Mutual Coupling Stereo Bass

[Cypherdelic]

Hi,

Im a little confused about coupling a subwoofer inbetween my stereo-front-standspeakers and how that affects SBIR and roommodes.

Gunter Nubert said, that two front subwoofers or two front standspeakers create a wavefront in which the waves join together and expand in the coupled direction (roomlength) only which leaves us basically left with only the frontwall and backwall to fix roommodes and SBIR with absorbers or using a DBA.

I dig deeper into that and found this explanation from QSC:

What does Subwoofers Mutual Coupling Mean - Blog | Resources

You will have noticed that in a large number of live events, sound engineers place subwoofers close together either on the floor, on the side of the stage, or at the top of the line-array rig. Are there any benefits in doing so, and what exactly happens? Before exploring the answer, let’s first...

blogs.qsc.com

Let’s now look at two different situations. First, if we setup two transducers next to each other at a distance of more than the half wavelength of the reproduced frequency, the average on- and off-axis measured output adds up to 3 dB (double the power). Now, with two transducers close enough to benefit from the mutual coupling effect, the measured output adds up to 6 dB.

So it happens wavelength dependant.

Lets say my L/R-front-speakers have a distance of 3m.
For lower than half the wavelength 2*3m = 6m = 57hz.

So at this distance they couple only lower than 57hz.

My idea is now to place a subwoofer inbetween with a distance of around 1,5m to each standspeaker.

This should increase the mutual coupling into a wavefront of the front-bass to frequencies up 114hz, so making them louder and also killing a lot of roommodes.

Also problems with SBIR reflections reduce since each speaker is still at the same loudness, but the total loudness is +6db for coupling two. I dont know exactly what it means coupling three.

Lets think about it. +3db for the energy of the sub (doubling power), and then another +3db twice (per coupling of three) or triple? Idk thats +9 or +12db on the aligned wavefront, while the SBIR of one single of them to the next wall is just +0db. So with slightly different distances to frontwall and sidewall of the L/R and to frontwall with the S and a little bit of absorbers, even if they not thick enough and only absorb 30%, the SBIR is so low on single frequencies compared to the loudness of the wavefront that the nulling or peaking reduces to minimum.

Thats a pretty nice thing and theres no concert or festival not using the same kind of technology to couple PA-subwoofers.

I didn't knew that we can use this technology at home to reduce audio problems with the room.

In fact every stereo-speaker with bass-woofers feature that out of the box and usually you should place them in the stereotriangle, not 3m distance like me stupor, of course. Wirt 1m to 1,50m distance from speaker to speaker and from speaker to listening position you have a really high frequency coupling of the woofers.

Its interesting that you can "bridge" the gap on higher distance with subwoofer placed inbetween.

Thats awesome.

 

[Curvature]

Hi,

Im a little confused about coupling a subwoofer inbetween my stereo-front-standspeakers and how that affects SBIR and roommodes.

Gunter Nubert said, that two front subwoofers or two front standspeakers create a wavefront in which the waves join together and expand in the coupled direction (roomlength) only which leaves us basically left with only the frontwall and backwall to fix roommodes and SBIR with absorbers or using a DBA.

I dig deeper into that and found this explanation from QSC:

What does Subwoofers Mutual Coupling Mean - Blog | Resources

You will have noticed that in a large number of live events, sound engineers place subwoofers close together either on the floor, on the side of the stage, or at the top of the line-array rig. Are there any benefits in doing so, and what exactly happens? Before exploring the answer, let’s first...

blogs.qsc.com

View attachment 467736


So it happens wavelength dependant.

Lets say my L/R-front-speakers have a distance of 3m.
For lower than half the wavelength 2*3m = 6m = 57hz.

So at this distance they couple only lower than 57hz.

My idea is now to place a subwoofer inbetween with a distance of around 1,5m to each standspeaker.

This should increase the mutual coupling into a wavefront of the front-bass to frequencies up 114hz, so making them louder and also killing a lot of roommodes.

Also problems with SBIR reflections reduce since each speaker is still at the same loudness, but the total loudness is +6db for coupling two. I dont know exactly what it means coupling three.

Lets think about it. +3db for the energy of each of three drivers, thats +9db and then another +3db twice (per coupling) that +15db on the aligned wavefront, while the SBIR of one single of them to the next wall is just +0db. So with a little bit of absorbers, even if they not thick enough and only absorb 30%, the SBIR is so low compared to the loudness of the wavefront that the nulling or peaking reduces to minimum.

Thats a pretty nice thing and theres no concert or festival not using the same kind of technology to couple PA-subwoofers.

I didn't knew that we can use this technology at home to reduce audio problems with the room.

In fact every stereo-speaker with bass-woofers feature that out of the box and usually you should place them in the stereotriangle, not 3m distance like me stupor, of course. Wirt 1m to 1,50m distance from speaker to speaker and from speaker to listening position you have a really high frequency coupling of the woofers.

Its interesting that you can "bridge" the gap on higher distance with subwoofer placed inbetween.

Thats awesome.

This kind of analysis is relevant at long distances in stadiums or acoustically large rooms.

At home, small room acoustics applies. I recommend Toole's book.

 

[staticV3]

PA techniques rarely work in domestic environments.

Subwoofers will radiate in every direction regardless of placement, so walls all-around will wreak havoc on any attempted beamforming that low in frequency, short of DBA that is.

Always best to double-check your theories against Room EQ Wizard's Room Sim tool, you'll likely find the optimal domestic placement doesn't resemble any PA technique at all.

 

[Cypherdelic]

It may look like that the loudness gain from mutal coupling +3db ( not the +3db from the double energy ) cames out of nowhere without increasing the total energy.
This is because the waves join an travel further into each other.

Imagine two watertaps next to each other dripping at the same frequency into water. The drops create waves when they hit the watersurface and expand 360°, like subwoofers.

Now when those waves hit each other inbetween the distance of the watertaps they dont create an interference pattern but they join into each other increasing the amplitudes of the coupled wave.

Always best to double-check your theories against Room EQ Wizard's Room Sim tool, you'll likely find the optimal domestic placement doesn't resemble any PA technique at all.

Its not about the optimal placements, its about the mutual coupling effect helps to tackle room problems.
The reference to PA was just an example.

 

[Curvature]

It may look like that the loudness gain from mutal coupling +3db ( not the +3db from the double energy ) cames out of nowhere without increasing the total energy.
This is because the waves join an travel further into each other.

Imagine two watertaps next to each other dripping at the same frequency into water. The drops create waves when they hit the watersurface and expand 360°, like subwoofers.

Now when those waves hit each other inbetween the distance of the watertaps they dont create an interference pattern but they join into each other increasing the amplitudes of the coupled wave.



Its not about the optimal placements, its about the mutual coupling effect helps to tackle room problems.
The reference to PA was just an example.

Read through these: https://www.audiosciencereview.com/forum/index.php?threads/analytical-analysis-room-gain.23211/

https://mehlau.net/audio/multisub_geddes/

 

[Cypherdelic]

Sry my calculations were a little wrong.
I do it again.

Each frontspeaker in BiAmp can draw up to 100w (200w) on the bassdrivers and the amp can deliver that too.
The subwoofer has the same energy than both of them alone: 200w... So thats +3db for the energy doubling.

Hope thats correct

If you place the frontspeakers directly to each other (joining the baffle) you get an additional +3db from coupling up to the upper bass range.
Also because of thedoubled baffle width you would get higher frequencies directed to the front (2pi instead of 4pi 360°) but thats just sidenote.

If you place them 1,71m apart this additional +3db only add up to frequencies ≤100hz.
If you place them 3,42m apart only ≤50hz.

Depending on the recommended placement of the frontspeaker manufacturer (i.e. 1m to 1,2m) this is already taking in account for by design.
So coupling is usually nothing to think about if you stay within the placement recommendation... Every stereo-frontspeaker with woofers feature that by design.

I dont apply something new or from different field like. Chances are high, that if you listen in stereo with your frontspeakers woofers at home that you already use this effect too. Its nothing new or special.

The thing is... When i placethem far away, like 3m ... I lose that +3db from the default coupling within 1-1,2m distance for the frequencies from 57,3hz (3m) to 172hz (1m manufacturer recommendation).

By introducing a subwoofer in the middle of them at around 1,5m distance to both of them i just reenable that coupling again up to frequencies ≤114,6hz.

In fact i gain only +3db from the energy increase of the subwoofers, because the +3db gain from coupling is ALREADY taking into accounted for in a normal placement within the recommendation. I just lost -3db placing them so far apart, which i regained.

So only +3db... But still its now behaving like a wavefront up to 114,6hz.

I have 30cm Sonorock at the backwall with 10cm air gap (70% of the hole backwall plus the open door at the backwall) and also 30cm Sonorock (nogap) behind the frontspeakers and some 20cm Basotect or 30cm Sonorock on the sidewall of the speaker and a tower from floor to ceiling in the frontcorners with absorbers too... So i got the SBIR pretty good under control. Also roommodes are not a big thing thanks to the absorbers taking a lot of energy out of bouncing waves behind the front and backwall, which are the most important ones because thats the direction all frontspeakers output alltogether (ideally in wavefront) and usually because of beeing close to the frontwall (ie 70-80cm) you get another +3db from that for lower frequencies, as you know thats why the recommendation goes to place a sub straigt as close to the frontwall as possible (louder and the SBiR nulling is out of output range, ie 140hz)

So when i place the sub in the middle of the front and move it as close to the frontwall as possible i get another additional +3db for the frontwall reflections of the sub for the hole coupled wavefront.

Important note: the distance or delay has to be adjusted to virtually be on a "line" with the speakers in timing.
This is neccessary for the coupling to happen.
When the sub is at the frontwall (ie 50cm) and the frontspeakers distance is 80cm to the frontwall,
then you do NOT set it up like usual at that distance so it would reach the listening position the same moment.
Instead you set the distance virtually to bring it 30cm closer than the real distance.... Or around 1ms earlier.
Only this way the coupling into a horizontal wavefront works out.

Also bringing the standspeakers woofers and the subwoofers drivers at the same vertical position (height) is helping the effect.

Now that the waves from the frontsystem form a mutually coupled wavefront through the length of the room were those absorber do their work at the front,back and corners, this is almost as good as a DBA, so standing waves quickly run out of energy within a few bounces. (Low RTA)

 

[NTK]

I am afraid the information from QSC is incorrect. When you add the sound waves from 2 incoherent/uncorrelated sources (the sound radiating from each source has no relationship with the other, i.e. basically 2 different random sources) of equal strength, statistically you will end up with a 3 dB increase in sound pressure level when in an anechoic/free field environment.

If the sources are coherent, which is the case here as both subs are playing the same signal, the combined sound pressure level will depend on the phase relationship between the 2 sounds. When they are perfectly in phase at the listening position, i.e. equi-distance from both subs and the sound radiating from the subs are perfectly time aligned, you'll get a 6 dB increase. When they are perfectly out-of-phase, you'll get total cancellation at the listening position.

Two coherent sources will give you lobing. Below is from a simulation of the sound radiation pattern from 2 sources in free field from 20 to 160 Hz (same strength, phase aligned, 3 m distance between sources). If you want to play with your own simulation, you can try the "Ripple Tank (2-D Waves) Applet" in Paul Falstad's site.

 

[Cypherdelic]

Thats right they must be in phase.

I thought about this too and that a rear sub can hardly be alligned with the fronts-wavefront.
So i gave up rear sub behind the couch...

I just stacked it with the frontsub
So i have my left standspeaker with active woofers, and my right standspeaker with active woofers.
And inbetween i have the (only LFE input) Teufel T10 (33hz -3db) now for LFE connected with Subout1 plus activated "double bass".
And the the Mivoc Hype 10 G2 is below it forming a stack with it, and that one is connected Stereo with a PreOut(L/R) on the AVR (just without LFE)
I dont need my external Basscontrol this way, because the Mivoc has crossover (lpf), I set it minimum 40hz) to not be too boomy.
Fronts set to full.

I used the room calibration to verify the Teufel-LFE is correctly measured in distance correctly and changed it accordingly be aligned (to form the wavefront).
Now because i dont have PC with REW to setup all the distances (time reference impulse return) according to meet the undelayed PreOut i tried something audible to align the Mivox with it.

Goa... And my fingertips... goa features very high BPM basslines... So i simply played high volume and then i felt with my fingertips on the bassdrivers if they are pushing out their driver same time.

Im not a perfect machine but i got it propperly time aligned the subs in the stack and the substack with the frontspeakers into a wavefront, now imhave a center sub-tower-speaker with two xones of the same size, one tube port, one flat... It was not good until I reduced all distances of the speakers to the lowest possible while keeping the same relative distance, so the LFE-Bass and the fronts, at their relative distance, with the surrounds also at their same relative distance, gets closer to the undelayed PreOut with the Mivoc on its passive Crossover (which should add around 1,5ms)... So its pretty close now.

Now i have not just one subwoofer adding out with 200W, but another one with 100W... More cone area.

Since im now again 3m far away from the LFE my cinebass is little weak... accueq want to give it +7db while i had it maxed gain already

But the musicbass is really great now.... Ultra dry and pretty precice... Not beeing to boomy, just a little low vibrant on the couch.
I need a bassshaker....

Can you show me such a screen for three sources?
That nearly fits my room (4,25m x 4,65m)... Oh no sry its 8x8... Im blind, the 4s on the yaxis were confusing me

 

[Cypherdelic]

Btw, the subs in your example are at 3m distance... Sure they cant couple into a wavefront for 160hz...
1,5m allows me up to 114hz mutally coupling.... What you think? Have you read what they have postulate.

Please show me THAT distribution what happens below half-width in distance.
3m * 2 = 6m... Only waves below wavelength-width of 6m mutaly couple with 3m distance. Thats the point.
What does it look like THEN... Show me that

Thats what we talk about... I know that you have lobing at higher frequency, i dont denie that.

Can you prove they are incorrect BELOW half-width distance?

 

[Cypherdelic]

You see the interference pattern between the subs which are expaning to the right?
Below the distance of half the wavelength, there is no interference as they align in phase and become one entity.

 

[Cypherdelic]

Thx. Thats, great for the lower frequencies, you see they join, so the lower the distance the higher the mutual coupling.
But of course 160hz for me is outside the outputrange of the middle substack tower, so the lobing applies from the 3m afar distanced frontspeakers.
Hmmm.
Only placing them closer together helps...

I have just that distance for the fronts in 4,65m x 4,25m room (concrete with floortiles), with a substack in the middle outputting 33-120hz reducing the distance for that range to 1,5m, its pretty strong wavefront, very dry bass.

 

[Cypherdelic]

Can you simulate that with another third speaker in the middle, how thats affects the lobing on the higher frequencies 120-160hz?
Please

This way we can see the difference in the lobing pattern when you halfen the distance between the speakers.
I hope this might help reducing the lobing holes.

 

[Cypherdelic]

If it helps with that i can turn up the crossover of the mivoc (preout-L/R) to the max, lowering the level to output higher for better coupling.
And for the Teufel T10 (via Subin) i could change the LPF for LFE to 250hz to let that output as high as it can go, at the moment ist set to 120.

Maybe this reduces the lobing for higher frequencies with the distance of only 1,5m.

Sry i was a little bit unfriendly. I didnt see the animation at first, thought it was only for 160hz. Now i see it.

It would be really helpful if could show it with 3 speakers (one inbetween) or if thats not possible just show two speakers with half the distance (1,5m).

Then please post both animations together so we can see the difference in the patterns from halfening the distance next to each other.
That would be so great. Please

 

[NTK]

If it helps with that i can turn up the crossover of the mivoc (preout-L/R) to the max, lowering the level to output higher for better coupling.
And for the Teufel T10 (via Subin) i could change the LPF for LFE to 250hz to let that output as high as it can go, at the moment ist set to 120.

Maybe this reduces the lobing for higher frequencies with the distance of only 1,5m.

Sry i was a little bit unfriendly. I didnt see the animation at first, thought it was only for 160hz. Now i see it.

It would be really helpful if could show it with 3 speakers (one inbetween) or if thats not possible just show two speakers with half the distance (1,5m).

Then please post both animations together so we can see the difference in the patterns from halfening the distance next to each other.
That would be so great. Please

Here is the simulation with 3 subs, with 1.5 m spacing between each. The lobing pattern is a function of the ratio of the separation distance and wavelength (δ/λ), which is usually expressed as kδ (where the acoustic wavenumber, k = 2π/λ). Therefore, you'll get the same pattern with half the spacing is halved and double the frequency.

However, my original point was that the explanations by QSC of the "mutual coupling" do not apply to listening rooms (and, I believe, is also generally incorrect). In home sized rooms, the LF response is completely dominated by the coupling of the subs and listening position to the room modes. The direct sound (as shown in the simulations) is usually only a minor part of the total sound you hear, unless you are using "near field subs" right next to you.

 

[Cypherdelic]

Ok.. thx.

I tried putting the substack on remote power control and checked if i hear difference with it on or off ... And guess what.
I barely did notice them at all, nothing more with them on.. Switching them on and off..just nothing, could nor tell if it was on until i saw the led of the powerplug, lol failed the blind test. F...xk. And then i realised couch vibrance is even less with them on, wtf.

So i changed that again... Haha.

Put the Teufel T10 as downfire with Sub1 in my back with the port down close the couch... Wow thats intense LFE now, and with double bass its adding a lot of vibrance to the music when i lay on the couch.

And i gave up connecting the other sub via PREOUT in the middle of the front

Instead, since it features high in and high out i connected it via the frontspeakers L bass-line (biamp) to the high in L and connected the high out L the frontspeakers.
So its percectly timed now with the L-frontspeaker, dont need to worry about about distance/delay for perfect timing anymore... And even when that subwoofer is powered off that high in/out passes the signal on to the frontspeaker, beeing just looped through.

And i placed it right to the side of it (the L-frontspeaker) on the stand... so that it effectively increases the baffle-width... (1mm gap only).

Now i checked again with the remote power control and i can clearly hear them both adding power. nice

I dont know why... But the mutual coupling with a middle substack i was taking about wasnt working at all. Sry. Please forgive me.
It really added nothing, not even 3db... Power got completely lost... I was totally wrong

Connecting a sub via preout is f*cks, you were so right. Using an active subs high in and out is much better if it features it.

 

[Cypherdelic]

You are just right... It just doesnt apply, even the +3db from adding 300W substack to 200w frontspeakers simply vanished somehow into nothing. I was wrong....
I dont have explanation... Putting the LFE back into my back and the other subwoofer closely next to the left speakers woofers with THAT speakers input looped through that speaker... That finally worked adding up the loudness, adding up couch vibrance having both subs on... And imcan feel the difference with each one shutting it off.

Man i dont know whats going on... I have no clues whatsoever.

Lol.. me stupor.

 

[Cypherdelic]

Here... To give you an introspective into my livingroom aka the all-purpose (music/cinema) listeningroom.
The As are absorbers... So... I got lot of those.
You see top X is the old stack with both subs that i tried and removed.
Moved one to the left, closely attached to the L speakers-baffle with that speakers input.
And the better (deeper frequency) teufel t10 got right behind the Couch as LFE (optionally with double bass from the AVR.
Now it works a lot better.

You see... its not a perfect square... The width at frontwall is 4,7m... The width backwall is 4,4m... The length is 4,25m.. That is not as bad as you might think.

If QSC were right, i should have heard the substack in the middle (it was already clipping, so loud)... But i couldnt tell blind if it was on or not !!! until i realised fhe couch was vibrating LESSwith them powered on, thats how i could tell the difference then blindly...heck. you were not only right.... It was louder without the subs... Maybe because of timing mismatch and deletion, idk, but it was a complete fail.

Now it works... Nowi can tell each one is working. The teufel is made for LFE... Only sub in, no crossover, lower frequency. The mivoc is made for music, stereo-RCA-Line-Ins, crossover and high-input/output, not so low frequency as the teufel.

Did it right now

I didnt realised the high in/out of the mivoc can take the speakers amp signal... Thats much better than misusing ZONE2-PreOUT for a frontsub.

 

[audiofooled]

To get this to work in room, there are some things to consider:

1. Floor bounce - pressure sources equidistant to the floor may not work in room as well as they would in half space.

2. It only works for a limited amount of time, for the close field complex behavior, where for the mono part of the signal you would get a lot of gain for transient signals which often puts in more energy than the room can handle. Center sub may have a difficult job of having equal amount of displacement as the sum of all radiators, plus frequency dependent phase offset with respect to room boundaries to create required cancellations.

3. Setup may be very inefficient in the far field. The key is you loose pressure as you would outdoors, perception being similar to listening to outdoor bass, for the transient behavior. Continuous signals could be more quiet than you think, bringing the available headroom in question when trying to achieve preferable tonal balance with regards to the rest of the spectrum. Done right, perception of it would be quasi anechoic, similar to headphone bass, with a clear neural stream and spatial separation of what happens "over there" and "over here"

4. Mains could be not in their best position to deliver satisfactory stereo field, listening triangle being smaller than conventional. They could also not be in their best position to have similar low frequency response for the left and right channel.

5. What happens in your room would be vastly different than mine.