There are several criteria to address during the basic construction phase. One of the most important is noise transmission through the walls.

Home Theater Design – Part 2

Steve Faber | 1 Touch Movie

by Steve Faber
www.1touchmovie.com

 There are several criteria to address during the basic construction phase. One of the most important is noise transmission through the walls.

Click here for Part 1


You're right in the middle of building your dream home theater. You've figured out the room size and shape that will sound good and accommodate your desired number of family and friends. You know what size screen to use to avoid eye fatigue but still deliver the real home theater experience. Now it's time to actually begin construction.

There are several criteria to address during the basic construction phase. One of the most important is noise transmission through the walls. The sound transmission coefficient (STC) is basically the ratio of how much sound energy at a given frequency that hits a wall surface actually passes through it.

There is a nice, little formula for STC that you will probably never use but here it is anyway: T = Wrec / Wsrc , where the "src" subscript denotes the sound energy impacting one side of a partition (wall, for our purposes) and the "rec" the energy received through the partition at the other side. The STC of your theater walls, and possibly the floors and ceiling, is important for two reasons.

First, 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. 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.

Second, you could probably enjoy your theater much more if it didn't disturb others in, or near, your house. If you keep sound transmission low, more of the sound will stay inside where you can enjoy it, and less will travel outside, where others may not. You want to be a good neighbor, don't you? Bass tends to travel right through most walls. If you really want to enjoy those new subwoofers you saved so long for, pay attention to proper room construction to reduce your home theater's STC.

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. The frequencies that most acoustic panels address are usually limited to those above 500Hz or so, depending upon the thickness of the panel. The frequencies you need to be concerned with the most regarding sound transmission are the bottom three or four octaves.

Another common and extremely effective technique to reduce sound transmission is the use of a staggered stud wall. 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 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.

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. With staggered stud construction you will have to build a sub-enclosure inside the wall to contain these types of devices. This is necessary because the entire wall basically shares one big stud bay. A path around your sound reducing technique, such as from an electrical box, is known as a "flanking path" and is obviously to be avoided. You can negate 80 - 90% of your techniques effectiveness at certain frequencies by providing the sound a flanking path.

Another very common sound transmission reduction technique 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. The drywall on the ceiling is not connected to the sheetrock for the walls with mud as in a traditional drywall installation. Special flexible caulk is used to allow the ceiling to move ever so slightly.

A third method of reducing sound transmission through the walls of home theaters is using two layers of drywall. This technique takes advantage of the "Mass Law", which states that the greater the mass of the panel, the greater the acoustical energy required to set it in motion. If you are going to use two layers of drywall, they 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 through its substantial mass. This material can also be used between the interior layer of drywall and the wall studs or, if used, resilient channel.

There is drywall available now that incorporates increased mass and viscoelastomeric properties that actually dissipate the acoustical energy as small quantities of heat. (Don't be alarmed) This type of drywall is available from companies such as Quiet Solution. It works very well but the downside is high cost. The Quiet Rock from Quiet Solution is approximately $150.00 for a 4' x 8' sheet. In addition special materials and installation techniques are required to maximize performance of the product.

Don't ignore noise sources from other areas such as washer / dryers and HVAC. These can intrude into your home theater and completely bring you out of that mood the film worked so hard to create. To reduce HVAC noise intrusion, try to locate the HVAC equipment as far away from the home theater as possible. This will reduce mechanical noises from your equipment that can be transmitted into your room. In addition, lined ductwork can be used to absorb HVAC noise. This ductwork can be fitted with 'S' shaped bends to further absorb noise.

If your HVAC contractor is unfamiliar with some of these techniques, many commercial HVAC contractors use these on a regular basis to reduce noise in their projects. The main cause of HVAC noise is the high speed movement of air through the registers into the room. Use more than the required number of ducts to increase the surface area of the vent. This slows down the airflow and reduces the noise level even further.

While on the subject of HVAC, it is desirable to have a separate HVAC zone for a dedicated home theater. When the doors are closed, the projector is running, power amps are driving hard and you have 5 friends over there is a lot of heat generated in that closed space. If you have a separate HVAC zone, the room will be much more comfortable. Otherwise, it will get quite warm.

It is desirable, to preserve good sightlines, that the rear row(s) of seating be elevated 6" - 12" higher than the row directly in front of it. This is easily accomplished with a riser constructed of 2 x 4s and ¾" plywood and carpeted to match or complement the room's decor. In addition to preserving good sightlines to the screen, the riser allows easy access for any wiring to the seats, such as necessary for power outlets, phone or internet jacks, secondary monitors or motorized seat wiring.

Make sure you have some sort of step lighting on the riser step(s) for safety. You don't want anyone tripping on the stairs in the dark. Rope lighting under the stair tread or standard flush, wall-mount step lighting works great for this purpose. The step lights should be on a separate lighting circuit to allow for independent control and dimming.

If you are not incorporating an existing video display, you will obviously need to decide on an appropriate one. You have three main choices; front projection, rear projection, or direct view. There are sub-categories within each of these.

A rear projection display may be an all-in-one, self contained box, such as you purchase from a store. You can also use a two piece design. This incorporates the same type of projector you would use on the ceiling, but it is fired from the rear onto a translucent screen. The net effect is of a huge, flat panel display. It's like hanging the largest plasma you've ever seen on your wall.

Rear Projection - multi-piece or one piece / single box, digital or CRT

Advantages

  • No projector in the room
  • Allows a large, theater size screen (when using a 2-piece design)
  • Keeps noise from the projector from intruding on the theater
  • Creates room for storage and easy rear equipment access
  • Looks great, with a huge image just floating on the wall
  • Fewer ambient light issues because the light behind the screen is completely controlled

Disadvantages

  • Requires 2' - 6' of depth, depending upon projector throw, screen size and mirror system, to accommodate a rear projection TV or mirror/projector assembly
  • The screen is more expensive than the screen required for a front projection application. In addition, depending upon available depth, a mirror assembly may be required. These are expensive as well and require either cleaning or a clean room to prevent dust build up.
  • CRT units require regular calibration to maintain their best picture
  • Digital projectors require periodic bulb replacement at $300 - $500/ bulb
  • A speaker cannot be placed directly behind the screen to promote proper audio localization.

Direct view - flat panel or CRT

Advantages

  • Less susceptibility to ambient room light
  • CRTs can have a fantastic picture, with deep blacks and a fantastic color palette
  • Thin (flat panel)
  • A flat panel can be concealed using a lift or motorized picture frame assembly. These solutions, while having a high WOW! Factor can be very expensive.

Disadvantages

  • CRTs are not available in very large sizes, typically 34" wide screen or 40" 4 x 3 aspect ratio
  • Flat panel displays are available in larger sizes but the screen size is still limited to much smaller than 100". The largest size available under $20,000.00 at this writing is 63".
  • Flat panel displays are currently expensive when sized for a theater application. (see above) • Some units require proprietary cabling from a control box to the display. These cables scan be very expensive in longer lengths.

Front Projection - floor or ceiling, digital or CRT

Advantages

  • There is a huge choice of different projectors available to fit every budget and application • The screen can be sized for a true theater effect
  • The picture quality can be both large and spectacular if using a good enough projector
  • An acoustically transparent screen can be used to facilitate proper speaker placement. Stewart Filmscreen and Screen Research make great screens for this purpose. If possible, the center channel speaker should be placed directly behind the screen. This will help promote proper audio localization.
  • The screen can be motorized in the event it is desirable to conceal it.
  • The projector can be on a motorized lift for concealment as well. Although prices have come down recently, the purchase, engineering and installation of a projector lift can still be very expensive.

Disadvantages

  • Front projection systems have a very high sensitivity to ambient light. Even with the advances in screen technology, too much light will destroy the system's ability to create black or dark colors. Sony has recently introduced a new screen, using proprietary technology, which may reduce or eliminate this problem in the near future. The disadvantage to the Sony screen is that it is available in a limited number of sizes and it is currently very expensive.
  • You need to have a projector mounted on the ceiling or floor, which may create aesthetic, headroom or furniture placement concerns.
  • Projectors can be noisy, some excessively so. To mitigate this, a shroud can be built around the projector. Any cover or shroud must be correctly ventilated or serious damage will occur to the projector.
  • CRT units require regular calibration to maintain their best picture
  • Digital projectors require periodic bulb replacement at $300-$500/ bulb

Once a video display has been selected, any required construction elements can be designed to accommodate it. It is important for any ceiling mounted projector, but especially for a ceiling-mount CRT, to make sure sufficient blocking is installed in the ceiling to support the projector and mount. Make sure it is located to allow for easy installation of the mount also. In some geographic locations (California, Japan, etc.) it is advisable to use a safety cable to prevent it from falling in case of a seismic event. Proper accommodations will have to be made for this as well. Check your local building codes for more guidance.

It is a good idea to run at least 1 cat-5 cable and a 2" conduit to any display location, in addition to any required video or control cables. This will accommodate future cabling standards and Ethernet for control or media delivery.

Next time we'll look at seating and speaker placement, interior room acoustics, and lighting.


Comments (0)

This post does not have any comments. Be the first to leave a comment below.


Post A Comment

You must be logged in before you can post a comment. Login now.

Featured Product

Art Diffusor® Model D by Acoustics First

Art Diffusor® Model D by Acoustics First

The ArtDiffusor® Model D (patent applied for) is the newest in the ArtDiffusor® Series. The Model D is an organic, quadratic diffuser; fusing diverse design concepts such as MLS, QRD, bicubic interpolation and boolean logic mapping. These rolling, asymmetric diffusers minimize specular acoustic reflections while improving sound clarity and ambience. Their design encourages developing customized patterns, allowing for a truly unique installation - every time.