38%, 47%, whatever it takes...

[beatlefan1970]

I know this has been done to death, so I'll try to keep it brief. The details:
My room is 27'L x 20'Wx 8'H. The four floor-to-ceiling corners each have (2) 4"x 24"x 48" 703 traps, I have six more of those traps around the room in ceiling/wall corners, and 8 or so 2" x 24" x 48" 703 hi-mid absorbers where needed, including covering a little less than half the width of the front wall. The ceiling is a foot of fiberglass in joists, covered with fabric, with a reflective perimeter 2' wide around the whole room. Concrete floor. It's an all-in-one recording/mixing/rehearsal space.
I went with the ol' 38% Strong Suggestion setting up the monitors, and it's working fine--it just puts me 10+ feet into the room. I'd like some of my floorspace back.
The question: within reason, does the 38% rule matter less in a room my size? I know I'll need to do some backing and forthing, re-place my first reflection panels, all that jazz, and I'm certainly not wanting to put my speakers right up against the wall. But if I could be, say, five or six feet into the room instead of nine...I've probably answered my own question--"screw around with it and see what works"--but any thoughts are appreciated.

[Nick Sevilla]

You can and should be able to move your speakers nearer to a wall.

The Trick is to know where your NODES are.

There is One main Node, which will determine the best and worst placement for your speakers.

These NODES are related to the axis that your speakers will be throwing their sound on.

Measure the length of the distance between the two walls that will be behind and in front of your speakers. then once you know this number, translate it into the FIRST frequency NODE. This is the FIRST frequency which will create the LARGEST node in the middle of your room. And the good thing is, IT DOES NOT CHANGE.

Formula : distance between walls / 565 ft/second X 1000. This 565 is the average of HALF the speed of sound.

Example for illustrations purposes : Say your room is 27 feet at the longest AXIS, and this AXIS is the ones your speakers will be throwing their sound along.

27 / 565 x 1000 = 47.787610619469027 Hertz. This is your FIRST MODE (NODE).

Which simply means that when you play back 47.787 Hertz through your speakers ANYWHERE IN THE ROOM, this frequency will be softest right in the middle between those two furthest walls.

More Modes (NODES) :

1st = 47.787
2nd = 95.575
3rd = 191.150
4th = 382.300
5th = 764.601
6th = 1529.203
7th = 3058.407
8th = 6116.814
9th = 12233.628

And the direct correlation between frequency NODES (nulls, low points) and the distance from one end of the room, named wall A:

1st = 13.5 feet (middle of your room,equidistant from wall A and B)
2nd = 6.75 feet from wall A
3rd = 3.375 ft from wall A
4th = 1.6875 ft from wall A
5th = 0.84375 ft from wall A
6th = 0.421875 ft from wall A
7th = 0.2109375 ft from wall A
8th = 0.10546875 ft from wall A
9th = 0.052734375 ft from wall A

When you find a NODE near where you want your speakers, then you know NOT to put your speakers there, nor your head either.

Your speaker placement will probably be in between Node 3 or 4.

And where you sit might be between Node 2 and 3. So you find what frequency will be the softest there.

Treat your room near those areas for those NODE Frequencies either with absorption for higher frequencies, or reflection for lower ones.

Do the math. Math is fun.

Cheers

[beatlefan1970]

That answer is above and beyond the call of duty, sir. Thank you very much!

[Nick Sevilla]

That answer is above and beyond the call of duty, sir. Thank you very much!

You're welcome. Now go move your gear about, and report back.

[GIK Acoustics]

That answer is above and beyond the call of duty, sir. Thank you very much!

Yes, very good advice!

The 38% rule is usually just a suggestion of a good first place to start, but a lot of rooms we see benefit from having the speakers even all the way up against a wall. Trying out multiple placements is annoying, I understand. But it doesn't cost you any money and can make very important changes in your room's response.

If you like, you can even run some testing to find optimal placement. While calculation can give you a round-a-bout estimation of the nulls and peaks, wall construction will give variances in those frequencies, testing can give you the exact frequencies and how strong they are, as well as how long they take to decay in the room. Here's a video we did covering how to test a room: http://www.gikacoustics.com/video_rew_r ... orial.html

[nag hammadi]

awesome response, nick.

so let me check my numbers....

my room is odd. it is 16 ft x 9 ft x 10 ft high.

so my nodes would be:

1- 28.318hz
2- 56.637
3- 113.274
4- 226.548
5- 453.097
6- 906.194
7- 1812.389
8- 3,624.778
9- 7,249.557


and feet from wall a:

1- 8'
2- 4'
3- 2'
4- 1'
5- 6"
6- 3"
7- 1.5"
8- .75"
9- who cares


is this looking about right?

[Nick Sevilla]

Pretty much... without me taking out my calculator...

You'll look at nodes 3-5 for placement issues.

Cheers

[jhbrandt]

Simpler put; Your listening position placement should be at a odd multiple of the room length. 3/8 (.375 X room length) of the room is a 'preferred' starting point. Testing is required to optimize.

+1 on Nick's post.

My other recommendations would be @ 2/7 (0.2857), 3/7 (0.42857), etc.

BUT remember, if it's not Baroque, Don't fix it. !!

Cheers,
John