The Listening Room Part I
Your Most Important Component
Article By David Smith
President And Chief Engineer, Snell Acoustics
Few people realize what a dominating effect the acoustics of their room plays in the sound quality of a stereo or home theater system.
While technology marches on and brings us DVD, Dolby Digital, DTS and MP3, in the end plain old room acoustics has a strong impact on the sound of your system. By making some simple improvements to your room's acoustics you can make a bigger difference in sound quality than thousands spent on fancy wire or more expensive components would bring.
First, what do we mean by room acoustics? All the surfaces, shapes and materials of your room add up to define its acoustics, or your perception of how sounds of familiar objects get modified in that space. A room is an enclosed, or semi enclosed space. Its boundaries are surfaces that are either smooth or rough and either absorptive or reflective to sound.
Do this test: Have a friend that is a non-stop talker (perhaps a spouse?) walk with you through out your home. As this person talks (they can be reading the dictionary or today's newspaper), and as the two of you walk from room to room, concentrate on the character of this persons voice.
Lets start here in the garage (loud and echoey), down the hall (bit of an echo), into the kitchen (louder here), into a bedroom (quieter but "chestier"?), into the tile breezeway (louder again and kind of hard to make out the words), into the living room (quieter again),
and so it goes.
In our hypothetical house tour we are sampling the effects on a human voice as the acoustics change from room to room.
Just like our talking assistant, our audio system would pick up the acoustic character of whatever environment it is placed in. Unfortunately, a lot of those environments would be bad for reproducing music or movies.
What Are We Looking For?
Music is often performed in reverberant spaces; spaces were the sound bounces around many times before slowly dying away. Whether auditoriums, concert halls or stadiums, these spaces are intentionally designed to have lots of reflective surfaces. The best concert hall can have surprisingly little absorptive material. But your living room needs more. Its dimensions are smaller and so reverberation
does not add the great sonic character of Boston Symphony Hall, it just adds sonic muddle. Besides, the recording that you paid good money for already has the sonic footprint of that large carefully designed performance space. Your living room doesn't need to add its footprint on top of that. In truth, your living room needs to absorb as much sound as possible. It needs to be acoustically
A good room generally has a "quietness" to it. Your can carry on a soft conversation and fully understand the person you are talking to. Street noise may make it in to the room but it is well damped and unobtrusive. This quality of quietness comes from having a good percentage of all surfaces covered with acoustically absorptive materials. These can be heavy drapes, thick carpets, stuffed pieces of furniture, etc. Openings to the rest of the house absorb sound too, because they let it escape down the hall where it is more likely to be absorbed than reflected back to you.
Different than absorption, but equally desirable, is the scattering of sound. A large flat surface bounces sound waves back as an undisturbed front. These large reflections may be heard as an echo, at the very least they up the sonic harshness, degrade stereo clarity and upset the balance of sound. Anything that breaks up the surface will take that sonic wave and scatter it in multiple directions. Not only does that reduce the severity of reflections, it tends to promote energy loss by giving the sound a longer path (hence more chances to hit an absorptive area) before it gets back to your ears, all good stuff.
So the two main components determining room acoustics are absorption and scattering diffusion.
Room dimensions are important also. Bass notes have a wavelength. Any continuous tone is a repeating series of pressure peaks and troughs, emanating out from the source at the speed of sound. If you could freeze a note in space and measure the distance from one pressure peak to the next, this would be what we refer to as the wavelength of that particular note. Bass notes have long wavelengths and treble notes much shorter. When the room dimensions (height, width, depth or diagonal) become an exact multiple of these wavelengths, then a new phenomenon occurs, the standing wave. Instead of the sound radiating out it takes on a stationary (standing) pattern of loud and soft points within the room. This leads to unevenness of bass. For frequencies where strong standing waves occur, the bass will have strong variation of level with position. Typically it will be strong in the corners or against the wall, weaker in the middle of the room. This has a very strong effect on the bass character of your speaker. Very regular rooms (each dimension the same) will be the worst case. Small rooms will push these standing wave frequencies up into the midrange and make them more noticeable, which is why your voice sounds very resonant in the shower. In the end there is little you can about this phenomenon short of ripping out walls and changing dimensions. Big opening to other rooms and not-so-rigid walls will help though. Sound rooms built into the cinder block basement tend to be the worst. In part two we will talk about using speaker placement to minimize this effect.
Assess Your Room
Now lets look around your listening room. Make a mental tally of what percentage of the rooms surfaces are hard and flat, what percentage have scattering surface irregularities.
Lets see, ceiling, hard and flat (bad). Sidewall hard and flat except for two large openings. Other sidewall has bookcase full of books and stuff (very good). Front wall has chimney and mantle (good). Back wall has staircase leading upstairs (good).
Similarly, assess the surfaces for absorptive materials:
Front wall has drapes by windows (good). Drapes are open revealing a lot of glass (bad). Nothing but sheetrock on that ceiling (bad). Thick carpet on the floor, with under padding (very good). Large sofa in front of side wall (good).
By now you should be starting to get a sense of what good and bad sounding rooms consist of. If you want a great example of a bad sounding room, pick up any architectural magazine. See that designer-look room with hardwood floors and nary a throw rug, minimalist furniture, expansive glass walls (nice view) and clean uncluttered walls? Bad acoustics, I guarantee it.
So what can we do with our room to improve its acoustics, short of bringing in the wrecking crew and starting over?
Add absorption. As a rule of thumb, try to cover 1/3rd of all surfaces with something soft. Purpose built audio absorbers are available but are frequently ugly and not the only solution. A large empty wall can have a rug hung on it. I knew a person that collected oriental rugs. They didn't want to walk on the silk rugs in their collection so they hung a few on the walls. (Hint: hide a layer of carpet backing behind it to further increase its absorptive properties.) At least put a large area rug over the center of that hardwood floor. Put a thick pad underneath it. Drapes for the windows add a lot of absorption, especially if they are heavy or thickly lined. Now is the time to buy that big 1940's look overstuffed sofa.
Add diffusion. Again, commercial solutions are available but there are many domestic solutions. One of the best diffusers of sound is a bookcase half full of books. Some
three dimensional art objects can diffuse sound. Decorative room dividers (the zigzag kind) can absorb or diffuse sound. Anything that breaks up that big expanse of hard plaster or drywall will help diffuse sound.
Locate These Objects At The Best Place
The drawing below shows the path of the primary bounces of sound in a typical rectangular room. Although placing these absorptive and diffusive sound objects anywhere in the room will have some effect, you can maximize the effect by placing them directly in the path of these primary sonic paths. This guarantees that strong hard reflections are scattered or absorbed. These early reflections are especially dangerous to the clarity of sound and to the stereo effect. Absorbing or scattering them will up the clarity of movies or music.
I hope I have given you some insight into what room acoustics are about, what you can do to improve yours and how that will improve any listening experience. If you've made it this far you should realize that any room can be improved, and you should have some ideas of what to try to improve matters. So get out there and fix your acoustics! Not only will your system sound better, but the room will be much more comfortable to be in, sonically speaking.
Or you could always move.
In Part 2 we talk about where to best place your
David Smith is President and Chief Engineer for Snell Acoustics, a manufacturer of high-end loudspeakers for music and home theater use.
143 Essex Street
Haverhill, MA 01832
Voice: (978) 373-6114