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From my amateurish observations, there seems to be a duality
of thought about the correct coupling of cone drivers in a line
array configuration. One school of thought seems to hold that
proper coupling occurs below C/Dia, where C=speed of sound
in inches and Dia=driver diameter. The other school holds that
coupling occurs below 1/2 the result of the above formula (both allowing for crossover rolloff). For example, two 12" cones
would be coupled around 1kHz in the first thoery, and around 500Hz in the second. I have heard arrays modeled on the C/Dia
method and didn't notice anything unreasonably uneven about
the sound quality. So, what constitutes correct coupling? At what points does theory become too compromised or too restrictive for practical application? Any takers for this one?
-Cuppa Joe
Follow Ups:
HiIf has a program which predicts the dispersion pattern then one can see that ideal coupling happens when the center to center distance from two point sources are less than 1/4 wl apart. Mutual coupling of woofer is limited to this region for example.
At 1/3 wl CC one see's a forward lobe, at 1/2 wl one has a figure eight pattern. Greater separations result in spider like polar measurements and as the wavelength spacing increases, the number of legs increase.Fwiw, this 1/4 wl CC requirement is half that cited by one of the big manufactures of concert line arrays, good thing they didn't model or measure the assertion.
CheersTom Danley
Perfect coupling tales place when either of two circumstances are present: the driver center to center is no more than 1 wavelength at the highest radiated frequency or the radiating area comprises at least 80% of the vertical plane. But practically speaking a wavelength to wavelength between radiating planes is usually adequate; integration may not take place until a wavelength or so from the radiating plane, but unless you're listening to it that close it matters little. Even when the radiating plane to radiating plane distance is over a wavelength integration takes place at some point.Because of driver frame widths perfect integration at 10 kHz, for example, is difficult with a 1.3 inch wavelength, but even if integration takes place 10 wavelengths from the radiating plane that's still only 13 inches.
Thanks, guys! The conclusions Bill cites are from an AES paper
by Christian Heil concerning the V-DOSC line array, if I'm not
mistaken. However, John Murphy of True Audio (creator of the
MacSpeakerz and WinSpeakerz programs) claims that true
coupling of two like drivers only occurs at 1/2 the wavelength.
Not being so well educated in engineering disciplines as I
should be for such a discussion, I sometimes allow opinion to sway my decisions. So, I guess that the real question is: What
occurs between the 1/2 WL and the full WL that divides the two
schools of thought? Both of you are welcome to take another
crack at it! -Cuppa Joe
"true coupling" takes place at the radiating plane with no null points between the sources. It's a nice thing to have at low frequencies, difficult to achieve at high frequencies,and all in all not worth losing sleep over in real world applications above 5kHz or so.
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