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Advice multiple subs/poor mans Geddes Method

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I have no idea how to go about modelling the room in simulation software.

The 50hz cancellation corresponds to the1/4 wavelength of the coffee table and the 80hz cancellation corresponds to the 1/4 wavelength to the ceiling according to this:


So I'm assuming a nearfield sub and one at height would help to mitigate both nulls - which is why I suggested a Micro SB3000 on the shelf behind me.

So my plan so far is:

When my second sub is back from warranty I'll play around with the position and see if the height idea works. I'll see if I can improve the frequency response by better placement of one of the subs. I have an overbuilt heavy duty shelf that could take the weight of the SB1000 Pro - still makes me a little paranoid having a 30lb lump hanging over my head though.

If that works, then I'll look at trying to improve output and extension. The SB1000 pros would then be roughly in the middle of the length way walls, I could the high pass them so they put out negligible volumes below 30hz. Then I could just about fit two PB2000/Monoprice 12 sized subs into the room - one hidden under a desk on the far left hand side of the room next to the left hand window - and one behind the sofa. I could tune these to deal with 30hz and below. The icing on the cake is that these should cancel out the 1/4 wavelength cancellations from the 6.4 meter lengthways wall to wall distance. I'm unsure how hard it would be to combine sealed and ported subs - so would definitely look at a miniDSP and MSO to do this.

Does this sound sensible or crazy? Should I just go and get an audio specialist to simulate the room for me?

I don't need a splitter unless I go to four subs abdo.
 
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Pogre

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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.
I've never used MSO, but I do have REW. I've also never tried to computer model my room either but I'd like to. I see REW has something along those lines but my room is so complicated I'm not even sure where to start. This is my layout, with the usual living room furnishings. "MLP" is part of a big sectional couch that bends in an L shape and up against the long wall opposite the den.

download_20200329_104128-1-1418x1983 (1)_copy_567x793_copy_425x594.jpg


Add to that, vaulted ceilings to boot. It looks like a potential nightmare to model. I've read that REW would likely be too limited to try modeling a room like mine.
 

Pogre

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I have no idea how to go about modelling the room in simulation software.
Me either! I'd be keen to learn tho if it doesn't require a degree and hundreds of dollars.
 

abdo123

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I've never used MSO, but I do have REW. I've also never tried to computer model my room either but I'd like to. I see REW has something along those lines but my room is so complicated I'm not even sure where to start. This is my layout, with the usual living room furnishings. "MLP" is part of a sectional that bends in an L shape and up against the long wall opposite the den.

View attachment 193946

Add to that, vaulted ceilings to boot. It looks like a potential nightmare to model. I've read that REW would likely be too limited to try modeling a room like mine.
Thanks to @NTK i have manged to simulate the room modes of my complex shaped room too!

You can read the story here: https://www.audiosciencereview.com/...ency-response-simulation-low-frequency.24850/

And the results here: https://www.audiosciencereview.com/...-your-in-room-measurements.13540/post-1110099

putting the subs in the 'correct' corners can make the world of difference.
 

abdo123

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I have no idea how to go about modelling the room in simulation software.

The 50hz cancellation corresponds to the1/4 wavelength of the coffee table and the 80hz cancellation corresponds to the 1/4 wavelength to the ceiling according to this:


So I'm assuming a nearfield sub and one at height would help to mitigate both nulls - which is why I suggested a Micro SB3000 on the shelf behind me.

When my second sub is back from warranty I'll play around with the position and see if the height idea works.

I don't need a splitter unless I go to four subs abdo.
your room is a shoe-box shaped so should be very easy to simulate.


to reduce seat to seat variation you want to place two subs each in a different pole of the mode.

To get rid of nulls you need to move either the subs or the listening position away from extreme ends of a pole.
 

Pogre

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Thanks to @NTK i have manged to simulate the room modes of my complex shaped room too!

You can read the story here: https://www.audiosciencereview.com/...ency-response-simulation-low-frequency.24850/

And the results here: https://www.audiosciencereview.com/...-your-in-room-measurements.13540/post-1110099

putting the subs in the 'correct' corners can make the world of difference.
I believe it. Thanks for the links.

It makes complete sense to me to try leveraging a room's acoustics instead of fighting them. Eq can only do so much, and the less you need the better.
 
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Ok, once I've got my SB1000 back from Chris at AmericanAudio - I'll run the subs in the positions I'm suggesting. I'll graph the frequency response for each position and then work out whether I can do this with acceptable results. The problem is, is that I can't put subs in all four corners due to the kitchen (the missus will notice if I replace the microwave with a subwoofer). I also cant put subs in all four mid points on the walls - because of a bloody great big fridge.

I can do:

1) Three corners or three midway points

2) Two small subs along perfect midway points along the length ways wall and two corners with big subs (top left hand corner and bottom right hand corner)

3) Two small subs along perfect midway midway points along the length ways wall and two less than perfect midway points with big subs along the width ways wall - each sub would be off the midway point by about a foot.

Those are my options without winding the other half up as I can keep the big subs acceptably discreet. I'm completely happy to spend days dialling the subs in as well, as I am used to bracketing high performance suspension which is a nightmare when you have high speed rebound, low speed rebound, high speed compression, low speed compression, hydraulic bottom out and spring rate to factor in.

This forum is awesome, cheers so far guys.

Cheers guys.
 
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abdo123

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Ok, once I've got my SB1000 back from Chris at AmericanAudio - I'll run the subs in the positions I'm suggesting. I'll graph the frequency response for each position and then work out whether I can do this with acceptable results. The problem is, is that I can't put subs in all four corners due to the kitchen (the missus will notice if I replace the microwave with a subwoofer). I also cant put subs in all four mid points on the walls - because of a bloody great big fridge.

I can do:

1) Three corners or three midway points

2) Two small subs along perfect midway points along the length ways wall and two corners with big subs (top left hand corner and bottom right hand corner)

3) Two small subs along perfect midway midway points along the length ways wall and two less than perfect midway points with big subs along the width ways wall - each sub would be off the midway point by about a foot.

This forum is awesome.

Cheers guys.
If your seating position is set in stone, focus on placing the subs to minimize the seat to seat variation of that particular area / seats.

the general guidelines are for rooms that are dedicated for home cinema with like 3 x 3 seats spread over the entire x, y, and z axes.

For a couch in a residential space you're likely only experiencing variation in one axis (x or y).
 
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Hah you nutters simulated a room using Python?

I'm going to have to show the other half this, as she's a Python programmer and data scientist. She'll love that! That's brilliant!
 
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If your seating position is set in stone, focus on placing the subs to minimize the seat to seat variation of that particular area / seats.

the general guidelines are for rooms that are dedicated for home cinema with like 3 x 3 seats spread over the entire x, y, and z axes.

For a couch in a residential space you're likely only experiencing variation in one axis (x or y).

Thanks Abdo! I would quite like to get a smoother bass response at the table if possible, as we like to listen to music on a friday evening whilst chatting and eating food in a civilised manner at the table. :)
 
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Okay guys - I played around with one sub and REW some more. Turns out the 49hz null was a crossover and level issue, it was broad from 49 to 80hz. It'still sort of there - but less so. I got rid of this by sorting phase, level and crossing at 80hz and pulling some peaks down further. The actual null now appears to be at 98ish hz - I'm not even sure it's worth trying to get rid of it? Some significant SBIR nulls higher up caused by running my speakers close to the wall as well.

No smoothing applied - as you can see the current working SB1000 pro rolls off around 25hz in this room.

Feel free to tell me I've done this badly, the single sub is very localisable for me.

Edited FR response.jpg
 

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I suspect I'm running the bass just a little too hot looking at the rough downward tend out to 200hz?
 

Bill Brown

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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?
No, it is simply that he prefers full range mains run full range (no crossover) as this increases the number of bass radiators.

There is no requirement they extend to 20 Hz. His designs, even with 15" woofers don't, but they can be run full range without strain.

I don't know much about receivers (only 2 channels to this point), but if your mains are small, they need a high pass somewhere (receiver set to "small"?). Likewise, the subs need a low-pass somewhere in the chain (mine are in the subs).

The measurements you posted give some information, but would be MUCH better if you changed the scale of the y-axis to about 50 db (maybe 40-100db?).

Best wishes,

Bill
 

Willem

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More and uncorrelated subs will improve the smoothness of the response, and over a wider area. Your room is not really that large, so you may not need a bigger additional sub - an extra small one may be enough: http://archimago.blogspot.com/2020/05/musings-measurements-subwoofers-to.html. Since you seem to live in the UK, a KEF Kube 8 may be all that is needed. However, if you also want more bass at lower frequencies, the best way would be to follow Geddes' own example, and add one big one located in a corner. This will cover the very lowest frequencies, whereas the other two will smooth the response. A KEF Kube 12b may do that very well for a modest outlay. Of course, if you want even more output and in a smaller package, you will have to look elsewhere and spend more. From what I understand it is advised not to mix ported and sealed subs. Beyond that, mixing subs should not be an issue.
 
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