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Placing of acoustic panels is probably half science and half listening. It is best to start with good acoustic materials
since implementing this with inferior materials can actually make things sound worse.
The biggest mistake that folks make is to use panels that are too thin. Thin panels only absorb the higher frequencies,
leaving the lower ones still bouncing around the room. Rooms treated in this fashion will sound dull and "boomy"
The idea behind absroptive panels is to absorbs sounds that reach your ears AFTER the sound that came directly from the speaker
cones.
The drawing above shows a listening room before and after acoustic treatment. The lower half of the drawing shows what
happens in an untreated room. We want to hear the sound that is directly radiated from our speaker, which is represented
by the blue line. At a normal listening distance, this sound takes about 5 ms to reach our ears. At frequencies above
about 125 Hz sound waves exibit "specular" behaviour. That is, they can be thought of as traveling like beams of light. Notice
the three red lines. These depict longer paths that the sound can take to reach our ears by
bouncing off of the room boundaries. What is important to realize here is that, since the path taken is longer, the
sound reaches our ears AFTER the sound that arrrived via the direct path. Fortunately, because the sound traveled twice
as far to reach our ears, its intensity is diminshes by 75 percent, which makes the task of absorbing it easier.
What this all amounts to is the production of echos in our listening room. We hear the
direct sound at 5 ms followed by four reflected versions (left
wall, right wall, front wall, ceiling) from 2 ms to 7 ms later. If you've ever struggled to understand someone who was
speaking in a room with an echo, you understand the effect that these reflections have on the sound that you hear.
These reflected sounds destroy the sense of soundstage and the detail in the music. The situation is even worse than depicted
in this drawing since it only shows the reflected sound from one of the two speakers. Also not shown, are reflection
paths that involve the sound reaching our ears after reflecting off of TWO walls. These sounds reach our ears
after the reflections shown in the drawing and further destroy the sens of imaging.
In treating our listening room, we want to concentrate on reducing those echoes that reach our
ears within the first 30 ms. We don't worry so much about reflections arriving after about 30 ms. By having traveled
a long distance, their intensity is diminished to below audibility. Typically, we want to place absorptive material at the
first and second reflection points, along the front wall and the ceiling where they can absorb the earliest (and hence loudest)
reflections. As shown in the upper drawing, when we do this, we are left hearing only the sounds largely coming directly
from the speakers. By they way, this drawing assumes that there is carpet on the floor at least in the area immediately in
front of the speakers. If not, there is another reflective surface where early reflections can be generated!
Here's a picture of my listening room to illustrate the placement of the panels. The most important panels are
the white ones placed at the first relection point. Of all the panels, the ones at the first reflection point make the
most striking difference. That's because, of all of the reflections, this one traveled the shortest distance and, hence is
the loudest. The soundstage becomes much deeper and wider and the localization of instruments is much easier.
The next location to try placing panels is behind the speakers. My room is particularly difficult
because of the fireplace that juts into the room from the rear wall. Effecitively, this give me two rear walls behind my speakers
and produces two sets of reflections from the rear wall. In a normal rectangular room, you should try absorptive panels
either behind the speakers, like the gray panels shown here or you can try placing the panels centered between and behind
the speakers. Here in my room, I also installed the blue panel hanging above the fireplace to dampen
early reflections coming from the fireplace.
Notice the ceiling. At this time, I only have a piece of foam to dampen the ceiling reflection. I'm
still working on what kind of absorptive device I'm going to put up there.
The green panel on the right side of my room is at the second reflection point for the LEFT speaker.
Adding a panel here increases the intelligibility of vocals and the overall clarity of the sound. However, this panel had
the least overall effect of all the ones I tried.
Another location to consider for absoptive panels is the rear wall behind your listening position.
If your listening chair is less than about five feet from the rear wall, you may also be getting reflections from the wall
behind you as well. You may get some improvement by locating a panel directly behind where you sit. This will remove
these reflections from the rear wall.
A Word of Caution I was especially careful in contructing the gray and blue panels that flank my working fireplace.
You must be careful placing panels around ANY working fireplace since the heat radiating from it can catch the fabric on
the panels on fire. The panels around my fireplace are only 3-1/2 inches thick so as to prevent them from overheating when
the fireplace is burning. If you don't know what you're doing (especially if you don't understand why thinner panels are
less likely to catch fire), either don't put panels around a working fireplace or consult a fireplace specialist before doing
so.
It's also a good idea to fireproof the burlap before using it to cover these panels. The fire in
the nightclub in Maine is an example of why you should do this. Jon has a recipe for doing this with a solution of boric
acid on his web site, or you could use this commercial product. A gallon costs only $31.00 and will treat 300 sq.ft. of fabric. I used it to treat ALL my panels!!!
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