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Old 28th May 2004, 06:25 PM   #1
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Default Multiple drivers - space them apart by how much?

Hi.. Sorry if this is a common question (or a daft one), I had no joy from searching on the subject (although the flameproof jacket is already on just in case!)

I see a lot of designs for speakers with multiple drivers, and discussion over box dimensions and types, but I've not come across much information about how far apart the driver units should be placed (other than what is prescribed in someone's drawings).. I've noticed, however, that this is roughly (or maybe exactly??) as far apart as multiples of the radius of the drive unit.. so if I had a 6" mid driver the distance from the centre of the tweeter to the centre of the mid is 6", or 9".. or similar..

Does this make sense, and is there more to it? (like positive/negative reinforcement of the wavefronts) Does this apply to MTM configs? ..to Port placement?
Any general rules....

...or am I barking up the wrong floorstander...

Thanks in advance..
G
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Old 28th May 2004, 06:28 PM   #2
sreten is offline sreten  United Kingdom
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Placement of drive units is generally as close as is practical.

Unless your going for looks of course .....

sreten.
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Old 28th May 2004, 06:34 PM   #3
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I see... so that'll be why I can't find any technical information or formulae on the subject..

Thanks
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Old 28th May 2004, 10:08 PM   #4
niacin is offline niacin  United States
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Default It's the wavelength at crossover

I've always heard that multiple drivers (especially line arrays) need a center-to-center spacing of 1 wavelength of the crossover frequency or less.

So if you have multiple 6' woofers spaced edge-to edge, you would need to cross over to the tweeter at 2k or less. (roughly).

If you take the crossover frequency too high, say 3.5K, the woofers will begin to interfere with each other and you will get increasingly complex patterns of "lobing" with little thin jets of sound shooting out at certain angles, while other angles have basically silence coming out.

Of course in the far field, in a reverberant indoor room, it could be argued that a lot of the lobing smears together and balances out, so I think you could "cheat" these figures a bit.
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Old 29th May 2004, 12:10 AM   #5
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I'm on my 6th line array project and I can tell you that the effects of lobing and comb filtering are vastly over stated. Scientifically they are there. Maybe using OB's with swept back wings makes a difference, but I've yet to attentuate the top end of the wide range mids that I use which go up to about 10Khz. The extreme top end may be attentuated somewhat by the effects of the array, but I add a super tweeter to fill in the top end. I use 4" drivers and on the 3 big arrays I've made, I used 4.25", 4.5", and 5" center spacing with only a slight difference in off axis response. From now on, however, I will put the drivers as close together as practical, so I can fit as many as possible in each array, since they're only $1.50 ea.

To me the very slight reduction in image focus of an array is more than offset by the "big" sound they create. Since you ask each driver to do very little, you can use very economical drivers and have super efficient speakers with great sound at a fraction of the cost a point source speaker with quality drivers.
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Old 29th May 2004, 12:16 AM   #6
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In David Weems' "Great Sound" book he describes discovering during the build of an MTM that the FR was better with the lower woofer farther away from the tweeter than the top one. I'm sure that was just for this particular design but it points out to me that it depends on the design and the drivers being used.
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Old 29th May 2004, 03:11 PM   #7
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Drivers should be spaced on center no more than one wavelength apart at the highest frequency being reproduced, but there are exceptions. The main deviation would be for an MTM configuration; in that case the outboard elements should be spaced no more than one wavelength on center at the crossover frequencywith respect to the axis of the center driver.

As to the line-array question, this still holds true. Spacing further than 1 wavelength doesn't particularly affect horizontal dispersion and comb-filtering concerns, that being addressed by the vertical driver placement, but it does affect the integration of the various point sources into a coherent line source. That doesn't properly happen with spacing over a wavelength, which means that the nearfield/farfield radiation pattern will be fouled up.
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Old 29th May 2004, 06:51 PM   #8
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OK.. so in a practical scenario, would you place the drive units close as possible and then tune the crossover accordingly?

I'm thinking of a set of folded transmission line speakers similar to the Capellmeister as a project, and was planning on an MMT configuration (i.e. inverted - with the tweeter below 2 mids), so there's more 'line' behind the mids for them to 'transmit' down.. Probably a daft idea but I can always remove the baffle and try something else I spose!
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Old 29th May 2004, 08:31 PM   #9
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Im not sure i understand how the wavelength thing works. Is there a set formula for driver spacing?
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Old 29th May 2004, 08:49 PM   #10
sreten is offline sreten  United Kingdom
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You are being confused by irrelevancies.

Given a minimum driver spacing this sets limits on c/o frequency.

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