https://www.theabsolutesound.com/articles/building-a-listening-room
EXCERPT:
"Planning the Room: Size and Dimensional Ratio
The first step in any room design is choosing the room’s size and dimensional ratios—the ratio of the length to the width to the height. This ratio has a very large effect on the room’s sound, particularly in the bass. Good room ratios spread out the room’s resonant modes more evenly, resulting in smoother and more linear bottom octaves. Explaining the importance of room ratios is beyond the scope of this article (The Complete Guide to High-End Audio’s chapter on room acoustics breaks it down), but know that the length-to-width-to-height ratio is the crucial starting point.
Fortunately, acoustic-design consultant John Brant has created an excellent tool on his website (jhbrandt.net) for evaluating room dimensions. The free downloadable spreadsheet performs a detailed acoustical analysis on any set of ratios that you enter. The tool plots all the resonance modes graphically, shows you if the resonance distribution meets the “Bonello Criterion,” and suggests ideal ratios, among other analyses. In addition to the dimensional-ratio spreadsheet, the site includes many other valuable resources for room design. Art Noxon, founder of Acoustic Sciences Corporation, inventor of the famous Tube Trap, and consultant on my room explains room ratios in the sidebar accompanying this article.
In practice, the listening room’s dimensions are also influenced by real-world considerations such as the amount of real estate you’re prepared to commit to the room and how the room fits into the floorplan of the rest of the house. It’s easy to forget that the listening room is just one part of a house and must integrate with the rest of that house in many ways. Keep in mind that good room ratios span a spectrum in which you’ll get good sound. Generally, the larger the room the better the sound (assuming good dimensional ratios); a large room spreads out the resonance modes more smoothly than a small room does, resulting in flatter bass. My room is 27′ long and 17′ wide, and the ceiling is 11′ tall. After living with the finished room for about three months at the time of this writing, I’m very happy with the size and feel of the space.
Once you’ve decided on the room’s dimensions, the next consideration is the room’s wall construction. There’s a wide spectrum of framing and construction techniques that improve the room’s sound quality as well keep sound in the listening room from getting into the rest of the house. You must decide how important this sound-proofing is to domestic harmony, and then choose the wall-construction technique that fits your budget and needs. I’ll share with you just a few examples of the vast range of wall constructions. But first you should know that a wall’s “transmission loss” (reduction in sound amplitude from one side of the wall to the other) is specified as an STC (“sound transmission class”) rating. The higher the number the greater the wall’s attenuation of sound. A standard 2×4 wood-framed wall (16″ on-center) filled with insulation and with 1/2″ gypsum board (drywall) on both sides is specified as STC-35—not a very high value. With music playing at a moderate level inside this room, standing just outside the room you would be able to clearly hear and identify the music. The next step up is to use 2×6 plate with 2×4 studs that are staggered. This costs next to nothing, but increases the STC rating. For a nominal additional cost you can use 5/8″ Type X Sheetrock on the wall outside the listening room and gain a few dB of additional transmission loss. Acoustic supply houses sell gypsum board composed of two layers of gypsum separated by a viscoelastic polymer (SoundBreak XP from National Gypsum, for example) that blocks more sound than does conventional drywall. You can also hang vinyl material inside the wall for even greater isolation. Double drywall adds to the transmission loss. I’ve mentioned this small sample of materials and techniques to illustrate that you can dial-in the amount of isolation you need, and balance it against your budget, with great precision. The gypsum board manufacturers (USG and National Gypsum, for examples) publish a wealth of useful information about the various wall-construction techniques and their sound-blocking performances. As Art Noxon explains in the sidebar, however, soundproofing and optimizing audio quality inside the listening room is more complex than simple soundproofing. The wall inside the listening room should be treated very differently from the wall outside the listening room, as we’ll see.
One of the problems of frame construction is that bass energy from inside the listening room puts the wooden-frame-and-drywall structure into motion—a bass impact, for example, makes the wall move. That wall motion, unfortunately, converts the stored mechanical energy in the wall back into sound after the transient is over (Fig. 1). Art Noxon has called this phenomenon “wall shudder.” Wall shudder colors the bass tonally because the walls will vibrate at their natural resonant frequencies, adding energy at that frequency. Moreover, the wall movement is chaotic. It doesn’t take much wall motion to hear tonal coloration because the acoustic output of a vibrating object is a function of the object’s excursion (how far it moves) and its surface area. With a large surface area such as a wall, even a very small excursion can produce an acoustic output.
Wall shudder also distorts music’s dynamics. Some of the transient’s acoustic energy is turned into structural resonance of the wall, diminishing the transient’s attack and thus the sense of suddenness and dynamic life. Then, as the wall releases that energy over time, the transient’s decay is smeared. The result is a distortion of an instrument’s dynamic envelope and thus a diminution of music’s dynamic expression. Moreover, wall shudder masks the delicate spatial cues that our brains need to form the sense of a fully developed soundstage, the space within it, and the impression of bloom and air around instrumental outlines. All these subtle forms of distortion add up and contribute to a hi-fi system sounding like a facsimile rather than like the real thing.
Another way in which listening rooms color the sound is familiar to anyone who has set up a full-range speaker: tubby and lumpy bass. The listening room selectively reinforces some frequencies and cancels others, with the frequencies reinforced and canceled determined by the room’s dimensions and the speakers’ and listener’s positions. This is one reason why some sort of bass trap is essential in every room.
To summarize, the three primary problems inherent in music-listening rooms are: 1) sound leaking from the music room into the rest of the house; 2) wall shudder; and 3) excess bass that requires bass traps in the listening room..."
