GOODSOUND!GoodSound! "How To" Archives

Published August 15, 2002

 

Listening to Your Room: Solving Acoustical Problems

All listening rooms have acoustical problems. The giant listening room known as Carnegie Hall has problems. While those problems might not be as severe as the ones in an average listening room, they do exist. Acoustical engineers have yet to figure out the perfect room in which to listen to music.

Some common problems in listening rooms include reflections of sound from one wall to another, ceiling-to-floor reflections, reflections coming from your sidewalls to your listening position, and out-of-control bass. Fortunately, you can minimize all of these problems by taking some simple, inexpensive steps.

The first, and probably most satisfying, method of correcting problems with the sound of your room is to get very wealthy, very quickly, and hire an acoustical engineer to build you a Taj Mahal of a dedicated listening room. Since that solution probably isn't going to work for everyone, try repositioning your loudspeakers and/or listening position. See "The Best Things in Life are Free: Speaker Placement" for more information on how to do this properly.

Not all problems can be fixed by rearranging speakers or your favorite listening position, however. Let's take a look at how some immovable objects, such as walls, and some irresistible forces, such as the laws of physics, can be manipulated in your favor.

Wall-to-wall echo, echo, echo

When you have two parallel, hard surfaces (like walls) facing each other, the soundwaves of your music can bounce back and forth between them like a tennis ball between Venus and Serena Williams. This bouncing creates an echo effect referred to as flutter echo. What you hear are echoes of your midrange and treble, especially, going back and forth and blurring the music -- fluttering with a metallic pinging.

A good way to check to see if your room might suffer from echo is to stand in its center and clap your hands together sharply. If you hear a ringing sound after the initial sound of the clap has disappeared, you and your room are being tormented by the heartbreak of flutter echo.

Luckily, this is the sort of heartbreak that's easily repaired. Take a look at those reflective walls of yours. If they're mostly bare, think about adding a tapestry or rug to one of the walls, or if you normally keep your windows uncovered, try pulling the drapes or curtains closed. Fabric can be a good absorber of unwanted sounds.

Which way is up?

If you're like most people, and most people are, your speakers will, by necessity, be on or near the floor of your listening room. If the floor is uncovered hardwood, for instance, you might also have problems with echo -- this time from sound bouncing back and forth between the ceiling and floor. Again, this can be easily remedied by having the floor carpeted. If this is too expensive or aesthetically unpleasing, try a throw rug or two, preferably made of natural fibers like wool.

Wool makes a better absorber of sound than the synthetic materials used in lots of inexpensive rugs and carpeting, because the fibers in a wool rug vary in length, thickness, and density. This variety helps diffuse the sound, and enables the material to absorb different frequencies of sound, whereas the uniform length and thickness of synthetic fibers enable them to absorb only one narrow frequency.

Think about this as well: If your loudspeakers are on the floor, they're going to reflect more sound, especially bass, than elevated speakers. Consider speaker stands if you've got problems with floor and ceiling reflections.

Another brick in the sidewall

A lot of people not only have to place their speakers on the floor, but near sidewalls (the walls to the sides of your speakers) as well. It can simply be the best, or sometimes the only, place to put your speakers. This placement can cause problems in the tone of the sounds you hear, unfortunately. Unwanted tonal coloration occurs in a couple of ways.

One common coloration occurs when the sound emanating from your speakers hits the sidewalls. It then is reflected to your ears a fraction of a second after the direct sound arrives. That means you hear the direct sound, and then a second, weaker version of it, reflected from your walls. If your walls absorb a bit of the high-end frequencies, that second (indirect) wave of sound will be colored differently from the first (direct) wave. What all of that means is that the high end, midrange, and low end of your music will be processed twice by your brain and colored by the delays in time and the absorption. Result? Music that sounds less defined than you might hope for from your music system.

Another way that your music can be colored by your sidewalls is by off-axis reflections from your speakers. Off-axis sound comes from the sides of your speakers. On-axis sound comes directly from the front of the speakers. Speakers are generally designed to fire music forward, out of the cabinets, but since no speaker design is perfect, sound always emanates to the sides as well.

These off-axis sounds often differ in tone from the on-axis sounds. They might have dips or peaks in frequencies that the on-axis sound doesn't have. When the off-axis sound reflects off of the sidewalls and then arrives at your ears just after the on-axis sound, you've got unwanted coloration of your music.

A simple way of alleviating problems like these can be to move your loudspeakers farther away from your walls. If that isn't possible, there are simple, affordable solutions. First, try the suggestion from above: Hang a tapestry or rug on the sidewalls between your speakers and your listening position. If that doesn't diffuse the problem to your satisfaction, you might want to consider adding some highly absorbent acoustical treatment to your walls. One of the most popular is Sonex. Even if you've never heard the name before, you're probably familiar with the look of this sound-absorbing stuff. Often used in recording studios to devour unwelcome echoes, it's made of gray, open-cell polyfoam. Tiles of Sonex look something like futuristic checkerboards with horizontal and vertical lines created by the sound-absorptive foam. Four 2' x 2' tiles will cost about $75, and can make a world of difference in dampening sidewall reflections.

Try putting four tiles, two on each sidewall, in various places on the walls, always between your listening position and the speakers. Listen to the same piece of music as you test the various locations. Very often, you'll find that just these four tiles will solve the problem. If they don't, however, you can either buy more tiles or you can build an inexpensive sound-absorbing panel of your own.

A typical absorber panel would be 4' x 8', built of 2" x 4" boards, fiberglass, polyester batting, and a cloth covering. Nail the 2" x 4"s (or 2" x 8"s to make a thicker, more absorptive panel) into a rectangle, with the short ends toward the ceiling and floor. Caulk the inside seams of the rectangle so that they're airtight. Staple a covering of tightly woven cloth over the back of the panel, then cover the cloth, on the inside, with polyester batting (available at fabric stores). Cut and fit a length of fiberglass material inside the panel. (Fiberglass and like materials come in varying thickness. If you're using 2" x 4"s, get 4"-thick fiberglass. The thicker the fiberglass, and your panel, the more sound it will absorb.) Cover the fiberglass with more of the polyester batting. (The batting helps ensure that the fibers in the fiberglass don't get outside the panel.) Now affix a decorative cloth covering over the front of the panel. You now have a sound-absorbing panel that can hang on your wall between the speakers and listening position, which will swallow those unwanted sounds as easily as Marlon Brando chowing on cheesecake. The total cost of two such panels should be under $70.

These sorts of panels can be placed anywhere in the room. If you have reflections coming from behind your speakers, as is sometimes the case with bipolar speakers (loudspeakers that deliver sound equally from the front and back), you can place panel absorbers behind each speaker.

If you find that you've got sound reflecting from your back wall (the wall behind you when you're in your listening position), try putting a sound panel back there.

Shutting the TV up

Objects that sit near speakers, such as television sets, can also be the source of those unwelcome reflections. Cover the TV with a cloth when listening to music. The same goes for amplifiers, if you have 'em near the sides or fronts of your speakers.

Sonic booms

If the bass from your system has a dense, boomy quality to it, again, try repositioning your speakers (and subwoofer, if you have one). If you've got everything placed as well as can be expected, you might need to take another step by adding a bass-absorbing panel, much like the one we described above. The difference is that a sheet of plywood or Masonite is nailed over the front of the panel. Drill .5" holes, spaced 1" apart, throughout the panel. Just like that, you've got a panel that will eat up a lot of that excess bass. If your bass is still too boomy, even after installing one or more of these panels, make those holes bigger to allow more bass into the absorptive fiberglass.

Remember: Before building or buying sound-absorbing panels, please check the positioning of your speakers and listening position. This is still the best, easiest, and cheapest (free!) way to get the best sound out of your system.


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