SBIR Matters for Subs Too!

SBIR Matters for Subs Too!

By Matthew J Poes

This is a quick write-up to show the effect of SBIR on low frequencies, highlighting something I found by accident, and how I addressed it. Recently, I was testing a large 18” subwoofer in my theater and temporarily had it placed in front of my screen, which is 30” from the actual front wall. This created a scenario where I had a subwoofer out into the room (a situation some believe leads to better sub performance). I decided to use this as a case study of why subwoofers should probably not be placed out into a room and should always be placed against at least one wall.


Speaker Boundary Interference Response
What is SBIR? SBIR stands for speaker boundary interference response and refers to the interference caused by low frequency sound waves bouncing off boundaries (e.g. walls, ceilings, and floors) and reflecting against the other waves from the source, causing either peaks or dips. Because of these responses, one unique attribute of SBIR (as compared to typical modal interference) is that resulting peaks and dips will always remain constant regardless of the listener’s position. The only thing that modifies the peaks and dips are moving the speaker.

The cause of the interference has to do with the way that acoustic waves interact with each other when their phase aligns or not. Look at this graphic to help understand how interference works in perfect 0-degree and 180-degree phase to each other:

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Figure 1: Interference graphic

As can be seen, when two waves are perfectly in phase (0 degrees or 360 degrees) the result is perfect constructive interference. In other words, the amplitude increases. When the two waves are completely out of phase (180 degrees) then the amplitude is decreased and the resulting sound is cancelled. Since a reflected wave travels a further distance, the phase of the wave will be different from the wave coming directly from the subwoofer. When the reflected and direct waves meet up they interfere with each other, sometimes constructively and sometimes destructively. Because low frequencies are steady state in small rooms by the time they hit our ears, we hear the bass with all the room reflections. This cancelation effect isn’t delayed, it’s a part of the fundamental signal we hear.


What does SBIR look like?
Let me first describe the setup in my room that caused this measurement. I turned off all correction, all EQ, and measured the 18” subwoofer along with my two 12” B&C Bandpass subs and my single 12” Dayton reference sub. When I saw the cancelation illustrated in Figure 2 (below), I assumed all four subs were interfering with each other and phase had been reversed. Because my front wall is a false wall, I forgot how far the subwoofer was from the front wall. I proceeded to switch each sub’s phase, singularly and in pairs, to see if that effected output measurements. While the response changed, a noticeable notch at 32hz never went away. Then I turned off the new subwoofer and the notch was gone. Of course! The 18” subwoofer measures roughly 28” long, and because of its in-room position, was actually sitting nearly 60” away from my room’s actual physical wall.

full.jpg

Figure 2: Uncorrected system response subwoofer front and center

Now you may be asking yourself: how did I know there was nothing wrong with the subwoofer? How did I know this was SBIR? The answer: simple calculations (but we need to keep a few things in mind). This interference effect depends on the acoustic distance to the walls, which can differ from the actual distance for a few reasons. One is that materials in the room can have an impact on the exact position of these interferences, but more commonly, the wall in our room may not be the hard barrier causing the reflection. It may be both a wall’s surface in a normal 2x4 wall (drywall on both sides) or it may be the cement outer wall in a basement room. In my case the cement front wall of the basement foundation is about 24 inches from the theater room’s inner wall. Due to some pipes my side walls are actually 36” from the cement foundation walls. The rear wall is a double stud wall with 4 layers of drywall that measures about 14” thick. All of this impacts the room’s acoustic dimensions, as compared to its internal actual dimensions. Remember, low frequency waves can travel through materials like drywall as a hot knife through butter.

full.png

Figure 3: Room simulation of SBIR

In Figure 3, I have created a simulation that seems to somewhat closely match the response I measured in my actual room. This is with the subwoofer placed dead center of the screen and with the LF source roughly 50-60”from the interior front wall. However, with a dip at 32Hz, it suggests the subwoofer is much farther from the walls, as if the room is much larger. I used the dimensions of the foundation walls instead and had a very close match. The proximity of the sub to each barrier caused a bunch of dips to all fall very close to each other. This compounded the dip and made it both very wide and very deep. Room modes caused the peak in that measurement, specifically the 70hz mode is a length axial mode in the room.


How did I fix the SBIR effect?
I was in fact able to largely fix this problem without moving the subwoofer behind the screen. I moved the subwoofer to the right side wall and oriented it to face the front wall. Why did this help? By moving the sub to a barrier, the distance between surfaces is no longer equal and the interference effects are spread out. This reduces the significance of their effect. Additionally, some of the interference effect is raised above the operating range by placing them against a wall. Turning the subwoofer toward the front wall placed it closer to the front wall, again moving the interference to a higher frequency. This is how it looked after I moved the subwoofer. To be clear, I did not change any settings in the DSP, nor did I adjust anything in the electronics. I only moved the subwoofer to the side of the room, and turned it around.

full.jpg

Figure 4: Uncorrected Systems Response with sub on right side wall

As you can see the response is much flatter. The dips and peaks are no longer on top of each other and the previously measured deep valley in the response is removed. Instead, the bass response is relatively flat and smooth.

Now let’s look at what a little EQ does:

full.jpg

Figure 5: Corrected Systems Response with sub on right side wall

As you can see, once some EQ is applied, the bass response can be made much flatter. The response falls within a +/- 5db window at nearly all frequencies relative to my intended house curve. It’s important to note that no amount of EQ could have filled in that dip in the bass response. That dip was a wide cancelation effect and the only solution was to move the subwoofer to a more optimal position. What was that more optimal position? It was pushing the subwoofer against the side wall and closer to the front wall.


Conclusion
Two things have been demonstrated in this exercise. First, subwoofers should not be placed far away from room boundaries. Second, the distance between a low frequency source and surrounding boundary surfaces should not be equal or multiples of each other. These compound cancelation effects, resulting in a large measurable trough in bass response. Hopefully this also helps to explain why full range speakers (and their typical position in a room) are perpetually suboptimal low frequency sources. The ideal placement of main speakers is rarely if ever the best location for bass reproduction. This is why I frequently say, subwoofers are audiophile. It’s a lack of subwoofers (separate LF sources which can be optimally placed in a room) which is not audiophile.
 
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Todd Anderson

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Great topic, Matt. Lots of actionable info for better sound! :T
 

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Wow, nicely done. Good topic and lots of good detail to go over. Thanks.
 

Matthew J Poes

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Thanks everyone. Hopefully the main takeaway that everyone gets is how important placement is. I think most people take subwoofer placement a bit for granted. SBIR is typically an issue with full range speakers, less so with subs. We typically place them near boundaries and we don't think much of this. When there are cancelation effects, it is often worth considering everything from proximity to boundaries to their orientation. A really long subwoofer might have a cancelation effect due to the LF source end's distance to the wall. Simply turning it around might fix that.

I will mention one thing that isn't in the article. It is my preference to place subwoofers near boundaries for the reasons shown here. However, there is another extreme that actually works pretty ok for some people. You can place the subwoofer well out into a large room. For some folks, this pushes the cancelation down below the operating range of the sub or happens to be the best blend of modal excitation and SBIR to give the flattest response. I know someone who does this for his room and while not my preference, I trust his reasoning.
 

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How do you define large room? >3000 cu ft?
 

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Thank you for this, Matt.

Locating a sub near boundaries (ceiling, walls AND floor) often creates peaks, which are more easily corrected than nulls. Multiple subs can create enough destructive interference to make many, many peaks, "smoothing" the room in the MLP (main listening position) and well outside of it.

The take-away: "This is why I frequently say, subwoofers are audiophile. It’s a lack of subwoofers (separate LF sources which can be optimally placed in a room) which is not audiophile."
 

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How do you define large room? >3000 cu ft?
Hi Todd,

That is a great question. It is actually defined by an acoustical property rather than an exact volume. Small rooms have modes, the bass anomalies we are all used to. Large rooms do not. Why? Because large rooms are such that their length and width (and often height) are larger than the longest audible wavelength (in fact are longer than the longest reproduced wavelength). Commercial theaters are large acoustic spaces, often the length and width is over 100 feet. Acoustically everything about a large room is different, reflections are different, absorption behaves differently, you name it. Small rooms typically have RT60 times well under 1 second (normal is actually around half a second). Large rooms typically have RT60 that is in excess of 1 second. A large room could have an RT60 of 1.5 seconds and sound just as dry as a small room with an RT60 of .5 seconds.

The point of mentioning large or small rooms in these articles is because of the lack of modes. Large rooms act much differently with regard to LF reproduction. The only concern is SBIR, but not modes.
 

Matthew J Poes

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Thank you for this, Matt.

Locating a sub near boundaries (ceiling, walls AND floor) often creates peaks, which are more easily corrected than nulls. Multiple subs can create enough destructive interference to make many, many peaks, "smoothing" the room in the MLP (main listening position) and well outside of it.

The take-away: "This is why I frequently say, subwoofers are audiophile. It’s a lack of subwoofers (separate LF sources which can be optimally placed in a room) which is not audiophile."
Oh that's a good one Dennis. I like that you guys are picking up on key take away differently than I may have intended. I actually added that line just minutes before posting the article. After I reread the article and decided if I had made the points I wanted to make, I decided it was worth throwing that view in. It's my opinion, of course. I don't happen to think you can get audiophile bass (which I define as accurately reproduced with a smooth response and low distortion) without the use of subs, since Mains typically place the LF sources in less than ideal locations. Even large line array subs, which cancel floor to ceiling modes, will still have SBIR effects and width/length modes.
 

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The way I see it, my take away, is that there is no downside to including subwoofers into the audio chain. There are only pluses.

People that state otherwise often have valid complaints, which are remedied with proper integration. All too often, the sub get the blame. If a properly working subwoofer is "making the sound bad", then there are set up issues that need addressed.
 

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The way I see it, my take away, is that there is no downside to including subwoofers into the audio chain. There are only pluses.

People that state otherwise often have valid complaints, which are remedied with proper integration. All too often, the sub get the blame. If a properly working subwoofer is "making the sound bad", then there a re set up issues that need addressed.
I very much agree with you. It is my opinion that when folks indicate finding the bass to integrate poorly or lack the quality they get from just a full range speaker, that this is caused by one of two things:
  1. A hole in the bass response caused by SBIR giving the false perception of cleaner/tighter bass
  2. Poor integration due to improper setup
What I never agree with is the notion that even the best full range speakers somehow have superior bass to that of separate subs. The biggest factor effecting bass sound quality is location and the optimal location for mains is not the optimal location for subs, that location imparts a significant amount of distortion. This also means that it makes no sense for someone to operate main speakers down very low and use subs only for the lowest frequencies and LFE channel. The most important range for a separate subwoofer to operate is actually between 50hz and ~150hz (room FS). Operating a sub below 50hz only and using the mains above that point is a bad idea (and this is not my opinion but something that is fully supported by science).
 

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I very much agree with you. It is my opinion that when folks indicate finding the bass to integrate poorly or lack the quality they get from just a full range speaker, that this is caused by one of two things:
  1. A hole in the bass response caused by SBIR giving the false perception of cleaner/tighter bass
  2. Poor integration due to improper setup
What I never agree with is the notion that even the best full range speakers somehow have superior bass to that of separate subs. The biggest factor effecting bass sound quality is location and the optimal location for mains is not the optimal location for subs, that location imparts a significant amount of distortion. This also means that it makes no sense for someone to operate main speakers down very low and use subs only for the lowest frequencies and LFE channel. The most important range for a separate subwoofer to operate is actually between 50hz and ~150hz (room FS). Operating a sub below 50hz only and using the mains above that point is a bad idea (and this is not my opinion but something that is fully supported by science).
Yes, I have been using subwoofers for decades and will not have a main system without them. To much is left on the table when you do not. Barring room size constraints (and even then, if there is room for two mains, a sub can be squeezed in) there is no good reason I can think of to not use subs.

Speaking of crossover, I generally like to go high, 120-150 Hz. This helps with mains floor bounce cancellations which generally occur below those frequencies. One little trick I picked up from Wayne Parham of Pi Speakers. Also, I place my dual subs behind the mains around 1/4 room points.

Right now, I am running an Emotiva XMC-1 pre/pro and Dirac Live is picking the crossover points, so I have no control over this at the moment. I am getting minimal cone movement out of my main midbass drivers, so it appears the xo is fairly high. REW shows a nice blend and my ears are happy.
 

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Thank you for providing some nice examples of what happens when you pull a sub to far from the front wall. You can see this issue even more prominently when you have rooms with relatively open backs or very deep rooms that open to a kitchen or rest of a basement.

I had some fun demonstrating this an fixing some issues Sonnie was having with his big DIY subwoofers I believe 4-5 years ago now when I was visited with a set of Catalyst 8Cs during the $2500 speaker shootout he, Wayne, Leonard, and Joe were conducting. By simply spinning around his subwoofers putting the woofers on the back, the huge hole in the upper bass range was pushed up out of the subwoofer range, with measurements and listening both affirming the improvement.

This can be extremely important to remember and test with any woofer you have around before you make plans for a physically deep subwoofer. Some subwoofers can leave the woofer 30-48" from the front wall, and you can count on that having a measurable impact. Grab another subwoofer and set the woofer out at the expected distance/location from the front wall (connections included), and listen or even better, measure.
 

Matthew J Poes

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Thank you for providing some nice examples of what happens when you pull a sub to far from the front wall. You can see this issue even more prominently when you have rooms with relatively open backs or very deep rooms that open to a kitchen or rest of a basement.

I had some fun demonstrating this an fixing some issues Sonnie was having with his big DIY subwoofers I believe 4-5 years ago now when I was visited with a set of Catalyst 8Cs during the $2500 speaker shootout he, Wayne, Leonard, and Joe were conducting. By simply spinning around his subwoofers putting the woofers on the back, the huge hole in the upper bass range was pushed up out of the subwoofer range, with measurements and listening both affirming the improvement.

This can be extremely important to remember and test with any woofer you have around before you make plans for a physically deep subwoofer. Some subwoofers can leave the woofer 30-48" from the front wall, and you can count on that having a measurable impact. Grab another subwoofer and set the woofer out at the expected distance/location from the front wall (connections included), and listen or even better, measure.
Thank you Mark. I appreciate you taking time to read the little article and comment. I like that you noted that detail about spinning the sub. It’s a trick I use often to position the actual Lf source closer to a boundary.

If nothing else a good actionable point is that when optimizing a system, try flipping the subs around.

I’m curious if you have ever seen a null as deep and wide as mine so low in frequency in the wild. Mine was caused by such an unusual set of circumstances. Who builds a false wall, sticks a sub in front of the false wall, and has such large chases behind their room shell walls? Add the symmetric placement and you have the perfect storm for a wide and deep null at a very low frequency. It practically wiped out all the bass. I certainly had never seen anything quite like it.
 

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Matt wouldn’t your response change if you changed the phase of the one sub your talking about in this article.
 

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Thank you Mark. I appreciate you taking time to read the little article and comment. I like that you noted that detail about spinning the sub. It’s a trick I use often to position the actual Lf source closer to a boundary.

If nothing else a good actionable point is that when optimizing a system, try flipping the subs around.

I’m curious if you have ever seen a null as deep and wide as mine so low in frequency in the wild. Mine was caused by such an unusual set of circumstances. Who builds a false wall, sticks a sub in front of the false wall, and has such large chases behind their room shell walls? Add the symmetric placement and you have the perfect storm for a wide and deep null at a very low frequency. It practically wiped out all the bass. I certainly had never seen anything quite like it.
Hi Matt,

I have seen very deep interactions before. Here's a really bizarre one from the often posted about theater of "thebland" on AVS. He has since sold the home and is planning a new theater, but this very old Dennis Erskine design had some unexpected results based on the full rear wall being an equipment room, and the wall being quite lossy into the room with the isolated wall behind. This created a case of relatively little reflected LF energy at the rear wall while the exterior/front wall behind the screen was very solid. You would normally think a rear corner of a dedicated theater might be a great place to add a ULF subwoofer which could be hidden in the rear corner. Since I had to visit the area anyway I brought a SubMersive and thought I'd check the relative efficiency of this location. Much to my initial surprise, due to the rear wall/equipment space configuration this location was worthless below 20Hz. This is what you get with 2 subs at the front (yellow) vs 2 subs stacked in the rear:

thebland-front-vs-rear.jpg


The key data to understand here is that in this room, 4 subs at the rear corner would deliver less energy in the teens than ONE sub up front. Ooops!

From these measurements I flipped the plan of attack and we used an array of 4 SubMersives up front do deliver the lowest frequencies and balance the rows, with a stacked pair of F2's at the rear, where you can see even the pair of acoustically identical SubMersive F2's delivered smoother response and equal or more energy above 20Hz than twice as many SubMersives at the front wall, giving us the energy we needed to even things out across the 3 rows of seats:

thebland-subs-F-R-start.jpg


I don't know if I have the measurements easily accessible from the initial testing, but as we slid a SubMersive from the rear corner toward the front wall, the dip progressively moved higher in frequency. This was a rather long room, hence the very low frequency notch. Fortunately as the frequency of the notch moves higher, you typically get a much more complex interaction with the other dimensions of the room causing things to average out more and be less dominated by a single bounce/cancellation. This is also why we see that very large rooms have fewer dominant modes or nulls as the modes are more closely spaced at a much lower frequency. On the opposite end of the spectrum are rooms with dimensions that are the same or multiples of each other, and even very long, narrow rooms where the lowest frequencies are bigger than the short dimensions, so the front to back length dominates.
 

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Matt wouldn’t your response change if you changed the phase of the one sub your talking about in this article.
That all depends on where that subwoofer is located. It might certainly change, but it likely will then cancel at other frequencies. There wasn't quite enough info about the multiple sub locations and their individual contributions to know for sure, but the simple examination is that if every sub exhibits that notch at the seats when measured alone, no amount of adjustments will fix it.

When overlaying responses, remember that two response curves can never add to more than +6dB above either one, and you only get the full +6dB when both are the same level and in phase at that frequency.

On a related matter where Matt posed the question of "how did I know there was nothing wrong with the subwoofer?":

It's always useful to have some basic understanding of how we can quickly measure subwoofers and how they are supposed to function. For a sealed subwoofer, if you ever need a sanity check, place the microphone about a finger's width from the dustcap, and take a measurement (setting levels not to send the microphone flying :doh:). Doing the same with a ported sub should also provide a smooth measurement with a notch in the response near the port tuning. A second measurement in front of the port will confirm the port is working as it should, and the issues you are seeing are from the room and not the subwoofer. If you can't easily get to the subwoofer at the moment, the next best test is to move the microphone say 3-5 feet in either the depth or width dimension (move farther for lower frequency issues, less for upper bass issues). Don't change both dimensions at once else you can't tell which is causing the issue.
 

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Great topic and great info by the way. I wanted to share I have one sub at the front of the room and one nearfield. We used to have carpet in the family room but when we got tile there was an echo with certain material. I contacted GIK and they suggested a throw rug and 3 GIK 244 bass traps since my couch sits by the back wall. I have bass traps along that wall. They also suggested bass traps right behind the main speakers to avoid SBIR. The idea per GIK is to extend low frequency absorption and tighten the bass response.
I have been thinking on ordering the traps to place behind the LR speaker but I am not sure. The reason is as a test when I removed the 244 bass traps that are at the back wall while playing content with UL frequency I could not hear a difference with the traps in place or off.
 
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I read a article not long ago about adjusting sub position/placement. It offered that elevating the sub off the floor would also make the response different. Claimed it was better in his room. Can you try that with your sub and see how it affects your response? I think he recommended at least the height of the sub as a starting point. It makes sense considering the floor and ceiling are also a boundaries.
 

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I read a article not long ago about adjusting sub position/placement. It offered that elevating the sub off the floor would also make the response different. Claimed it was better in his room. Can you try that with your sub and see how it affects your response? I think he recommended at least the height of the sub as a starting point. It makes sense considering the floor and ceiling are also a boundaries.
What were his conclusions?

Interesting idea... but not so easy for heavy subs! @Matthew J Poes , have you tinkered with elevating subs?
 

Tonto

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He claimed that all subs should be off the floor. Decoupled as well. Claims it makes a big difference.
 

Matthew J Poes

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What were his conclusions?

Interesting idea... but not so easy for heavy subs! @Matthew J Poes , have you tinkered with elevating subs?
I won't even touch the idea of decoupling subs. That's a totally loaded topic. I'm not aware of any published scientific articles on the benefits of decoupling subwoofers to improve performance nor can I think of good reasons why it would matter. I'm not against the idea, but i can't put that idea in the same place as the acoustic issues of SBIR effects.

As for elevating subs, like moving them in the X and Y position, moving them in the Z position is important as well. It wasn't an idea I had ever given thought to in the past, elevating subs is not exactly easy to do. However, Dr. Geddes suggested with his multi-sub approach that one of them should be placed in a different height position from the others (significant, many feet off the ground, not just decoupled). If you play around with the room modeling software in REW you can see the benefits. It's especially beneficial to maximize the excitation of height modes, which helps smooth bass further in all planes. In other words, it's certainly a good thing to change the elevation of subs. In my own room, one of my subs is placed 18" higher than the other's. I've played around with placing subs on tall stands in the past as well, seems to have the expected benefit when integrated with the rest of the subs correctly.

Placing a single subwoofer off the floor would likely cause a worse response however since (specifically with SBIR) it would lower the cancelation frequency, sometimes known as the Allison effect) from ground and ceiling bounce. This is why towers and bookshelves typically have a notch in their response. If you plan to play around with the height of a sub, I would personally suggest doing so carefully, and preferably in multiples. Better to have subs close to the barriers to minimize the SBIR effects.

I'm open to doing an experiment however if this would be of interest. I won't do it with the 18" sub that was the premise of this article, but I have some 12's I could do it with.
 

Matthew J Poes

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Great topic and great info by the way. I wanted to share I have one sub at the front of the room and one nearfield. We used to have carpet in the family room but when we got tile there was an echo with certain material. I contacted GIK and they suggested a throw rug and 3 GIK 244 bass traps since my couch sits by the back wall. I have bass traps along that wall. They also suggested bass traps right behind the main speakers to avoid SBIR. The idea per GIK is to extend low frequency absorption and tighten the bass response.
I have been thinking on ordering the traps to place behind the LR speaker but I am not sure. The reason is as a test when I removed the 244 bass traps that are at the back wall while playing content with UL frequency I could not hear a difference with the traps in place or off.
I think treating the front wall for SBIR is a great idea. There is no denying that multiple subwoofers can be deployed in a fashion to mitigate SBIR effects, but in many cases they show up anyway or show up higher than the subs operate. Then what? That is where classical acoustic treatment is necessarily. I agree with GIK's idea and do so in my own room.

The warning I would give is that I have not found that a single bass trap on the wall behind each speaker to be adequate to fully address SBIR effects. There are a number of reasons for this:
  1. Bouncing off the front wall is just one source of the SBIR cancelation and peaks that you see, the side wall, ceiling, and floor are other sources. It's very common that the placement is such that the floor bounce and front wall bounce collectively cancel near each other which widens the dip (lowers the Q). It can obliterate the bass from the speaker.
  2. Bass waves are huge and so absorbing just a small portion of them right behind the speaker may not do a lot
  3. The bass waves tend to build up greatest pressure in the corners and these contribute to the SBIR effect as well.
When we take all of the three points above together it tells us one terribly depressing fact. Absorbing low frequencies across the entire front walls and corners is especially important. And wait...it gets worse, the back wall (the one behind you) also contributes to SBIR and so guess what, we need to absorb bass on that wall. So now we suddenly realize this terribly hopeless situation where in an ideal world every wall would provide bass damping.

Still, it is my opinion that if we avoid the ol' "perfection is the enemy of good" and instead focus on the real tangible improvements that adding some damping can provide, treating the area like GIK said is a good plan. I suggest using a bandwidth limited bass trap with the most LF absorption they offer, like the monster bass trap. I also suggest treating the corners if possible. Anything in the corner is fine, but I like these best http://www.gikacoustics.com/product/gik-acoustics-soffit-bass-trap/
Finally, I suggest adding one or two additional bass traps, bandwidth limited, on the rear wall. I think these three locations are ideal ways to improve the bass response. More is better, but this is a really good start.

In my own room I treated the entire front section of the room, about 30" deep. The side and front wall, floor, and ceiling all have 2" to 4" of acoustic absorption with a membrane. Corners have piles of leftover absorber material, but that will be replaced with large corner bass traps. The rear wall of the theater has a membrane bass trap and currently has one of the large corner bass traps that will eventually go in the front corner. I hope to experiment with ceiling bass traps as well. I intentionally left the portion of my ceiling where ceiling bounce is created untreated so I can add them later. That is why my ceiling diffuser/absorber is not at the first reflection point.
 

Asere

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I think treating the front wall for SBIR is a great idea. There is no denying that multiple subwoofers can be deployed in a fashion to mitigate SBIR effects, but in many cases they show up anyway or show up higher than the subs operate. Then what? That is where classical acoustic treatment is necessarily. I agree with GIK's idea and do so in my own room.

The warning I would give is that I have not found that a single bass trap on the wall behind each speaker to be adequate to fully address SBIR effects. There are a number of reasons for this:
  1. Bouncing off the front wall is just one source of the SBIR cancelation and peaks that you see, the side wall, ceiling, and floor are other sources. It's very common that the placement is such that the floor bounce and front wall bounce collectively cancel near each other which widens the dip (lowers the Q). It can obliterate the bass from the speaker.
  2. Bass waves are huge and so absorbing just a small portion of them right behind the speaker may not do a lot
  3. The bass waves tend to build up greatest pressure in the corners and these contribute to the SBIR effect as well.
When we take all of the three points above together it tells us one terribly depressing fact. Absorbing low frequencies across the entire front walls and corners is especially important. And wait...it gets worse, the back wall (the one behind you) also contributes to SBIR and so guess what, we need to absorb bass on that wall. So now we suddenly realize this terribly hopeless situation where in an ideal world every wall would provide bass damping.

Still, it is my opinion that if we avoid the ol' "perfection is the enemy of good" and instead focus on the real tangible improvements that adding some damping can provide, treating the area like GIK said is a good plan. I suggest using a bandwidth limited bass trap with the most LF absorption they offer, like the monster bass trap. I also suggest treating the corners if possible. Anything in the corner is fine, but I like these best http://www.gikacoustics.com/product/gik-acoustics-soffit-bass-trap/
Finally, I suggest adding one or two additional bass traps, bandwidth limited, on the rear wall. I think these three locations are ideal ways to improve the bass response. More is better, but this is a really good start.

In my own room I treated the entire front section of the room, about 30" deep. The side and front wall, floor, and ceiling all have 2" to 4" of acoustic absorption with a membrane. Corners have piles of leftover absorber material, but that will be replaced with large corner bass traps. The rear wall of the theater has a membrane bass trap and currently has one of the large corner bass traps that will eventually go in the front corner. I hope to experiment with ceiling bass traps as well. I intentionally left the portion of my ceiling where ceiling bounce is created untreated so I can add them later. That is why my ceiling diffuser/absorber is not at the first reflection point.
Matthew I currently have 3 GIK 244 Bass Traps Full Range on the back wall right where the couch sits. GIK had suggested Full Range for best results. http://www.gikacoustics.com/product/gik-acoustics-244-bass-trap-flexrange-technology/ When you mentioned Bandwidth Limited trap is that same as Full Range or not? I ask because if you look at the link I sent you it gets confusing (at least for me) because it mentions they have Full Range, Range Limiter and Scatter Plate.
 

Matthew J Poes

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Matthew I currently have 3 GIK 244 Bass Traps Full Range on the back wall right where the couch sits. GIK had suggested Full Range for best results. http://www.gikacoustics.com/product/gik-acoustics-244-bass-trap-flexrange-technology/ When you mentioned Bandwidth Limited trap is that same as Full Range or not? I ask because if you look at the link I sent you it gets confusing (at least for me) because it mentions they have Full Range, Range Limiter and Scatter Plate.
Well this is just an issue of GIK vs my perspective. I don't necessarily think that tons of full range absorption is always good. I think it depends on the speakers and room size. As such, especially in modest or smaller rooms and with speakers that have good controlled dispersion, I tend to feel that you mostly want bass absorption. However, if you sit against a rear wall you want full band absorption. If you have speakers that are very wide dispersion and the polar response is bumpy, you want more full band absorption.

There range limiter also boosts bass absorption, which helps improve the SBIR issue, so I also think that is a benefit. You need so much bass damping that any little boost helps.

So I meant range limiter and the Gik vs Matt advice is just a slight difference of opinion. Both will be good, we just disagree slightly over which will be best.
 

Asere

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Well this is just an issue of GIK vs my perspective. I don't necessarily think that tons of full range absorption is always good. I think it depends on the speakers and room size. As such, especially in modest or smaller rooms and with speakers that have good controlled dispersion, I tend to feel that you mostly want bass absorption. However, if you sit against a rear wall you want full band absorption. If you have speakers that are very wide dispersion and the polar response is bumpy, you want more full band absorption.

There range limiter also boosts bass absorption, which helps improve the SBIR issue, so I also think that is a benefit. You need so much bass damping that any little boost helps.

So I meant range limiter and the Gik vs Matt advice is just a slight difference of opinion. Both will be good, we just disagree slightly over which will be best.
I went ahead and asked GIK if he suggests Full or with the Limiter for the front wall. He said Full to better extend and tighten the bass.
To get a better idea say for example. I use Audyssey DEQ and as you know the bass gets boosted at levels below reference levels. Having said that the bass boost sometimes depending on content can be overwhelming to others (not for me though lol). If I use a Range Limiter it will absorb even more of the lower frequencies vs if I use Full?
 
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