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.{mospagebreak}
Now it was Time to Actually Install this Stuff
I am not very good at construction projects, but as it turned out, this was easy, although time consuming.
So, I had all these things to stick on the wall and ceiling. I needed adhesive. After getting a recommendation from SONEX, I went to our local Home Depot and bought some Liquid Nails®. I had no idea how much I would need, so I got six tubes to start with. I ended up using about 20 tubes. Fortunately, the product is less than $2 per tube. I started out with the Small Project version of the adhesive, but then switched over to the Heavy Duty version, which requires a caulking gun (about $5).
This adhesive smells terrible and will make you cough and wheeze. It has several volatile solvents in it, so be sure to have good ventilation. Also on hand were some paper towels, plastic bags to throw away empty adhesive tubes, a sharp tool to puncture the adhesive opening, tape measure, a roll of thread, and some pushpins.
You have to be careful with the Melamine panels, as they are delicate and easily torn or punctured (polyurethane panels are sturdier). I thought I might put a few up temporarily with pushpins, but that just tore small holes in the panels. Pushpins will work fine as temporary mounts with the polyurethane panels though. So, I just held them up on the wall with some tape to see where I wanted them. You could also put the pushpins into the wall at the bottom and set the panels on them temporarily.
I started with the front of the room. The panels come in pairs, with the two panels facing each other. You peel them away and set them aside. Each member of the pair is a mirror image of the other, so they are not identical.
Being the compulsive person that I am, I decided to arrange them geometrically, alternating the mirror image units rather than just putting them up in a random fashion. I don't think this affects the absorption, but it is a more pleasing appearance, because the mirror image differences are noticeable.
I set them on the floor against the wall to see how the spacing would come out across the width of the wall, and ended up spacing them about 6" apart and at different heights, since the wall is angled.
I used one of the SONEX gray panels (2'x2') that I had previously, to add some color to the wall, breaking up the monotony of the white panels. I began at the ceiling, and worked my way down, stacking two 2'x4' panels one above the other.
Next, I did the side walls, followed by the rear wall.
One tube of adhesive was enough for about three 2'x4' panels, making a box with an enclosed X on the rear, and a 1/4" bead of adhesive, as shown in the diagram on the right. The glue pattern is shown in red. Put the glue about 2" in from the edge of the panel so that it does not seep out on to the wall when you apply the panel on the wall, or when you have to slide the panel around a bit to get it into final position. Once the panel is applied to the wall, it is stuck tightly after only about 30 seconds, so you can't dilly dally in getting it into final position by sliding it an inch or so to one side or vertically. I used black thread strung out with the pushpins, to make a straight line across the wall so I could line up the panels when sticking them on the wall.
Then came the bass traps. They are 2' high, so I used several of them in each corner to reach the ceiling. I had to cut the finishing section, using an electric knife, so that the last one would fit.
I also used bass traps along the soffit angles. There are angled traps available that are specifically for this purpose, but I wanted something deep, and the bass traps were perfect. I had to cut a corner section from the rear of each bass trap, so that it would fit over the molding that was already there. Using the X enclosed box of adhesive pattern on each rear surface, I held them against the wall for about a minute such that the adhesive would stick enough to the wall that they would not fall off after I removed the pressure.
Finally came the ceiling diffusors. Velcro® pads are included, and I was so irritated with the bad smell from the adhesive, I decided to use the pads instead of glue. All you have to do is remove the protective sheet from one side of the pad, stick it on, and do the next one. I used four pads, one in each corner. I had my black thread and pushpins to mark a straight line, removed the protective sheet from the other side of the pads, and stuck them on the ceiling.
Here is a photo of the front wall, after the panels, bass traps, and diffusors were installed. Diffusors are on the angled part of the ceiling as well as the horizontal part of the ceiling. They were spaced 10" apart, except for where the recessed lighting was located, where I left more space open. On the angled ceiling, the diffusors were placed side to side, while on the horizontal ceiling, they were placed corner to corner. The underside of the soffit on the left is not covered with panels. The computer on a roller cart is for bench testing, as is the microphone stand seen on the right.
You can see the panels on the walls, bass traps in the corners and soffit angles, and diffusors on the ceiling. I would estimate that about 65% of the surfaces were covered with panels or diffusors, leaving 35% of the wall and ceiling surface uncovered.
The rear of the room now looks like this. Notice that the panel and trap placement are not symmetrical. This is because the room is not symmetrical.
Here is a view of the front ceiling, and you can see details of the diffusor panels. They consist of small blocks of different heights. A close-up of a diffusor is shown in the second photo.
The photo below shows the SONEX panel mirror images of each other. In a box of eight panels, you get four of each.
The left wall has foam panels up to the soffit, and the panel texture is evident. You can see the bass traps in the soffit angle, and how they are trimmed on the rear so that the corner molding will fit behind them.
In this photo of the front right corner, ceiling, and part of the right wall, you can see that the diffusors are side to side on the angled ceiling and corner to corner on the horizontal ceiling. There are several gray 2" thick SONEX absorption panels on the ceiling at the rear of the room, as I plan to put slate tiles in front of the doors. The SONEX here will help to absorb sound reflected from the tiles. In the bottom right corner of the photo, you can see how I cut the SONEX panel to fit around the air conditioner. I used a sharp knife to do this.
A photo of the right rear corner (right when facing the front, as in the right rear surround), is shown below.
You can see that the panels are not the same height, and the corner bass traps do not come all the way to the floor, as there is an electrical junction box at the bottom that I did not want to cover up. I put in the wall panels before the bass traps, so I made sure that I was leaving enough space for the corner traps to fit in. I glued the first bass trap against the soffit at the top, then put two more underneath the first one.
Now, the left rear corner.
There are actually two corners here. I placed bass traps fully in one of them and only part of the other corner where the burglar alarm and motion sensor are located. You can see that I also placed foam panels on the doors, as shown below in more detail.
The Resulting Sound
There was a huge sound change as a result of the room treatment. I can hear details in the music that were masked before by the multiple arrival of the sound. Bass is now very tight, and it is much more localizable to where the subwoofers are sitting, and this surprised me. I knew it would be less boomy, but did not realize I would be able to tell where those low frequencies are coming from.
The soundstage is now much narrower, and this may seem like a downside, but all it means is that the walls adjacent to the speakers are no longer a significant part of the sound that is arriving at my ears. It is mostly direct from the speakers.
The clap echo is completely gone, yet the room is still alive.
I have to turn the volume control up more than before, because the treatment is absorbing a significant portion of the sound. This I consider a bit of a downside, because the amplifiers and speakers are working harder now, to give me the same loudness I was used to. It means you had better have a big power amplifier if you treat your home theater and like to watch movies at high volume.
I am left with a bit of a hump in the 80 Hz – 150 Hz region that I could reduce by putting in something like ASC Tube Traps, which are designed just for this sort of thing. However, I am expecting to receive the new card for my Lexicon MC-12 that auto-EQs the room up to 220 Hz, so I will wait to see if that does the job.
Conclusions
Room treatment may be last on your list of things to do for your home theater or audio room, but it is something you definitely should get to sooner or later because it really makes a difference.
It can be expensive, but you don't have to do extensive treatment to get results, and you don't have to do it all at once. There are other options for those who prefer something a little less industrial in appearance, such as using curtains, and DIY projects with things like Corning fiberglass panels that you can cover with material of whatever color that suits you. We will include all these ideas in subsequent articles.
Reviewer: John E. Johnson, Jr.
