You all know Groundhog Day, right? No, not the Bill Murray movie — the rather spurious method of determining the duration of chilly weather. All indications are that it has no scientific origin, but probably came out of repeated casual observations as a basic “rule of thumb” to help simplify a complicated thing like predicting the onset of spring. There are many other cases of “rule of thumb,” and even some in our very own industry.
While I would never advocate anything but solid engineering practices while seriously designing a high-end home theater, rules of thumb can be super-useful for situations like a client meet-and-greet where you thought the conversation was going to be about lighting, but suddenly the client started talking about home theater and how big the room is and how many seats they want and how big a screen they want…and suddenly you’re referring everything to “the engineers,” and that doesn’t make you look very prepared or competent. Now you’re wondering why you can’t just talk about lighting like you thought you were going to…
So, I’m here to save the day with some simple home theater design rules of thumb that will help you out of the jam and, unlike groundhogs, won’t leave you in worse shape than just guessing.
#1 How big should my screen be?
Quick answer: Screen width = 0.6 x room length
How did we get there? Based on the available resolution of 4K video, you can set up a screen that is big enough to cover a horizontal subtended viewing angle of 50 degrees. A bit of trigonometry will show you that the screen should be 0.93 x the viewing distance. A good place for the primary row is 0.68 x the room length in order to avoid serious standing wave nulls, and be at a reasonably even distance to all the speakers — 0.68 x 0.93 = 0.63. With rounding, that makes the screen width as 0.6 x room length.
Now…all rules of thumb break down if you don’t follow basic assumptions, and this one is no different. If the room is 50 feet long, and you’re only using the last 15 feet of it as the “home theater” (e.g., the rest is bar and game room), no, you don’t make the screen 30 feet wide.
See what I mean? This isn’t sufficient for engineering the room (you need to be a lot more precise), but it can get you out of a tight spot in hurry.
Before we move on, I want to interject that our industry is falling back into the bad habit of talking about screen size in terms of diagonal instead of width. Stop that! Diagonal measurements are a cheat from the early days of TVs to make them appear larger. Anyone who is serious about big screens talks screen width. Whoever heard of a movie theater with a “43-foot diagonal” screen? No, movie theater screens are 40 feet wide or 60 feet wide or…you get the picture.
#2 How bright should my picture be?/How big a projector do I need?
Quick answer: Screen width x screen width x 11
How do we get there from here? The basic standard for illumination on a screen is 20 fL. Starting out with a 1.0 gain screen with an aspect ratio 1.78, you take the square footage of the screen (WxW/1.78), multiply it by 20, and you have pretty close to the projector lumen count for a 1.78 imaging panel. Now, I’m going to keep saying this: Do-not-use-this-to en-gin-eer- the-room. Once you get down to it, you may have a different shape screen, or a different gain screen, or some other unusual parameter that adjusts the numbers. That’s what engineering is for, and the client doesn’t need to know this. “Your projector needs to be about this big” is good enough for a client meeting.
#3 How loud do my speakers need to play?
Quick answer: For a 300 ft2 (or 3000 ft3) room, the speaker is going to play 7 dB less loud at the main seating position than the 1m anechoic sound pressure level specification.
• For a 500 ft2 (or 6000 ft3) room, the speaker is going to play 10 dB less loud than the 1m spec.
• For a 700 ft2 (or 9,000 ft3) room, the speaker is going to play 13 dB less loud than the 1m anechoic spec.
• For a larger room, call me, because I want to work on your project!
How do we get there from here? For starters, the burst (peak) SPL requirement for proper dynamics in a film soundtrack is 105 dB per speaker at the seats. You take the speaker’s 1m anechoic peak SPL and subtract the above values based on the room size. That will give you the speaker’s burst output at the seats in that room. If the number is equal to or larger than 105 dB, then the speaker will play loud enough.
For example: Brand X model Y has a burst SPL of 112 dB at 1m anechoic. The room is 3000 ft3. 112–7=105 dB, so that speaker is fine.
For this one to work, you’re going to need a little background info on the products you sell. That background info is the peak SPL at 1m (3 feet) anechoic. This is something you need to know, people! Not all you need to know, but definitely this much. It’s not necessarily the same as power handling, although you can use power handling and sensitivity to sort of get in the same Zip code as peak SPL in a pinch. The better approach is to deal with a speaker company that will give you the peak SPL data, such as Perlisten, Genelec, Pro Audio, and, well, my company Grimani Systems does, too. There are others.
Now…the more engineering-minded of you out there are going to want to know how I got those loss numbers. You’re going to notice that they’re not inverse square law loss. That is because that loss is for an anechoic or non-reflective environment. Home theaters are neither of those. It’s too complex to go into here, but the loss data involves the typical dispersion of speakers, the typical reflected energy, and the typical listening distance in a home theater environment of the specified size.
I’m going to keep reminding you that these are rules of thumb and not to be used for engineering, because I don’t want to hear in the halls of CEDIA, “But Anthony told me to just subtract…” Part of engineering is to make sure you properly specify the size of speakers and sound system you need. If you under specify, which is the more common mistake, then something could melt and lead to costly repairs. If you over specify, you’re wasting money and size, and the client may very well go with a lower bid from your competition.
#4 How much acoustical treatment do I need?
Quick answer: About 15% coverage of room surfaces with absorption and about 10–15% with diffusion
How do we get there from here? The premise is that there is a target amount of reflected energy in the room that is pleasant — not too much, not too little. It also assumes a mostly carpeted floor and comfy seating. If those factors vary, the amount of acoustic treatment will also need to vary. This is not a substitute for a calculation of the room’s decay time and spectrum.
Also by Anthony Grimani: Simplify Your Work, Not Your Theaters
#5 How many seats can I have?
Quick answer: Number of rows = Room length x 0.09
How do we get there? We don’t want the front row to be closer than 0.45 x room length from the front wall. That leaves 0.55 x room length for all the rows. A nice recliner chair takes up about 6 feet of depth. 0.55/6 = 0.09
For the width, the number of seats across is a bit trickier. You would normally have a fixed reduction of 7.6 feet from the room’s width. This is necessary for two 6.5-inch widths of stretched fabric dress, two 36-inch aisles, and one 6-inch arm of a chair (long story…). That all adds up to 91 inches, which is equivalent to about 7.6 feet.
Once that’s taken care of, each lounger is about 2.33 feet wide (22-inch pad+6-inch Arm => 28 inches/12 = 2.33 feet).
Subtract 7.5 feet from room width, then divide by 2.33 for number of seats across.
For example, a 20-foot wide room would yield 20-7.6 = 12.4 feet.
12.4/2.33 = 5.3. So you can put five seats with 22-inch pads and 6-inch arms across a 20-foot home cinema with acoustical treatments, stretched fabric dress, and comfortable aisles.
#6 How tall should the risers (and how high should the ceiling) be?
Quick answer: 18 inches per riser
How do we get there? That seems like a lot, but when you run the calculations on a large 4K projection screen at the appropriate viewing height, you typically need that much height for the risers to avoid blocking sightlines. Note that this is going to require extra height in the room, otherwise people are going to bump their heads on the ceiling at the back. Plan to enter the room at the back and have the home theater slab dug out 36 inches from the floor at entry level, like a swimming pool. This gives you two risers for three rows of seats, with the back row maintaining full height to the ceiling. Adjust as needed for the number of rows.
#7 What is a good shape for my room?
Quick answer: Room length x 1.2 to 1.35 of room width
How do we get there from here? Very long, narrow rooms do not sound good for a number of reasons, such as the necessity for the speakers to cramp the listeners. If a ratio of about 1.2 to 1.35 times length-to-width is maintained, appropriate spacing can be achieved.
Actually sizing the room during engineering is way more complicated than this, because you have to take into account standing waves. This ratio just gives you a starting point for a generally pleasing and good-sounding room, not necessarily the best modal distribution.
More from Anthony Grimani: Where to Place the Center-Channel Speaker When You Can’t Place It Behind the Screen
#8 How high should my screen be?
Quick answer: The screen mid-point should be no more than 3.25 feet + Viewing Distance *0.17 above the floor at the front row (all in feet).
How do we get there? This one is based on comfort and avoiding neck strain, pain, and headaches. In general, we go for no higher than 10 degrees up to the middle of the screen at the front row. SMPTE specifies 15 degrees as worst-case. Using a bit of trigonometry, add in the typical seated eye height of 3.25 feet, and there you go. Watching movies is fun, and we don’t want to injure anyone or cause them physical discomfort in the process. They may not even realize that looking up at the picture for long periods may be harmful in the long run. (Look how many people put their TVs above the fireplace.) You should look out for them.
There you have it. Eight rules of thumb to address some of the most commonly discussed topics in a home theater meeting. Hopefully, this will keep you ahead of the client…and avoid any unexpectedly “chilly weather.”
Anthony Grimani is co-founder of Grimani Systems loudspeakers (www.grimanisystems.com), and president of PMI Engineering (www.pmiltd.com) and MSR Acoustics (www.msr-inc.com).
Chase Walton contributed to this column.
