Room Acoustics: Acoustic Treatments - page 3

Acoustical Treatments - Absorption
Okay, what if your situation is such that you can't rearrange your room or system or you've already tried the speaker aiming and positioning as best your situation will allow? And, you can still tell from moving in and out of the critical distance that more acoustical treatment needs to be done? One suggestion would be to enlist the help of your significant other to "confirm" the need for absorptive or diffusive materials.

Over the years I've found that women are much more critical of perceived "distortion" or "unclear sound" in the midrange frequencies. The effect can be more pronounced on female vocals of her favorite artist backed up by well-recorded and somewhat complex musical arrangements. If you can demonstrate to her that moving forward into the critical distance area (adjusting the volume down to compensate for the nearer distance) will yield clearer, more distinct overall musical presentation, you've made a pretty convincing case for acoustical treatment. Now you need to get clever and figure out how to incorporate acoustical treatment into your room/system interface in the most amenable way possible.

First some facts:

Note that 1" acoustical panels only work down to 1000Hz which is above most of the primary vocal frequencies and thus just about useless in affecting vocal intelligibility. Two inch material gets down to 500Hz or so and 4" down to 250Hz.

So what should we use? According to F. Alton Everest from "The Master Handbook of Acoustics" a bass or baritone voice can get down to 80Hz or 100Hz respectively. So, technically speaking, even the 4" thick acoustical absorption won't get you down in frequency as far as you want to go. In practice though, absorbing down to 250Hz will take care of most of the sound reflections that affect intelligibility and sound imaging for human beings. So 4" material is the sweet spot for thickness. Two inch material works OK too, but for good quality work you really need to absorb down to 250Hz.

This is why Tony's second slide is so useful; by standing 2" thick foam off of a surface with, for instance, a couple of 2 x 2s, you can actually get much the same effect as using 4" thick material in the first place. Not identical, but similar.

So how to get the most bang for the buck and still pass the WAF test? A recommendation would be to use some of the more common pre-made 2" x 2' x 4' acoustical panels offered by a variety of manufacturers. These panels come with the correct acoustical fabric which allows sound waves to pass through and have their energy dissipated in the fibrous absorption material behind.

The best-bang for-the-buck part comes if you can place that 2" thick panel over a 16" section of wall where the significant other has allowed you to remove the sheet rock between studs while filling the gap in the wall cavity with high density mineral wool or fiberglass insulation (3 pounds per cubic foot). Now you effectively have 6" of acoustical absorption which will truly absorb reflections across a wide bandwidth of frequencies and down to ~125Hz.

That last phrase in italics is important. If you use only 2" thick acoustical absorbers, for instance, mounted directly to sheetrock you are only attacking those frequencies down to 500Hz. So you have, in effect, used a high frequency tone control on the reflections to 500Hz while ignoring those reflections from 125Hz to 500Hz. This type of set-up will change the spectral balance of the reflections heard and thus the overall sound of the system as heard at the listening position. And it is virtually impossible to know if the end result will be satisfactory to you the home theater owner without trying it.

Acoustical Treatments - Diffusion
Absorption is not the only way to effectively deal with reflections. Breaking up the direction of a reflection so that the frequency doesn't carom off a wall at a predictable angle is the job of a diffuser.

As noted in the first slide, diffusors can be either purpose built panels or perhaps something more common to a home like a bookcase. How then does one know when to use an absorber vs. when to use a diffuser?

In many, if not most instances in either home theaters or dual-use rooms it usually works out that the front left, center and right channel reflections which bounce off the walls, ceiling and floors are attended to by the use of absorptive materials. In the rear area of the theater it is a combined use of both absorptive and diffusive materials which can provide the dual-purpose role of attenuating the effect of slap echoes from directly behind the listening position (absorptive), while at the same time, adding to the immersive effect from the surround speakers which comes most easily by the judicious placement of diffusors.

A common solution for the rear of a theater is place absorptive material in the center area and diffusors on either side. This will knock down the slap echoes directly behind the listening position while allowing the surround speakers, which are mounted on the left and right walls to fire back into the diffusors, giving the desired immersion effect. For a 7.1 system place monopole left and right rear surrounds on either side of the absorptive material, at least 3 feet apart.