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In Reply to: RE: Basement 2 Channel Room Build Out Dimensions - Help posted by xodeuce on June 11, 2009 at 11:11:55
All formulas for room mode calculation are based on the assumption that the room is empty (no furniture), has no wall openings (doors, windows), and has perfectly rigid and reflective walls.
Large absorbing furniture is capable of shifting mode frequencies and lower mode levels. Large reflective furniture is capable of splitting up modes, hence generating two modes instead of one. Wall openings are structural weaknesses and locations of pressure maxima and minima are shifted.
When using room dimension optimizers one should keep in mind that such optimizers try to place ALL modes that are possible in a room on the frequency scale according to certain criteria. The only location where all of the possible modes can be excited is a corner. The only location where all of the modes excited in that manner can be perceived is a corner. Hardly a practical setup. Everywhere else you will excite only a fraction of the possible modes and of those you will perceive only a fraction. The chances that those remaining modes are spaced according to the optimizing criterion is close to zero.
What you can do is to optimize room dimensions for pre-determined positions of loudspeakers and listening chair.
In rooms with non-parallel walls axial and tangential modes are not excited, only the oblique ones (which involve all six room boundaries). You still get strong pressure maxima and minima, but rather randomly distributed across the room.
The only way to know beforehand how the mode distribution in a given room looks like is finite element analysis.
De Melo et al.(2007), “Sound absorption at low frequencies: room contents as obstacles”, J. of Building Acoustics, vol. 14, no. 2, p.143
Bork (2005), „Modal analysis of standing waves (in German)“, Fortschritte der Akustik, DAGA ’05, 31. Jahrestagung für Akustik (Deutsche Gesellschaft für Akustik), München 2005
Welti (2006), “Low-frequency optimization using multiple subwoofers”, J. of Audio Eng. Soc., p.347
Toole (2006), “Loudspeakers and rooms for sound reproduction – a scientific review”, J. of the Audio Engineering Society, p.451
Bolt (1939), “Normal modes of vibration in room acoustics: experimental investigations in nonrectangular enclosures”, J. of Acoust. Soc. of America, vol. 11, p.184
Van Nieuwland (1979), “Eigenmodes in non-rectangular reverberation rooms”, Noise control engineering, Nov., p.112
Klaus
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