• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). There are daily reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

Advice multiple subs/poor mans Geddes Method

airmail

Member
Joined
Mar 19, 2022
Messages
22
Likes
10
Hi all,

So I've got the go ahead to do this from the missus and despite her thinking that I'm nuts, she loves the living room home theatre I put together during lockdown.

My setup is currently a Denon 3700 and an NC252 amp driving a 5.2.2 setup including a pair of Wharfdale Linton Heritage speakers, a Wharfdale Evo 4.c centre channel, q acocustic 3010 surrounds and Dali upfirers. My subs are two SVS SB1000 pros up front between the mains, I couldn't fit a larger sub to the left or right so went with two smaller subs for output.

Couch is right up against the back wall so I have a 120*60cm GIK acoustics Monster Bass trap on the back wall.

Anyhoooo, I have two massive nulls at around 50 and 80hz when measuring using a UMIK and REW.

It's my understanding that the Geddes method requires both 20hz capable mains and an AVR that can route large Mains to LFE in double bass mode but LFE to the mains as well - so a handful of boutique AVR's?

Here the distortion of my Lintons looks like it would be acceptable down to about 40-45hz? I haven't learnt to read these distortion graphs yet so feel free to shoot me down.


So the "shower-thoughts" solution I had today is to run my mains in small at a 40 or 50hz crossover and:

1) Get a set of BK XLS200's, place on back wall - y-splitter from the mains pre outs into a minidsp and then from that into one of the low-level connections on the sub. Use mini-dsp to roll off the low end of the XLS200 so they don't drive into distortion trying to keep up with the SB1000 pros. Connect in the mono LFE as well, but avoid the mini dsp so they can play the full 20-120hz LFE channel range - but perhaps it isn't necassary to bypass the mini-dsp? This way, the nulls get cancelled on both the mains and the LFE channel.

2) Get two more SB1000 pros - do the same as above but with no mini-dsp due to the onboard DSP - means that I can't have separate high pass filters for each channel.

3) Get two more SB1000 pros - use "high-level" from mains to first back wall sub and use onboard DSP. Send LFE to second back wall sub to cancel nulls in LFE channel.

4) One SB3000 micro and one SB1000 pro - do something similar to any of the above but stick the micro on a shelf that's on my back wall.

Thoughts?

I'm favouring the idea of two more SB1000 pros - because if my experiment fails - I can just stack them ontop of my current SB1000 pros, get a mini DSP and get more extension out of them.
 
Last edited:
OP
A

airmail

Member
Joined
Mar 19, 2022
Messages
22
Likes
10
Also, as Geddes calls for lots of LFE sources and runs his mains full range. I could potentially go a little more nuts with my modified method and turn off the AVR's onboard low pass subwoofer filter - run the mains preouts into ALL of the subs through a minidsp and go nuts with sperate overlapping cross overs and EQ for the mains and a separate EQ and 120hz roll off for the LFE channel. The problem I see with doing this is that it introduces more DSP delay, which would be a problem on the subs located at the front of the room. Which is why I'm thinking of only sending mains information to the rears and sending LFE content to all four of the subs.

I guess I could just be normal and use the 80hz thx crossover? Just doesn't seem as much fun in doing it that way and from what I have read, it seems like the Geddes method brings some benefits in terms of total output and frequency smoothness.

I think I might have had too much wine, so be patient with me.
 
Last edited:
OP
A

airmail

Member
Joined
Mar 19, 2022
Messages
22
Likes
10
For info, in room response of the Lintons means they roll off naturally at around 30hz when I measured using REW.
 

raindance

Addicted to Fun and Learning
Joined
Sep 25, 2019
Messages
759
Likes
671
The monster bass trap likely won't do anything to the measured frequency response at the frequencies you have nulls at. Where is it mounted - sounds like it isn't in a corner? It needs to be in a location where bass builds up, typically a corner or a wall-floor or wall-ceiling transition. At such low frequencies, placement and EQ will do the best job. Typical low cost bass traps will do little except shorten the decay time of certain bass frequencies.

One of the nulls may be SBIR determined by either speaker to front wall or sub to wall distance. Sitting close to the back wall, especially in room center, will cause a large bass boost at the seating position and there will be deep nulls also; ie it's not great for even bass. A little asymmetry might help in the layout. Of course I don't know what it looks like...

How did you measure? Single location or averaging across multiple?

Also running the mains with LFE included will just downgrade power handling. I doubt any benefit will be worth it. Stick with the 80Hz crossover, but experimenting with a little lower or a little higher may help with nulls.

Post a sketch or picture so we can see what you have.
 

Sancus

Major Contributor
Forum Donor
Joined
Nov 30, 2018
Messages
2,321
Likes
5,860
Location
Canada
The "Geddes method" is sort of a mess that differs depending on which document you read or which video you watch, I wouldn't try to follow it to the letter. The placement advice can be helpful, but it really boils down to: With multiple subwoofers, you want them as decorrelated as possible, which means they have different peaks and nulls. If they have the SAME peaks and nulls, they're too close together or misplaced. The specific frequencies aren't important.

So here's my advice:

1) I don't think you need more subs right now. You do need a MiniDSP 2x4HD, so get one of those if you don't already have one.

2) Measure your speakers alone. Do either of them show the 50hz or 80hz nulls? If they do, then I would advise a crossover higher than this, 80-100hz, to hopefully eliminate them. In general, speakers with 8" and 6.5" woofers(the center) are not very well suited to playing home theatre SPLs below 80hz anyways.

3) Regardless of what crossover you pick, learn to use Multi-Sub Optimizer and do 1 sub out to the MiniDSP using it as a splitter. Punch in the values from MSO. There are some helpful miniDSP tutorials, a video, and the manual for MSO is good as well (if long).

4) Run Audyssey after the MSO optimization. Then measure to see where you are after that.

If your problems are solved, you're good to go. If you're still not happy with the bass measurements after that point, you could consider adding 1 or 2 more subs. I was able to achieve excellent results over 3 seats even with just 2 subs though. YMMV. But in general MSO produces much better results than any manual process or AVR room EQ.
 

Pogre

Active Member
Joined
Mar 5, 2022
Messages
209
Likes
250
How big is your room? A pair of SB1000 Pros would be pretty tame unless your room is pretty small.
 
OP
A

airmail

Member
Joined
Mar 19, 2022
Messages
22
Likes
10
Can I measure up the room properly tomorrow and get you guys a drawing? Can't go sticking bass traps in corners as that will annoy the misus proper, the monster trap was acceptable on the back wall because that back wall is big flat and ugly and I've sort of blended it in with shelving. It did help with some psychoacoustic reflections I was getting off the back wall.

Room is quite large though, 14 foot by 25 foot rectangle roughly with an 8.2 foot floor to ceiling height - short axis used as it has a small kitchen in the corner (London apartment). Walls are all hugely thick solid concrete, so annoying the neighbours isn't an issue (I know upstairs and downstairs) and they've never heard a thing or at least have assumed that any rumble is jets taking off from Heathrow. Walls are 50 percent glazed with massive floor to ceiling windows, which made the room sound bright until I damped it down a bit.

I'll use REW to measure in multiple positions perhaps next weekend as one of the SVS SB 1000 pros is getting replaced as it's a warranty job, so this might be in a week or two!

Regarding the output of SB1000 pros I could then sell the replacement as a brand new sub or get my dealer to upgrade both - just can't think how I'd fit a bigger sub in without offsetting the media cabinet and annoying the wife - or running a large ported right next to the couch and getting more localisation issues. I would love bigger subs but they are a huge pain to fit in the room in terms of how it's setup, I'd have to go tall with multiple drivers - or get a huge ported sub to the right hand side between one sidewall and the couch. Unfortunately I loved my Lintons too much to make space by getting rid of them when I decided I wanted some subs - they'll get prised out of my cold dead hands - the only speaker that hasn't pissed me off massively without EQ in this glassy room.

When I was running dual subs (and one hadn't died) - the system was capable of hitting 20hz in room at levels that caused me to worry I'd crack the windows - both subs set up at -14db over the standard -10db in the SVS app so I'm assuming I'm getting about 4db of output gain over a single SB1000 Pro. My curve followed a rough Harmann approximation - 2/3db boost in the bass region gradually tapering off into the midrange. The only issue was the gigantic nulls.
 
Last edited:

Pogre

Active Member
Joined
Mar 5, 2022
Messages
209
Likes
250
I have a similar space and had a pair of SB1000s (not pro) and they were woefully undersized and underpowered for the space. I moved to larger ported subs and holy crap, I had no idea what I was missing! Huge, huge difference and it wasn't all just boom and bang. I had it stuck in my head that sealed subs were better for music. I was wrong. My ported subs sound just as good and musical, better even, because they're big enough for the room. I am absolutely a ported sub convert now.

A pair of PB1000 Pros might be sufficient for that space, but I'd even consider the 2000 series. The PC2000 would have a smaller footprint then the PB and be much more suited to your large room. You'd need like 6 or 8 SB1000 Pros to match the output of two PC2000 Pros. They're simply too small for that room. You're already in the SVS ecosystem and they have a great trade up program...

Oh, and I wouldn't even consider the Micro. That thing is a lifestyle product that prioritizes aesthetics over sound. They aren't even true subs. Maybe if you installed it right under your seat...
 

FrantzM

Major Contributor
Forum Donor
Joined
Mar 12, 2016
Messages
3,168
Likes
5,024
Hi

And Welcome to ASR.
If it is your first trial keep it simple.
I , am a fan of the geddes method, it provides good results. You need however to keep it simple and ... it will take some time to get the best results. Please, keep it simple. DO not make it more complicated than it has to be.

You may need a miniDSP 2x4HD, for the subwoofers, do not put it in the main speakers path... Resist, Don't.
....
The "Geddes method" is sort of a mess that differs depending on which document you read or which video you watch, I wouldn't try to follow it to the letter. The placement advice can be helpful, but it really boils down to: With multiple subwoofers, you want them as decorrelated as possible, which means they have different peaks and nulls. If they have the SAME peaks and nulls, they're too close together or misplaced. The specific frequencies aren't important.

So here's my advice:

1) I don't think you need more subs right now. You do need a MiniDSP 2x4HD, so get one of those if you don't already have one.

2) Measure your speakers alone. Do either of them show the 50hz or 80hz nulls? If they do, then I would advise a crossover higher than this, 80-100hz, to hopefully eliminate them. In general, speakers with 8" and 6.5" woofers(the center) are not very well suited to playing home theatre SPLs below 80hz anyways.

3) Regardless of what crossover you pick, learn to use Multi-Sub Optimizer and do 1 sub out to the MiniDSP using it as a splitter. Punch in the values from MSO. There are some helpful miniDSP tutorials, a video, and the manual for MSO is good as well (if long).

4) Run Audyssey after the MSO optimization. Then measure to see where you are after that.

If your problems are solved, you're good to go. If you're still not happy with the bass measurements after that point, you could consider adding 1 or 2 more subs. I was able to achieve excellent results over 3 seats even with just 2 subs though. YMMV. But in general MSO produces much better results than any manual process or AVR room EQ.
+1

Was about to reply when I saw this. Written in a much clearer way that I could muster. That’s what I did, except too lazy and too pleased thus far to go MSO …
 
Last edited:
OP
A

airmail

Member
Joined
Mar 19, 2022
Messages
22
Likes
10
Hey guys,

Thanks for the replies and welcoming me to ASR :) Can I say I love this hobby? Love having something (speakers) in my life that doesn't seem like a disposable product that I will replace next year for the next great thing.

I'll try and dig out my measurements tomorrow, during lockdown I used a method that I use for setting up racing suspension - called bracketing where you try extremes of settings and move inwards to a central setting eg 0 phase, then 180 phase, then 10 phase, then 170 phase. I did this for each sub after level matching them, for distance and phase and watched the REW curve - taking log notes as I did it. Once I did that I pulled down the peaks using the on board DSP. Off the top of my head this got me roughly +/-5db curves with the odd complete and total null that was there no matter what I did. One sub is in a corner and one is mid way along the same wall - there was a huge improvement over just using one sub even if they are located on the same wall.

There's two issues here:

One is total output, that's why I tried to keep them close enough to get a bit of a boost - the other is frequency response.

Would I lose SPL by moving one sub to the back mid wall?
 

Grur

Member
Joined
Feb 19, 2019
Messages
81
Likes
33
I have a similar space and had a pair of SB1000s (not pro) and they were woefully undersized and underpowered for the space
While more headroom is a good thing, I'm not sure about 'woefully' undersized. Audioholics gave the SB1000 pro a medium room size rating, which is 1500 to 3000 ft3, and the OP's room is around 2800 ft3. The pro also has 1-2 dB more output than the non-pro, due to the slightly more powerful amp. But I agree the 1000 is not a big room sub and might be best kept to rooms under 2000ft3 or so.
 
Last edited:
OP
A

airmail

Member
Joined
Mar 19, 2022
Messages
22
Likes
10
Room is actually a little smaller than that as it's 21 foot by 14 foot. Not sure what the huge amount of glazing does as well? I've heard windows act like huge bass traps but this may be wrong?

Apologies for my chicken scratch handwriting - I'm hungover.

FD6B796F-0E74-4F00-B96D-68D75F8B8580.jpeg


I could fit a ported sub down the right hand side of the sofa, in the back corner - and then I could offset my media cabinet and fit a ported between the media unit and the left hand Linton.

It's just setting my aesthetics OCD off as the thing would be taller than the nice linton stand and would block the side view of it by being jammed right up against the Linton - also the offset cabinet will make me go "hngggggggggggggggggggg" everytime I see it. This missus loves it, not only for the sound but partly because the setup looks cool and retro as well - I don't want to test my luck so far guys by getting a great big 200lb weapon of bass destruction. I could potentially in two subs the size of a Monoprice THX 12 or an Arendal 1961V in with some aesthetic compromise.

Could I get a huge but relatively pancake flat cabinet made up that would fit under the sofa, with a ridiculous array of 15 inch JBL drivers?

Use is 50/50 Home Theatre and Music, the rest of my family are all professional musicians so music is also a huge part of my life.
 
Last edited:
OP
A

airmail

Member
Joined
Mar 19, 2022
Messages
22
Likes
10
So looking at the audioholics bassaholic spreadsheet found in the subwoofer comparison thread, it seems even the THX 10 inch unit has considerably more output than the SB 1000 pro - those aren't too huge and wouldn't require me offsetting the media cabinet.

Those are pretty cheap over here in the UK compared to SVS, 425 quid versus 630. I could sell mine, replace them with the THX 10 units - I can just about fit two units along the front wall and another two on the back wall either side of the sofa (or one in back right corner and one on the left hand sidewall) and have the budget left over for a minidsp?
 
OP
A

airmail

Member
Joined
Mar 19, 2022
Messages
22
Likes
10
Also,

I could fit a larger PB2000/PB3000 class ported sub down the right hand side of the sofa, but there would be nowhere else in the room to fit a second.

So I would be looking at having to run the large ported subwoofer full range and the SB 1000 pros as mid bass units and roll them off so they can keep up with such a large ported without driving to distortion.
 

Pogre

Active Member
Joined
Mar 5, 2022
Messages
209
Likes
250
While more headroom is a good thing, I'm not sure about 'woefully' undersized. Audioholics gave the SB1000 pro a medium room size rating, which is 1500 to 3000 ft3, and the OP's room is around 2800 ft3. The pro also has 1-2 dB more output than the non-pro, due to the slightly more powerful amp. But I agree the 1000 is not a big room sub and might be best kept to rooms under 2000ft3 or so.
For some reason I thought I'd read vaulted ceilings and had 4000³+ in my head. My room has similar, but bigger dimensions, vaulted ceiling and multiple openings to other rooms. I calculated just under 6000³ in total for me, so yes, they really were woefully undersized for my room. My room is quite a bit bigger tho...
 

Pogre

Active Member
Joined
Mar 5, 2022
Messages
209
Likes
250
Also,

I could fit a larger PB2000/PB3000 class ported sub down the right hand side of the sofa, but there would be nowhere else in the room to fit a second.

So I would be looking at having to run the large ported subwoofer full range and the SB 1000 pros as mid bass units and roll them off so they can keep up with such a large ported without driving to distortion.
I was a bit off in calculating your room size. I still think ported would be better for more output and headroom, but you could get away with a pair of PB1000 Pros if you can manage to make them fit.

I've never set up a mid bass module but if you have capable subs, and SVS make very capable subs, I don't think you'd need one.
 

abdo123

Master Contributor
Forum Donor
Joined
Nov 15, 2020
Messages
6,182
Likes
6,157
Location
Brussels, Belgium
I don’t think MSO is worth it for any frequencies above 100Hz or when you have less than four subwoofers.

It’s much more important to model the room modes on a computer and use the model to figure out where to place your two existing subs.
 
Top Bottom