Google "live end/dead end room acoustics".
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The front wall reflects sound waves, either from the speakers themselves (not so much if you have front firing boxes, lots if you have bi-polar) or bouncing around the room, first reflections principally off the back wall. Deadening the wall behind the speakers allows you to hear the direct signal from the speaker more clearly, without pollution from these reflections.
With front firing boxes you can achieve almost the same effect by deadening the wall behind the listener as this will kill not only the first reflections from the speakers but also deaden second reflections off the side walls.
Personally I find the sidewall reflections most objectionable and focus on reducing them. They do the most damage I think. Most of the benefits from deadening the wall behind the speakers can be obtained by just pulling your speakers out into the room about 5ft. Putting diffusing (not deadening) stuff on the wall behind the listener can also be very effective in quieting reflections.
FWIW, I, and I think many others, prefer the effect of diffusion more than deadening. Some reflections are a good thing. In real life they contribute to the spaciousness we enjoy. A lot of recorded music can be on the dead side and benefit from these reflections.
Thanks guys. Makes a lot sense.
Newbee, my room layout/restrictions limit my ability do everything I want to with acoustic treatments. I cannot treat sidewall reflections because there is a door and a window there that the family needs access to. I cannot put my speakers too far into my small room for the same reasons. And the wall behind my head is actually only 1/3 height with large openings to my large family room and kitchen.
Again, thank you for the input!
Tboooe, You can successfully deal with sidewall reflections by using a severe degree of toe in, i.e. setting the axis of the speakers to cross in front of your listening chair. If you do that, you can also increase the space between the speakers and your listening position, if room size allows. Something to play with on a rainy day. :-)
Tboooe - now you know the science, an alternate solution to pillows - a trip to Winners to purchase a couple of cheap canvas prints.
Then go to a material store and purchase some polyester batting - it's better than fiberglass insulation - less irritating - place this behind the canvas (hold in place with some fishing line) and mount them on the wall - play with placement - it can make a difference.
Another option is to adjust speaker placement from the front wall so that the reflected sound waves are "aligned" with the speaker drivers such that it augments the original signal and not degrade it.
Whilst this is not the ideal solution - i.e. it can create a little bit of an echo depending on room characteristics - it can be more pleasing than having the reflected signal degrade the source signal effecting the spatial imaging.
I opted for the speaker placement solution because of many factors (WAF & furniture) and the front of my speakers are only 39.5" from my front wall - which is not too greater protrusion into the room.
The other issue my room had was a significant echo at the front wall - which I fixed with a pair of 15" x 86" baffles placed in the corners at a 30 degree angle to the side wall. For these made a 1.5" frame with a canvas front and polyester batting.
Just a thought
What's happening is that; when the reflections are close to the original sound source, the timing between the original sound and the reflected sound is too close for the brain to process as separate sounds, so it sounds smeared (not correct). By absorbing some of the early initial reflections your allowing yourself to hear things more clearly.