● Room Dimensions
When you listen to music through a high-end audio system (whether it's just for music or as part of a home theater), you'd like the loudness of each note to be just as the musician played. For example, if an acoustic bass player plays a enticing riff that descends in pitch, and if the musician plays each note equally loudly, you'd like to each of those notes to be properly reproduced at the same volume. In this way, the musical dynamics of the performance are properly reproduced.
Now, imagine that you have carefully selected the components of your audio system. You've paid attention to choosing "neutral" components that properly reproduce the volume of notes at each pitch, just as the musician played them. You still may not hear in your home the notes properly reproduced at the same volume. And that's because your room will make certain pitches louder or softer than normal.
Rooms resonate just like a pipe organ (or a partially filled soda pop bottle when you blow across the neck). The basic room resonances are determined by the length, width, and height of the room, just the like pitch of the pipe in the organ is determined by the diameter and how long it is. (There are other, more complex resonances, but we have found that it is usually most important to focus on the most basic ("axial") resonances because they are the highest amplitude ones.)
Each dimension of the room determines a set of frequencies (i.e. pitches) at which the room will resonate: the fundamental frequency, and all of its harmonics. For example, a room that resonates at 50 Hz, will also resonate at 100Hz, 200Hz, and so forth. Because there are three dimensions in a rectangular room with a flat ceiling, there would be three sets of resonant frequencies.
When a note is played at or near one of the resonant frequencies for the room, the note will be heard louder or softer than normal, depending upon where in the room the sound is made (i.e. where the speaker is) and where the sound is detected (i.e. where your head is).
Our goal in choosing the room resonances is to cause all of the resonant frequencies in the three sets to be evenly distributed in the bass region. In particular, we want to avoid a lot of overlapping frequencies or a large range of frequencies where there are no resonances. We do this by choosing the room dimensions (length, width, and height), ideally without constraint.
At the same time that we calculate the room dimensions, we determine the recommended optimal speaker locations and listening (and perhaps viewing) locations. We choose these locations by considering a wide variety of factors including: number of occupants, overall room size, bass resonances, acoustic reflection points, and comb-filtering effects.
When all three dimensions are flexible, we can choose one dimension based on considerations other than the room resonances. We can then consider room resonances in choosing the other two. For example, we can choose the first dimension based upon desired listening position or based upon the desired overall room size or furniture positions. (This is important for home theaters, because we would choose the first dimension by considering view distance and possibly sight lines if there will be multi-row seating.)
Sometimes one or even two room dimensions are fixed by non-acoustic considerations. Maybe the ceiling height can't be changed easily. Or bearing walls can't be moved without a major expense. In this case, we can still improve the acoustics of the space to the degree that we still have flexibility.
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