The most dramatic change in bass caused by subwoofer placement has to do with the interaction of the subwoofer and the room itself. Because of the relationship between a room’s dimensions and the wavelength of sounds, certain frequencies are reinforced and certain frequencies are canceled at different areas within your home theater (or any other room).

Home Theater Design - Part 3

Steve Faber | 1 Touch Movie

--- Part 3 ---
by Steve Faber, www.1touchmovie.com

The most dramatic change in bass caused by subwoofer placement has to do with the interaction of the subwoofer and the room itself. Because of the relationship between a room's dimensions and the wavelength of sounds, certain frequencies are reinforced and certain frequencies are canceled at different areas within your home theater (or any other room).

Click here for Part 1
Design Considerations
Click here for Part 2
Construction


This time we'll examine seating and speaker placement and some interior room acoustics. These are obviously extremely important to get the best sound quality and preserve the true theater experience. In a dedicated room you have the advantage of being able to place the seating and speakers in the optimum locations.

Typically, today's dedicated theater will be configured for 7.1 channel surround, requiring a left, center, and right front speaker and at least four surround speakers. Some of the new multi-channel surround algorithms are extremely effective at constructing a realistic audio environment to the listener. When constructing your home theater, you should definitely take advantage of this. 7.1 or 6.1 surround is available on even modestly priced equipment today, so the impact on your budget should be minimal.

The four surround speakers will be arranged with one speaker immediately adjacent to the primary listening area and the remaining two directly behind it. If using an A/V receiver with 6.1 capabilities split the single rear channel and use two speakers unless you are on an extremely tight budget. If you have multiple rows of seating, you may have two or more sets of side speakers. These would be placed adjacent to each row of seating. If you are using multiple rows with only one set of side speakers, you can place these either between the rows, or optimize their placement for the primary seating locations.

With the advent of the newer, digital surround formats, directors and sound designers have the ability to place sound track elements exclusively in any one speaker. This opened up a new world of realism. The 7.1 channel formats allow the placement of sounds directly behind the listener. In the past, these could only be obtained by providing identical sound of equal amplitude in both the left and right surround speakers. The problem arises when the listener is not located precisely between both speakers. In this case, the closer speaker predominates, destroying the rear center image and shifting it to the side.

The front speakers should be placed to optimize the front sound stage. The most important speaker in your system is the center channel speaker. About 60% - 70% of the typical motion picture sound track emanates from this speaker. The center channel speaker has been called the dialogue speaker. In the beginning of home surround systems 15 - 20 years ago, the center channel speaker tended to be a small afterthought. Don't consider it in this fashion. The center channel speaker's purpose is to anchor dialogue and sound track elements to the screen no matter where in the room the listener is sitting. Almost everything that is supposed to come directly from the screen will come from this speaker.

The left and right speakers are to give the sound track a sense of width and to allow for sound pans laterally across the screen. Much of the music in a sound track also comes from the screen. You can use full range, tower type speakers or smaller, satellite speakers. You can use an in wall center and box style left and rights or vice versa. The most important thing is that all the front speakers are timbre or voice matched to each other. This matching preserves the integrity of the sound stage and makes lateral sound pans consistent. To achieve this matching the speakers should be from the same manufacturer and engineered to work together in the left, center, right (LCR) application. When using an in-wall speaker in combination with box style speakers, some equalization may be required to compensate for the in-wall's tendency to boost the lower frequencies. This low frequency boost is caused by the wall and is known as a boundary effect. Many quality speaker manufacturers take this into account when designing their in-wall speakers and compensate to deliver a flat frequency response. You'll or your theater designer will have to determine this depending on the specific speaker combination you choose.

Optimum front channel speaker placement is to use a perforated screen (front projection applications only) and place the center channel speaker directly behind the screen. The next most advantageous placement is to have the center channel speaker directly above or below the center of the screen, as close to the screen as possible. The last choice is to have the center channel speaker above the screen in the ceiling and use a controlled directivity speaker to direct the out put to the listener. There are some applications where the in-ceiling application is the only alternative due to construction or aesthetic concerns. In-ceiling placement however, is not really desirable for a true, dedicated home theater.

The left and right speakers can be located either directly behind the screen when using a perforated screen. The can also be adjacent to the screen when using either a perforated or standard screen. Try to keep the left and right channel speakers no more than 36" away from the screen. It probably goes without saying, but both the left and right speakers should be the same distance from the screen. It is best to have the LCR speakers on the same horizontal plane if possible. If this is not possible, such as when using floor standing speakers, try to ensure there is less than 24" of vertical separation between the LCR speakers.

It is a common fallacy that "bass is non-directional, you can put your subwoofer anywhere you want". While it is true that, when properly crossed over, a subwoofer is basically impossible to localize, the tonal quality and output of the subwoofer(s) can be dramatically affected by their placement. Bass is reinforced by room boundaries, so placing a subwoofer at the intersection of two walls will give a bass boost, and the placement of the sub at the intersection of three walls (a corner) will give an even more substantial boost.

The most dramatic change in bass caused by subwoofer placement has to do with the interaction of the subwoofer and the room itself. Because of the relationship between a room's dimensions and the wavelength of sounds, certain frequencies are reinforced and certain frequencies are canceled at different areas within your home theater (or any other room). This cancellation and reinforcement is caused by standing waves. A standing wave is caused when a sound wave at a particular frequency is the same length or a ½ or ¼ multiple of a room dimension. The waves at the frequencies where this occurs have a tendency to remain in one location. In engineering parlance this is known as "having a fixed distribution in space".

The frequencies at which this cancellation or reinforcement occurs are called room modes. The ideal placement of your subwoofer(s) will excite as many of these modes as possible. Exciting only a few modes causes the frequency response to be uneven, with deep valleys and tall peaks at certain seating locations. The more modes that are excited, the closer the peaks and valleys are together on the audible frequency spectrum. This achieves a smoother frequency response at a given location in the room. Research has shown that using multiple subwoofers, usually two or four, maximally excites the room modes and gives more even bass response throughout the room. The most advantageous placement , exciting the maximum number of room modes, is front corner placement (2 subwoofers) and on the floor at the centers of each wall (4 subwoofers).

Correct subwoofer placement is extremely important due to the fact that modal problems can cause certain frequencies to cancel at certain locations within the room. These "nulls" cannot be fixed with an equalizer, only by relocating the subwoofers. Trying to fix the nulls with an EQ will only overdrive the speaker and the amplifier and cause distortion or damage. You can, however minimize the nulls through correct subwoofer placement, and then EQ down the peak resonant frequencies. Minimizing these modal problems by using multiple subwoofers will yield much better, tighter bass at all seating positions in your theater.

Next time we'll continue into more interior room acoustics and cover some lighting design concerns for your home theater.


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