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What You Should Know About Home Theaters and Sound Control
After months or years of dreaming about and designing your home theater set up, you finally make your decisions and go out and purchase all of your electronic equipment. You bring it all home and set up the speakers, run the cable and hook up the new plasma TV. You sit down in your new plush chair and hit play ready to finally enjoy all your hard work.
That's when you realize your new system does not have the same sound quality as it did in the retailer's showroom. You double check your connections and equipment and the settings are all correct and that's when you realize that the acoustics of your room are just as important to the overall sound quality as the audio equipment that is being used.
Sound engineers and audio experts have known for years that the physical attributes of the listening room are critical, unfortunately this can be overlooked by many do-it-yourselfers that are trying to set up the perfect home theater.
Many electronics manufacturers try to make consumers believe that high cost electronics will guaranteed great picture and sound. Unless you have considered the room that you are using during your design, you could end up very disappointed with even the very best systems or electronic packages available. The fact is that the home theater system cannot be separated from its room. Hardwood floors, large windows and drywall do affect the sound quality of your system.
The good thing is that with proper home theater design and construction these pitfalls can be avoided. To start a proper acoustic home theater design, you must first begin with the construction of your walls, floors and ceiling. Nobody wants to build a home theater that they cannot enjoy at above average listening levels, everyone wants that thundering bass. Also nobody wants their spouse, kids, parents or neighbors complaining every time they sit down to watch a movie that it is too loud in adjoining rooms.
1. Mass and Vibration Isolation Reduce Sound Transmission
There are many simple and inexpensive ways to make improvements during the planning stages. Consider using staggered studs in your walls. Basically, use two rows of studs instead of one, with those on one row offset from those on the opposite row. Be sure to fill the bays or cavities between the studs with acoustical grade fiberglass insulation.
Because mass and density block airborne sound transmission, it's often recommend that you add a layer of a mass loaded vinyl sound barrier to the studs. This type of sound barrier typically weighs one pound per square foot and is 1/8" thick. Be sure to caulk all of the seams of the vinyl sound barrier as well as the perimeter of the wall with a water based acoustical caulk. Keep in mind during this installation that a one percent opening will let through fifty percent of the sound, so even a small crack can have a severe impact on sound transmission.
2. Receptacles Can Create Sound Leaks
Use fire stop putty pads around receptacles to block sound from leaking into other rooms. Fire stop putty pads are widely used for their ability to add an additional barrier around metallic and non-metallic electrical boxes. The extra barrier provides installation, which is what effectively blocks the sound leaks. Putty pads are relatively simple to install.
Simply wrap the pads around the back of the box, as well as the three sides that face away from the stud. The pad must be applied to the box and then either lapped over the front of the tile plate (if one is used) or excess material must be worked toward the front of the box and built up into a compressible gasket much like the pinched area of a pie crust. This gasket compresses between the front edge of the box and the drywall.
3. Have a Plan for Vibration
At this point you have added mass to the wall and sealed the walls as tight as possible, now it is time to start planning out your vibration isolation.
A few alternative methods to consider:
The traditional way of doing this is to use resilient channel to offer some separation between drywall and studs. Resilient channels, simply put, are metal brackets that you can install between the drywall and the studs. The downfall to this is that there still is a metal to metal or metal to wood connection for sound transmission.
A newer method to isolate, or "float," the drywall is to use resilient isolation clips and hat channel to hang the drywall. The resilient isolation clip features a rubber, or neoprene, mount on it to better control the structure borne sound transmission.
The last alternative to consider for your walls and ceilings is using two layers of drywall instead of one; keep in mind that mass blocks airborne sound transmission. If you are doubling up your drywall adding a layer of Green Glue, which is a visco-elastic sound dampening compound, between the two layers of drywall will go a long ways in increasing the STC (Sound Transmission Class) of the wall or ceiling. Green Glue is available from most acoustical material distributors.
After you have carefully planned out your wall, ceiling and floor construction, start thinking about your wall and ceiling finishes. The science of room acoustics within a small-enclosed space can be quite complicated; in an effort to keep things simple we are going to focus on two techniques that if followed properly will keep you out of trouble: Point of Reflection and Reverberation Levels.
1. Point of Reflection
The first thing that must be determined is the first point of reflection from your speakers. The first point of reflection is the first place on the wall that the sound bounces off of after coming out of the speakers. Obviously this is different for every home theater, but you must determine where the sound waves that are emitted from your speakers first come into contact with the side or back walls of your room. These points of first reflection are important because they are the most significant component of sound reproduction. Reflection points are important because of their relatively large amplitude and a direct point of contact, and often affect the feeling of depth within your room.
There are two ways to control the first point of reflection. The best home theater designs will use a combination of the two; sound absorption and sound diffusion. The reason this is so important is that you do not want to be listening to sound or voice from your film that originated two or three seconds before hand and is now bouncing around the room still audible distorting the sound waves that are currently emanating from the speakers. This causes the sound to sound muddled or distorted.
Sound absorption is most commonly added to a room in the form of fabric wrapped fiberglass wall panels. Not only are these sound absorbers but they are also very decorative additions to the look and feel of the room. Absorption sound panels are available in multitude of shapes, colors and patterns to offer a very customized look to any home theater.
Fabric wrapped panels allow the sound waves to pass through the sound transparent fabric covering the panels and come into contact with the fiberglass. At that point the absorptive surface of the dense fiberglass panel dampens the sound energy so that only a small portion of that energy is reflected back into the room. The panels turn the rest of the sound energy into heat or thermal energy which is then dissipated through the air.
Typical acoustical wall panels will have sound absorption values or Noise Reduction Co-Efficient of .85 - 1.0, which means they will absorb 85% - 100% of the sound energy that comes into contact with the panel and not reflect that sound back within the room.
Sound diffusion is a technique designed to scatter sound waves. The idea here is to take the sound waves that are coming at your first point of reflection and break them up, or scatter them, into smaller components and equally disperse them throughout the room. Sound diffusion can take many forms, depending on the desired look and feel of the room. Sound diffusers are going to multifaceted squares or rectangles and are commonly available in fabric finishes, wood finishes, and paintable finishes. Also bookshelves and wooden columns can be used as sound diffusers.
2. Reverberation Levels
After controlling the first point of reflection, the last thing to consider is the overall reverberation level or ambient noise level within the room.
Carpeting and a drop tile ceiling will help with this, however these those alone will not solve the problem; additionally, many custom home theater builders do not want the look of drop tile ceiling. The sound absorption techniques described above, fabric wrapped acoustical panels, can be added to the sidewalls and back walls to create an acoustically effective and aesthetically eye pleasing wall treatment.
If additional sound absorption is needed, ceiling treatment can be accomplished by adding panels directly to the ceiling with Z-clip attachments or acoustical foam ceiling tiles can be glued directly to drywall ceilings that offer both contoured designs or patterns combined with high sound absorption values.
Bass traps can also be added to the corners of the room, if the room sounds too "boomy" or if you desire to reduce the bass or low frequency within the room.
The amount of sound absorption or reverberation control needs to be carefully evaluated on a room by room basis, too much absorption and your room will create acoustically dead, or sound very cavernous room. On the other hand, too little absorption and room will be too live or reverberant.
It is always best to consult with an acoustical consultant, systems integrator or acoustical material supplier to determine proper sound absorption for your individual home theater design.
