Home Toys Article
- February 2005 -
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The main rule is to stay away from using the same length for two or more of the room's dimensions. The very worst shape is a perfect cube. Each dimension (length, width, height) of the room causes specific audio frequencies to be reinforced and others to be cancelled.
Soundtracks in modern motion pictures are delicately crafted masterpieces. The director & sound designer weave together an intricate tapestry of different sound effects & dialogue to pull you into their story. Many of the requisite sound track elements are very subtle. To experience these subtleties, it is essential the theater's noise floor is very low. Lowering the ambient noise requires acoustic isolation techniques to reduce sound transmission from the exterior environment.
If the noise floor is not suitably low, the volume of the small sounds must be increased to compensate. If the sound system has good dynamic range, (the ability to reproduce very quiet sounds and very, very loud sounds without distortion) increasing the volume of the quiet sounds will render the louder portions of the sound track painfully loud. If it does not, the sound system will become overdriven and distort. THX reference level is 105db, which means the loudest sounds will be quite loud but the very subtle sounds will be lost in a room with even a medium level of ambient noise.
As a reference, a Lexus LS-430, one of the quietest luxury cars, has an ambient noise level while cruising of about 63db. A living room is usually quieter still, about 50db. Strive to get the ambient noise in your theater to around 35db. Suspension of disbelief also requires the elimination of exterior sounds that will distract the viewers. Nothing is worse than hearing the neighbor's argument during a touching emotional moment in the movie.
The other goal to strive for in reducing sound transmission is to keep the sound of your theater from disturbing others. This especially true if you enjoy action/adventure movies with very dynamic sound tracks. Bass from your cool, new subwoofers can penetrate walls very effectively. Your neighbors and other family members may not appreciate this as you enjoy the new Star Wars box set until 1am on a weeknight.
The best construction technique for eliminating sound transmission is thick, concrete walls, underground, if possible. Even using this technique on some of the walls, such as in a daylight basement situation, will improve the STC of the room. Failing this, there are other approaches that will reduce sound transmission. Interior acoustic panels are not for reducing sound transmission. They are for treating the room's interior surfaces to improve internal room acoustics. This deals with the behavior of sound inside the room and will be addressed shortly.
One of the easiest, and most common sound transmission reduction techniques is to mount the drywall on resilient channel, also known as "c-channel", "hat-channel" and "R/C channel" or "R/C". This is thin metal that is affixed to the wall studs and ceiling joists. The drywall is then mounted to the resilient channel, effectively decoupling it from the room's structure. Sound is then not transmitted as effectively from the drywall to the room's structure or vice versa.
When locating electrical outlets, vacuum outlets, or any other wall penetration, be sure two penetrations on opposite sides of the wall do not share a common stud bay. A penetration on opposite sides of the wall, in a common bay, gives sound an unobstructed path from your media room to the adjoining room or vice versa. The outlets should be sealed with insulating foam to further reduce sound transmission.
Staggered stud wall construction is an extremely effective technique to reduce sound transmission as well. This technique keeps opposite walls from sharing studs. Dedicated studs will reduce sound transfer from the media room's drywall, into the wall studs and into the adjoining room's drywall. Drywall is a great sound radiator, and will re-radiate the sound from the studs into an adjoining room with amazing effectiveness. When constructing the wall, use top and bottom plates at least one size larger than your studs. For example, if you are using 2x6 wall studs, use a 2x8 for the top & bottom plates. Use at least R-19 batt insulation inside the wall. Lay it horizontally and weave it between the studs.
A greatly effective method for reducing the STC (sound transmission coefficient) of the wall is to use two layers of drywall. These should be of different thicknesses. A layer of 5/8" and a layer of ½" works well. Using different thicknesses keeps the two layers from sharing a common resonance frequency. If they do share a common resonance, that frequency will have a greater propensity to travel through the wall. To really increase the effectiveness of this technique, sandwich a layer of sound deadening barrier between the two layers of drywall. This is a very heavy, acoustically dead material that will effectively kill any resonance in the drywall. This material can also be used between the interior layer of drywall and the wall studs or, if used, resilient channel.
An often overlooked area of ambient noise in home theaters is the noise contributed by the HVAC system. This can be quite intrusive and seems to rear its ugly head at the most inopportune times. Fortunately, there are simple, inexpensive techniques to reduce it. One of the main causes of HVAC noise is created by rapidly moving air. Slowing down the HVAC airflow into the theater, while maintaining the same volume of air is a key to reducing noise. Increasing the number of or size of the registers is the best and simplest way to accomplish this. Try to locate the theater as far away from the HVAC equipment as possible to minimize noise transmitted directly from the equipment through the walls. This is often a more difficult task. To minimize the mechanical sounds conveyed from the HVAC equipment through the ducts, use lined ductwork with smooth radiused bends. Commercial HVAC contractors tend to be more familiar with this type of insulated ductwork than those that do strictly residential projects.
One of the main concerns, obviously, is the sound quality inside the room. It is very important the room is designed to minimize problems due to the shape and size of the room itself. This is so important because problems caused by the rooms shape or dimensions are very difficult to solve. The main rule is to stay away from using the same length for two or more of the room's dimensions. The very worst shape is a perfect cube. Each dimension (length, width, height) of the room causes specific audio frequencies to be reinforced and others to be cancelled.
This reinforcement / cancellation is caused by the relationship between the wavelength of the frequency in question and the room's size in one or more dimensions. These are known as room modes. If the wavelength, or an exact fraction of it, such as ½ or ¼, is the same as the length of one of the room's dimensions, that frequency will be reinforced by forming what's known as a standing wave. The wavelength of a frequency is calculated by dividing the speed of sound over the frequency in question. From this, you can see the lower frequencies are the most problematic because their wavelengths tend to correspond to room dimensions in typical sized rooms.
The effects of standing waves are one of the most vexing problems confronting theater room designers. They will cause certain locations in the room to be very loud at certain frequencies while having virtually no sound at others. Careful selection of room dimensions, seating and speaker, chiefly subwoofer, locations all help minimize the problems caused by standing waves.
As mentioned previously, the first thing to look at is the dimensions of the room. The ratio of any dimension should not be the same as, or an even multiple of, any other dimension. You should not be able to multiply a dimension by an even number to get any other dimension. Examples of poor ratios (height:width:length) are: [1:1:1], [1:2:4], [1 : 1.5 : 2], [1 : 1.4 : 2.8], etc. Some often used ratios proven to work are:[1 : 1.3 : 1.7], [1 : 1.7 : 1.9], [1 : 1.7 : 2.3]. As an example, using one of the better ratios would lead to a theater with a 10' ceiling, 13' wide and 17' long. Rooms with non parallel sides can be very effective at eliminating standing waves. These types of rooms tend to be much more complex to calculate room modes. They are beyond the scope of most basic acoustic modeling software.
Commercial grade modeling software works very well for this but can cost over $3,000. The better commercial software is supported by most commercial speaker manufacturers, who supply data about their speakers to include in the program's database. This type of software is rarely used in residential applications. Most custom installers simply do not have it or the measuring equipment required to use it correctly. Some do, however, or the acoustic modeling can be contracted to an outside acoustical consulting firm if that level of sophistication is required.
Once dimensional and construction issues are taken care of, Seating, screen, and speaker placement and interior acoustics are considered. These are no less important, they may actually be more so, but they are also much easier to change, if necessary.
These issues will be visited in a future article. You should have enough information to begin to address the most common acoustical problems encountered in home theater design and construction.
Bio: Steve Faber has almost 15 years in the custom installation industry. He is a CEDIA certified designer and Installer 2 with certifications from both the ISF and THX. His experience spans many facets of the industry, from the trenches as an installer and control systems programmer, and system designer, to a business unit director for a specialty importer of high end audio video equipment, a sales rep for a large, regional consumer electronics distributor, and principal of a $1.5M+ custom installation firm. He currently is senior design engineer for On Screen Entertainment in Redmond, WA. He is on the web at www.1touchmovie.com.