Bring Your Home Theater to New HeightsBuilding Your Seating Platform (Your Secret Weapon Against the Evil Room Mode)
Solving acoustic issues in residential rooms can be a daunting challenge, particularly when it comes to solving bass response problems. The good news is that bass response problems can be solved, and one of your secret weapons against uneven bass response can be a seating platform. The basic construction of a seating platform is very similar to a stage, but there are some important considerations and decisions you'll have to make before you begin. Don't forget, the purpose of a seating platform is to raise the seating height of a second (or third or fourth) row of seats providing a clear line of sight to the screen. Remember, while providing enough height for seating, you also need to leave adequate headroom as well. If you are planning on installing tactile bass units, or would like the option to do so later, read the Special considerations for tactile equipped platforms
Before you can determine the correct height for your seating platform, you need to mount your screen for the best vertical viewing angle. The vertical viewing angle is defined as the angle from eye level of the front row's center seat to the top of the highest protected image, or the top of the screen. The SMPTE (Society of Motion Picture and Television Engineers) notes that most viewers will experience physical discomfort when this angle exceeds 35 degrees. Various studies have suggested the optimum angle to be in the area of 15 degrees, which corresponds to the human cone of vision. Ultimately, you want the center of the Screen to be slightly above eye level and the top of the screen to fall in the range of 10 to 20 degrees (when possible). At the same time, you don't want the screen too close to the ceiling, or you'll get a light reflection Following these general guidelines, you can size the vertical height of your screen and, for our platform, take the measurement from the floor to the bottom of the screen.
Since the entire purpose of the raised seating platform is to provide a clear view of the screen from a second row of seating, the first step is to determine what that height should be. The easiest way to determine the minimum height of the platform is with a state drawing. On your drawing you want to mark the location of the bottom of your screen and head height for the front row. Since we're all different heights, and seating height can vary, you might want to sit in your theater seat and measure the distance from the top of your head to the floor. If you don't have your chairs, you can use an avenge height of 42". Now draw a straight line from the bottom of the screen to the back of your room, passing just over your head height mark (front row). Measuring 39" to 40" down from this line will be the minimum height requirement for your seating platform. Ultimately, if you have the actual measurement for your seats, you will be able to plan more accurately.SECTION 2
From the angle of the line, you can easily see that the farther back from the front row of seats, the higher your platform will have to be. In fact, if you end up putting your seats farther back than you planned ahead for, you'll have a line of sight problem.SECTION 3
It is a good idea to know exactly what seats you will be buying for the theater before you decide on your seating platform dimensions. When laying out your seating positions, you need to make certain you plan clearance for foot rests to extend in front and recliners to tilt back comfortably.
If your seating platform height is more than about 8" or 9" high, you'll also want to consider steps. In some locations, local building codes will demand steps for certain heights. Check with your local building department for this information in your area. If you build steps, your tread (the part of the step you step on) depth should be between 10" and 11". When building steps, once again, your seats come into play. You'll need to leave enough room between the steps for your front row seats. You don't want to have to change the step width once the carpet is down!
It's also a good idea to provide some kind of low level lighting at the steps to prevent accidents in a dark room. If you want to install rope lighting under the step edge, building your steps so the treads overhang by at least 2"
Now that the critical dimensions are known, you can start building your platform. In an ideal world, the platform should extend all the way to the back of the room and fill the width of the room. While this will decrease the possibilities of a "trip accident," as you'll see later, it will also be your stealthy bass response fix-it.
Like the stage, the room should be completely sheetrocked before you build, with any gaps between the floor and the sheetrock completely filled with a 50 year flexible caulk. Unlike the stage, however, the platform can contact the walls. For this project, we'll assume we've determined that a platform height of 9-1/2" is suitable.
The first step is to frame the perimeter of the platform using pine boards. Depending on the height of your platform, you may find that you will use a different size. We used a 2x8, which is very common. In this example, your 2x8s will sit directly on the floor and, because you don't want any rattles, you should lay down a layer of 30 lb. roofing felt over the floor. First, install a 2x8 against the back wall, gluing and screwing the 2x8 through the sheetrock to the studs. Before you screw it in, be sure it is level from side to side. Next, attach the two side 2x8s to each side wall, checking for level. These side pieces should be 5-1/2" shorter than the planned depth of your seating platform. When installing the sides, you may need to shim between the floor and the bottom of the 2x8, or even trim some of the 2x8 back to maintain level from front to back. The sides should also be glued to the drywall and screwed through to the studs.
With the sides and back in place, you can install the 2x8 across the front of your platform. When you do, make certain the top of the front 2x8 is even with the tops of the two sides, then check for level and make any adjustments in the middle that are needed for a level platform. Again, you'll want to screw and glue the front to the two sides. You now have your basic box frame in place.
Next, run 2x8s from the front to the back (as shown) inside the box frame. These 2x8s should be 12" on center and should be screwed and glued to the front and back of the box frame. Important: The 2x8 joists must run from front to back as shown!
Now would be a good time to think about a few "convenience" items. First, it is a good idea to plan for an electrical outlet in the face of your platform. You'll want to install a single gang box in the face of the front riser and run the electrical cable for the outlet. Second, a phone jack would be a rather convenient accessory also in the face of the platform. If you have any plans to surf the internet or play video games, you'll want to install a 3" diameter PVC pipe from your equipment rack to the face of the platform riser. The PVC should terminate at a double gang box in the face of the riser. Also, don't use any 90 degree corners if you install PVC. IF you do, pulling wire later could be a challenge.
We don't fill the platform with sand, but every space between every 2x8 must be completely filled with fiberglass insulation. For this you want to use the non-backed fiberglass batt. Whatever thickness you use, just make certain you've completely filled the space
. Johns-Mansville has formaldehyde free fiberglass batts that are excellent for this application.SECTION 4 : The guts of your seating platform
The platform must be very solid and must not add rattles to the room. For the first layer of decking, run a bead of construction adhesive down the top edge of the joists. Using 3/4" tongue and groove plywood, install your first layer of decking. The decking should be screwed down to the joists and should extend 2" beyond your framing to the front. With the first layer down, cover it completely with 30 lb. roofing felt. Next install your second layer of decking using 1/2" plywood and screwing to the plywood below. When you install the second layer of decking, overlap the seams from the first layer. Finally, lay down another layer of 30 lb. flooring felt and screw down the final layer of decking using 3/4" tongue and groove plywood. You'll have a solid, echo-free platform.SECTION 5
Now let's solve a few bass response problems using all of that fiberglass you just stuffed in your platform.
One of the characteristics of axial modes is that the frequencies at which the modes occur will always be at their peak pressure at the boundaries of the room. For length modes, the high pressure will be at the front and back walls. For width modes, it will be at the side walls, and for floor/ceiling modes, it is at the floor or ceiling. If you have a real time analyzer, you can place your microphone at the back wall, and you'll be able to see all the frequencies causing length modes (whether or not the mode is a peak or null at the listening position). If you want to determine the frequencies for all axial modes, you would place the microphone at the corner near the floor. Now let's use that information to reduce or eliminate those modes.
In the locations shown in blue, cut holes completely through the decking of your platform. The holes should be centered between your joists and be 4" x 10" in size. Be absolutely certain you do this in the corners! Get several pieces of light weight black fabric and place the fabric in the holes, spread out flat, to hide the fiberglass from view. After your room is carpeted, insert standard (or decorative) HVAC registers over the holes. They have no HVAC function, but some of the high pressure of the modal frequencies will be reduces. You can add more registers later if you need to, but you won't now exactly what is needed until you finish your room, install your furniture, and then measure the room's response with a real time analyzer. If there are any pesky problems remaining, you can also build useful columns, as you learned from our last issue (HTB, July 2003, Going Vertical) to create a perforated resonator for specific frequencies.TECH TALK : Special Considerations for Tactile Equipped Platforms
There are a few differences in building a platform for Tactile Sound Transducers. The goal is to make the riser tight like a drum skin, solid on its outer edge only. The technique will make the tactile sound transducer perform efficiently and will also allow for irregular floors and wood materials so it will lay down better. While the riser will be strong and as solid as regular wood floors, more than enough to support the seating and the viewers without making noise and feeling weak, it will also allow the Tactile Transducer to perform efficiently.
For starters, only the outer joists should touch the floor. Also, the inner joists should be 2 inches smaller than the outer joists (i.e., using 2x8s for your frame and 2x6s for your inner joists). The inner joists can be spaced 16 or 24 on center and will be 2" off the floor and level with the top of your frame. The raised inner joists help in rurning the needed wires under the structure and prevent them from pinching as well as allowing the riser to float as much as possible. Tie the two center most joists together with a bridge pieces (see illustration). The TST will be mounted to the bridge, not the plywood flooring. This cross member bridge will make the transmission of energy as efficient as possible by making two joists move instead of just one. For your deck, use only one layer of 3/4" plywood. All wood joists should be glued with liquid nails or an equivalent before being screwed together to prevent creaking.
For this application, it is very important that you don't attach your platform to a wall or another riser. That would detract from the efficiency of the tactile effect and transmit unwanted vibrations to the walls. Isolation feet or other materials are another great way of improving the tactile response even more by decoupling the riser from the floor. Keeping the platform floating and unattached from the walls will give you a very professional look with the high performance that will allow you to gt every ounce of your tactile transducer and maximize the tactile effect.BIO
Dennis Erskine is a member of SMPTE, AES, ALA, and IEEE. He and his firm specialize in the design of home theaters, listening rooms, and other acoustic spaces. Dennis can be reached at firstname.lastname@example.org or visit his website, Design Cinema Privee Inc, at www.designcinema.com