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Getting the Right Acoustics for Your Listening Room

by rives August 24, 2004
Contributors: Christopher Huston

"The room is the first thing we start with and the last thing we think about." --unknown

The above statement is so often true. It's unfortunate because the room, as we often refer to it as the "invisible component" can easily make or break the system performance. Think about it, an amplifier company makes a state of the art amplifier capable of reproducing a signal with no more than 0.01 Total Harmonic Distortion, dynamic headroom above 110 dB, and gold plated connections to insure the best possible path for the signal. Then we put this amplifier together with speakers and other fine performing electronics into a room that delivers a bass boost of 12 dB at 80 Hz, a huge dip at 300 Hz, and another strong peak around 5 kHz to 10 kHz. The phase distortion alone is well into double digits and all due to room interaction.

So if the problem is so obvious, why is it so often ignored? I have surmised several facts and myths that could be the reason(s):

  1. Acoustical room treatment is ugly. Myth. While it's true there are some pretty unsightly acoustical designs out there, and many of the less expensive off the shelf products make one wonder if the person designing them was blind, room treatments need not be ugly. The fact is room acoustic treatment can take on almost any form one desires. Many designs are now built into the walls and ceilings and covered in fabric. The result is aesthetically AND sonically pleasing.
  2. To treat my room properly will cost a fortune. Fact or Myth. It depends on the room and all other requirements. Some rooms don't need a lot of treatment, others do. If you go to a company that sells acoustical panels they may try to figure out how many will fit in the room to determine what you need, rather than calculate how much is sonically required. If the later were used it would probably be much less expensive. It also depends on how far you want to take the room sonically. To do some basics is generally inexpensive. To create a world class listening room can be very expensive.
  3. I would like to try some acoustical treatment, but I have no idea where to start. It's really an "acoustical jungle" out there. If I ask for 5 opinions I usually get 10 and am more confused than before I even asked the question. Fact. This has been our experience as well. Acoustics is both art and science. The science is not that difficult but the art is purely experience. Opinions even from renowned experts vary greatly.

Designing Rooms With an Educational Perspective
Our company designs listening rooms and home theaters from an acoustical perspective. We always try to educate the client along the way, but our most important objective is to understand the client's needs. Thus, as a listener, you have to understand your needs. Sounds easy, but often it isn't. We always tell a client there are three things to consider when undertaking acoustical treatment and design for a room. They are: Sonic benefit, Aesthetics, and Budget. These three things work in a balance and every client has a different threshold for each. Whether you are going to do the design and installation yourself or hire a professional group to do these things for you, you still need to understand your own limits for each of these aspects.

In this article we will examine the fundamentals of taking on the very basic acoustical approaches. These will not take you to the magical sonic bliss that a well engineered room can deliver, but it will go a long way. I have seen many $100k systems plunked down in a room with little or no thought to acoustics, and if they just got these basics right they would be much happier with their system. What can even be more humorous is listening to the person with a terribly bright room, filled with glass and terrible long reverberation times in the high frequencies, and they recognize the ill sound. Their response: "I think it's the CD transport. I should upgrade it."

If you are going to undergo this yourself I've already told you the first thing to do, the second is to read this article, and the third is read some more. At the end of the article I've provided a reference list that can be very helpful.

Everybody's Got an Opinion...
One thing I will recognize is the various expert opinions. I too have my opinions that I feel are correct from experience but acknowledge others for some reason or another have deviated from my forthright path (I'm being facetious). In this article we will focus on 2 channel listening environments. Perhaps in a future article we will discuss multi-channel and home theater. With 2 channel listening I tend to lean towards a live room, not be confused with bright. By live I mean I want to retain the kinetic energy of the speakers moving air through the room-the production of sound. This varies from some that would rather deaden the sound field. The one thing to keep in mind is balance. If you have a live room, all frequency bands need to be kept in balance in terms of frequency response and reverberation times (or reflected energy retained).

Beginning with Acoustical Measurements
Any acoustical engineering group is going to have a way of acoustically measuring a room. Before I go any further, I would strongly recommend that anyone taking on acoustical treatment do this as well. Now the equipment used by an engineering group is likely to be expensive, cumbersome, and have a steep learning curve. I wouldn't recommend you go buy this type of equipment, but if you have no idea the current performance of the room, it's unlikely you are going to go very far successfully. Buy the outrageously inexpensive Radio Shack SPL meter (the analog version if you can get it) and a test CD that has 1/3 octave test tones and measure the response of the room. Keep in mind that these meters are C weighted which means they roll off in the high and low frequencies. You either need to manually compensate for this with a spreadsheet or buy a test CD that already is calibrated for these meters, such as the Rives Audio Test CD 2 (a shameless advertisement). This will give very basic information on relative frequency response-always a good place to start.

Now before we get into measurements let's look at the room. The most significant thing you can do is place your speakers and listening position properly. So let's start here. How many people have a rectangular room with a flat ceiling and doors that allow it to be completely closed? That few, huh? Many listening rooms are shared environments. They are the family room, the den, and in today's homes may be very open. They may open into the kitchen or some other area. All of these things can compromise acoustics, but if dealt with diligently they do not have to be a major problem.

Speaker Placement
When I look for the best place for the speakers there are two things I'm interested in. One is a good back wall behind the speakers to retain the kinetic energy coming from the back wave of the speakers. If I don't have this I will lose a lot of bass energy. Glass behind the speakers is generally a bad idea as it leaks bass badly, but sometimes it's the only realistic position. The second thing I look for is relative symmetry. I don't want a hard wall on one side and a massive opening to a kitchen on the other. It will be virtually impossible to get a sound stage and imaging with this. Now there are a few papers in the resource section that will help you more. Two I would like to point you to: the Cardas website and the Rives Audio white paper on speaker positioning. You may actually want to try both methods. The Cardas set up may not work if you have an unusual shaped room. The Rives method will work in virtually any room.

Philosophically I always believe that the bass response is the foundation of the entire musical presentation. If the bass is wrong everything falls apart. If the bass is right, it makes fixing whatever else might be troublesome much easier. In small room acoustics the most common problem is what is known as room modes. Room modes occur because there are boundaries, namely the walls, ceiling and floor, that reinforce certain frequencies. Rather than go into detail on what frequencies might be accentuated and why, we will discuss what can be done. For those that want the details, please see the reference list, there is ample information on room modes available on the world wide web. The best thing to do is first measure the response of your system at the listening position. See if there are certain frequencies that are accentuated. A theoretically perfect room will still only be accurate to within +/- 3 dB, but most average rooms will have deviations of more than 20 dB. (There is an article in the reference list on how to take measurements) Once you know the response you can assess what the biggest problems are.

There are a variety of ways to deal with bass problems. Most problems are accentuations in certain frequencies. Every now and then there are "holes" or "suck outs" of one or more frequencies. These are less common, the ear adapts to these holes rather well, and quite frankly there's little you can do about them-so we won't discuss those here. As to the boosted notes, we can deal with these. The first approach should be speaker and listener position. There are a variety of resources at the end of the article to help you with that, but there are a few important points to note. First, proper positioning for best bass response and midrange is likely different. Frequently we have to compromise. At this stage of the game, we will optimize for bass, and likely back off this position later in our approach. Now, unfortunately, I'm going to have to put in a math equation for you-but it's an easy one so don't stop reading. Let's say we have a boost at 80 Hz. 80 Hz has a wavelength of a little more than 14 feet. The ¼ wavelength is about 3 ½ feet. If we place the speaker 3 ½ feet away from the wall the 80 Hz cancels itself out at the speaker position and thus reduces its effect in the room. You may have solved one of the biggest problems in your listening environment and it cost you the Radio Shack meter and one Test CD. A total outlay of about $60.

Handling Tougher Bass Problems
Let's say the bass problem is more extensive, or for an aesthetic reason you can't move the speaker. There are two basic approaches in this instance: passive methods which include a variety of treatments or electrical correction. Passive methods come in a variety of methods, but in general you either need a properly tuned capacitive type of bass trap or a Helmhotz resonator. Both are somewhat large, need to be custom made, and it's advisable that you have an acoustical engineer do the design, so that if it doesn't do what you want it to do, they can pay for the correction. The second method, electrical correction, is far more practical for most instances. In fact we designed such a device because we were not satisfied with any of those that were on the market. This device is the PARC, a parametric adaptive room compensation system. Basically it's a parametric equalizer that can attenuate the boosts caused by the room. What's even more interesting is that by its very nature it counter balances the phase shift caused by the boost in the room. It also offers the advantage of being completely programmable and can be moved to a new home, room, or change with any other significant changes in the acoustics.

Whatever method you choose to get the bass right-do just that. Get it right and everything else will work better and the sound quality will come together much easier.

Treble and Critical Midrange
Now it's on to the treble and critical midrange. We will deal with the treble first. Recall we have placed our speakers for optimum bass performance, however this may not be the optimum midrange and treble position. For now, I'm going to suggest we leave the speakers there. Before we do anything, let's just listen to some music. Don't get overly analytical here, just listen. Is it enjoyable? Are there some things missing, or things that are over emphasized? Well let's go back to our trusty Radio Shack meter and do another test. How is the treble performing with respect to the rest of the frequency response? Is it elevated or depressed. If it's elevated, that's easy to deal with. We need to add some absorption material in the room to tame the treble. Okay, have you every seen that funny shaped foam in the audiophile stores for absorption? Should we use that-no? Too ugly. I agree. Let's buy some Owens Corning 703 pressed fiber board that's 1 inch thick. We can wrap it in a nice natural fiber fabric. It's light and now with our favorite fabric it looks great. You can of course buy these already made up from a variety of companies (see the resource list) but doing it yourself is not too difficult. Now we need to know where to put them. Generally we want to insure that the first reflection points are tamed. This is the point on the wall, that if you had a mirror on a side wall the listener would look into the mirror and see the speaker. The sound reflects just as the line of site. So if the wall is bare there, we need to absorb there. The same is true for the floor. If you are on hardwood floors, which are generally good surfaces for a sound room, you need an area rug around the listener position to absorb the reflected sound. I generally recommend a heavy pile wool rug-now you have an excuse to get that Chinese rug you've been wanting. What about the ceiling? This is an important topic, and I'm going to save it for a little later, let's move on to the midrange.

The midrange is the most critical area and often the most difficult to get right. The midrange has the vocals, strings, a large portion of the piano, guitar-really critical stuff. Well, let's start the same way as we did before. Let's listen to some music. I generally recommend some female jazz vocals that have an upright bass and piano. What does her voice sound like? Is it clear, really clear? Can you hear every detail of her breath and singing? If so, you probably don't need to do a thing. Chances are, however, that it's not that clear. Do her notes carry on too long? Is the intelligibility lost to some degree? Are you scratching your head wondering and not really sure about these last two questions? Here's a simple experiment to help you understand the issue of intelligibility. Have a familiar female person stand at your speakers and talk to you while you are in the listening position. Now have them walk closer and closer to you. You can do this exercise in a number of rooms. If you have a sun room with a lot of glass and hard surfaces, be sure to do it there. You will lose intelligibility in a room like this and will immediately know what you are trying to avoid.

How do we fix this midrange issue? Generally diffusion works well. So how about some of those large wooden slat things around the room? I get the same reaction with the foam I mentioned earlier. How about a nice book shelf with books? This works great for diffusion and absorption. I frequently place books or album shelves in the audio room even when it's a dedicated room. They look good and give the place a nice warm and relaxing feel. Books behind the listener work quite well. Now what if behind the listener opens into another room? This can make things more complex. What is that next room like? Is there a way we can acoustically divide the two rooms? These issues can get complex quickly. But having a lot of absorbing material in the space behind the listener and the next room can create somewhat of an acoustical division. However, if you do have this situation and you are serious about your music you may want to hire an acoustical group to look at the situation, measure the room response, and come up with a definitive plan.

Ceilings and Diffusion... Often "Overloooked"
Last area I want to discuss is the ceiling, often overlooked and vitally important. If you have large cathedral ceilings this can be good, but often times it's a major problem. Usually if there is a back slope behind the listener it's reflecting energy back at the listener. This, just like our sidewalls needs to be absorbed. Again, the fiberglass solution works well, but there are some even more attractive solutions. RPG makes a "BASWaphon", which is an acoustically absorbing wall. It looks just like sheet rock and can even be painted to match the wall color in your house. However, rather than reflecting energy it is highly absorptive down to 400 Hz. The downside to this is that it is expensive and does have to be professionally installed-it's not a DIY project. This is a good example of how budget and aesthetics need to be kept in balance.

Another common way people believe in adding diffusion is plants. It's true they do add diffusion, but not really very much. Diffusion works on the basis of pressure changes. Plants are in free space and really don't change pressure very much. If you have plants, they aren't going to hurt your sound, but it's unlikely they will help it very much either.

Is the ceiling flat. That's a huge flat untreated surface and we would like to break up the surface energy. There are a variety of ways to do this. One of the most complex, but beautiful ways is a coffered ceiling. We've designed these for traditional homes where we wanted no acoustical treatment to show. For a more modest approach we can use our fiberboard panels again wrapped in ceiling white fabric and attached to the ceiling with "roto-fast" screws. If we cut the fiber glass into 2 foot squares and lay them out with about a 2 foot space between them there is a very nice symmetrical pattern. Of course it depends on the décor, but it can actually enhance the room.

Well that's all for this month. Please join us for future months. Till then, enjoy the music!

Resources and education in Acoustics: www.rivesaudio.com/educate

Contact Rives Audio: [email protected] or 800-959-6553

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