Mr. Blandings Part 2 - May 1999
Architectural Design
by
May 15, 1999
May 15, 1999
In Part One, I described how I went about selecting my audio and video components. Once the equipment list was complete, I began the room design. The goal was to capture the motion picture theater experience as closely as possible. As you'd expect, a home theater's architectural design must be based on the incestuous relationships among the screen, the projector, and the seating positions. But overwhelmingly, it's an acoustical design challenge. Let's start with the video.
The Video
I chose to install two rows of theater seats, three in the first row and three or four in the second. I knew that to achieve a forty-degree field-of-view, the first row must be positioned so that the viewer's eyes would be about ten feet from the screen. The second row must then be spaced back to accommodate guests' legs and offer enough room to move past seated viewers. The projector manual quickly revealed that for my screen size the projector lenses had to be located roughly at the back of the first row; I had to mount the projector on the ceiling. The projector manual also revealed that the screen's vertical position would make it difficult for second row viewers to see. First row heads would be in the way. I opted for stadium seating; the first row would be a full twelve inches below the second row. This would also allow me to avoid staggering the second row, so I could save the money required for a seventh seat.
The Audio
The acoustics weren't quite as simple. There are two acoustical considerations: transmission and reflection. And within reflection, there are two attributes: quantity and quality. (There are also some unique audio considerations; I'll get to that, too.) Transmission refers to how much sound escapes from the theater to disturb other people in the household. The key to limiting sound transmission is to block it with mass. High mass doesn't couple very well with air (they have very diffe rent acoustical impedances). No fewer than two layers of alternating acoustical impedances between the inside and the outside of the theater should contain the sound.
The best known method for this type of construction is to use 2x6 floor plates and ceiling plates with two sets of 2x4 studs on each edge. Spacing for each set of studs would be the usual 16 inches, but the first set of studs would be offset by 8 inches from the second. When gypsum wallboard is applied to each face, there's no physical connection from one face to the other. So as sound causes the interior face of the wall to vibrate, there would be very little coupling to the exterior face of the wall to create sound on that side. Installing two layers of 5/8-inch thick, type-x gypsum with staggered seams on each face (rather than one layer of conventional half-inch wallboard) would significantly increase the mass and would dramatically reduce coupling and transmission. The same technique can be used for the theater ceiling to reduce vertical transmission.
I had a slightly different but related approach in mind. I wanted a separate space that would wrap around three sides of the theater to hide all the wiring and the electronic components, especially the front speakers. So I chose to construct a room within a room (see figure 1). The outer room would have conventional 2x4 construction, but receive two layers of 5/8-inch type-x gypsum on each face of the walls. The ceiling would be a separate structure and would not be connected physically to the beams and members that supported the sub-flooring for the floor above. The ceiling, too, would receive two layers of 5/8-inch type-x gypsum.
The inner room would be constructed mostly with conventional 2x4 walls with one layer of 5/8-inch type-x gypsum on each face. The walls closest to the screen (and would be in the subwoofer's near-field) would be constructed with 2x6 plates and studs for additional stiffness. The speakers would be recessed directly below the screen in a large panel constructed from two layers of 3/4-inch particleboard, glued and screwed together. To further stiffen the inner room walls, a non-formaldehyde, expanding-foam spray would be injected between all the studs before the second face of gypsum was installed. High stiffness would help avoid wall resonances that would affect the in-room frequency response. All wallboard would be screwed; no nails would be used. All very rigid. All high mass.
The doors would be the thickest, solid-core, pre-hung type commercially available and would be sealed with flexible, silicone-rubber, weather-stripping when closed. The floor wouldn't be an issue. To avoid all ambient light, I chose to make the windowless theater a part of the finished portion of the basement. The floor would be an inert slab, just like in a motion picture theater. And although the theater's back wall would appear to be conventionally framed, there would be a cinderblock foundation wall behind it. Less than one-hundredth of the sound should escape from the room (over 40 dB of attenuation).
(Hint: don't waste money on conventional thermal insulation (for example, Fiberglas) within a theater's walls. It may absorb high frequencies when exposed, but little of that part of the spectrum gets through wallboard, particularly of the type and thickness I've described.)
Reflectivity
Now that I trapped all that sound, I had to address the issue of reflectivity. Home theaters should be acoustically dead. A film's acoustical environment is created on the soundstage and is reproduced by five channels of audio. The room should not contribute, so the theater had to be as acoustically absorbent as possible. I wanted to capture the look of a classic film palace, so using many heavy velour drapes to reduce wall reflectivity made aesthetic and acoustic sense. I then chose deep, open -pile, wall-to-wall carpeting which would also absorb sound. And the high-back, rocking, theater seats, covered in the same velour as the drapes, would further absorb sound. That should sufficiently reduce the "quantity" of reflections, mostly in the upper midrange and top end, of course. (Technically speaking, I'm reducing the T60 time, the time it takes for a sound to decay 60 dB with respect to its initial amplitude.) Now for the most difficult property: the quality of reflections.
I'm using the word "quality" to simplify a troublesome problem. Reflections in any room will reinforce one another (in-phase reflections) or destroy one another (out-of-phase reflections). This causes bumps and dips in the in-room frequency response, which may damage intelligibility or simply spoil the spectral balance of the sound. A room in the shape of a cube is the worst case; all resonant frequencies (stable in-phase and out-of-phase tones) are the same because the wall-to-wall and floor-to-ceiling distances are the same.
To minimize these problems, the trick is to create as many acoustical paths of different lengths and directions as possible. This tends to smooth out the in-room response and makes the sound most pleasing. So I made the interior of the theater as complex as aesthetics would allow. Front-to-back, side-to-side, and vertical dimensions are all quite different. In the front of the theater, surrounding the screen, there's an angled, shell-like shape with staggered wall sections. Arrayed around the sides and back of the theater, there are eight two-foot wide, rectangular extensions supporting half-round columns. The floor has four different elevations with respect to the ceiling. Many path lengths. Many angles. And, I hoped, a relatively smooth acoustical environment.
Dimensions and Arrangement
You probably noticed from my drawing that my theater is wider than it is deep. This might seem counter-intuitive, so I'll explain. Simply speaking, when sound travels through the air, its amplitude diminishes by one-half (6 dB) every time the sound path doubles in length (attenuation in dB = 20 x log (R1/R2)). If the sidewalls (and consequently the surround speakers) are made close to the seating, a guest seated at the end of a row would be much closer to one surround speaker than the other. The closer surround speaker would seem much louder than the other and surround effects would be badly unbalanced. But if the sidewalls and surrounds were farther away from the seats, the distances between an end-seated guest and the two surrounds would be more similar, the sound amplitudes would be closer to one another, and surround effects would be better balanced. So the object of this arrangement is to maximize the sweet spot within the seating, to make the audio presentation as good as possible for everyone. To think about it in another way, consider the seats toward the center of a large movie theater. Erect a wall behind row ten. The relative proportions are similar to my home theater.
The front speakers would be mounted just below the screen to create the illusion of sound emanating from the screen. The left and right speakers would be spaced as far apart as the structure would allow, but not beyond the edges of the screen. And all the front speakers would be recessed. Why? When sound travels away from the front face of a speaker, it wraps around the speaker's sides and bounces off the wall behind. The reflection will then join the direct sound. Because the reflection's path is a different length than the direct path, the reflection will cause reinforcement and cancellation with the direct sound at frequencies related to the path length difference.
This arrangement acts like a comb filter, making the in-room frequency response a bit jagged before any of the other room reflections ever come into play. By recessing the cabinet and placing the front of the speaker system (the plane of propagation) even with the wall, this comb filter effect is minimized. The in-room response is smoothest. (I would compromise this arrangement very slightly by toeing-in the left and right speakers to ensure that all seating positions were within the speakers' high frequency horizontal dispersion angle.) Note that recessing was not an option for the surround speakers since they propagate sound from three sides.
You may have read elsewhere that frequencies below about 100 Hz are difficult to localize. I certainly agree. So conventional wisdom suggests that you can put the subwoofer anyplace in the room. I don't agree. One must remember that sound from the subwoofer must seamlessly blend with sound from the other speakers. If there's a physical separation, they probably can't. Allow me to try a simplified explanation. The wavelength of 120 Hz (the typical crossover frequency between the subwoofer and the other speakers) is about 9.3 feet. If the difference in path length between ear-to-subwoofer and ear-to-front speaker is an odd multiple of half of that wavelength (4.7, 14.0, 23.3, etc.), any 120 Hz signal will cancel acoustically.
Both the amplitude and the phase response in the crossover region will be adversely affected since any other path length difference will affect the blend in a different manner. So I decided to mount my subwoofer between the center speaker and the right speaker (it could just as well have been between center and left). The subwoofer would also be recessed, so the propagation plane of the subwoofer would be the same as the other front speakers. The front speakers and the subwoofer would then stay in phase acoustically as well as electrically. The resultant response would be smoother and more accurate. (The decoder delays the surround channels. This tends to maintain reasonably close phase coherence, and my architecture maintains as close to a good phase fit as possible over the seating area. But clearly, the surrounds are not as critical as the front speakers are.)
Problems and Mechanicals
One last note concerning audio/acoustics. One potential flaw seemed unavoidable. Two columns supporting one of the steel I-beams that held the home's floor structure above limited the theater's depth. So I knew that the back row of seats would have to be mounted fairly close to the rear wall. Seats in this position would probably have a greater reinforcement of the bass than those in the front row. I would have preferred to have left three to five feet behind the back row, but that was impossible. It'll be interesting to measure the effect. Okay, let's move on to some mechanicals.
The cavity on either side of the screen would accept a drapery stack when the front drapes were open. Three of the drapes would be on remotely operated motorized tracks: the front drapes; the drapes that cover the equipment; and, the drapes that hide the entrance to the theater. The others would be manually operated.
To support the 125-pound projector, I designed and fabricated an assembly made from steel Unistrut and 3/4-inch plywood. It would fit precisely between the TGIs (prefabricated wooden I-beams) used to support the theater's ceiling. When this assembly would be mounted, four threaded rods would be bolted to the Unistrut, and the gypsum wallboard would be screwed in place with the rods extending down through snug holes. Trimming the rods and installing the projector bracket would come later.
I wanted to avoid any rattles and buzzes that would be stimulated by the sound, so I chose a heavy, 19-inch, electronics rack made by Emcor to house the equipment. Those electronic components that didn't offer rack-mounting kits (like the Sony DVD player) would sit on steel shelves within the rack. An open-backed alcove to the left of the seated viewers was designed to accept the rack. This would have the added benefit of isolating any fan noise from the theater. For film storage, I designed a complimentary, shelved closet to the right of the viewers. The shelves would be constructed from 3/4-inch particleboard supported at four points, heavy enough to avoid rattling even without software in place. Both openings would be covered with draperies during any presentation. When seated, the theater would appear perfectly symmetrical.
Atmosphere
The lighting was to be subtle. Four recessed top-hats would be installed over the steps that lead down from one seating level to another. The lighting would be on a remotely controlled three-way dimmer, so appropriate instructions were given to the electrician when the switch boxes were wired. When the drapes were closed, covering the electronics' panel lights, and the ceiling lights were dimmed to black, the room would be as dark as a coffin. This is critical to achieving a true projected black. There would be no ambient light to reflect off the screen. The other advantage is that the projector would not have to be run hard to achieve a great presentation; this would extend the tubes' lives. But a front projector does generate a little stray light.
Stand alongside a screen and look back at a front projector's lenses. Even though your eyes are not in the path of the projected image, the three guns do not appear black. So given the chance, this stray light would illuminate the walls around the screen. This is something to avoid. Take a look at the color scheme in your better movie theaters. The area around the screen is flat black. The ceiling is flat black. The walls, carpeting, and seats are usually in dark colors (blues, greens, burgundies). So that's the approach I would take. The wall behind the screen, the shell around the screen, and the ceiling would all be painted flat black - three coats. All the hardware (supply grills, return grills, lighting fixtures) would be painted flat black. The seven-inch raised "stage" in front of the screen would be carpeted in a light-absorbing black. The seats, the remaining carpeting, the walls, and the draperies would be finished in a dark, rich burgundy. (The egg-shell finish burgundy paint would have to be hand matched to the fabric.) The columns were to be painted in egg-shell finish black with gold capitals and bases. With all lights out and a picture on the screen, nothing would intrude into the viewers' fields-of-view.
Wrapping Up
Let me digress for a moment to say that some of the choices I made would be rather difficult to execute in an existing home. I should mention that the outer room was fabricated during the construction of a new home for which we had been saving for over twenty years. And that gave me unusual opportunities. Two extra courses of cinderblock were added to the foundation to accommodate extra basement ceiling height. The stadium seating elevations (I should really say depressions, since they're pits) were accomplished with a special pour as the basement slab went in (but the same effect may be accomplished by building the back row of seats up if the ceiling height permits). I had the heating/air-conditioning contractor put in supplies and returns based on where I expected the inner room's walls to be, a return just over the equipment rack to exhaust heat, and had him add a dedicated control zone for the theater. All connections from the forced-air system were made through large loops of flexible ductwork to attenuate noise. The security system contractor was instructed to put smoke detectors on the inner and outer room ceilings and a motion detector in the inner room to supplement the burglar alarm system. And the electrician was asked to install a dedicated 30-amp outlet to power all the theater electronics when he put in the switches, conventional outlets, and light fixtures.
But the techniques I've described in this part may be applied to any installation. I hope you found them helpful.
(If you have any questions or comments for the author, say hello to Mr. Blandings here.)
(Hint: don't waste money on conventional thermal insulation (for example, Fiberglas) within a theater's walls. It may absorb high frequencies when exposed, but little of that part of the spectrum gets through wallboard, particularly of the type and thickness I've described.)
Reflectivity
Now that I trapped all that sound, I had to address the issue of reflectivity. Home theaters should be acoustically dead. A film's acoustical environment is created on the soundstage and is reproduced by five channels of audio. The room should not contribute, so the theater had to be as acoustically absorbent as possible. I wanted to capture the look of a classic film palace, so using many heavy velour drapes to reduce wall reflectivity made aesthetic and acoustic sense. I then chose deep, open -pile, wall-to-wall carpeting which would also absorb sound. And the high-back, rocking, theater seats, covered in the same velour as the drapes, would further absorb sound. That should sufficiently reduce the "quantity" of reflections, mostly in the upper midrange and top end, of course. (Technically speaking, I'm reducing the T60 time, the time it takes for a sound to decay 60 dB with respect to its initial amplitude.) Now for the most difficult property: the quality of reflections.
I'm using the word "quality" to simplify a troublesome problem. Reflections in any room will reinforce one another (in-phase reflections) or destroy one another (out-of-phase reflections). This causes bumps and dips in the in-room frequency response, which may damage intelligibility or simply spoil the spectral balance of the sound. A room in the shape of a cube is the worst case; all resonant frequencies (stable in-phase and out-of-phase tones) are the same because the wall-to-wall and floor-to-ceiling distances are the same.
To minimize these problems, the trick is to create as many acoustical paths of different lengths and directions as possible. This tends to smooth out the in-room response and makes the sound most pleasing. So I made the interior of the theater as complex as aesthetics would allow. Front-to-back, side-to-side, and vertical dimensions are all quite different. In the front of the theater, surrounding the screen, there's an angled, shell-like shape with staggered wall sections. Arrayed around the sides and back of the theater, there are eight two-foot wide, rectangular extensions supporting half-round columns. The floor has four different elevations with respect to the ceiling. Many path lengths. Many angles. And, I hoped, a relatively smooth acoustical environment.
Dimensions and Arrangement
You probably noticed from my drawing that my theater is wider than it is deep. This might seem counter-intuitive, so I'll explain. Simply speaking, when sound travels through the air, its amplitude diminishes by one-half (6 dB) every time the sound path doubles in length (attenuation in dB = 20 x log (R1/R2)). If the sidewalls (and consequently the surround speakers) are made close to the seating, a guest seated at the end of a row would be much closer to one surround speaker than the other. The closer surround speaker would seem much louder than the other and surround effects would be badly unbalanced. But if the sidewalls and surrounds were farther away from the seats, the distances between an end-seated guest and the two surrounds would be more similar, the sound amplitudes would be closer to one another, and surround effects would be better balanced. So the object of this arrangement is to maximize the sweet spot within the seating, to make the audio presentation as good as possible for everyone. To think about it in another way, consider the seats toward the center of a large movie theater. Erect a wall behind row ten. The relative proportions are similar to my home theater.
The front speakers would be mounted just below the screen to create the illusion of sound emanating from the screen. The left and right speakers would be spaced as far apart as the structure would allow, but not beyond the edges of the screen. And all the front speakers would be recessed. Why? When sound travels away from the front face of a speaker, it wraps around the speaker's sides and bounces off the wall behind. The reflection will then join the direct sound. Because the reflection's path is a different length than the direct path, the reflection will cause reinforcement and cancellation with the direct sound at frequencies related to the path length difference.
This arrangement acts like a comb filter, making the in-room frequency response a bit jagged before any of the other room reflections ever come into play. By recessing the cabinet and placing the front of the speaker system (the plane of propagation) even with the wall, this comb filter effect is minimized. The in-room response is smoothest. (I would compromise this arrangement very slightly by toeing-in the left and right speakers to ensure that all seating positions were within the speakers' high frequency horizontal dispersion angle.) Note that recessing was not an option for the surround speakers since they propagate sound from three sides.
You may have read elsewhere that frequencies below about 100 Hz are difficult to localize. I certainly agree. So conventional wisdom suggests that you can put the subwoofer anyplace in the room. I don't agree. One must remember that sound from the subwoofer must seamlessly blend with sound from the other speakers. If there's a physical separation, they probably can't. Allow me to try a simplified explanation. The wavelength of 120 Hz (the typical crossover frequency between the subwoofer and the other speakers) is about 9.3 feet. If the difference in path length between ear-to-subwoofer and ear-to-front speaker is an odd multiple of half of that wavelength (4.7, 14.0, 23.3, etc.), any 120 Hz signal will cancel acoustically.
Both the amplitude and the phase response in the crossover region will be adversely affected since any other path length difference will affect the blend in a different manner. So I decided to mount my subwoofer between the center speaker and the right speaker (it could just as well have been between center and left). The subwoofer would also be recessed, so the propagation plane of the subwoofer would be the same as the other front speakers. The front speakers and the subwoofer would then stay in phase acoustically as well as electrically. The resultant response would be smoother and more accurate. (The decoder delays the surround channels. This tends to maintain reasonably close phase coherence, and my architecture maintains as close to a good phase fit as possible over the seating area. But clearly, the surrounds are not as critical as the front speakers are.)
Problems and Mechanicals
One last note concerning audio/acoustics. One potential flaw seemed unavoidable. Two columns supporting one of the steel I-beams that held the home's floor structure above limited the theater's depth. So I knew that the back row of seats would have to be mounted fairly close to the rear wall. Seats in this position would probably have a greater reinforcement of the bass than those in the front row. I would have preferred to have left three to five feet behind the back row, but that was impossible. It'll be interesting to measure the effect. Okay, let's move on to some mechanicals.
The cavity on either side of the screen would accept a drapery stack when the front drapes were open. Three of the drapes would be on remotely operated motorized tracks: the front drapes; the drapes that cover the equipment; and, the drapes that hide the entrance to the theater. The others would be manually operated.
To support the 125-pound projector, I designed and fabricated an assembly made from steel Unistrut and 3/4-inch plywood. It would fit precisely between the TGIs (prefabricated wooden I-beams) used to support the theater's ceiling. When this assembly would be mounted, four threaded rods would be bolted to the Unistrut, and the gypsum wallboard would be screwed in place with the rods extending down through snug holes. Trimming the rods and installing the projector bracket would come later.
I wanted to avoid any rattles and buzzes that would be stimulated by the sound, so I chose a heavy, 19-inch, electronics rack made by Emcor to house the equipment. Those electronic components that didn't offer rack-mounting kits (like the Sony DVD player) would sit on steel shelves within the rack. An open-backed alcove to the left of the seated viewers was designed to accept the rack. This would have the added benefit of isolating any fan noise from the theater. For film storage, I designed a complimentary, shelved closet to the right of the viewers. The shelves would be constructed from 3/4-inch particleboard supported at four points, heavy enough to avoid rattling even without software in place. Both openings would be covered with draperies during any presentation. When seated, the theater would appear perfectly symmetrical.
Atmosphere
The lighting was to be subtle. Four recessed top-hats would be installed over the steps that lead down from one seating level to another. The lighting would be on a remotely controlled three-way dimmer, so appropriate instructions were given to the electrician when the switch boxes were wired. When the drapes were closed, covering the electronics' panel lights, and the ceiling lights were dimmed to black, the room would be as dark as a coffin. This is critical to achieving a true projected black. There would be no ambient light to reflect off the screen. The other advantage is that the projector would not have to be run hard to achieve a great presentation; this would extend the tubes' lives. But a front projector does generate a little stray light.
Stand alongside a screen and look back at a front projector's lenses. Even though your eyes are not in the path of the projected image, the three guns do not appear black. So given the chance, this stray light would illuminate the walls around the screen. This is something to avoid. Take a look at the color scheme in your better movie theaters. The area around the screen is flat black. The ceiling is flat black. The walls, carpeting, and seats are usually in dark colors (blues, greens, burgundies). So that's the approach I would take. The wall behind the screen, the shell around the screen, and the ceiling would all be painted flat black - three coats. All the hardware (supply grills, return grills, lighting fixtures) would be painted flat black. The seven-inch raised "stage" in front of the screen would be carpeted in a light-absorbing black. The seats, the remaining carpeting, the walls, and the draperies would be finished in a dark, rich burgundy. (The egg-shell finish burgundy paint would have to be hand matched to the fabric.) The columns were to be painted in egg-shell finish black with gold capitals and bases. With all lights out and a picture on the screen, nothing would intrude into the viewers' fields-of-view.
Wrapping Up
Let me digress for a moment to say that some of the choices I made would be rather difficult to execute in an existing home. I should mention that the outer room was fabricated during the construction of a new home for which we had been saving for over twenty years. And that gave me unusual opportunities. Two extra courses of cinderblock were added to the foundation to accommodate extra basement ceiling height. The stadium seating elevations (I should really say depressions, since they're pits) were accomplished with a special pour as the basement slab went in (but the same effect may be accomplished by building the back row of seats up if the ceiling height permits). I had the heating/air-conditioning contractor put in supplies and returns based on where I expected the inner room's walls to be, a return just over the equipment rack to exhaust heat, and had him add a dedicated control zone for the theater. All connections from the forced-air system were made through large loops of flexible ductwork to attenuate noise. The security system contractor was instructed to put smoke detectors on the inner and outer room ceilings and a motion detector in the inner room to supplement the burglar alarm system. And the electrician was asked to install a dedicated 30-amp outlet to power all the theater electronics when he put in the switches, conventional outlets, and light fixtures.
But the techniques I've described in this part may be applied to any installation. I hope you found them helpful.
(If you have any questions or comments for the author, say hello to Mr. Blandings here.)
SITE | CONTACT US | TERMS OF SERVICE | PRIVACY POLICY
MORE IN THE CRAVEONLINE FILM CHANNEL : Comingsoon | Shock Till You Drop
