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

Matthew J Poes

Matthew J Poes

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306
#1
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.

 
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shadyJ

Speaker of the House
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#2
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

Full Audioholic
Ratings
306
#3
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

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#4
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

Full Audioholic
Ratings
306
#5
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

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#6
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

Full Audioholic
Ratings
306
#7
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 Slumlord
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#8
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

Full Audioholic
Ratings
306
#9
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

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#10
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
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4,808 25 9
#11
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.
 

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