How far out into room for rear ported speakers (1 Viewer)

[ChrisAG]

I've heard of the 1/3 rule, where the speakers are 1/3 into the room from the front wall, and the listener is 1/3 from the back wall, but this is not always practical. Right now my MS-906's sound great at 18" from the wall (measured from the rear of the speaker), but I'm wondering if this is enough. Due to furniture considerations it isn't possible to move them out much more. My rear bookshelf 902s are only six inches away from the back wall - less than I want, but they sound OK when my receiver's bass management is set to "Small." Again, it is not possible to move them out much more. Any thoughts, especially for the front?

 

[Bob McElfresh]

Hi Chris.
Here is the trick: You want the back-fire sound to hit the walls and bounce so it reaches your ears some time after the front fire sound.
Some PsychoAcoustics research has found some interesting things about a sound, followed by an echo. They looked at what the ear perceived based on the ammount of delay for the echo:
0-5 miliSeconds: The ear registers a single sound.
6-11 miliSeconds: Here is where the ear registers the distortion, but cannot tell anything about placement and assumes 1 sound source.
12-20 miliSeconds: The ear registers a single sound, but tells the listener that the sound is several feet farther away than it really is.
> 20 miliSeconds: The ear recognizes an echo.
So pulling the speakers into the room, you want to get that 12 miliSecond delay to give you the sense of "spaciousness" or the effect of the walls of the room falling away.
Sound travels in your living room at about 1.1 feet per milisecond. So you can try to measure the distance to the rear walls, then the distance to your ears and calculate the delay using this number.
Hope this helps.

 

[Rick Radford]

Bob,
That's really interesting info.
But if my math is correct, I'd hafta pull the speakers in 5.5 feet to get that 12 ms delay (5.5 x 2 x 1.1 = 12.1)... in which case, I imagine most HT rooms don't have that much space to play with.
at least, my little 11.5x12 room doesn't!
------------------
--RR

 

[ChrisAG]

Hi Bob,
So do you mean that if we can't achieve 12-20 miliseconds for spaciousness, the next best thing would be 0-5 miliseconds, because anything in the middle is registered as distortion and would thus sound worse?
Chris

 

[Bob McElfresh]

Yes, it does seem like a lot but the effect is staggering.
I had to strip everything out of my living room to have wall-paper stripped, re-texture and painted. I managed to move back the HT equipment and nothing else for a few days.
With the speakers pulled into the room about 4 feet, all positioned correctly and level-adjusted, I watched several movies. But one disk, U.S. Marshals stood out.
There are several scenes with a phone call that toggle between a open-swamp area, and a closed-in, stuffy office. When the scene would switch, it felt like the walls of the room exploded outwards. I thought my ears were going to pop. Then the walls rushed back in for the office scenes.
So, do you need 5 feet of space? No. But a good 3 feet of space is better than 18 inches. Most sound-tracks have a complex mix of sounds. And the sound engineer can help this effect by simply putting in a delay for the L/R copy of a sound. So when the center speaker fires a sound, a delay in the L/R will produce a similar effect.
ChrisAG: you said

the next best thing would be 0-5 miliseconds, because anything in the middle is registered as distortion and would thus sound worse

Yes, this is true.
People do not coat their rooms with sound-absorbtion material. But they do treat some spots with absorbers/diffusers. Why?
The books I have read on room treatments & acoustics talke about an issue known as "Early Reflections". These are sounds that come out of the front/sides of a speaker and reflect from a wall/ceiling to hit your ears with a delay. Panels are used at the reflection spot to reduce the single-bounce reflections that have the 5-12 miliSecond delay.
But they leave other places un-treated because some delay & distortion for these longer bounce reflections actually sound good.
So some room-acoustic theories and some psycho-acoustic theory (called the "Hass Effect") seem to support each other.
If you want to learn more, try these references:

• Widescreen Review #15 - This is well worth the $16 for the back-issue reprint. Even if it was written in 1995, it has 2 very good articles about the effect of delays and reflections for the "new" Dolby Digital 5.1 format.
• The "Hass Effect" - search the internet for some papers discussing this.
• A book: "Sound Studio Construction on a Budget" by F. Alton Everest. This book is focused on sound studios, but it takes several sized rooms and tells you how to treat them acoustically to prevent early reflections. Chapter 10 is about Home Theater rooms, but you need to read the first 3 chapters to understand their logic.

Hope this helps.