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When analyzing a critical
listening room, it is important to examine the ratios of the room's dimensions
in addition to the room's reverberation time and any potential problematic reflection
(such as from concave, reflective surfaces or peaked/domed ceilings). Certain
room dimension ratios are better suited than others for achieving a rich acoustic
The following Room Mode Ratio Diagram is an acoustical tool
developed by Richard H. Bolt. The diagram displays the ratios that are known (based
on empirical and mathematical tests) to enhance room acoustics. If a particular
room's dimensions are outside of the preferred ratio range, acoustical treatments
(such as absorption) that mitigate the negative effects of less-than-ideal proportioning
will become more critical.
The goal is to find ratios that allow even distribution of sound across the frequency
spectrum. When this happens, tones are reinforced equally and the least amount
of coloration occurs. Colorations largely determine the quality of sound in a
smaller room (such as a studio or home theater). These colorations, caused by
room resonances (modes), can degrade the quality of music and speech, creating
an unnatural and monotonous emphasis at certain frequencies.
are caused by reflections between room surfaces. There are three types of modes
in a rectangular room: axial (sound waves reflecting between two parallel surfaces),
tangential (sound waves reflecting between four surfaces), and oblique (sound
waves reflecting between all six surfaces). Axial modes have the most influence
on the acoustical characteristics of the room. Oblique modes have less effect
than the other two.
In a studio or home theater, a mode is heard as a coloration of the desired sound
by emphasizing the modal frequency. If a certain modal frequency is isolated from
its neighbors (more than 20 Hertz separation), its effect is more likely to be
noticeable. If modes are widely separated, then the sound will be abnormally weak
at the notes between the modal frequencies. At frequencies above 250 Hertz individual
modes are seldom distinguishable. Speech coloration below 80 Hertz is rare because
so little speech energy is in that part of the spectrum. If there is no spacing
between modal frequencies (called a coincident) they would tend to overemphasize
the sound at that frequency.
When determining optimal dimensions and
analyzing room modes, it is best to seek the advice of a professional acoustical
here to find an acoustical consultant in your area.
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