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In this article, we’re going to talk about the ideal room sizes and dimensions. This is such an important issue because nearly everybody gets it wrong, and if you get it wrong in the beginning, getting good sound in that room is always going to be a struggle.
How do we determine what size room to build? Using a room that’s available is not necessarily the right answer. First, we have to determine usage. We have to decide what we are going to do musically in this room. Is it two-channel playback? Is it home theater? Is it a control room? Is it a listening room? Because each usage dictates energy requirements and how large a room it should be. Each usage dictates the treatment that we’re going to put inside the room. So the first thing we have to define is the usage.
Speaker size to room size
Let’s take the example of room usage. Let’s do two-channel playback because that’s a popular one. Okay, so what are we going to do for room size and volume with two-channel playback? First question we need to answer is how much energy are we going to put in the room? How do we determine that? How big are our speakers? We need to match the room size that we have available, that we’re considering, to the speaker size.
Are we going to put a 7-foot-tall speaker in an 8-foot tall room? No, we want to put a smaller speaker in an 8-foot-tall room because 8 feet is a really bad room dimension for acoustics but it’s the standard distance that we have here in North America. We have to be very, very careful about matching the amount of energy placed in the room to the room size and volume. Are we going to put a speaker with a 12-inch low-frequency driver in a room of 1,500 cubic feet? We do if we want to have so much low frequency energy that it will drive us out of the room. If we figure out what those ratios are, what the speaker size and the room volume and size need to be, then we can figure out the actual room height, width and length we need.
Let’s take a standard size speaker that’s 4 feet tall. It’s a good size speaker to work with — and quite honestly, you really don’t need anything larger in any size room but that’s a discussion for another time. The bottom line here is once we have matched the speaker to the room size and volume, then we can ask how much distance we need for the energy the speaker produces inside the room so we have fewer acoustical issues.
Sound pressure management and reflections
The way we reduce the number of acoustical issues is by managing sound pressure and reflections. We need space to do that.
Each speaker radiates energy into space. Each speaker, depending on the speakers’ design, has radiation requirements. When I tell people that the width of the room is too small for their speakers, it’s because I know the narrow room width is going to combine with the radiation pattern of the speaker to create reflection issues. We need to define distance in terms of what the speakers’ requirements are, not what you think looks and works best. This is the speaker’s room, not yours!
It takes time to figure all of these things out. Change one thing and you have to change all the others. Once we have our speaker chosen, then we need to pick out the room size that will work for speaker height, radiation and diameter of all low-frequency drivers. It is a marriage where no partner can be allowed to dominate.
Room height is critical also
Room height is critical also because floor and ceiling reflections are the first ones we hear. People don’t understand this. They think it’s the sidewalls. No. You’re sitting on a floor. You can’t get much closer to the surface than sitting on it. You are sitting directly under that 8-foot ceiling. Look up, it’s right above you! So the reflections from the floor and ceiling must be managed.
Look to the ceiling
The ceiling is the next reflection you hear after the floor. So, we need to work with that distance next and that is also determined by speaker height and size of the room.
Room length is very, very critical for pressure. Not so much for reflections but for pressure, so we need the right length. Why do we need the length? Because most tower speakers are capable of producing bass frequencies down to at least 30 Hz. A 30-Hz sound wave is almost 37 feet long. In order for that energy to comfortably fit in a room, we have to have a room length that is some multiple of that lowest wavelength that the speaker produces. A 40-foot-long room and a 30-Hz wave works perfectly.
Look at the lowest wave like that the speaker produces, we need to have distance to allow for that wave to fit into the room dimensions. The smaller the room, the larger the low frequency pressure issue is. The larger the room, the more the issue shifts to reverberation time or reflections. We need to manage both of those issues and choose length, width, and height to first match our speaker and to deal with the pressure issues that the speaker produces.
Those are all the criteria that we need to use to choose a room size. Once we have our gear figured out that we like and want to use, then we go to work figuring out the room size and volume that the speaker requires, not what we feel is best.
What are the correct room sizes?
There are none. Everything is dependent on the amount of energy you’re putting into the room. That said, there are ideal room size starting points. Well, there are if you match the pressure that the speakers are producing to that room size. There is no one-size-fits-all. You have to look at the gear, you have to look at the energy produced by the gear and you have to fit it in the right box which is your room.
Good starting points we like to recommend when listening room dimensions can be chosen or adjusted are 17-foot width, 10-foot height and 23-foot length. Why did we pick those ratios? Because they minimize low-frequency pressure issues that need to be treated, and they also minimize reverberation time issues, which are reflection-based. Is this the correct room size for all situations? Absolutely not. It would not be a good home theater room, but it would be a good studio control room. What’s the difference between home theaters and studio control rooms? There’s more energy in home theater rooms, from more speakers, so the space requirements increase.
To summarize: You must match the size and energy-producing capability of your speakers to the room size and volume, then you must minimize the low-frequency pressure issues in the room, and then you must manage the sound reflections. All of this depends on gear, room size and room volume. There are no easy answers, but there are many options and possibilities.