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Discussion Starter · #1 ·
So help me understand this chart. If I place a subwoofer smack in the middle of the room, it should in theory take care of that 21Hz and 62Hz null? I have 2 subs. I thought of placing one in the middle of the room and the other 3.5 feet away from one corner to take care of the 84Hz null.

Is this correct? I don't want to break my back unless needed. lol
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The sub in the middle of the room will be at the null location for the 1st and 3rd modes (21Hz and 63Hz), so those modes will be cancelled. But that's also the peak location for the 2nd and 4th modes (42Hz and 84Hz), so those modes will be very strongly energized (peaks will be really loud). Placing your second sub in one of the nulls of the 84Hz mode won't cancel it because that's being sabotaged by the other sub at one of the peaks of the 84Hz mode. The two subs are working to opposite effect at 84Hz.

Can you post a rough diagram of where the seating will be in the room relative to the walls?
 

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Discussion Starter · #3 ·
The sub in the middle of the room will be at the null location for the 1st and 3rd modes (21Hz and 63Hz), so those modes will be cancelled. But that's also the peak location for the 2nd and 4th modes (42Hz and 84Hz), so those modes will be very strongly energized (peaks will be really loud). Placing your second sub in one of the nulls of the 84Hz mode won't cancel it because that's being sabotaged by the other sub at one of the peaks of the 84Hz mode. Can you post a rough diagram of where the seating will be in the room relative to the walls?
Hmmmm interesting! I decided to place my 1 row of seating (5 seats) at 2/5th from the front wall at approximately 10' 9"... I think that's the best position I can do because the rear of the room has a 9 foot pool table. lol

Let me show you a picture.

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Thanx for the pics; very helpful.

Imagine a swing at a playground is room mode. You don't want that swing to resonate (move back and forth). There are two ways to accomplish that. Prevent any of the kids from touching the swing, so it never moves to begin with (movement never occurs). Or you can have 2 kids push equally hard from opposite sides of the swing. Any movement the swing would have made is cancelled because of equal but opposite pressure. Same end result with either approach.

Placing the source(s) of bass in a null of a mode prevents that mode from being energized to begin with. The peaks & nulls associated with that mode never occur. Or you can place 2 subs equally on both sides of a null, cancelling that mode (and any peaks & nulls associated with that mode).

Since you have 5 seats going across the width of your room, it would make more sense to place the subs to cancel width modes by employing both approaches described above. Common advice for dual sub placement is to centre them at the ¼ and ¾ points of room width. This puts them in the nulls of the 2nd width mode, so that mode is cancelled. It also puts them equally on both sides of the 1st and 3rd width modes, so those modes are cancelled. That means the peaks & nulls at 21Hz, 42Hz and 62Hz are gone, for ALL listeners across the width of the room.

In the graph you posted, notice that nulls always fall at even divisions (half, quarters, sixths) of room length. Easy to avoid them by placing the listeners' ears at one of the odd divisions (thirds, fifths) of room length; as you already have. Since all the listeners are the same distance from the front and back walls, they will experience the same length mode peaks. While you can't boost your way out of a null, you can pull down peaks with EQ. Best part: fix the peaks in one seat fixes them for all seats.
 

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Discussion Starter · #5 ·
Thanx for the pics; very helpful.

Imagine a swing at a playground is room mode. You don't want that swing to resonate (move back and forth). There are two ways to accomplish that. Prevent any of the kids from touching the swing, so it never moves to begin with (movement never occurs). Or you can have 2 kids push equally hard from opposite sides of the swing. Any movement the swing would have made is cancelled because of equal but opposite pressure. Same end result with either approach.

Placing the source(s) of bass in a null of a mode prevents that mode from being energized to begin with. The peaks & nulls associated with that mode never occur. Or you can place 2 subs equally on both sides of a null, cancelling that mode (and any peaks & nulls associated with that mode).

Since you have 5 seats going across the width of your room, it would make more sense to place the subs to cancel width modes by employing both approaches described above. Common advice for dual sub placement is to centre them at the ¼ and ¾ points of room width. This puts them in the nulls of the 2nd width mode, so that mode is cancelled. It also puts them equally on both sides of the 1st and 3rd width modes, so those modes are cancelled. That means the peaks & nulls at 21Hz, 42Hz and 62Hz are gone, for ALL listeners across the width of the room.

In the graph you posted, notice that nulls always fall at even divisions (half, quarters, sixths) of room length. Easy to avoid them by placing the listeners' ears at one of the odd divisions (thirds, fifths) of room length; as you already have. Since all the listeners are the same distance from the front and back walls, they will experience the same length mode peaks. While you can't boost your way out of a null, you can pull down peaks with EQ. Best part: fix the peaks in one seat fixes them for all seats.
Awesome analogy with the swings! It's all starting to make sense. Ok, so I will make sure the center of the drivers of both subs are at 6' 9" (1/4) from the side walls. How far from the front wall should the driver then be?

BTW, thanks for your help and thoroughness!
 
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How far from the front wall should the driver then be?
It's not going to make a huge difference. BTW, the subs don't have to be at the front wall. They can be at the back wall or just behind the seating. As long as they're centered at the quarter points of room width. Keep that in mind in case your subs end up blocking part of the screen.

Also, you can extend the mode cancelling to the 4th and 5th width modes (83Hz and 104Hz) by centering the woofers of your L/C/R speakers at the 1/6, 3/6 and 5/6 points of room width. The reason I mention woofers is because we are addressing bass frequencies. If you look at the modal graph you posted, that means the 2 subs go in the 2 nulls of the 2nd width mode (black trace) and the 3 front speakers go in the 3 nulls of the 3rd width mode (red trace).
 

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Discussion Starter · #7 ·
It's not going to make a huge difference. BTW, the subs don't have to be at the front wall. They can be at the back wall or just behind the seating. As long as they're centered at the quarter points of room width. Keep that in mind in case your subs end up blocking part of the screen.

Also, you can extend the mode cancelling to the 4th and 5th width modes (83Hz and 104Hz) by centering the woofers of your L/C/R speakers at the 1/6, 3/6 and 5/6 points of room width. The reason I mention woofers is because we are addressing bass frequencies. If you look at the modal graph you posted, that means the 2 subs go in the 2 nulls of the 2nd width mode (black trace) and the 3 front speakers go in the 3 nulls of the 3rd width mode (red trace).
Oh wow! If I'm not mistaken, 6' 9" from the side walls would place the subs exactly as "end tables" on both sides of the row of seating! I would have to double check when I get home from work. This would be perfect then because they cover part of the screen bezel when up front. If they can be placed right next to the edge of the row of seats, that would also enhance the near-field bass?

As for the LCR, yes! I think I could definitely place them at that distance you mentioned.

This means that effectively, all modes have been resolved along the entire row of seating?!?! 🙏🏼🙌🏼
 
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This means that effectively, all modes have been resolved along the entire row of seating?!?!
Not all, but the first 5 width modes. That goes a long way towards getting smoother bass response AND seat-to-seat consistency across the row. The room correction system in your receiver will thank you. Plus, it's free (doesn't cost anything to move subs and speakers around).
 

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Discussion Starter · #9 ·
Not all, but the first 5 width modes. That goes a long way towards getting smoother bass response AND seat-to-seat consistency across the row. The room correction system in your receiver will thank you. Plus, it's free (doesn't cost anything to move subs and speakers around).
Ok buddy, I had to run home. Couldn't wait! LMAO

At 6' 9" off the side walls, the subs will be perfect on both sides of the seating area.

My question now is, if I place the sub driver exactly at the 10' 3" mark from the front wall, will that be even better?

In other words, 6' 9" off the side wall and 10' 3" from the front wall, thus placing it at the 4th mode null (yellow trace).

If not recommended, then I'll just slide it a bit closer to the front wall to about 9' 7" from the front wall, but even with the seats.
 
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In other words, 6' 9" off the side wall and 10' 3" from the front wall, thus placing it at the 4th mode null (yellow trace).
Should help. Even if the subs are crossed over lower, they should still be putting out some energy around 84Hz (4th length mode). Connect both subs to the same subwoofer output on your receiver so that they receive identical signals (level & delay). Cancelling anything (not just room modes) works best when the two items are opposite AND identical. IF the subs aren't identical (same gain, same delay), then the mode cancelling won't be as clean.
 

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Discussion Starter · #11 ·
Should help. Even if the subs are crossed over lower, they should still be putting out some energy around 84Hz (4th length mode). Connect both subs to the same subwoofer output on your receiver so that they receive identical signals (level & delay). Cancelling anything (not just room modes) works best when the two items are opposite AND identical. IF the subs aren't identical (same gain, same delay), then the mode cancelling won't be as clean.
Sweet! I have them connected via a MiniDSP HD 💪🏼

Now that I have the LCR and subs optimally placed for my room, I will try and build me the bass traps and acoustic panels this weekend
before I begin to calibrate everything with REW.

I think I will just leave the subs 9' 7" from the front wall and not at the 84hz null point, so they can be right next to the chairs.

BTW, the center channel is the only speaker I can't get the drivers to align with the center of the room because the middle of the center channel is a horn tweeter. The 4 drivers are offset, 2 on each side of the center tweeter. It's ok though, can't have my cake and eat it too. lol
 

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I have them connected via a MiniDSP HD
I would remove it from the signal path and replace it with a simply Y-splitter.
I will try and build me the bass traps and acoustic panels this weekend before I begin to calibrate everything with REW.
When looking at the REW measurements, remember that your square room means that the length modes and width modes are almost identical. So despite using the subs to minimize the width modes, those same frequencies might still show up as peaks & dips from the length modes.
The 4 drivers are offset, 2 on each side of the center tweeter.
The 4 woofers are aligned with the midpoint of room width, 2 equally on each side of the null. Same as all 4 being in the null. 2 kids pushing the swing from opposite sides gives you the same result as none of the kids touching the swing.
 

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Discussion Starter · #13 ·
I would remove it from the signal path and replace it with a simply Y-splitter. When looking at the REW measurements, remember that your square room means that the length modes and width modes are almost identical. So despite using the subs to minimize the width modes, those same frequencies might still show up as peaks & dips from the length modes. The 4 woofers are aligned with the midpoint of room width, 2 equally on each side of the null. Same as all 4 being in the null. 2 kids pushing the swing from opposite sides gives you the same result as none of the kids touching the swing.
Hmmm What a mess and nightmare then! Those will still show up as nulls and peaks. Seems like an almost impossible scenario then to try and EQ the system.

And reference the MiniDSP HD, many people swear by it! I was going to use it to time align my subs to the front speakers. Are you saying I shouldn't bother and just connect the subs straight to the AVR's sub output 1 with a y-splitter? 🤷🏻‍♂️

Now I really feel like you are starting to lose me when I was just starting to make sense of it all. 🤣
 

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Discussion Starter · #15 ·
Why? Yup, for the reasons explained earlier.
I re-read your reasoning but just can't figure out why in my setup, I wouldn't benefit from using a MiniDSP and should just use a y-splitter to connect the 2 subs. Not saying you are wrong, just that I am not catching on as to why. I would like to understand because that's how one learns.
 

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I re-read your reasoning but just can't figure out why in my setup, I wouldn't benefit from using a MiniDSP and should just use a y-splitter to connect the 2 subs.
When you want to cancel 4, you don't use -3 or -5, you use -4. The more identical the two items, the better the cancelling. Using a simple Y-splitter will make sure that both subs are getting identical signals. Then it's just a matter of gain matching the subs so that they have identical outputs. IF it makes you feel better to have the miniDSP in the signal path, then leave it in there.
 

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Discussion Starter · #17 ·
When you want to cancel 4, you don't use -3 or -5, you use -4. The more identical the two items, the better the cancelling. Using a simple Y-splitter will make sure that both subs are getting identical signals. Then it's just a matter of gain matching the subs so that they have identical outputs. IF it makes you feel better to have the miniDSP in the signal path, then leave it in there.
@sdurani Oh I see what you are saying. I will get a y-splitter.

BTW, correct me if I'm wrong, but if I was to place 1 sub in the middle rear wall and a second sub at the middle of the left or right wall, I would be creating a virtual sub right in the middle of the room and thus getting rid of both the 1st and 3rd harmonics nulls (red and blue trace)? Effectively creating absolutely a SWEET SPOT smack in the middle of the room?

If I can do this, then I can move my seating back to the middle and get closer to a 45 degree viewing angle instead of 60 degree I currently get from my current seating distance. It can be fatiguing at times on the neck.
 
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