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Room acoustics for home audio

How to find and treat your room's trouble spots

We generally think of the speakers in our stereo or home theater systems as the final link in the audio chain — and the one that makes the biggest difference to our ears. But there's much more to the sound we hear than just where you place your speakers in a stereo or home theater setup, and what comes out of them. You might not even realize it, but your room plays a rather large part in the sound that you hear from your system. And as with any other component, there are steps you can take to improve your room's performance.

Room Acoustics Sound energy is in the form of invisible waves. Since our hearing can perceive sounds from 20-20,000 Hz, we're talking about wavelengths that range from 11/16" at 20,000 Hz, to over 56 feet at 20 Hz.
Room Acoustics Most people are more comfortable thinking about music as notes or tones, rather than frequencies. Middle "C" on a piano is 262 Hz. Low "E" on a bass guitar is 41 Hz. Cymbals can go out to 15,000 Hz.

Why your room matters

The sound that you hear in any room is a combination of the direct sound that travels straight from your speakers to your ears, and the indirect reflected sound — the sound from your speakers that bounces off the walls, floor, ceiling, and furniture before it reaches your ears.

Reflected sounds can be both good and bad. The good part is that they make music and movie dialogue sound much fuller and louder than they would otherwise. If you've ever played your speakers outdoors where there are no walls to add reflections, you've probably noticed that they don't sound as good — thin and dull, with very little bass. Reflected sound can add a pleasant spaciousness to your sound.

The bad part is that these same reflections can also distort sound in a room by making certain notes sound louder while canceling out others. The result may be midrange and treble that's too bright and harsh or echoey, or bass notes that are boomy, with a muddy "one-note" quality that drowns out deep bass. Because these reflections arrive at your ears at different times than the sound from your speakers, the three-dimensional "soundstage" created by your speakers and the images of the instruments and singers may become vague or smeared.

Room acoustics In addition to the sound from your speakers, you hear reflected sound from your room's four walls (above left). Your room's ceiling and floor contribute reflected sound, as well (above right).

Basic tips on taming your room's reflections

As we've discussed, reflected sound is necessary for music and speech to sound natural, but too much can rob your system of sound quality. The two main ways to control reflected sound are by absorbing or by diffusing (scattering) these reflections. We'll get into some more in-depth solutions a bit later in the article, but for now, here are a few simple tips for getting better sound in your room:

  1. One of the easiest ways to improve your sound is to move your chair or sofa away from your wall and out into the middle of your room. You might also want to try positioning it closer to or farther from your speakers, and listen to see where your audio sounds best in your room.
  2. If you have a large expanse of glass in your listening room, like a picture window or French doors, try installing drapes over them to absorb reflections.
  3. Along the same lines, if you have wood or vinyl flooring, try placing an area rug to help absorb some of those harmful reflections.
  4. Bookshelves can help break up or diffuse reflections. Try placing a bookcase or two filled with odd-shaped books to the sides or in the back of your listening room to see if your sound improves.
  5. Finally, be on the lookout for "acoustics-savvy" components, such as powered subwoofers with built-in bass equalization and home theater receivers with automatic speaker calibration. They can help digitally correct for room problems when the above solutions aren't an option given your room's layout or décor.

If these simple fixes don't cut it, and you still want to improve the sound in your room, then read on. We'll look at the science behind reflection, absorption, and diffusion, as well as give you some more in-depth tips on how to find and fix your room's trouble spots.

Room Acoustics

Reflections

Room Acoustics
Room Acoustics Room reflections add different sonic effects depending on their volume and how long they are delayed:
1) Some blend in seamlessly with the direct sound.
2) Some add spaciousness and increase image size.
3) The most damaging are heard as distinct echoes.

One of the reasons that the effects of room reflections are so noticeable is that our ears — actually, our entire auditory system, which also includes the brain — are amazingly sensitive at locating the source of a sound. Even with your eyes closed, you can usually locate the position of someone speaking to you in a room. Your brain uses timing differences between the original and the reflected sound to locate the source.

But our ears aren't perfect. Sounds that arrive at our ears soon after the direct sound are perceived as being part of the original sound. As the graph at the right shows, early reflections that are not too loud or delayed too long will not only increase the loudness of the sound, but can actually add a pleasant spaciousness. In fact, part of the reason that the surround speakers in a surround sound system can create such a believable impression of spaciousness is that the signal fed to the surround speakers includes a 15-20 millisecond delay.

As you can see, when it comes to sound there are two factors: loudness and length of delay. If the reflection is too loud, or if the delay between the original sound and the reflection lasts too long, you'll generally hear a distinct echo. That's why it's much more difficult to locate the source of a sound in a highly reflective room with uncontrolled echoes, or outdoors in an open field, where the only reflective surface is the ground.

There are several different ways that room reflections can interfere with your enjoyment of music and movie sound. Let's start by talking about the unique set of reflections that develop based on the size, shape, and dimensions of your room.

Problem 1: Standing waves and room resonance modes

Any time you have a pair of parallel reflective surfaces (like room walls, or the floor and ceiling), you're going to experience some degree of a phenomenon known as "standing waves." Standing waves distort the bass and lower midrange frequencies from 300 Hz on down.

Room Acoustics Standing waves are created when sound is reflected back and forth between any two parallel surfaces in your room. They affect frequencies below 300 Hz.
Room Acoustics A room's primary or "axial" resonance modes are based on the room's three main axes: length, width, and height. These resonance modes create bass peaks and dips of up to 10 dB throughout the room — so the volume may sound twice as loud in some areas as opposed to others.

One way to understand the effects of standing waves in a room is to think of how a microwave oven works. The high-frequency microwaves generated to heat the food on your plate are reflected over and over inside the oven compartment. As these reflections collide, some are reinforced while others are cancelled, creating areas of varying microwave intensity. This translates into definite hot spots and cold spots in your plate of food, from steaming to lukewarm to cool.

The sound from your speakers acts in much the same way. It's reflected back and forth, over and over between the parallel surfaces in your room: the side walls, the front and rear walls, and the floor and ceiling. This creates areas of differing sound pressure or loudness: the "hot" and "cold" spots. And while some reinforcement is necessary, too much can distort the sound. To use the microwave example again, it would be like having too few waves to heat your food in the center, and waves so intense that they heat your food to burning around the edges.

You can easily hear these standing waves if you play some music with a lot of bass, like pipe organ music or reggae, and take a walk around your room, listening at different spots: the middle of the room, near the walls, and in the corners. You'll probably notice that the bass sounds stronger near the walls and especially in the corners, where standing waves tend to collect. These are specific types of standing waves which are called "room resonance modes."

Finding your room's resonance modes
It's actually pretty easy to calculate the axial resonance modes for your room. Knowing the frequencies of these axial modes will provide valuable information about how your system and room are interacting, specifically on bass notes in the under-300 Hz range.

First, get a tape measure and measure the length, width and height of your room. As an example, we'll use these typical room dimensions: 21 feet long x 12 feet wide x 8 feet high.

The formula for finding axial room resonance modes:

Room Acoustics In the example above, we've calculated our sample room's main resonance mode for length. The room's length is 21 feet, so plugging in 21 for our distance variable in the equation, we get a resonance frequency of 27 Hz.

Our sample room has a length of 21 feet, so plugging 21 into the formula gives us our axial resonance mode for length.

Room Acoustics Resonance modes occur when the distance between the room's walls equals half the wavelength of the sound, and at multiples of half a wavelength. Notice that there are always sound pressure (volume level) peaks at the walls.
Room Acoustics The circled frequencies will be reinforced by the room. Frequencies appearing in more than one column will receive added emphasis, causing even more sound coloration. In this example, you can see trouble spots at 141 Hz, 188 Hz, and 282 Hz.

So, the main mode for the length axis of the room falls at 27 Hz (it's actually 26.9, but we're rounding to the nearest whole number). This means that although you'll still be able to hear deep bass sounds from your speakers below 27 Hz, your room cannot provide any reinforcement of frequencies much below 27 Hz.

In addition to this fundamental mode at 27 Hz, there will be other weaker modes at multiples of the fundamental mode (2x27, 3x27, 4x27, etc...). So, along with the first mode at 27 Hz, there will be other resonance modes at 54 Hz, 81 Hz, 108 Hz, etc....

Now we can use the same formula for the room's width and height. Plugging the 12-foot width into the formula gives us a fundamental mode at 47 Hz, with multiples at 94 Hz, 141 Hz, 188 Hz, etc.

Using the formula again, our fundamental 8-foot height mode is at 71 Hz, plus multiples at 141 Hz, 212 Hz, etc. It's a little easier to see what's going on if we arrange our room modes into a table (see right).

There's actually more to the story than just the axial modes involving two walls, described above. There are also tangential resonance modes involving four room surfaces, and oblique modes involving all six surfaces. These other room modes don't affect the sound as strongly, but as we've mentioned before, all reflections affect the overall sound.

How to deal with room resonance modes
So now that you know what room resonance modes are and how they can distort your system's sound, what can you do about them? In many cases, not much. These room modes are based on your room's dimensions, which are difficult to change. Even bass-loving audiophiles will hesitate to move a wall just to hear more accurate low frequencies. And room treatment products that are great for controlling treble reflections with short wavelengths don't work at all on long-wavelength bass reflections.

If you haven't chosen your listening room yet, or if you're building a listening room in a new home, here are some things to keep in mind concerning room resonance modes:

  • Certain room shapes are fundamentally bad from a room-mode standpoint. A cube is one of the worst shapes for a room — each resonance mode gets triple emphasis. You'll also hear more standing wave distortion in rooms with two equal dimensions, or rooms with dimensions that are multiples, ie. 8' x 16' x 24'.
  • If you're building a house or finishing a room, here are some room dimension ratios that are superior soundwise:
Room Acoustics Applying the 1 : 1.4 : 1.9 room dimension ratio (see table) to a room with an 8-ft. ceiling yields dimensions of 8'H x 11.2'W x 15.2'L.
  • In general, the smaller the room, the more its resonance modes will color bass response.
  • A high, sloped ceiling tends to scatter ceiling mode effects.
  • Common types of wall construction such as drywall or wood paneling on 2x4s will absorb a significant amount of added bass reflections in the under-125 Hz range (see table below).

If you're trying to fix problems with standing waves in an existing room, then you might be able to lessen the problem with some of these tips:

  • Standing waves are always stronger next to walls. If your chair or sofa has its back against a wall, moving it out away from the wall should reduce standing wave boominess.
  • Try moving the position of your chair or sofa closer to or farther from your speakers to get out of a standing wave hot spot.
  • Room corners are notorious collection points for standing waves. If your room has an 8-foot ceiling, professionally designed bass traps can help reduce or eliminate these standing waves. This is accomplished by soaking up the bass reflections created by the 71 Hz fundamental resonance mode of the 8-foot ceiling.

Problem 2: Flutter echo

Probably the most common and immediately noticeable room problem results from having parallel surfaces (walls, floor and ceiling) with a hard finish that reflects sound. The resulting effect is called "flutter echo," a ringing reverberation that remains after the direct sound has stopped.

If you've ever stood in an empty uncarpeted room or hallway, and clapped your hands, you've heard flutter echo. The original clap sound is reflected back and forth between two surfaces. Because the wavelengths of mid- and high-frequency sounds are so much shorter than those of bass notes, the reflections bounce around very directionally, like reflected light. The resulting sound is this ringing flutter echo rather than the boomy standing waves described previously.

Flutter echo affects music by blurring transients (fast musical attacks) and adding an unpleasant harshness to the midrange and treble. Flutter echo and other side wall reflections affect sounds above 500 Hz, and are a major reason why the same pair of speakers will sound different in different rooms.

How to deal with flutter echo
To treat flutter echo you need to control the reflections on one or both of the parallel surfaces. This usually means applying some sort of sound-absorbing or sound-diffusing material to the side walls between the speakers and your listening position. Likewise, carpeting or acoustic ceiling tile will reduce floor/ceiling flutter echo. Check out the table in the next section for a better idea of how different materials absorb sound.

The movie industry certainly understands how sonically damaging reflections can be. Think about all the reflection-absorbing surfaces in your neighborhood movie theater: heavy drapes all around, upholstered chairs, and a human audience — that's right, our bodies act as sound absorbers too.

Room Acoustics

Absorption

The sound produced by your speakers, as well as its reflections from your room's walls, ceiling, floor and furnishings, is actually sound energy, or acoustical energy. These sound waves cause air particles to vibrate, and when they vibrate against our eardrums, we hear sound.

A basic law of physics states that energy can neither be created nor destroyed, but can be converted into another form. If it's impossible to simply destroy all these unwanted sound reflections, how can we control them? This is where the concept of sound absorption enters the picture.

If you've ever been inside a recording studio, radio or TV station, concert hall, or music practice room at a school or music store, you've probably seen some type of sound-absorbing material. Applying absorptive material to walls and other reflective surfaces is the primary method for taming unwanted reflections.

Dense, porous materials like polyurethane foam and fiberglass tend to be the most popular choices. These materials absorb sound by converting the acoustical energy into heat. This happens when the air particles are driven into motion by the sound waves, then attempt to pass through the dense sound-absorbing material, resulting in a very small amount of heat-generating friction.

Whether we're talking about common room materials (see table) or professionally designed room treatment products, a material's ability to absorb sound varies according to the frequency of the sound. As the table shows, soft, fibrous materials like carpet and drapes will absorb most reflected sound above 500 Hz, yet have little or no effect on reflections below 125 Hz.

Room Acoustics The illustration above left shows that a 1" thick fiberglass panel provides excellent absorption of sounds above 500 Hz, but that controlling lower-frequency reflections requires the use of thicker panels. As an alternative to thicker fiberglass, the illustration above right shows how creating an air space between the panel and wall surface increases low-frequency absorption.

This makes sense when you remember the huge differences in the wavelengths of high- and low-frequency sounds. Fibrous materials, which are so effective at absorbing 1000 Hz sound waves a little over a foot long, can do very little when it comes to 125 Hz wavelengths that are 9 feet long. These long-wavelength reflections simply pass right through these soft materials with almost no resistance.

The table below also shows that drywall and window glass provide significant absorption in the 125 Hz range. This conversion of acoustic energy is accomplished in a different way than that of the soft, fibrous materials described previously. When a low-frequency sound wave strikes drywall or a window, those surfaces convert some of the sound energy to motion; they actually flex a tiny amount, thus absorbing some of the acoustic energy.

Room Acoustics Notice the increase in the absorption of reflected sounds — especially for sounds at or above 1000 Hz (1kHz) — when the fabric is folded into drapes.
Room Acoustics The sound-absorbing effectiveness of some common room surfaces. Fibrous materials like carpet and drapes provide significant absorption above 500 Hz, but have little effect on lower frequencies. Conversely, window glass and drywall can absorb bass frequencies, but are very reflective above 500 Hz. The most successful approaches combine materials like these with professionally-designed room treatment products.

Tips on absorptive treatments
Although absorptive treatments are very effective at taming flutter echo and mid- and high-frequency reflections in general, they won't cure all room acoustics problems. In fact, using too much absorptive material can itself cause problems. Your goal should be to balance the amount and frequency of the absorption in your room to achieve some bass and high-frequency absorption. Typically, bass absorption is the more difficult to achieve.

Here are a few tips and ideas to keep in mind concerning sound absorption:

  • Before turning to professional room treatment products for absorption, try to get the most out of ordinary room materials (see table).
  • Large expanses of glass such as picture windows or French doors should be covered with drapes.
  • You don't have to treat every surface in your room. There are a few key spots which, if treated, will give you maximum sound improvement for your investment. Check out our tips at the end of the article to find your room's reflection trouble spots.
  • The pad beneath a carpet contributes to its sound-absorbing ability. While your first considerations should be durability and comfort, it's worth knowing that an "open-cell" pad such as foam rubber will absorb more sound than a "closed-cell" pad.

Absorption is an important ingredient of room treatment, and is especially effective at treating side wall reflections. But absorption is not the only answer, and in many situations, it's not the best choice. In a small listening room, overuse of absorptive material for reflection control can result in a room that is too acoustically "dead." And although some music lovers may think of professional recording studios — which have been heavily treated with absorptive materials — as an acoustic model, keep in mind that studios are able to add artificial reverberation through electronic signal processing. And as a rule, music lacking the richness contributed by the room effect is less involving.

For example, if your system was in a room with thick carpeting on the floor, acoustic tile on the ceiling, and heavy drapes covering much of the wall surfaces, you would have nearly all of the high-frequency reflections being absorbed and nearly all of the bass sounds being reflected. The sound in this room would be unpleasant: thick and boomy in the bass with little or no sense of spaciousness. An over-absorptive room can also make spoken dialogue sound unnaturally dry.

At the other extreme, a room with painted drywall on the walls, drywall or plaster on the ceiling, linoleum over concrete on the floor, and no sound absorbing drapes or rugs, would sound extremely bright, thin, and echoey. Too many echoes can negatively affect movie dialogue, making it more difficult to understand.

Fortunately, there's another way to control room reflections: diffusion. Read on to find out more.

Room Acoustics

Diffusion

Treating your room with absorptive materials can get you most of the way there, but if your movies and music still don't sound quite right, then diffusion is another option. Diffusion is the scattering or redistribution of acoustical energy. The advantage of diffusion is that because the sound energy is scattered rather than absorbed, that energy isn't lost, thereby maintaining more of a "live" sound in your room.

It's difficult to describe this type of effect because it's completely rooted in advanced mathematics. Concert halls, recording studios, and broadcast facilities that use diffusion rely on on professional products based on mathematical number theory. And though diffusive materials are more difficult to implement in a home listening room than absorptive materials, it is possible.

Tips on diffusive treatments
Diffusion products can be used to treat many of the same problems that absorption is used for. Again, diffusion will rid your room of echoey reflections without replacing them with acoustic deadness.

Here are some situations where diffusion works particularly well:

Room Acoustics A bookcase filled with odd-sized books makes a very effective sound diffusor.
  • If your room already has built-in absorption in the form of carpeting, drapes, or acoustic ceiling tile, diffusion may control side wall reflections better than adding more absorption.
  • You may already have a good natural diffusor in your home without realizing it. A bookcase filled with odd-sized books makes an effective diffusor.
  • In a home theater system using traditional bookshelf speakers for surrounds, place diffusors in the middle of the back wall and aim your surrounds toward the diffusors at a 45° angle (see below).
  • One of the best-sounding setups for music or home theater is to use absorptive material on room surfaces between your listening position and your front speakers, and treat the back wall with diffusive material to re-distribute the reflections.

Room Acoustics If you're using conventional (non-dipole) surround speakers in a home theater system, you can achieve much of the diffuse sound of dipole speakers by treating your rear wall with diffusors and aiming your surrounds at them.

Locating Your Room's Reflective Trouble Spots

If you've made it this far, then you've learned how room reflections affect the sound of your stereo or home theater system. And you've learned how absorptive and/or diffusive materials can help control those reflections. Now the fun begins. You're going to learn how to locate your room's reflective trouble spots.

Room Acoustics At frequencies above 500Hz, sound waves bounce off reflective surfaces (like walls) as predictably as light bouncing off a mirror. In the above illustration, sound from a speaker is reflected off a side wall toward the listening position.

Mirror, mirror on the wall...
Along with standing waves, the reflections that distort sound the most are the loud reflections that bounce once off the side walls, ceiling, and floor on the way to your listening/viewing spot. These strong reflections are called "early" reflections. Controlling the intensity of early reflections is crucial to achieving optimum sound.

So, how do you figure out exactly where these reflections are coming from? An easy, accurate way to locate the precise points of sound reflectivity on your walls, ceiling, and floor is to use a mirror. You'll need a friend or family member to act as an assistant. Here's how it's done:

  • While seated in the listening position have your assistant slide a small mirror (8" x 10" works well) along the left wall at the height of the tweeter. Your assistant should start across from the left speaker and move slowly toward the listening position. As the mirror is moved toward you along the wall, you will at some point see a reflection of the left speaker in the mirror. Mark the spot on the wall where the tweeter reflection appears with a piece of tape. As the mirror continues moving toward your listening position, you will next see a reflection of the right speaker. Mark the location of the right tweeter reflection spot on the wall with another piece of tape.
  • Repeat this procedure on the right wall to locate the corresponding two reflective positions there.
  • Now it's time for your rear wall. Use the mirror technique again to find the point of reflectivity for each speaker on this wall and treat it with absorptive material.
  • If your floor is carpeted, you needn't worry about floor reflections. But if the floor is a hard surface like hardwood, tile, or linoleum, use the mirror technique to find the points of reflectivity. Move the mirror along the imaginary lines connecting each speaker with your listening position. Again, you will be locating one point along each line roughly midway between the speaker and the listening position.
  • Finally, repeat the wall procedure on the ceiling. Have your assistant move the mirror along the imaginary lines on your ceiling that would connect each of your speakers to your listening position. You should be able to locate one reflective point on each line about midway between the speakers and the listening position. Mark each of these spots with a piece of tape.

Early sound reflections from the points you located are adding significantly to the sound you hear at your listening position. They cause some sounds to be canceled out while others are amplified, resulting in smeared stereo images.

Room Acoustics If you can see it, you can hear it. Wherever you see your speaker reflected in the mirror, that's a point of reflection that should receive absorptive, or in some cases, diffusive acoustic treatment.

How to fix your trouble spots
The solution to this problem is to treat these points of reflectivity with some form of absorptive material. Panels made of 1" fiberglass or foam (polyurethane or melamine) installed on the walls do an excellent job of absorbing these reflections. Be sure to use enough absorptive material so that it extends at least 18" on either side of the marked locations. The material should also be at least as high as the tops of the speakers for best reflection control.

The wall behind your listening area may also require treatment, though if it's several feet away, reflections probably aren't a serious problem. This rear wall is the surface that would benefit most from diffusion products or a bookshelf. You can also try using thicker material, or providing for an air space between the material and the wall, to get improved low-frequency absorption.

The floor between your listening spot and your speakers can also be a source of reflected sound. An easy, good-looking way to treat floor reflections is to cover the points of reflectivity with a reasonably thick area rug. And if you found any reflective points on your ceiling, then you should apply absorptive material to the ceiling extending at least 12" on either side of each marked location.

Summing up
The next time you sit down to watch or listen, think about the ways — good and bad — that your room may be affecting the overall sound of your system. A basic understanding of room acoustics (and speaker placement) can help you maximize the performance of any audio or home theater system.

For more room acoustics information:
The Master Handbook of Acoustics, Fourth Edition, by F. Alton Everest. A wide-ranging look at room acoustics, both in the home and the studio. Includes detailed explanations and some math, yet is very accessible. Complete with essential information for building or remodeling a listening room. Highly recommended. 592 pages.

The Complete Guide to High-End Audio, Fourth Edition, by Robert Harley. 552 pages.

Special thanks to RPG for their assistance. Their website is www.rpginc.com

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