By Roy Johnson, loudspeaker designer, Green Mountain Audio, Inc.
We all have been in rooms with exceptionally bad acoustics and heard what they do to conversation and any music being played. We know these rooms would benefit from something -- be it additional furnishings, carpets, and drapes or a total redesign from the ground up. In this article, however, we focus on the typical 'living area' to explain 'good acoustics' as well as to describe common acoustical problems and their solutions.
Echo, echo, echo
When anyone is told a room has bad acoustics, perhaps the first thought is that it has echo. Because of everyday experience, we associate echo with voices. Indeed, the word 'echo' originated in Greek mythology when the nymph, Echo, was punished by being deprived of speech, except for the ability to repeat the last words of another. Rooms also have echo in the bass and treble. We do not hear those in the same way, and so solutions are not obvious.
Concert halls and public places always have been designed with certain acoustic properties. In the 1930's, the sound recording industry for music and movies undertook intense research to determine which echoes made the best rooms for recording, monitoring, mixing, and home playback. They performed many detailed measurements, and yet, today, acoustic design remains a complex art and science.
Nonetheless, the 1930's research identified a strong listener preference for how long a sound should linger. This 'reverberation time' depended upon many factors -- the cubic volume of the room, type of sound (music, speech, movies), and the particular tone being heard, from low bass to the highest treble.
For our purposes, it was determined that a medium-sized living room would have a voice-range echo that dies away after only one-half of a second. In the low bass, that time needed to be nearly one second, and in the high treble, about one-third of a second. Computer measurements are now performed by acoustic consultants for these 'RT-60' decay times, frequency by frequency. For those designing a listening or home theater room from the ground up, we suggest reading the Master Handbook of Acoustics by F. Alton Everest before you visit the architect or builder.
First know that no recording engineer expects listeners to be in a bare room full of echoes. The presence of furniture and carpet are the two most important factors in controlling them, followed closely by walls with art and bookshelves, and then windows with drapes. When those are all present in the room, making good sound from speakers becomes much easier. The room is also far more comfortable for conversation and outside noise is lessened. Although the room is gorgeous in the photo below, music and movie reproduction in that sort of room will never be satisfying to someone who has experienced the difference. (But may we visit?)
In the bass we do not hear echoes or reverberations as such. What we hear is too much bass in general and some bass notes that have oddly disappeared. How many of us have listened to music that we know has great bass, only to reach for the volume or equalizer knobs when we did not hear what we expected to hear, thinking it was a problem with the speakers? Certain sizes and shapes of rooms cause the most serious problems with bass: small rooms; rooms that are highly symmetric; in sealed rooms, and in basements. In these areas, bass tones return from each wall and meet your ears. There, they either add together -- a condition which produces too much bass; or they subtract from each other, a condition which cancels out certain notes.
The particular tones that add or subtract are determined by the room's physical measurements that fit those wavelengths, and the outcome is compounded because these rooms do not allow the bass tones to die away quickly enough. This leads to problems with 'standing waves', where it is pretty obvious if you stand or sit over there, there is no bass, or too much bass when you are near a corner of that room- far too much bass. Now, in living rooms open to the rest of the home, and in loft-style apartments, we have the problem of the high ceilings allowing too much low bass to escape. There, even large speakers will have their owners asking, "Where's the bass?" In the most severe cases, the room's shape has to be modified. Here are the common situations and how their problems are addressed without structural change.
- An L-shaped room or one with many openings allows bass to escape around the corner.
- A large bookcase or cabinet can be used to extend the length of the wall past the corner of the 'L.' When placed around the corner, it will be its front-rear thickness that makes that wall longer, which retains more bass in that portion of the 'L'.
- Your results will depend on the overall shape and size of such a room and it is possible that the speakers should really be placed along one of the sidewalls. Placed down the length of the 'L' on the end wall, one might only hear bass when fully inside that portion of the room, depending on the length of that section of the room.
- A small, closed room retains too much low bass.
- Add bulky furniture to absorb bass and to break up low-bass waves.
- Open a door -- even a closet door -- at least a few inches. This will vent some bass and together with the clothes in the closet, absorb some bass. One could put a louvered door on that closet, or perhaps the door to that otherwise-sealed room.
- Add what are called 'bass traps' in the corners.
- A cubical room creates bass that varies greatly in loudness from note to note.
- Break up its shape with bulky furniture.
- Keep the speakers a few feet out from the wall behind, while sitting on either side of the halfway-point down the length of the room.
- Avoid woofers and subwoofers mounted at floor height.
- Put bass traps in the corners.
- High ceilings reduce the bass and a low ceiling holds in too much bass.
- For a high-ceiling room, place bookshelf speakers on stands taller than usual.
- In a low ceiling room, add bulky furniture and do not sit near the halfway point in the room.
- A long, narrow room produces strong bass at each end, with little in the very middle.
- For speakers placed at the far end, pull them out 6'+ (2m+) from the wall behind.
- Sit just past the halfway point.
- Add bulky furniture along the sides of the room.
- Put bass traps in corners.
- An ideal, but sealed, room generally has 'Golden-Ratio' proportions between its height, width, and length, approximately 1:1.6:2.6. Those ratios are only guidelines, as the final result is greatly influenced by the materials used to construct the room, the furnishings, and decor. Definitely avoid whole-number ratios, such as 1:2:3 or 2:3:4 (which also equals 1:1.5:2), or 1:1:2. Those are the ratios which create severe bass problems, from too much to too little bass, depending on the bass note and your location.
- Flexible walls and floors, large windows, and suspended ceilings allow bass to pass right through.
- In an old home, it may be possible to add support posts below the floor joists, especially under the speakers and again in the middle of the room to stiffen the floor. Large windows are a challenge and it would be best to compare the solutions offered by the acoustic-control companies in the links above.
- Bass-retention of suspended ceilings is greatly improved with a heavy layer of vinyl behind the panels. Visit AcousticalSurfaces.com for more information.
- Perhaps add at least one subwoofer, placed on an isolation pad. Visit Auralex.com for more information.
- Rooms with cement floors and walls retain too much low bass.
- Add bulky furniture.
- Add thick carpet with pad. A carpet of natural fibers has much better sound absorption than one of synthetics.
- Open a door, or a closet door, as above.
- Put bass traps in the corners.
- Add a suspended ceiling designed by an acoustics company, such as Auralex.
Note that when low bass is absent from a room because of very high ceilings or the room opens to the rest of the home, no amount of bass boost in the amplifier will bring it back. If the bass is missing in what appears to be a 'normal' room, it could be that the bass reflections off the wall behind you are canceling the bass coming from the speakers. In those rooms, bass traps placed in the rear corners actually increase the bass you hear because they capture much of what would otherwise reflect and cancel.
Midrange and treble acoustics
Furnishing the room and using carpet or area rugs will reduce voice-range and treble echoes. Bookshelves here and there break up echoes between two parallel sidewalls. Think of two barber-shop mirrors reflecting back and forth to infinity. Remove just one of those flat mirrors and the reflections will stop.
For voice and treble control in rooms with large, bare walls, nicely-styled acoustic absorption and diffusion panels can be mounted, perhaps up high on the walls. Absorption panels remove sound from the room while diffusion panels scatter the sound. There are needs for both. Noted manufacturers in the USA include Acoustic Sciences, Auralex, SRL Acoustics, Real Traps, and RPG. Each will tell you how many panels would be needed, or how few, based on your room dimensions and photos. Tell them where the speakers will be and where you intend to sit.
We are especially sensitive to voice-range reflections off of the surfaces that are closest to the speakers. Those reflections are reduced by positioning the speakers and possibly treating the sidewalls close to them. For details, read Placing Speakers.
A quiet room is relaxing. You also hear much more from your system while playing it more softly. Noises from outside most easily come through window glass. A double or triple layer of glass absorbs the high frequency hiss of traffic, but does nothing to muffle the sound produced by aircraft, for example. One can install sound-absorbing drapes. For extreme situations, there are soundproof windows.
Noise also passes through cracks under doors. Just a 1/4" (6mm) gap transmits 90 percent of the sound! A rubber floor-sweeper attached to the bottom of the door will help close that gap. The door itself is a problem when it has a hollow-core (lightweight) construction. A rubber seal around the door jamb always helps this situation. When compressed by the door, it will also reduce the door's vibrations caused by the stereo and you will hear better sound.
When the sound is coming up through the floor, the only 'easy' solution is a thickly padded and dense carpet. The carpet should be of natural fibers for the best absorption. Other solutions require alterations to the floor.
If you share a common wall with your neighbors, you can dampen the common wall with acoustic panels. Also consider closing and covering your windows, making sure they have good weather stripping.
To prevent bass vibrations from being transmitted, place an inexpensive foam-rubber/plywood platform under your speakers and/or subwoofers. Since that foam platform will also improve the sound for speakers placed on wooden floors, consider buying the neighbors a pair so their speakers' bass does not bother you. They will also experience better sound!
You will enjoy your system the most when the speakers are not up against hard, reflective surfaces, either right behind them, between them, or to either side. The music and movie experience is completely different and better when the speakers can be free by at least enough room to walk around the speakers, even if that is a tight fit for someone.
When the basics of room acoustics and speaker placement are addressed, your home feels more comfortable and you become more deeply lost in your music and movies.
We are happy to help our owners determine the best speaker layout for their room. There is no charge for this service. Please upload a room diagram and some photos to our dropbox. Even if you do not own our speakers, and no matter the shape of your building or the size of your room, you can enjoy great room acoustics. Occasionally, Designer Roy Johnson accepts contract projects related to room acoustics. Please contact us to discuss your needs.