Geddes approach to Multi-Sub setup Instructional Video by Matthew Poes

Matthew J Poes

Matthew J Poes

Senior Audioholic
As the title states this is a recording of a talk I gave on the Geddes approach to subwoofer setup. It covers both theory and execution. While the content had direct input from Earl Geddes, the views expressed are my interpretation.

 
S

shadyJ

Speaker of the House
For the record I will say that the asymmetric placement of the Geddes approach makes a bit more sense to me than Harman's approach of adjacent ends of the room. Symmetric placement can trigger the same room modes, whereas the modes ought to be randomized. I get why Geddes insists on full-range main speakers, but that makes bass management a bit complex, furthermore it calls for main speaker bass response that is equal to a subwoofer's performance, and how many speakers can actually handle that? Very few.

I think a new approach is required that takes the room out of the equation. I am thinking that there could be some kind of cavity that the listener could place their head in that is lined with subwoofer cones on the inside, and this would give you very precise control over the low-frequency sound, since the woofers are in such close proximity to the ears. Sure, it would totally interfere with sound coming from the main speakers, but that is a small price to pay for perfect bass.
 
Matthew J Poes

Matthew J Poes

Senior Audioholic
For the record I will say that the asymmetric placement of the Geddes approach makes a bit more sense to me than Harman's approach of adjacent ends of the room. Symmetric placement can trigger the same room modes, whereas the modes ought to be randomized. I get why Geddes insists on full-range main speakers, but that makes bass management a bit complex, furthermore it calls for main speaker bass response that is equal to a subwoofer's performance, and how many speakers can actually handle that? Very few.

I think a new approach is required that takes the room out of the equation. I am thinking that there could be some kind of cavity that the listener could place their head in that is lined with subwoofer cones on the inside, and this would give you very precise control over the low-frequency sound, since the woofers are in such close proximity to the ears. Sure, it would totally interfere with sound coming from the main speakers, but that is a small price to pay for perfect bass.
Bass augmentation optimizer. Woofers attaches to a headband that the listener wears while listening to their music. Sophisticated dsp will be used to assure integration. We could totally sell that!

Really the Geddes approach for bass management isn’t that complex, it’s just not the legacy we work from and thus not a common accepted approach. I can do it natively with my Onkyo but couldn’t do it natively with a product I recently reviewed.

For those wondering what James is talking about, while watching my talk will help, briefly I’ll mention that Geddes dislikes the use of a crossover in the bass management of the mains and subs. There is no hard turnover between sub and main. Instead he prefers shallow low pass filters and the natural highpass of a sealed woofer main speaker (so no external or active highpass filter of any kind). This has a number of benefits but also puts specific requirements on the system. The mains need to be able to operate full range and hit reference levels without exceeding xmax. You need sufficient amplifier to do that while operating full range. Basically almost no speakers on the market can do this and no receiver has sufficient power. Geddes approach was designed around his systems approach, which makes use of mains which use pro drivers in small enclosures. My speaker (Geddes Abbey) has a 12” B&C with 11mm of xmax and 1000 watts rms power handling in. 1.25 cubic foot sealed enclosure. It’s sensitifity is 95dB or so and it rolls off in room around 60hz or so. Simulations show that a 1000 watt input won’t drive it past xmax so in practice it shouldn’t be possible to overdrive the speaker if it operates full range (no highpass filter).
 
gene

gene

Audioholics Master Chief
Administrator
For the record I will say that the asymmetric placement of the Geddes approach makes a bit more sense to me than Harman's approach of adjacent ends of the room. Symmetric placement can trigger the same room modes, whereas the modes ought to be randomized. I get why Geddes insists on full-range main speakers, but that makes bass management a bit complex, furthermore it calls for main speaker bass response that is equal to a subwoofer's performance, and how many speakers can actually handle that? Very few.

I think a new approach is required that takes the room out of the equation. I am thinking that there could be some kind of cavity that the listener could place their head in that is lined with subwoofer cones on the inside, and this would give you very precise control over the low-frequency sound, since the woofers are in such close proximity to the ears. Sure, it would totally interfere with sound coming from the main speakers, but that is a small price to pay for perfect bass.
Really, you're going you have listeners place their head in a prefixed cavity? LOL.

I've seen Geddes take on multi-sub and agree with some aspects of it but I do find some of his logic not too palatable. I do agree with the benefits of true fullrange towers as bass sources in a multi-sub system.

Absolute phase? You can't hear absolute phase. Time alignment does matter and is easily seen when you integrate mult-sub and observe frequency response with subs time aligned vs not time aligned.

4 corner placement is usually the best approach for LF gain factor and good seat-seat consistency if you also have the ability to apply global EQ, and time alignment. 4 midwall placement theoretically is better consistency from seat to seat but the overall system efficiency isn't as good as 4 corner.

I disagree about symmetric locations should be avoided. That's simply NOT true. There are good rules of thumb, like 4 corner placement or 1/4L front wall that work extremely well, especially with the assistance of global EQ.

At the end of the day, what matters is how the mains with ALL active subs integrate at the primary seats. Since speakers are min phase devices, you can use super position and add one sub at a time to get the integration good, then apply global EQ. The ONLY time I advocate using a LPF at 150Hz is when you're using tiny satellite speakers with subs located towards the front speakers. Ideally you'd want ALL subs playing a summed mono signal with the same XOVER point but it usually involves more tweaking if you're not in a symmetric room.

For more advanced users, that want to combine 2CH listening with home theater, I prefer integrating truly fullrange towers with multi-sub.

Personally I like having fullrange towers with LFE integrated but it requires speakers of similar output as your subs and enough amp power to keep up with your subs. The other problems is MOST processors (and ALL AV receivers) don't route LFE to the main channels IF you also have a subwoofer channel engaged. It's tragic that the major brands don't offer this despite me lobbying for years to at least offer it as an advanced calibration feature.

In my situation, I'm left turning the subwoofer channel off and running my additional subs through the main L/R preouts and doing local bass management and delay control.

Why do I bother? BC the bass from my tower speakers is epic. I've never heard better bass from any other products including running bass managed satellite speakers with multi-sub. The key is the integration of my towers at and above 80Hz with the satellite speakers that you just can't achieve with most satellite speakers and multi-sub. IF you can integrate full range towers with multi-sub, it can be an amazing experience but it's very difficult to set up properly and requires the ability to measure, analyze, EQ, and repeat.

Modeling your room in REW is only useful if you have a defined rectangle or square shaped room. Most people don't. I honestly gave up on modelling as it's so much easier to just place the subs in an actual room and pull measurements. Room modeling only get's you so far especially comparing an undamped room to one loaded with furniture.
 
Matthew J Poes

Matthew J Poes

Senior Audioholic
Really, you're going you have listeners place their head in a prefixed cavity? LOL.

I've seen Geddes take on multi-sub and agree with some aspects of it but I do find some of his logic not too palatable. I do agree with the benefits of true fullrange towers as bass sources in a multi-sub system.

Absolute phase? You can't hear absolute phase. Time alignment does matter and is easily seen when you integrate mult-sub and observe frequency response with subs time aligned vs not time aligned.

4 corner placement is usually the best approach for LF gain factor and good seat-seat consistency if you also have the ability to apply global EQ, and time alignment. 4 midwall placement theoretically is better consistency from seat to seat but the overall system efficiency isn't as good as 4 corner.

I disagree about symmetric locations should be avoided. That's simply NOT true. There are good rules of thumb, like 4 corner placement or 1/4L front wall that work extremely well, especially with the assistance of global EQ.

At the end of the day, what matters is how the mains with ALL active subs integrate at the primary seats. Since speakers are min phase devices, you can use super position and add one sub at a time to get the integration good, then apply global EQ. The ONLY time I advocate using a LPF at 150Hz is when you're using tiny satellite speakers with subs located towards the front speakers. Ideally you'd want ALL subs playing a summed mono signal with the same XOVER point but it usually involves more tweaking if you're not in a symmetric room.

For more advanced users, that want to combine 2CH listening with home theater, I prefer integrating truly fullrange towers with multi-sub.

Personally I like having fullrange towers with LFE integrated but it requires speakers of similar output as your subs and enough amp power to keep up with your subs. The other problems is MOST processors (and ALL AV receivers) don't route LFE to the main channels IF you also have a subwoofer channel engaged. It's tragic that the major brands don't offer this despite me lobbying for years to at least offer it as an advanced calibration feature.

In my situation, I'm left turning the subwoofer channel off and running my additional subs through the main L/R preouts and doing local bass management and delay control.

Why do I bother? BC the bass from my tower speakers is epic. I've never heard better bass from any other products including running bass managed satellite speakers with multi-sub. The key is the integration of my towers at and above 80Hz with the satellite speakers that you just can't achieve with most satellite speakers and multi-sub. IF you can integrate full range towers with multi-sub, it can be an amazing experience but it's very difficult to set up properly and requires the ability to measure, analyze, EQ, and repeat.

Modeling your room in REW is only useful if you have a defined rectangle or square shaped room. Most people don't. I honestly gave up on modelling as it's so much easier to just place the subs in an actual room and pull measurements. Room modeling only get's you so far especially comparing an undamped room to one loaded with furniture.
Thanks for adding your thoughts Gene.

Your approach to bass management is basically what I do. The Onkyo I have has a mode called double bass, but I don’t think it redirects the LFE to the mains, just sends bass to both the subs and large mains.

To be able to bypass undesirable processing I have actually bypassed bass management completely and operate the subs by simply telling the system it has no sub and sending it all to the mains. I then split the outputs between my subs and actual mains. It works fine.
 
gene

gene

Audioholics Master Chief
Administrator
Thanks for adding your thoughts Gene.

Your approach to bass management is basically what I do. The Onkyo I have has a mode called double bass, but I don’t think it redirects the LFE to the mains, just sends bass to both the subs and large mains.

To be able to bypass undesirable processing I have actually bypassed bass management completely and operate the subs by simply telling the system it has no sub and sending it all to the mains. I then split the outputs between my subs and actual mains. It works fine.
Double bass simply sends the main front speakers bass to the sub but the LFE does NOT get rerouted to the mains unless you turn the sub channel off. As far as I know, only custom processors like Datasat, Storm Audio and Emotiva give options to route LFE to the mains and to the subwoofer channel(s) simultaneously.
 
Matthew J Poes

Matthew J Poes

Senior Audioholic
Double bass simply sends the main front speakers bass to the sub but the LFE does NOT get rerouted to the mains unless you turn the sub channel off. As far as I know, only custom processors like Datasat, Storm Audio and Emotiva give options to route LFE to the mains and to the subwoofer channel(s) simultaneously.
That’s my understanding as well.

I tried to convince another boutique manufacturer of affordable gear (that I believe we are both fans of) to provide that level of flexibility into their processor. Unfortunately they felt that such flexibility is too confusing for the average user and makes the product too difficult to support. I wish they would offer some kind of pro-setup mode that is harder to access, but still give us that flexibility.

Can we start a petition here on Audioholics to get this changed?
 
lovinthehd

lovinthehd

Audioholic Jedi
I'd love to see more options with bass and LFE management, including varied crossover slopes. There is always going to be the issue in consumer electronics as to usability by the average user. Be nice if the basic menu could start with a beginner vs advanced menu for setup....might help all of us.
 
Matthew J Poes

Matthew J Poes

Senior Audioholic
I'd love to see more options with bass and LFE management, including varied crossover slopes. There is always going to be the issue in consumer electronics as to usability by the average user. Be nice if the basic menu could start with a beginner vs advanced menu for setup....might help all of us.
In my conversation with the individual responsible for both writing the bass management software in the processor as well as handling a great deal of the technical assistance calls, it seems that giving people access to advanced options is too lenient. Basically, people who don't have advanced capabilities have false illusions of grandeur (don't we all?) and end up going down that path anyway. As I understand it, they did in fact put up barriers to accessing the more advanced bass management option, but people chose it anyway, then complained about boomy bass.

The implementation of multiple low-frequency sources requires a certain base level of knowledge. As I point out in the video, it really isn't that complicated, it's more a tedious process. Even a single subwoofer with two full range speakers is still three low-frequency sources. Integration requires a bit of skill, knowledge, and potentially, measurements. If measurements are taken, that user has to know how to take them properly for this purpose, how to interpret them, and how to make use of that knowledge to integrate everything. Unfortunately, I read a lot of guides on bass integration that seem to stem from inaccurate information and assumptions. I think a lot of ideas that became standards today were chosen because they were easily accepted by consumers and simplified the concept of a home theater. Not because they were ideal.
 
gene

gene

Audioholics Master Chief
Administrator
That’s my understanding as well.

I tried to convince another boutique manufacturer of affordable gear (that I believe we are both fans of) to provide that level of flexibility into their processor. Unfortunately they felt that such flexibility is too confusing for the average user and makes the product too difficult to support. I wish they would offer some kind of pro-setup mode that is harder to access, but still give us that flexibility.

Can we start a petition here on Audioholics to get this changed?
I've been petitioning Denon and Yamaha for years. No luck. If you want that, you will have to goto products from Storm Audio, DataSAT and Emotiva (they listen). Sucks but it took me years to get Yamaha to change their 90Hz crossover for bass management. Maybe some day....
 
gene

gene

Audioholics Master Chief
Administrator
In my conversation with the individual responsible for both writing the bass management software in the processor as well as handling a great deal of the technical assistance calls, it seems that giving people access to advanced options is too lenient. Basically, people who don't have advanced capabilities have false illusions of grandeur (don't we all?) and end up going down that path anyway. As I understand it, they did in fact put up barriers to accessing the more advanced bass management option, but people chose it anyway, then complained about boomy bass.

The implementation of multiple low-frequency sources requires a certain base level of knowledge. As I point out in the video, it really isn't that complicated, it's more a tedious process. Even a single subwoofer with two full range speakers is still three low-frequency sources. Integration requires a bit of skill, knowledge, and potentially, measurements. If measurements are taken, that user has to know how to take them properly for this purpose, how to interpret them, and how to make use of that knowledge to integrate everything. Unfortunately, I read a lot of guides on bass integration that seem to stem from inaccurate information and assumptions. I think a lot of ideas that became standards today were chosen because they were easily accepted by consumers and simplified the concept of a home theater. Not because they were ideal.
This is why for 90% of installs, it's best to run satellite speakers all around, multi-sub and 80Hz crossover. It really is the easiest thing to setup and understand for most consumers. Integrating tower speakers with subs is very tricky business.
 
T

theshade

Audioholic Intern
As the title states this is a recording of a talk I gave on the Geddes approach to subwoofer setup. It covers both theory and execution. While the content had direct input from Earl Geddes, the views expressed are my interpretation.

Regarding multisub, welti says that the subs should be similar thus I would like to implement a sub approach more in line with geddes with the caveat that I cannot run my mains full range as they are ported genelec 8030s. Can I run the mains full range if it is with a jbl lsr310s(studio sub with high passed outputs) then use the sub out of an avr to a minidsp where I would connect other subs. I recognize that this is not truly Geddes as he does not advocate crossovers.

In the geddes approach, where should people put their strongest performing sub? Is it the first one in a corner closest to the mains. I have searched and watch videos both of geddes and your video and posts but I cannot find detailed instructions. I have seen others who interpret geddes as placing their best sub first closest to the mains in the corner and their worst sub last.

Thanks!
 
Matthew J Poes

Matthew J Poes

Senior Audioholic
Regarding multisub, welti says that the subs should be similar thus I would like to implement a sub approach more in line with geddes with the caveat that I cannot run my mains full range as they are ported genelec 8030s. Can I run the mains full range if it is with a jbl lsr310s(studio sub with high passed outputs) then use the sub out of an avr to a minidsp where I would connect other subs. I recognize that this is not truly Geddes as he does not advocate crossovers.

In the geddes approach, where should people put their strongest performing sub? Is it the first one in a corner closest to the mains. I have searched and watch videos both of geddes and your video and posts but I cannot find detailed instructions. I have seen others who interpret geddes as placing their best sub first closest to the mains in the corner and their worst sub last.

Thanks!
That level of detail isn’t something you would get out of Geddes. His approach was advocated based on speakers that performed like his.

His idea wasn’t about mixing strong and weak. It was about their optimal range of operation. He would advocate for the ULF subwoofer to be in the corner. Then to put the others in other locations as per the instructions. As for that corner needing to be closest to the mains. Sure, but that kind of misses the point. It wouldn’t absolutely have to be that corner. It could work in a rear corner too. For most people it makes the most sense to use one of the front corners.

as for crossovers. To be clear, his mains were large sealed mains with a -3dB point close to 80hz. So there was a natural crossover. What you could do is apply a 2nd order highpass crossover At 80hz in the case of your speaker. He might also advocate to try stuffing the ports. It would help ensure the slow remains shallower. The problem with smaller speakers like those is that the woofers may not take to the lack of port well that they may require some excursion control. Knowing Genelec, they might also have a lot of response shaping for the ported setup. So if Genelec doesn’t offer a sealed mode, personally I would leave it as is and highpass it.

his idea of operating the mains as full range was just a way to increase LF sources. It wasn’t a requirement to work right. If your system is flexible and your speakers Have the dynamic range to handle this, try overlapping substantially the subs and mains. If you can set a low pass of 80hz and a high pass of 50hz, try that. It gives you much of the same benefit.

another thing that would be consistent with the approach and that I have done is to vary the low pass filter on each sub. I might do the sub closest to the mains as 100hz. The next closest at 80hz, and the third closest at 70hz. Something like that. The goal being to have it so the sub closest to the listener has the lowest crossover. It could involve a lot of trial and error.
 
T

theshade

Audioholic Intern
That level of detail isn’t something you would get out of Geddes. His approach was advocated based on speakers that performed like his.

His idea wasn’t about mixing strong and weak. It was about their optimal range of operation. He would advocate for the ULF subwoofer to be in the corner. Then to put the others in other locations as per the instructions. As for that corner needing to be closest to the mains. Sure, but that kind of misses the point. It wouldn’t absolutely have to be that corner. It could work in a rear corner too. For most people it makes the most sense to use one of the front corners.

as for crossovers. To be clear, his mains were large sealed mains with a -3dB point close to 80hz. So there was a natural crossover. What you could do is apply a 2nd order highpass crossover At 80hz in the case of your speaker. He might also advocate to try stuffing the ports. It would help ensure the slow remains shallower. The problem with smaller speakers like those is that the woofers may not take to the lack of port well that they may require some excursion control. Knowing Genelec, they might also have a lot of response shaping for the ported setup. So if Genelec doesn’t offer a sealed mode, personally I would leave it as is and highpass it.

his idea of operating the mains as full range was just a way to increase LF sources. It wasn’t a requirement to work right. If your system is flexible and your speakers Have the dynamic range to handle this, try overlapping substantially the subs and mains. If you can set a low pass of 80hz and a high pass of 50hz, try that. It gives you much of the same benefit.

another thing that would be consistent with the approach and that I have done is to vary the low pass filter on each sub. I might do the sub closest to the mains as 100hz. The next closest at 80hz, and the third closest at 70hz. Something like that. The goal being to have it so the sub closest to the listener has the lowest crossover. It could involve a lot of trial and error.
Thanks for the detailed instructions. I was confused because some of what is available online says that the ultra low frequency sub or best sub is in a corner near the mains while Geddes himself says in his pdf on multi sub to "Pickthesubclosesttothemains,thismustbeaHFsub,i.e.itmustoverlapthemains." I interpreted it to be a small sub that could do mid bass. Thank you for enlightening me. Now I know that the ultra low frequency sub would be the one in a corner and overlapped higher with the mains.

I may not be able to do separate high pass low pass on the genelecs if I would use my av receiver.

I have another question though. I watched numerous audioholics videos and you guys advocated similar subs on 4 corners over the 4 midpoints. If the subs are not similar, will the weakest sub act as a bottleneck to maximum performance? What sub can be considered good enough? I know that from your reviews here the svs pb 1000 pro is good enough but it is way overpriced in my country at more than $900. The polk hts-12 costs about $540 dollars. Is it good enough? Multiple threads in different forums and in audioholics videos as well say that multisub non megabuck subwoofers are better than one super sub. However you and gene also said in your criticism of atmos that the push for multi channel lowers the performance of bought speakers. As resources are finite, people tend to spend less per speaker. I also remembered you sort of defending geddes subs in another forum as some people misconstrue the quality of his subs as they are cheap. What is considered good enough for subwoofers if implemented multi sub? I guess my fronts (genelec 8030c) are good enough although spl limited.

Once again, thanks a lot for taking the time to answer. Even though this is one of my first posts, I have learned a lot from your posts in this and other forums with regard to the geddes approach and the abbey speakers. I also enjoyed your videos.
 
Matthew J Poes

Matthew J Poes

Senior Audioholic
Thanks for the detailed instructions. I was confused because some of what is available online says that the ultra low frequency sub or best sub is in a corner near the mains while Geddes himself says in his pdf on multi sub to "Pickthesubclosesttothemains,thismustbeaHFsub,i.e.itmustoverlapthemains." I interpreted it to be a small sub that could do mid bass. Thank you for enlightening me. Now I know that the ultra low frequency sub would be the one in a corner and overlapped higher with the mains.

I may not be able to do separate high pass low pass on the genelecs if I would use my av receiver.

I have another question though. I watched numerous audioholics videos and you guys advocated similar subs on 4 corners over the 4 midpoints. If the subs are not similar, will the weakest sub act as a bottleneck to maximum performance? What sub can be considered good enough? I know that from your reviews here the svs pb 1000 pro is good enough but it is way overpriced in my country at more than $900. The polk hts-12 costs about $540 dollars. Is it good enough? Multiple threads in different forums and in audioholics videos as well say that multisub non megabuck subwoofers are better than one super sub. However you and gene also said in your criticism of atmos that the push for multi channel lowers the performance of bought speakers. As resources are finite, people tend to spend less per speaker. I also remembered you sort of defending geddes subs in another forum as some people misconstrue the quality of his subs as they are cheap. What is considered good enough for subwoofers if implemented multi sub? I guess my fronts (genelec 8030c) are good enough although spl limited.

Once again, thanks a lot for taking the time to answer. Even though this is one of my first posts, I have learned a lot from your posts in this and other forums with regard to the geddes approach and the abbey speakers. I also enjoyed your videos.
Thank you again for the compliments.

let me start by asking what country you are located in. I sometimes have contacts I work with in other countries who could figure out a good solution.

The Polk seems ok for the money. Not sure how it performs. Hard to say without seeing measurements.

our criticism of ATMOS or really of ever higher channel count is a bit different than advocating multisub. Basically, the sound quality of a speaker is largely dictated by the speaker. Not the number of channels. So better speakers cost more money and thus take resources away when you have to distribute them across more channels. By comparison, bass quality is largely dictated by the summed effect of the subwoofer and Room. Cheap subwoofers can be made to sound better than expensive ones if the expensive one is used in isolation and has a terrible response at your listening position. By comparison the multisub could sound better by virtue of being smoother and more consistent.

but 4 subs gives about 4dB of gain over a single sub. So if the individual sub is more than 4dB off your needed output level and 1 super sub would meet your needs, you may have a problem. In this case, a $2000 subwoofer very well may exceed that Polk by more than 4dB. In that scenario, I suggest two $1000 subs which are within 3dB of your needed output.

when specificying equipment you really need to think about this as a kind of performance metric. What do you need the system to be able to do. From there you have to scrutinize the equipment to be sure that the sum of those parts meets your needs. In this case, if 1 of those Polks is close to meeting your needs, 4 should be fine. But if 1 is them is well short of your needs, 4 may not give enough boost to be helpful.

oh one last complexity you consider. The 4dB is a kind of general rule when sources sub randomly. You get 6dB for coherent sources, 3dB for random sources when you double them. Going from 1-2 is 3dB. 2-4 should be another 3dB but you often don’t see a perfect amount. So to be safe, I sat 4. But let’s say it’s actually going to be 4-6dB. Depending on the size of the room, there will be a point where the sources some coherently. You get 6dB for each doubling of sources. That means you get 12dB more output. Where is that? Probably below 30-35hz in most rooms. Could be as high as 40hz in smaller rooms. So keep in mind that if the area where the Polk falls short is the deeper bass, you will see about 12dB of additional output. However, if a sub is ported and tuned to 30hz, there is so little output at 20hz that the 12dB of boost might not matter. The sub may be well into distress at that point. I recently worked on a system where below 25hz the system was producing 10% THD at just 75dB in room. There is no saving that system. He needs a different sub for clean output below 25hz. And it’s worth noting, that was a $10,000 subwoofer. So price doesn’t always equal quality.
 
T

theshade

Audioholic Intern
Thank you again for the compliments.

let me start by asking what country you are located in. I sometimes have contacts I work with in other countries who could figure out a good solution.

The Polk seems ok for the money. Not sure how it performs. Hard to say without seeing measurements.

our criticism of ATMOS or really of ever higher channel count is a bit different than advocating multisub. Basically, the sound quality of a speaker is largely dictated by the speaker. Not the number of channels. So better speakers cost more money and thus take resources away when you have to distribute them across more channels. By comparison, bass quality is largely dictated by the summed effect of the subwoofer and Room. Cheap subwoofers can be made to sound better than expensive ones if the expensive one is used in isolation and has a terrible response at your listening position. By comparison the multisub could sound better by virtue of being smoother and more consistent.

but 4 subs gives about 4dB of gain over a single sub. So if the individual sub is more than 4dB off your needed output level and 1 super sub would meet your needs, you may have a problem. In this case, a $2000 subwoofer very well may exceed that Polk by more than 4dB. In that scenario, I suggest two $1000 subs which are within 3dB of your needed output.

when specificying equipment you really need to think about this as a kind of performance metric. What do you need the system to be able to do. From there you have to scrutinize the equipment to be sure that the sum of those parts meets your needs. In this case, if 1 of those Polks is close to meeting your needs, 4 should be fine. But if 1 is them is well short of your needs, 4 may not give enough boost to be helpful.

oh one last complexity you consider. The 4dB is a kind of general rule when sources sub randomly. You get 6dB for coherent sources, 3dB for random sources when you double them. Going from 1-2 is 3dB. 2-4 should be another 3dB but you often don’t see a perfect amount. So to be safe, I sat 4. But let’s say it’s actually going to be 4-6dB. Depending on the size of the room, there will be a point where the sources some coherently. You get 6dB for each doubling of sources. That means you get 12dB more output. Where is that? Probably below 30-35hz in most rooms. Could be as high as 40hz in smaller rooms. So keep in mind that if the area where the Polk falls short is the deeper bass, you will see about 12dB of additional output. However, if a sub is ported and tuned to 30hz, there is so little output at 20hz that the 12dB of boost might not matter. The sub may be well into distress at that point. I recently worked on a system where below 25hz the system was producing 10% THD at just 75dB in room. There is no saving that system. He needs a different sub for clean output below 25hz. And it’s worth noting, that was a $10,000 subwoofer. So price doesn’t always equal quality.
I'm from the Philippines. The pollk hts-12 is recommended in audioholics budget subs under $500 but has no measurements. It is nice to know about that 12db more output if four subs in lower bass. I already have 2 cheap but good diy subs that is most likely better than the polks but way more ungainly in size. That is why I am asking about bottlenecks. If I add a sub or two that is worse in performance. However, you also suggested 2 $1000 dollar subs which sort of suggest the minimum quality needed. I was thinking of the geddes approach with one great for the ulf and other not as good if they would bottleneck as the third sub would just be added after the response of the mains and two subs is equal and to fill in the dips or excite more room modes. If combining subs different in maximum output is possible then I could wait and just save up for even a used svs pb13 ultra then cheaper subs like the polk. If there has to be a minimum bar regarding quality then 2 svs pb 1000 pro even if they are overpriced and i have to save longer. As the Geddes summas also act as other low frequency sources, that is 3 front and center + his subs = 6 low frequency sources. I already have 2 diy subs. Thanks!
 
Matthew J Poes

Matthew J Poes

Senior Audioholic
I'm from the Philippines. The pollk hts-12 is recommended in audioholics budget subs under $500 but has no measurements. It is nice to know about that 12db more output if four subs in lower bass. I already have 2 cheap but good diy subs that is most likely better than the polks but way more ungainly in size. That is why I am asking about bottlenecks. If I add a sub or two that is worse in performance. However, you also suggested 2 $1000 dollar subs which sort of suggest the minimum quality needed. I was thinking of the geddes approach with one great for the ulf and other not as good if they would bottleneck as the third sub would just be added after the response of the mains and two subs is equal and to fill in the dips or excite more room modes. If combining subs different in maximum output is possible then I could wait and just save up for even a used svs pb13 ultra then cheaper subs like the polk. If there has to be a minimum bar regarding quality then 2 svs pb 1000 pro even if they are overpriced and i have to save longer. As the Geddes summas also act as other low frequency sources, that is 3 front and center + his subs = 6 low frequency sources. I already have 2 diy subs. Thanks!
There is a minimum quality. Basically, as you add subs, you have to set the volume such that you get up to the level you need to be at. At a certain point what happens is that you are having multiple subs working together, but during dynamic peaks, the weak link overloads. When Geddes developed this approach, he wasn't telling people to go buy cheap subs and add them to a great sub and see what happens. It was about using 3 full range speakers, 2 midbass modules, and 1 ULF subwoofer. All speakers could output equal amounts at say 50hz, and none of them would overload under normal conditions at maximum input power. His bandpass subwoofers, for example, didn't hit xmax until after more than 1000 watts (unless a lot of EQ was added to extend their response). Same for the mains, they won't exceed xmax until after 1000 watts is applied, and nobody is going to do that. The enclosures were all very small and that helped ensure linear operation at high output, just not a lot of deep bass. I mean, to give you an idea how good these were, I actually got passing results at 12hz from a 12" his 4th order bandpass subwoofer. It wasn't very loud, but it passed. Most subwoofers cannot do that.

So all that to say, if you have two DIY subs with significant output capability and you plan to add 2 more subs without such output, I think that is risky. But you could always try it if you think it would be feasible to sell them or use them elsewhere if it doesn't work out.
 
T

theshade

Audioholic Intern
There is a minimum quality. Basically, as you add subs, you have to set the volume such that you get up to the level you need to be at. At a certain point what happens is that you are having multiple subs working together, but during dynamic peaks, the weak link overloads. When Geddes developed this approach, he wasn't telling people to go buy cheap subs and add them to a great sub and see what happens. It was about using 3 full range speakers, 2 midbass modules, and 1 ULF subwoofer. All speakers could output equal amounts at say 50hz, and none of them would overload under normal conditions at maximum input power. His bandpass subwoofers, for example, didn't hit xmax until after more than 1000 watts (unless a lot of EQ was added to extend their response). Same for the mains, they won't exceed xmax until after 1000 watts is applied, and nobody is going to do that. The enclosures were all very small and that helped ensure linear operation at high output, just not a lot of deep bass. I mean, to give you an idea how good these were, I actually got passing results at 12hz from a 12" his 4th order bandpass subwoofer. It wasn't very loud, but it passed. Most subwoofers cannot do that.

So all that to say, if you have two DIY subs with significant output capability and you plan to add 2 more subs without such output, I think that is risky. But you could always try it if you think it would be feasible to sell them or use them elsewhere if it doesn't work out.
Thank you for clarifying! This clears up a lot of things for me.
 
W

wgb113

Enthusiast
I’m struggling to apply all of the knowledge out there around room modes and multi-subs to my future square (14 x 15 x 9) square room for a 2 channel setup with one listening position. Everything I’ve seen up to this point seems concerned with smoothing out the bass over a listening area, typically in a HT environment.

Is the multi-sub approach where someone in my position should start in order to get the smoothest bass possible? Or will it just be setting aside a weekend of trial and error through positioning and measurements?

I appreciate the work @gene has done on the series around bass and rooms with @Matthew J Poes and Anthony Grimani.
 
ryanosaur

ryanosaur

Audioholic Spartan
Setting up multiple Subs can still be worthwhile, but it has been argued by many that if you are only worried about a single seat then you can forgo them for one well placed Sub.
That said, I have a small room which I use for my personal listening room and theater... one seat... 2 Subs. :) To me it's worthwhile.
Fortunately, your room is not truly square. Where an inch can make a significant change in acoustic performance, you've got a whole foot to play with by your measurements. ;)
Probably the easiest way to hear how bass will perform in your room is to use the Subwoofer Crawl. It is crude, but effective.
Also, if your room is rectangular and sealed from the rest of your home, you can fall back on the Welti-Devantier approach to using symmetrical placements like opposing corners, 1/4-wall, 1/2-wall, etc. The key thing about this latter approach is that it is predicated on pairs of subs added.
My understanding of Geddes, on the other hand, is that he encourages removing Subs from the front wall, especially if your Mains are capable of full range, and placing them one at a time in the best measuring spots around the room.
Perhaps I got lucky in my room, using a strategy based on Geddes and the subwoofer crawl combined. I have not sat down to measure everything as I should to prove my setup, but the "before" graph from my Audyssey App is pretty good.

More importantly, and easily overlooked is also how you set up and place the rest of your speakers and LP. It is very important to keep your mains off the front wall by at least a foot (and I also prefer to keep mine out of the corners by at least 2.5'), toe in, etc. Likewise, Center placement should be strongly respected and is just as important as the L/R. Keeping the Baffle of the Center at least 1 foot away from the screen, aiming it towards your shoulders (I never like being directly on axis, but within 15º is usually very good)...
In the end, when your room is well adjusted and you are getting good measurements from the speakers, adding the subs one at a time and taking measurements to confirm their placement as you go would be the Geddes approach. Or place a pair or 2 of Subs symmetrically in your room a la Welti-Devantier.

Lastly, considering your question here, are you set up with REW and a Umik1 to do your own measurements?
 

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