory using the room correction software on my Meridian (provides a graphical interface w/ frequency sweep, waterfall and impulse response graphs).

I have on sub on the front wall, about 3 feet in from the corner, and the other sub on the rear wall, about 3 feet in from the other corner:

Thanks for posting your this. By sitting in a cancellation zone, bass reinforcement at problematic frequencies will definitely be diminished at the expense of SPL.  Losing 5dB of SPL equates to loss of about 71% of acoustic power -- probably reflecting cancellation of the primary waves from the subs.  

I suspect much of the audible/measurable output at the listening position with the subs firing simultaneously with inverted phase is reflected off the walls, floor and ceiling.  This isn't necessarily a bad thing -- because of the multiple paths between the subs and the various reflecting surfaces the peaks and nulls will be much more spread out which is desireable and will lead to a flatter response.

Used wtih room correction, which can easily take out peaks, the better arrangement will probably whichever offers higher absolute (not relative) SPL at the deepest dip.  In other words, consider mean SPL running subs in-phase of 90dB with peaks of +12 and dips of -6dB.  Running subs out of phase results in mean SPL of 85dB with peaks of +8dB and -4dB.   The in phase arrangement still has better minimum SPL of 84dB versus 81dB.  Of course without room correction to take out the peaks, the better measurement and sound will be obtained with the subs running out of phase.

[John Casler]

John,

I am confused on the polarity/phase issue. Maybe you can educate me...  

 ...

    Well I'm not a teacher but I have taken a few journeys I can share.

If you have two subs at the same distance from a particularly point in space (such as the listening position) and the output of one sub is inverted in phase relative to the other, why would their outputs not cancel at that point ? It shouldn't matter whether the two subs are right next to each other or on opposite ends of the room. You seem to imply that in your current arrangement the outputs of the front and rear subs sum, rather than cancel, at the equidistant listening position. This is what I would expect with the front and rear subs wired with the same polarity, not with inverted polarity.

Yes that is what I am saying.  Look at it like this.  What you really want is the front direct wave to reach you with the best signal possible.

If the subs are equal distances from the listener, and "IN PHASE" they are both moving toward the listener in unison.  That is as the front subs woofer cone moves towards the listener, the rear subs woffer cone moves toward the listener.

This means the wave enregy is "launched" toward each outher and will arrive at the listener at the same time.  The collide at the listener and their sonic force being equal will effectively "neutralize" the other.

If the subs are wired "out of phase", then as the front woofer moves toward the listener, the rear woofer moves away from the listener and the summed effect is that the it creates the greatest movement of air, because the woofers work in unison, to move the air in the center of the room between the two Subs.

Obviously if the distances are not equal between the front and rear subs, cancellation or summation will occur at varying degrees at different frequencies, depending on location.

That is true and in some rooms strangely enough the opposite might prove to be better.

But if you can set up a system that has the same symetry I have where you are either equidistant from both woofs, you will see what I mean.

You can easily see it if you just take a "single" woofer and track its energy.

First it is launched at the listener.

Then it passes the listener and strikes the rear wall.

It bounces back toward the listener again and if it encounters specific frequencies as it passes the listener what happens?

It creates a "dip" in response.

It continues on to the front wall and again bounces of it.

It then begins traveling toward the listener again, but as it passes the speaker, if it coincides again with specific r=frequencies being produced at that very moment, it "sums" with those frequencies and when they reach the listener the SPL is higher and we have a "peak".

Air is a compressable fluid, which allows for the creation of sound waves at various frequencies.  I know it is rather complex to ponder, but reducing pressure in one direction is the same as providing pressure in the opposing direction.

I'm not trying to be confrontational in the least, and I'm sure your system sound fantastic. Rather I find real world room bass acoustics fascinating and challenging, and am trying to learn as much about it as possible.

Not taking your comments as confrontational and I wish you could hear the system.

And the key phrase is "real world" bass. I have an inate sense of what is happening physically to the sound in my room, and find that this type of system works well.

When you have your new room set up and get another pair of subs, it won't take too much trouble to try my set up, since it only requires a measuring tape and reversing the speaker wire.  If you're an equal distance between the subs, with the rear "out of phase", I think you'll get the same sonic picture.

There is always the chance that room dimensions will creep in and screw things up if you don't have perfect symetry, but is hasn't on mine.

But in case I didn't make the point totally that doing a "push pull" mirror image in the room will cause "ALL" basic room nodes, modes or what have you to be the exact opposite forces applied to them when you use mirror image, push/pull.  

This means the smoothest response over the greatest area (not that, that is a goal of mine, since all I care about is what happens "in the sweet seat") should be when using this system.

It will be fun to see if you get the same results

[JohninCR]

John & JGubman,

Now you guys have me confused.  Are you both saying that you have subs that are equi-distant from your listening position firing out of phase, at the same time or with the rear subs on a time delay?

If they are firing at the same time and out of phase, I don't understand why you aren't getting a dipole null effect at the seating position.  What is the sound like at the perimeters of the room?

[ekovalsky]

John & JGubman,

Now you guys have me confused.  Are you both saying that you have subs that are equi-distant from your listening position firing out of phase, at the same time or with the rear subs on a time delay?

If they are firing at the same time and out of phase, I don't understand why you aren't getting a dipole null effect at the seating position.  What is the sound like at the perimeters of the room?

That is my thinking too.  I've always thought that

1.  Equal distance + equal phase = summation of response = increased amplitude

2.  Equal distance + opposite phase = cancellation of response = decreased amplitude

This certainly is the case with two front subwoofers only.  I obtained this graph by measuring my two front subs, one at a time, from the listening position, one with positive and one with inverted polarity (by switching speaker wires on the in phase and one out of phase (speaker wires swapped on right channel only).  Listening with this setup with Ethan's Realtraps test tones and with music, it was very obvious that there was a marked reduction in SPL at the listening position compared with both subs wired with the same polarity, or even just one sub by itself -- confirming that cancellation was indeed occuring.  This is analogous to what happens at the sides of dipole bass radiators.  

John seems to be suggesting that the opposite is occuring in his setup

If the subs are equal distances from the listener, and "IN PHASE" they are both moving toward the listener in unison. That is as the front subs woofer cone moves towards the listener, the rear subs woffer cone moves toward the listener.

This means the wave enregy is "launched" toward each outher and will arrive at the listener at the same time. The collide at the listener and their sonic force being equal will effectively "neutralize" the other.

If the subs are wired "out of phase", then as the front woofer moves toward the listener, the rear woofer moves away from the listener and the summed effect is that the it creates the greatest movement of air, because the woofers work in unison, to move the air in the center of the room between the two Subs.

Oh well, my wife just went to sleep some time to go listen     Anyway my head was about to explode from all this acoustics stuff !  

[JohninCR]

He must be using the time delay.  I'll just assume so for now.  That way I can get some sleep, otherwise I'll be up all night trying to figure it out.

[jgubman]

Mine are both at the same time, one in-phase the other reverse wired. I'm not exactly equidistant, I probably sit closer to the rear subwoofer.

W/ both of the subs wired the same, the average SPL is higher, but the dips  and peaks are more exaggerated. I get a smoother response (at least according to the Meridian's graphs) w/ the subs wired out of phase (and the output level of the sub raised).

[John Casler]

John & JGubman,

Now you guys have me confused.  Are you both saying that you have subs that are equi-distant from your listening position firing out of phase, at the same time or with the rear subs on a time delay?

If they are firing at the same time and out of phase, I don't understand why you aren't getting a dipole null effect at the seating position.  What is the sound like at the perimeters of the room?

       Great stuff here.

Hi John,

Yes I am saying my subs are run out of phase in the rear, but because they are eqidistant from my listening position I don't need any delay.

I think the dipolar null you are speakeing of is more the radiation pattern related to the front and rear to the sides, not what happens between the drivers.

In fact a dipole can be a single driver.  

Imagine this.  You are actually inside a "huge" dipole sub sitting in between the woofers.  One in front of you, and the other behind you.

As the front woofer pushes, the rear woofer pulls.

That wouldn't nullify the bass, it would make it more efficient.

But if, still sitting in the Sub, you reversed the polarity of the rear sub and they fired in unison.  The wave from each would meet at your ears and cancel each other.

I think many think (mistakenly) that as two waves (in phase) collide that the pressure goes up, but that (I think) is not the case.  They simply cancel each others force as they pass through each other.

And that is another point.  They don't cancel the energy all together, but only at the point of collision.

I do have to warn you....I could be wrong, since I am not an acoustician, but I seem to have the results to prove it.

If I am sitting in a "null" and am only hearing reflected bass, that would be an incredible turn of events, especially with the quality of bass I am hearing.