The best listening room I've had the pleasure of being in was a custom designed room using the Cardas Golden Ratio for its dimensions, as Davetherave describes. The dimensions of this room were: 16' wide x 26' deep x 10' high.
If I were designing a room from scratch with no limitations, these are the dimensions I'd start with. At the same time, I've been in many listening rooms that had excellent sonics but were far removed from any of the various "ideal" dimensions. The main ingredient to success always seems to be in preventing the dimensions from being multiples of each other as much as possible, even if this means making the room a bit smaller in one dimension or another. It also helps to get as much depth as possible to support the bass. My current room is 14.75 x 26 x 8 and I'm delighted with the sound of this room. Also, be aware that it is possible to make the walls too rigid. It can be surprising difficult to tame excessive bass nodes in a room, particularly in a basement with concrete floor and exceptionally rigid walls. Often it's better to let those walls flex to naturally dissipate the bass from the room.
As I recall, the additional build specifications for the first room above included: floated concrete slab floor, interior walls isolated from exterior walls (shell construction), doubled interior sheetrock walls on z-bar, dedicated HVAC designed for low air flow and exceptionally low noise, electrical system separate from the rest of the house with each outlet on a dedicated isolated-ground line. The interior used very minimal acoustic treatment, relying mainly on half-height record shelving along the side walls, wool carpets and furniture.
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If I were designing a room from scratch with no limitations, these are the dimensions I'd start with. At the same time, I've been in many listening rooms that had excellent sonics but were far removed from any of the various "ideal" dimensions. The main ingredient to success always seems to be in preventing the dimensions from being multiples of each other as much as possible, even if this means making the room a bit smaller in one dimension or another. It also helps to get as much depth as possible to support the bass. My current room is 14.75 x 26 x 8 and I'm delighted with the sound of this room. Also, be aware that it is possible to make the walls too rigid. It can be surprising difficult to tame excessive bass nodes in a room, particularly in a basement with concrete floor and exceptionally rigid walls. Often it's better to let those walls flex to naturally dissipate the bass from the room.
As I recall, the additional build specifications for the first room above included: floated concrete slab floor, interior walls isolated from exterior walls (shell construction), doubled interior sheetrock walls on z-bar, dedicated HVAC designed for low air flow and exceptionally low noise, electrical system separate from the rest of the house with each outlet on a dedicated isolated-ground line. The interior used very minimal acoustic treatment, relying mainly on half-height record shelving along the side walls, wool carpets and furniture.
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