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Room Acoustics: Acoustic Treatments - page 2

By Patrick Hart

A Common Living Area/Home Theater Layout
Before you all give up and figure the whole room acoustics deal is hopeless let me give you my current solution to reflections in my home theater:

The key here is to be able to picture how the reflections like you see in Tony's slide might change once you move the speaker system around in the room a bit. I want to also stress, as you study the pictures, that I've done as much as I can with proper speaker placement and positioning. We have added the dual-drape system, but so far I have not had to propose more drastic changes to my significant other's carefully appointed living room in which my A/V set-up is the barely-tolerated intruder.

About my set-up > The last two homes I've had leant themselves to corner placement of the home theater. I figure for the vast majority of us we're making due with a home theater in the living room and that usually means there's a fireplace right smack dab in the middle of the main wall. No problem. Placing my beloved (and huge) 40" Mitsubishi tube TV in the corner does two very important things for us at the same time. See the first pictures below.

1Sem3.jpg  2Sem3.jpg


Most obvious in the long shot from the right side of the system is that the Infinity Modulus satellite speakers hanging outboard of the TV on the optional Infinity bar have their front baffles almost parallel with the right window (with the tan shades drawn). Guess what? No sidewall(axial) reflections. The same is true for the left satellite. Note also that not only are both left and right satellites pointed inward toward the listening position on the couch but they are tilted down toward the listening position as well. (See Tony's doorstop trick from Part 1 of this series.)

This positioning would shift a good portion of the first reflection frequencies, from the ceiling bounce of the left-center-right speakers, to behind the listening position so you can't hear them. The tilt also makes the upper ceiling and lower floor bounce angle different (diffusion) for the same frequency.

A 1000Hz reflection heading toward the ceiling will hit the ceiling and end up hitting the listener's head without any control at all. Tilting the speakers down reduces the level of this reflection. The same 1000Hz frequency going to the floor or toward the glass coffee table will hit the floor and the coffee table having traveled a shorter distance (than the ceiling 1000Hz frequency). It will be absorbed in the Persian area rug in front of the TV. And for the coffee table I will tell you of a trick later in this article.

Meanwhile, at the listening position, the 1000 Hz tone is heard directly. There is about 3 milliseconds of delayed 1000 Hz bouncing off other surfaces and arriving at the listening location. So it's important to either absorb these sound-smearing tones or divert a large percentage of them out of the listening area.

I mentioned the coffee table. This hard surface right between you at the listening position and the left-center-right speakers is a real reflective liability. (See the second set of pictures.) Since the speakers are now pointed inward and downward toward the listening position the highly reflective glass coffee table will have a lot of mid and high frequencies bouncing off the glass before they arrive at the listening position. This is very similar to the "console bounce" recording engineers have become aware of when they put small studio monitor speakers on the mixing console's meter bridge. Blurred sound and a masking of intelligible vocals can result.

4Sem3.jpg  5Sem3.jpg


Fortunately the answer, for my humble abode anyway, is pretty easy. I want to put my feet on the coffee table when I kick back to watch a movie so I just throw my heavy Mexican blanket (absorption) on the table and the significant other is truly pleased with my thoughtfulness. Works Great!


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