Design the Ultimate Home Theater- On a Budget Page 2
Oddly enough, the first step in gleaning every last ounce of performance from your home theater may be choosing your chair. After all, your seating position will have a big effect on your sonic interaction with the system. Think about it: The chair's height determines where your eyes and ears will be relative to the speakers and screen. The chair's size dictates how many people can fit side by side or in rows (allow for leg room) and where they'll be in the room. There are certain seating locations you may want to avoid (see Tip #3), so the chair's dimensions can, in a very real way, determine the baseline requirements for a room's length, width, and height. When you design a room from scratch, it may be easier to first determine what chair you're going to use, then design the room around it. Otherwise, you might find that your options are limited to specific (and possibly uncomfortable) sizes.
Many of these parameters are difficult to explain without an in-depth discussion of room acoustics, which we'll get to in a minute. To start with, here are a few pointers from Russ Herschelmann. First, a chair should be comfortable but not so comfortable that you fall asleep. Also, chairs with high backs may support your head, but they also reflect the sound that's coming from the front of the room back at your ears immediately after you hear the initial sound. This blurs the sounds together and makes them change in timbre. The high back may also block the surround sound that's coming from the rear speakers. Many designers can shorten a chair's back, if the rest of the design works for you. Dolby Labs brags that the chairs in their listening room absorb as much sound with a person in them as they do on their own. I have no idea how they measured that, but I'll discuss the importance of absorption later on.
Russ discourages the use of reclining chairs, since reclining changes the position of your head, which changes the sound. From a purely technical standpoint, ottomans seem like a better choice, as the position of your head stays fixed. Your family may not be willing to give up the recliner, though. We've been using a pair of comfortable reclining chairs on loan from Acoustic Innovations in our current listening room. In theory, we should probably use something more acoustically neutral. When I mentioned this to my fellow listening-room users, they suggested that I might have better luck sneaking explosives into Los Angeles International Airport. Needless to say, the comfy chairs (left, top photo) are staying in the current room, but we plan to outfit the new room with nonreclining, low-backed chairs from Irwin Seating (left, bottom photo).
Tip #2: Calculating Axial Modes
Start with half of the speed of sound and divide it by each of the room's three dimensions. The speed of sound is roughly 1,130 feet per second; half of that is 565. In a room that's 18 feet long by 14 feet wide by 8 feet high, for example, the fundamental resonances are:
565 / 18 = 31.4 Hz
565 / 14 = 40.3 Hz
565 / 8 = 70.6 Hz
Take each fundamental resonance and multiply it by integers from 1 to about 9, depending on when each dimension's resonance gets to about 300 Hz. Again, you can often acoustically treat frequencies above 150 Hz.
As an example, the dimensions 10 feet by 15 feet by 20 feet will create the resonances shown in figure 4. The figure shows that the length and width resonances repeatedly coincide with the height. This will create tremendous peaks throughout the system's low-frequency response, resulting in boomy bass. This uneven frequency response is less desirable for a home theater.
On the other hand, figure 5 shows the axial modes of a room with the dimensions 10 feet by 14 feet by 19 feet (keeping the same ceiling height as the earlier example but shortening the length and width by only 1 foot). Notice how the resonances are more uniformly distributed, with fewer large gaps and no coincidences. Ideally, the resonances will be separated from each other by at least 5 percent or more. There should also be no substantial gaps (greater than 25 Hz) between resonances.
You can download an Excel spreadsheet that calculates axial modes from our sister publication's Website: www.guidetohometheater.com. Click on the Sweet Spot link.
Length: 31.4 Hz x (1,2,3...) = 31.4, 62.8, 94.2, 125.6, 157, 188.4, 219.8, 251.2, 282.6
Width: 40.3 Hz x (1,2,3...)= 40.3, 80.6, 120.9, 161.2, 201.5, 241.8, 282.1
Height: 70.6 Hz x (1,2,3...)= 70.6, 141.2, 211.8, 282.4