If you’ve ever heard a rattling noise when a passenger jet starts its engines, you’ve heard resonances. You may have also noticed that certain frequencies seem louder than others. The sound of a jet engine starting up, where it goes from very low frequencies to midrange frequencies with engines idling, is very much like a sweep tone, used frequently in both electrical and acoustic audio testing. You may have noticed this on your home theater speaker system, where sounds at certain frequencies cause something in the room to rattle. You may have also noticed that certain low frequencies seem louder than others. This is what are called room modes, a room’s resonant frequencies.
A room’s resonant frequencies vary with the shape and dimensions of the room. THX has a spreadsheet for installers that calculates where those resonances are going to be, and several programs are freely available to make these calculations. You can find out the resonant frequencies of your room using a DOS-based app (that works fine under Windows XP) like ModeCalc. Tony Grimani took the class through an exercise where we calculated the room modes (resonant frequencies) for first through fourth-order harmonics for each of the three dimensions of the room. We put these in tabular format, and were looking for groupings of frequencies that were very close to one another. Groupings can cause an amplification effect at a nearby frequency if two or more harmonics are close to one another. To avoid this problem, Grimani recommends a room where not one dimension is evenly divisible by another. For example, an 8x16x20 (1:2:2.5) room would have this issue, whereas a room with dimensions of 9x16x30 (1:1.78:3.33) would have fewer problems in this area. Several acoustics sites on the web point to these aspect ratios (h:w:l) as being better dimensions. Here are a few:
Every room has room modes. They’re unavoidable. There are no perfect dimensions for a room, only better and worse choices. This isn’t to say that a room with dimensions of 8x16x30 cannot be made to sound pretty good; it’s just that the job will be made more difficult by standing waves. The resonant frequencies will become important later on when the designer is trying to get the room’s equalization (EQ) as flat as possible. Continued…