| Re: Tuning below Fs with JBL 2226H? [message #75318 is a reply to message #75317] |
Tue, 22 January 2013 10:21   |
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Wayne Parham
Messages: 18686
Registered: January 2001
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Illuminati (33rd Degree) |
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Lobing from acoustically distance sources is undesirable, that is true. It is to be avoided where possible, and outdoors or in very large areas, you want point source summing, which is accomplished by keeping source locations within 1/4λ, just as you've said.
But this is impossible to achieve indoors. Boundaries are further than 1/4λ away, so point source summing loses meaning. The reflections are out of phase with the direct sound. Even a single source gets interference from its reflections, which are nearly as loud as the direct sound. The result is deep notches at some points in the room, with hot spots in between them.
Sound energy distribution is like a checkerboard of hot and dead spots, and the pattern is frequency specific. At low frequencies, in what is called the modal region, there are large, widely spaced nodes. As frequency goes up, the number of nodes increases and distances between them become smaller. Eventually - at the Schroeder frequency, approximately 200Hz or so - the nodes become so closely spaced they cannot be distinctly identified. This is where the energy distribution becomes more of a statistically averaged sound field.
Room modes are the reason to use a multisub configuration. It is done to smooth the sound field at low frequencies by making sound in the modal region act more like the statistical region. For more information, see the link below:
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