Room dimensions - AVS Forum | Home Theater Discussions And Reviews
Forum Jump: 
 7Likes
  • 1 Post By Red MC
  • 4 Post By sdurani
  • 1 Post By sdurani
  • 1 Post By Danboy24
 
Thread Tools
post #1 of 7 Old 10-21-2019, 05:48 AM - Thread Starter
Member
 
Join Date: Mar 2015
Posts: 20
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 14 Post(s)
Liked: 3
Room dimensions

I have a room which is currently 6.68M wide by 3.58M deep. Originally I was looking to extend the shorter dimension by around three meters but this will make the room square and I understand this is really bad for acoustics?

Bearing this in mind I was thinking about extending by just 2.00M so I end up with a room 6.68M wide by 5.58M deep. Do you think this will be OK acoustically?
Danboy24 is offline  
Sponsored Links
Advertisement
 
post #2 of 7 Old 10-21-2019, 02:07 PM
Senior Member
 
Red MC's Avatar
 
Join Date: May 2016
Posts: 425
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 295 Post(s)
Liked: 238
Compare these modal distributions:

https://amcoustics.com/tools/amroc?l...&h=244&r60=0.6

https://amcoustics.com/tools/amroc?l...&h=244&r60=0.6

Typically, the strongest room modes are the lowest order axial modes associated with the length and width of the room. In the current room, they will be around 26 Hz and 48 Hz. The 1-0-0 mode at 26 Hz shouldn't be a big problem with music because there's not much content down there. But for HT, it might cause some boom with LFE. I would expect the 0-1-0 mode around 48 Hz to be more of a problem, and it's close to the 2-0-0 mode at 51 Hz, so they may be perceived together as one really strong mode.

By changing the room depth to 5.58M, the 0-1-0 mode moves down to 31 Hz. It could still contribute to room boom down there, but since it's no longer adjacent to the 2-0-0 mode I would expect it to be an improvement, at least in the low bass range.

Higher up in frequency, it's hard to say without knowing the ceiling height. I assumed an 8 foot ceiling when I plugged in your numbers. It looks like the axial modes are a little more evenly distributed in the larger room. However, it also looks like the oblique modes are a little more evenly distributed in the smaller room. Some modes will be stronger than others due to loudspeaker location. Between that and the ceiling height, it's not clear to me which is going to be better above 70-80 Hz.

If your room is rectangular and closed, I suggest you plug the correct ceiling height into the calculator and then adjust the 5.58M dimension up and down within the limits of what you can build to find the room depth that results in the most even spread of modes. On the other hand, if the room is not rectangular, or is part of an open floor plan, or has a vaulted ceiling, then a room mode calculator is only going to be accurate for the lowest order modes.

Note that broadband bass trapping isn't very effective at attenuating strong room modes in the low bass, but works well higher up in the bass range.
If you use a subwoofer, you can place it at one of the nodes of a room mode and thus avoid exciting the mode. If you have two subwoofers, you can also cancel room modes. So, I use a combination of using multiple, strategically placed subwoofers to deal with the low bass and bass trapping to deal with the upper bass.
Danboy24 likes this.

HT: Dynaudio C2, Contour S CX, 2x BM14S, Aperion surrounds, Simaudio Titan, Marantz AV8801, Oppo 103, Linn Majik DS, and a Pioneer Kuro
Stereo: Dynaudio Focus 160, Simaudio W-5 LE or Luxman M-600A, Linn Akurate DSM, 2x Rythmik F12G
Other interests: motorcycling, skiing, being active
Red MC is offline  
post #3 of 7 Old 10-28-2019, 11:32 AM - Thread Starter
Member
 
Join Date: Mar 2015
Posts: 20
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 14 Post(s)
Liked: 3
Thanks for the comprehensive reply Red, and apologies for it taking me so long to respond.

I know a bit about AV but clearly not enough as I have no idea what the various Modes are that you mention or what the web page is actually showing me

The room height will be 2.32M and the width is fixed at 6.68M. With this is mind what would you say would be the most favourable depth of room? Or is it not that simple?

Last edited by Danboy24; 10-28-2019 at 12:18 PM.
Danboy24 is offline  
Sponsored Links
Advertisement
 
post #4 of 7 Old 10-28-2019, 02:06 PM
AVS Forum Addicted Member
 
sdurani's Avatar
 
Join Date: Oct 2001
Location: Monterey Park, CA
Posts: 28,140
Mentioned: 218 Post(s)
Tagged: 0 Thread(s)
Quoted: 7355 Post(s)
Liked: 6417
Quote:
Originally Posted by Danboy24 View Post
I was thinking about extending by just 2.00M so I end up with a room 6.68M wide by 5.58M deep. Do you think this will be OK acoustically?
Yes, there is over 1M of difference between width & length to keep it from being square.
Quote:
Originally Posted by Danboy24 View Post
I know a bit about AV but clearly not enough as I have no idea what the various Modes are that you mention or what the web page is actually showing me
If you blow across an empty bottle, you can get the air in there to resonate (make that booooh sound). If you enlarge the dimensions of that small chamber to the size of your room, the air in there will still resonate (of course at different frequencies than the bottle). These resonances are called room modes or standing waves. We're mostly concerned with room modes occurring in the low frequencies (in the bass range). Modes result in peaks & nulls at fixed locations throughout the room, which means different listeners might hear wildly different bass response depending on where they're sitting.

For example, your 6.68M width will result in modes at 26Hz, 52Hz, 77Hz, 103Hz, etc. The first 4 of these are mapped on the graph below. Each problem frequency is colour coded, so you can see where its peaks & nulls fall across the width of your room.



You can cancel the first 3 of these width modes (26Hz, 52Hz, 77Hz) simply by placing a pair of subs at the ¼ and ¾ points of room width, thereby greatly minimizing their peaks & nulls. Listeners sitting across the width of the room will hear much more consistent bass response from seat to seat.

The problem with having a square room is that you have the same dimensions for width & length, which will result in width modes that are the same as length modes. So you place your subs as described above to get rid of peaks & nulls at 26Hz, 52Hz and 77Hz. When you go to measure, you find out that you still have problems at 26Hz, 52Hz and 77Hz. How can that be when you just got rid of them? Turns out the subwoofer placement got rid of the width modes but not the length modes, which are at the same frequency.

So, room designers prefer to have room dimensions that are different enough to have width modes and length modes separated (not at the exact same frequency). Enlarging your room length by 2M to 5.58M will result in modes at 31Hz, 62Hz, 92Hz, 123Hz, etc. You can see these on the graph below. Again, each problem frequency is colour coded, so you can see where its peaks & nulls fall along the length of your room.



Notice that the problem frequencies across the width of your room are separated enough from the problem frequencies along the length of your room that they won't overlap.

Also notice in the graph above that the midpoint of room length is the worst place to sit (acoustically), since the problem frequencies are either loud peaks or deep nulls (no moderation). By comparison, sitting at 2/3 back from the front wall has most of the problem frequencies at roughly the same level.

The extra room length will be helpful and won't make the acoustics worse. So do it.

Sanjay
sdurani is offline  
post #5 of 7 Old 10-28-2019, 03:03 PM - Thread Starter
Member
 
Join Date: Mar 2015
Posts: 20
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 14 Post(s)
Liked: 3
Thanks Sdurani

Like Red's before that is an excellent explanation. Thank you

You mention I tried to allow more than 1M difference between the length and width keeping the room from being square. However when I use the Amroc software and make the dimensions 6.68M x 6.12M there seems to be very good separation between the width and length modes. Would you agree this is a bit of a sweet spot? 5.4M also seems to offer very good separation, however between the two I think I'd prefer to go for the larger dimension.

As far as avoiding converging modes I see from Red's post that I should definitely avoid opposites combining width and length axials? Is it also best to try and get as much separation as possible between axials, tangentials and obliques?

Lastly how did you work out the position of the sub woofers at the 1/4 and 3/4 width of the room? Is this because it coincides with the 52hz low frequency?

Last edited by Danboy24; 10-28-2019 at 03:18 PM.
Danboy24 is offline  
post #6 of 7 Old 10-28-2019, 03:35 PM
AVS Forum Addicted Member
 
sdurani's Avatar
 
Join Date: Oct 2001
Location: Monterey Park, CA
Posts: 28,140
Mentioned: 218 Post(s)
Tagged: 0 Thread(s)
Quoted: 7355 Post(s)
Liked: 6417
The first width mode for 6.68M is 26Hz. The first length mode for 6.12M is 28Hz. Doesn't look like very good separation to me.

There are two ways to cancel a room mode. Place the source of bass (subwoofer) in one of the nulls of that mode, which will prevent that mode from resonating. Or, if that null location is not possible, then place a pair of subs on either side of that null (apply equal pressure from both sides). Imagine you're at a playground and you don't want a swing to move back and forth (resonate). You can achieve that two ways. Don't let any of the kids touch the swing (prevent that mode from resonating). Or have two kids push the swing equally hard from opposite directions (apply equal pressure from both sides). Either way, the swing doesn't move (mode does not resonate).



If you look at the graphs I posted, you'll see that the 1st width mode (blue trace) and 3rd width mode (red trace) both null at the midpoint of room width. The 2nd width mode (black trace) nulls at the quarter points of room width. Placing a pair of subs at the quarter points of room width accomplishes two things. The subs are in a null (in this case, both nulls) of the 2nd width mode, which prevents that mode from resonating. But since the subs are applying equal pressure from both sides of a null of the 1st and 3rd width modes, they are also cancelled. So the quarter point placement uses both approaches to mode cancelling. And when you cancel a mode, all the peaks & nulls associated with that mode are gone. Much smoother bass response (fewer/smaller peaks & dips) as well as better seat to seat consistency.

Here's a more technical explanation from Floyd Toole:

Red MC likes this.

Sanjay
sdurani is offline  
post #7 of 7 Old 11-17-2019, 08:49 AM - Thread Starter
Member
 
Join Date: Mar 2015
Posts: 20
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 14 Post(s)
Liked: 3
Thanks sdurani

It does look like the smaller room dimensions of 5.4M offer better separation so this is the room size I'm going to go for (and it's cheaper to boot)

I can't do anything about the ceiling height (there are rooms above). As a result it doesn't seem acoustically advantageous to only increase the room in the X and Y direction.
sdurani likes this.
Danboy24 is offline  
Sponsored Links
Advertisement
 
Reply Audio Theory, Setup, and Chat

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off