• Welcome to The Audio Annex! If you have any trouble logging in or signing up, please contact 'admin - at - theaudioannex.com'. Enjoy!
  • HTTPS (secure web browser connection) has been enabled - just add "https://" to the start of the URL in your address bar, e.g. "https://theaudioannex.com/forum/"
  • Congratulations! If you're seeing this notice, it means you're connected to the new server. Go ahead and post as usual, enjoy!
  • I've just upgraded the forum software to Xenforo 2.0. Please let me know if you have any problems with it. I'm still working on installing styles... coming soon.

Distance Calculation for Treatment of Back Wall

FredtheFilmFan

Active Member
So, wise and wonderful purveyors of acoustic knowledge, what is the formula to determine if a wall is far enough away from my seating to leave it untreated? The wall is about 25 feet from my speakers and about 14 from the seats, so round trip from speakers to wall to seats is about 39 feet.
 
The speed of sound is 1128 feet per second. So, the time it takes for the sound of the speaker to reflect off the wall and reach your ear is 35mS, which is beyond the threshold for most people recognize it as a reflection (echo) rather than part of the original sound. So, you do not absolutely have to treat it.

That said, you will still benefit from putting some diffusion on that wall.
 
Thanks Flint. I have one absorber there right now and I didn't know if I should be concerned about making more. I have some clap echo issues in the room so I thought that may be part of the problem. I think its the stairwell that is at the back of the room and parallels the HT area. Also, at the front of the room and to the left there is an opening to another room and a hallway, so I'm sure that doesn't help either. Every time I start messing with my ht it makes me want to build a new house so I can have my perfectly dimensioned room with a proper door and sound isolation.
 
I see a DIY article on diffusers, but in the pics of your HT Flint, the diffusers are curved, I'm assuming, wood. I like the idea of the curved wood as I may be able to put some kind of graphic on them. Directly behind my ht seating is the kids play area. The absorber I have there has a piece of fabric over it with a Disney Cars movie pic, so it doesn't look like an absorber... kinda.

So, long question longer, which is preferable, curved or crazy blocks?
 
Ideally? Crazy blocks are best. Those are three dimensional quadratic diffusors which evenly reflect sound in all directions. Mine are not full diffusors, just curved reflectors which offer some two dimensional diffusion to nearly eliminate slap echo and I made them open in the back and filled the cavity inside with fiberglass and spaced them off the wall to make them also mid-bass absorbers. Also, the curved shape makes them absorbers as the mathematical frequency of the shape & size, but I'd have to look up the math on that.

You could also more easily make an angled reflector which reflects the sound 15 degrees from the direct reflection (away from the listening position) which will also cut way down on slap echo. The width and height of the reflector determines how low in frequency it will affect the sound.
 
I copied these notes from another person/site ??? for block diffuser.

Hi. I layed out the measurement grid from the BBC doc if anyone wants to build these "block" style diffusors. Here's the grid (height ratio of blocks):

0 3 4 1 2 3 3 1 4 2 3 3
3 0 1 4 2 1 1 3 3 2 1 1
3 1 1 3 1 3 2 2 1 0 2 2
2 2 2 2 0 4 3 2 3 2 1 1
3 3 1 1 3 1 1 3 4 3 1 3
2 3 2 1 2 0 3 2 4 2 1 0
2 3 2 1 3 1 2 2 3 1 3 4
2 0 2 4 4 0 1 2 1 4 2 2
3 4 1 0 1 3 3 1 0 2 3 3
1 3 3 1 2 4 1 2 0 1 3 1
2 1 2 3 1 3 3 2 4 2 3 4
2 4 2 3 3 1 1 2 0 3 1 0

And here's a list of how many of each length block you will need when precutting the wood:

Count of number:

number Total pieces of this length (height actually) of wood

0 13 (no cuts here if you're going flush with the surface)
1 38
2 38
3 40
4 15

Grand Total 144 pieces (131 if 0's aren't used)

I assume these will work with 1x1 or 2x2 inch squares (cut longer depending on the graph number- ie. graph designating number 3 would mean use a 1x3 inch block or 2x6 if using 2x2's)...I'll have to break out the saw!
The cheapest commercial diffusor you can get in the market is the Skyline type diffusors sold by Hiway Laser. If you went to the KLIAV show in July you would notice how these diffusors look like and how they place them in one of their demo room. You need to get minimum 8 pieces of 2'x2' in order to perceive an appreciable sonic difference.



http://www.bbc.co.uk/rd/pubs/reports/index.shtml
http://www.acousticsfirst.com/artdif.htm
http://www.rpginc.com/news/library.htm
 
Back
Top