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How close do woofers have to be placed to each other to achieve acoustic coupling?
In 4350, 4355, and 4435, the woofers are about as close to each other as they can be physically, but in, say, Project May, the distance between the woofers centre-to-centre is about 75 cm.
Put differently, at which frequency does acoustic coupling cease to work as a function of the distance between the woofers?
Ripped from Genelec's site:
Multiple Subwoofers
When two or more subwoofers (except the 7050A as it does not have a sum output) are positioned close to one another mutual coupling is the fortunate by-product. This is due to the long wavelengths, associated with low frequencies, causing constructive superimposition. For mutual coupling, the subwoofers must be placed within 1/2 a wavelength of one another (85Hz upper crossover frequency 1/2 wavelength is approximately 2m). For example, two subwoofers give a 6dB increase in acoustical output at the listening position - see table below.
Ripped from Electro-Voice documentation:
When a common signal is applied, a 6-dB increase in maximum acoustic output occurs. The cones “mutually couple” and act as one cone with twice the area (therefore twice the efficiency) and twice the power capacity. The additional cone area provides 3 dB more output and the additional power capacity accounts for the remaining 3 dB.
Specifically, mutual coupling occurs at frequencies whose wavelengths are longer than one-quarter the center-to-center distance between the cones. The highest frequency at which mutual coupling occurs is calculated from the following equation:
f = 3,000 / DMAX
where DMAX (inches) is the distance between the cones, and f (Hz) is the highest frequency at which coupling occurs. When DMAX is greater than one-quarter wavelength, or at frequencies much above f even when closely spaced, the increase in acoustic output is limited to the 3-dB power-handling increase.
Reference Low Frequency Augmentation - Subwoofers starting on page 26 in the JBL Professional Sound System Design Manual - 1999 Edition (Pt.2):
And associated graphics- Very useful.
http://www.woodartistry.com/linkwitzlab/x-sb80-3wy.htm
Mike.e
That's a strong case to run subs in pairs. Nice little gain 6db. Even at 7 ft spacing with 1/4 using the EV work that's at 35hz and below which is where your really going to need it without going up to high and giving you too much gain.
Rob:)