SMALL ROOM LOW-FREQUENCY ACOUSTICS
As a result of its size and geometry, a room excessively amplifies sound at certain frequencies. This is the result of standing waves (acoustic resonances/modes) of the room. These are waves whose original oscillation is continuously reinforced by their own reflections. Rooms have many resonances, but only the low-frequency ones are discrete, distinct, unaffected by the sound absorbing the material in the room, and accommodate most of the acoustic energy build up in the room.
In a typical room similar in size to a home-theater, a listening room, an office or an owner’s suite in a yacht the resonant frequencies of these standing waves fall in the bass frequency region. Provoking these low-frequency resonances to result in boomy noise in a room.
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Mode Shapes and Resonant Frequencies
At certain frequencies the dimensions of a room are integer multiples of the wavelength of the tones corresponding to those frequencies. This causes the reflection of the wave from the walls reinforce each other and establish standing waves in the room. These frequencies are called resonant frequencies (known also as characteristic frequencies) of the room and their corresponding standing wave pattern are called mode shapes of the room. The resonance frequencies and the corresponding mode shapes depend primarily on the shape and size of the room. The images on the right depict 4 of such standing waves in a rectangular room with a closet at one corner.
Figure 1 depicts the sound transmission, over the frequency range of 20 to 80 Hz, from a source located in one corner of a 7 by 5 by 3 meter room to a microphone located at the opposite corner. The marked influence of the room on the low-frequency sound transmission, in terms of favoring some frequencies (resonances) over the others, is quite apparent. Moreover, the first few (specially the first) room resonances are more discrete, distinguishable, and contain most of the bass acoustic energy.