In Search Of The Audio Abode---The Hi-fi House Page 3
Dividing 20Hz into 550 will show that opposite walls must be 27.5' apart to fully support that frequency. Dividing 12.8 into 550 will show that the longest dimension of our "nice, compact little listening room" will support 43Hz---okay for your average British speaker system, but a shameful waste for Infinity bass towers.
Suppose you want to go for a conservative 35Hz low-end limit? 550 divided by 35 is 15.7'---not at all unusual for a room in an average house. But...this is almost twice the height of the average ceiling, and that's bad news. The room will have the same standing waves in two directions. But all is not lost, because those ideal dimensional ratios are just that: ideal. Any one of them can be halved or doubled without changing their 1/3-octave relationship, and without seriously impairing the smoothness of the room's bass response.
Doubling the length dimension, to 25.6', would extend the room's response to an impressive 21.5Hz, but would make for a very narrow room, more like a wide corridor than a room. But doubling the 10' dimension would provide a more reasonable shape (20' by 12.8') and still give flat response down to 27Hz. Of course, you can always go for broke, doubling both the original dimensions to give a 20' by 26' space with a 21.5Hz lower limit. But you aren't likely to find a room like that in a 3-bedroom ranch home. On the other hand, you may get lucky.
Generally, flat ceilings of 9' or more will be found only in residences more than 50 years old or in very expensive newer homes, and the extra foot or so will make relatively little difference in a properly proportioned room's low-end limit anyway. But so-called open or vaulted ceilings, which are enjoying a vogue these days in "contemporary" homes, are a special case calling for special attention. Because they are of indefinite height and their undersides are not parallel to the floor, they generate less-pronounced standing waves than a flat ceiling. Their worst liability is that they tend to produce lots of quick-return echoes that smear detail. These are easily suppressed with strategically placed panels of decorative-surface fiberglass on the underside of the roof. Otherwise, a vaulted ceiling can be considered as a flat ceiling whose height is the average of the ceiling height. That is, if the walls are 8' high and the peak of the ceiling is 14', it can be viewed as an 11-footer for purposes of calculating the other dimensions.
If you care about such niceties as imaging and soundstaging, your loudspeakers must "see" a symmetrical room, and your listening seat must be symmetrical to your loudspeakers. And symmetry here refers to acoustical reflectivity as well as to dimensioning. The room cannot have a quaint little alcove to the left of where you plan to put the speakers, or picture windows on one side with bookshelves along the other. House buyers tend to be attracted by such things for visually aesthetic reasons, but they are the major cause of image biasing---a tendency for all program material to "pull" toward the left or right when the balance control is centered. The control can center the image, but it will need readjusting for different programs, and the system will never deliver imaging specificity that is any better than vague. (A mono signal source is an ideal test for imaging specificity. The "image" should be extremely narrow, and remain motionless at all times.)
With a two-channel system, acoustical symmetry is of much less importance behind the listening seat, because the differing patterns of reflections will be beyond your angular range of directional acuity. You can have a bare wall at the left rear and floor-to-ceiling drapes at right rear without undue effect on imaging or balance, but it's still best to try and avoid large surface areas of right/left asymmetry. And it is necessary to avoid these if your system is outfitted for surround sound.
All accesses to the room should be on midwalls rather than near the room corners. Most bass-energy buildup due to standing waves concentrates in the corners, and substantial leakage from any one will cause loss of bass. And if you plan to use Tube Traps for controlling standing waves (which I most highly recommend), you'll need unbroken corners for these, because that's where they work most efficiently. A midwall access need not be closeable if it is small relative to the length of its wall, but if it is wider than one third of its total wall length, it should have heavy sliding or folding doors, and these should be kept closed for serious listening.
If there is a fireplace, it should be in the middle of its wall, so that if that wall turns out to be the best one to have the speakers backing against, the fireplace can be between them. (Watch the distance, though; too close and you'll fry the backs of the enclosures or, worse, set the rear fabric grille on fire.) You can expect to get a mild cavity resonance from the fireplace opening, which can be suppressed with large blocks of fiberglass when the fireplace isn't in use. (Fireplace-stuffing blocks can be made from several layers of 4" fiberglass with fabric covers, like large sofa cushions. An earth-tone fabric will work better, design-wise, than a Paisley.) When the fireplace is in use, you must learn to be tolerant of imperfection.
Window asymmetry will be a problem if the room isn't a wing, and there may be conflicts of exterior view vs interior light vs acoustical symmetry. If the windows are in the wrong places, they should be as small as possible. Picture windows and floor-to-ceiling sliding doors may look lovely, but they're as reflective as anything you can get, and at least some of them will probably need to be covered with heavy pull drapes while you're listening. So much for that fabulous view of the mountains! Another alternative is free-standing fiberglass panels, which can be kept in a closet when not in use. It has very low WAF (Wife-Acceptance Factor). Maybe you can persuade her to listen with her eyes shut.
L-shaped rooms make almost classically bad listening spaces, because they add another set of standing waves to the equation, and produce a very asymmetrical soundfield. Don't dismiss them on sight, though, because if their dimensions allow it, you can always partition-off one leg of the L.
Although there would seem to be compelling reasons for listening in a room having no parallel surfaces, the fact is that they just don't work very well. Acoustical spaces with converging walls, a sloping ceiling, and perhaps a curved rear wall are ideal as performing halls, because they tend to have fewer "dead spots" than rectangular halls. Many well-heeled audiophiles, bent on building the perfect listening room, have made the mistake of assuming that what works in a 60,000-cubic-feet space will work in a 600-cubic-feet one. It won't. An auditorium is so much larger in every dimension than the lowest audio frequency that standing waves are virtually out of the question. And the multiple reflections are so randomized by the time they reach the listening seats that they have no effect on the perceived frequency response.