In Search Of The Audio Abode---The Hi-fi House Page 4
If the floor plan is rectangular and the ceiling slopes, treat the latter like a vaulted ceiling, using its average height for your dimensional-ratio calculations. The slope should, however, be in the direction you'll prefer to face when listening, otherwise you'll have a reflective asymmetry that will cause left or right image biasing. (The only solution then will be to cover the entire ceiling area with absorptive material.)
So-called "open" house designs, with few interior dividing walls, may look nice and airily spacious, but they're terrible for audio, because the relative lack of boundary spaces acts much like the classically horrible L-shaped room. It may work out, but again, because the results are completely unpredictable, the chances of it doing so are very poor. The most usual audio casualty of open designs is bass performance, which will be thin in your listening room and superb when heard from another room.
Low frequencies---the reason you might opt for the big Duntechs instead of Celestion 700s---must be confined to the listening area if you want to hear them properly. And large doorways (or the lack of interior walls) are not their only escape route. Bass is surprisingly powerful, which is why it takes a lot of power and a large radiating area to reproduce it. This is also why it is easily lost through the room boundaries if these are not extremely rigid.
If the boundary surfaces, including the floor and ceiling, are capable of flexing under pressure, the bass will flap them back and forth like drumheads and pass right on through them to the great outdoors (or to other rooms in the house). In other words, the ideal listening room should be made of masonry or concrete on all six sides. This will be the hardest of all listening-room requirements to meet, because there is absolutely no other reason except acoustics for building a room this way. As a matter of fact, finding such a room is so unlikely that it is impractical even to call it a requirement. Some compromises are almost inevitable here, particularly with the ceiling. Just remember, though, that the more of the boundary surface that is rigid, the better your low-bass performance will be.
The worst possible case is a ground-floor room in a frame house with a basement. All six boundaries will flex, and that fabulous bass will probably be almost as loud in the farthest bedroom as it is in the listening room.
If the house is of masonry, LF loss to the outdoors will be minimized, but its transmission through the house will still depend on how many shared (with the rest of the house) walls are of wood frame. Since all interior walls tend to be frame, the best you can hope for is one shared wall, as when the room is a wing with masonry on three sides.
Masonry homes are rare in many parts of the country, but basements usually aren't. As for those people who live in earthquake zones, where building codes prohibit both basements and unreinforced masonry in residential construction, I can only offer my sympathy and point out that you should expect to give up something in exchange for that heavenly climate. (And you may still be able to find a home that was built before those code restrictions went into effect.)
The basement alternative:
A properly partitioned basement space can make an excellent listening room, if its dimensions are such that three walls can be used as-is with the addition of only one partition. This still won't confine bass as well as six masonry boundaries, but it will be a lot better than six flapping wood-frame surfaces.
The problem with a basement space is, of course, the furnace, which is always in the basement when there is a basement. A furnace rumbles, sometimes quite loudly, and some gas-fired ones whistle too. And as soon as heat starts to circulate, there is the added noise of the air blower or water pumps. So for maximum rigidity and isolation from furnace noise, the listening-room partition should consist of two layers of sheetrock, hung on two rows of staggered 2-by-4 studs with the intervening space stuffed with fiberglass (fig.5). If noise is still a problem despite the partition, it may be necessary to separately enclose the furnace, too.
Fig.5 Top view of a partitioning wall with high sound-attenuation characteristics.
The walls of this enclosure should be at least 20" from the sides of the furnace all the way around, extend from floor to ceiling, and have a fairly heavy 2'-wide door, at least as wide as the furnace unit, giving access to the front of it. And because a lot of sound can pass through a tiny space, all openings should be sealed, as should the perimeter of the door. Naturally, this will create a new problem: It will cut off the furnace's air supply, causing poor combustion which will generate lots of poisonous carbon monoxide, and it will prevent the chimney from efficiently venting the deadly gas. Some older forced-air heating systems don't even have a main return for the circulated air, relying instead on leakage under the basement door to get the circulating air back to the furnace. In either case, it will be necessary for you to open some sort of large air path between the top of the baffle and the floor above it.
This could be as simple as putting a large (maybe 6" by 20") rectangular hole in the ceiling above the furnace enclosure and covering it with a grille---if you don't mind letting all that blower noise into the floor above. If you do, find a spatially central place on the upper floor, some distance from the furnace, install the return grille there, and run ductwork back to the furnace enclosure.