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Acoustical Room Treatment: Treating the Secrets Test Lab

Part I

September, 2004

John E. Johnson, Jr.

 

Introduction

I am sure that just about every Secrets reader has a home theater or hi-fi system of some sort. Expensive, modest, or just basic . . . but something.

After getting all that stuff, a DVD player, receiver, extra power amplifiers, speakers, cables, maybe a projector and screen, you thought you were done, right? Well, I thought I was done, until I realized that in spite of having very good equipment in our test lab and other rooms, something didn't sound quite right. It wasn't as focused as I wanted. Maybe a little boomy, even with servo-feedback subwoofers. Some of the sound was coming from the walls near the speakers, rather than mainly from the speakers.

So, what was the problem? Lack of acoustical room treatment.

Recordings are made in rooms that have reflective surfaces, more or less. Some have lots of absorption panels, some not so many. But, there is always at least a little reflection.

When you listen to these recordings in your own home theater or audio room, living room, den, whatever, you pile the reflections that the recording has onto the reflections of the sound in your room after it comes out of the speakers.

This does two basic things. One is the high frequencies are bounced around so that any one sound arrives at your ears several times very close together. This mushes up the detail. Secondly, the room serves to accentuate the bass, making it boomy, like being inside a big bass drum.

Neither of these room effects are desirable to high fidelity sound reproduction.

So, what is the solution? Apply acoustical room treatment.

The Available Measures

Anechoic chambers are the extreme end of room treatments, and are really only useful for testing the absolute frequency response of speakers. They are not for listening to music, because we are not used to having no sound reflections. In fact, it is downright uncomfortable to be in an anechoic chamber.

What we want in our own home theaters and audio rooms is not anechoic, but rather, a controlled amount of absorption and reflection (diffusion), and that is what are going to talk about in this series of articles. Some of the commercial solutions are expensive, and some are not expensive. You can also make your own, and we will cover those too. The goal of this article is not to convince you to buy any particular brand or product, but to just give you our experiences in treating the Secrets lab.

There are lots and lots of products out there to help you make your home theater sound great. Basically, they fall into two categories, absorption and diffusion.

Absorption products remove some of the sound that hits them by absorbing it and converting it to heat. They include such things as foam panels, fiberglass panels, and bass traps. Foam panels have different surface designs. Some have pyramids or other shaped protrusions. This increases the surface area and deflects unabsorbed sound from one protrusion to another, instead of reflecting the sound back into the room. Depending on the depth of the panels, it will absorb sound of different frequencies to different degrees. Thicker panels absorb more sound.

Usually, the panels are more effective at higher frequencies than lower ones, so you need additional absorption devices to handle the low frequencies. This is where the bass trap comes in. Bass traps usually go in the corners, where bass tends to get amplified, like a megaphone. They can be just thick foam triangles, or cylindrical units.

The amount of absorption by room treatment panels can be measured and specified. Some of the terms used include the coefficient of absorption and sabines. The coefficient of absorption states how absorptive the product is, with the higher the coefficient, the more absorptive. The number represents the percentage of sound that is absorbed. A standard of 500 Hz is often used, since the coefficient changes with the frequency. A sheet rock wall might have a coefficient of 0.02 (i.e., it absorbs 2% and reflects 98%), while a 2" thick absorption panel could be 0.6 at 500 Hz and 0.1 at 100 Hz . A sabine is the coefficient of absorption multiplied by the square foot of surface. (One square foot of material that absorbs 100% of the sound that hits it represents 1 sabine.) So, with the sheet rock wall 8' high by 12 feet wide, that is 96 square feet, multiplied times 0.02 equals 1.92 sabines. A single 2" thick 2'x4' absorption panel would be somewhere around 5 sabines.

The idea is to reduce the reverberation time in the room, and there is a formula which states that the reverberation time in seconds is equal to 0.05 times the volume of the room in cubic feet divided by the total number of sabines in the room.

If you put absorption panels on all the room surfaces, excluding the floor of course, the room would probably be pretty dead, meaning that there are very few sound reflections. This is unpleasant. But, leaving a wall or ceiling untreated still results in that smearing of sound. So, this is where you use diffusors. These reflect the sound, rather than absorbing it.

Diffusors are usually in the shape of panels with mathematically designed surfaces that bounce the sound around and release it in a random fashion. This lets the room remain alive, but you don't hear distinct echoes. The flutter echo, clap echo, or slap echo is one test that is used to determine if your room has a problem. You just clap your hands together and listen for the echoes. If there is a problem, you will hear the clap reverberating.

Our test lab had boomy bass, unfocused sound, smeared sound, and plenty of clap echo. So, I knew I had to do something. Not just because it is a test facility, but, I wanted it to sound good too, for when I sat down and just listened to music.

Treating the Secrets Lab

Our main test lab is about 18'x20' with 6'-8' ceilings at the soffits, and a 10' ceiling in the center of the room.

I looked around at many different products. There are lots of them out there, including those from SONEX, RPG Acoustics, Auralex, Tecnifoam, Echobusters, Acoustic Sciences, and others.

I settled on SONEX foam panels and bass traps, and RPG diffusors.

The question was which surfaces to treat. After getting some suggestions - each of the companies offers all the advice you want - I decided to treat all the walls with SONEX foam absorption panels, covering most, but not all of the surface, and treat the ceiling with RPG diffusors.

So, after settling on that, I had to decide on which model of panel and diffusor to use.

SONEX carries several types of foam panels, including various thickness and surface designs. I settled on what they call SONEXclassic. They also offer SONEXone, SONEXsuper, SONEXmini, and SONEXvalueline. They are different prices and are for a variety of applications.

I purchased the SONEXclassic, in white, 2" in thickness. I chose white because I did not want the lab to look like a dungeon. It is like photographic darkrooms. Most of the darkrooms I have seen are painted black. That is not necessary if you control the light in the room.

I have several lighting systems in our lab. One nice thing about the white, is that it only comes in Melamine, which is fireproof, compared to the polyurethane panels in the other colors. Here is a picture of the front of the room, before treatment, where the main speakers are located.

You can see that there is equipment all over the place in this room, one of the hazards of being the editor of an A/V publication. The room was just begging for proper treatment, and I am surprised it took me this long to realize that.

This photo a panoramic one, having been stitched together from four separate photos using some panorama software, so there is distortion in the room lines. But, you can see the basic room layout. The left side has two soffits, while the right side has only one. I had some basic wall treatments, including some SONEX foam panels on the side walls, and I used an egg shell mattress pad for the front main wall. It worked OK, but as I discovered, not nearly well enough. There was no treatment on the ceiling, except for a couple of corner foam triangles, and this resulted in significant clap echo.

There are no windows in this room, and only one opening on the right side where an air conditioner sits.

You can see the three lighting systems, each on solid state rheostat controls. One is on the angled part of the ceiling, one underneath the first soffit, and a third in the two corners at the ceiling line. I use the three systems to obtain a particular mood for listening. I love the two corner lights the best, and they will be used as bias lights once our projection screen is installed, as they will be behind the screen. I have all lights on when I do bench testing, as I am getting older, and every watt counts when I want to see all those details in the products.

I also purchased some SONEX bass traps, which are polyurethane, 12"x12"x24". Each bass trap has 12 sabines of absorption below 500 Hz, which is six times what the 8'x12' sheet rock wall has.

I tested polyurethane in the bass traps by setting a scrap piece to the flame on a stove. It smoldered a bit, then went out. It did not flame at all, due to chemical treatment at the SONEX factory.

Here is a photo of the rear wall, where the doors are located. Again, it is a stitched panorama, so ignore the crooked room lines.

I have two corner foam triangles there, like in the corners of the front wall. Basically, I hereby confess that this is pretty pathetic treatment. I thought that I had done enough, but now realize that it was not nearly what I should have had in there. Look at those dipolar ribbons in the corner. How stupid I was to not put corner treatments behind them.

Anyway, I had to decide on how much of the room surface to cover, and then calculate the number of panels to buy, knowing that the foam wall absorption panels are 2'x4' and the ceiling diffusor panels are 2'x2'. The bass trap calculation was based on its 1'x1'x2' dimension.

I purchased two packages of SONEX wall panels, with eight in each package ($213 per package), and five packages of SONEX bass traps, with two to a package ($54 per package). I ordered the traps in beige. I also purchased six packages of RPG Skyline LP diffusors, with two to a package ($160 per package). As it turned out, I needed a few more bass traps and diffusors, and simply ordered them later.

Click Here to Go to Part II.

Copyright 2004 Secrets of Home Theater & High Fidelity
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