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Old 25th September 2005, 02:25 AM   #11
j.burtt is offline j.burtt  United States
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That is exactly what I mean Andy. Is it safe to assume a 14" wide 18" tall baffle would roll off at a lower frequency than a 9" wide 12" tall one ?

I guess this is probably one of those things left to trial and error.
I could always run the X-over a little higher if I needed to.

James
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Old 25th September 2005, 02:54 AM   #12
Andy G is offline Andy G  Australia
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that seems a reasonable assumption to me.

you can add wings or a short section at 90 degrees (good for stability too) and lower the roll-off freq that way as well. Just don't make the wings go back too far or you start creating a hollow.

T and E !! use some cheap mdf for trials, go to nice timber later !
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Old 25th September 2005, 08:49 AM   #13
derf is offline derf  United Kingdom
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At what size does an open baffle become an infinite baffle, is there some kind of consensus on the size?
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Old 25th September 2005, 10:23 AM   #14
poptart is offline poptart  Canada
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IMHO there has to be no rear wave in the listening area at all if it's going to be called "infinite" baffle. If a "really large" baffle qualifies as "infinite" then the term loses any distinct meaning. I can see what you're saying though, at some point there's likely not much audible difference between a giant open baffle and a wall.
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Old 25th September 2005, 11:30 AM   #15
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When does open baffle become big enough to practically be considered infinite baffle?

When the baffle is big enough to have no dipole cancellation within its bandwidth. Lets say you consider the bandwidth going down to 20 Hz.

Using the formula fequal = 0.17 x 344/D
transposed gives

D = (0.17 x 344)/fequal

fequal = 20 Hz
hence

D = 2.9m

The baffle if flat would have to be 5.8m wide, and wouldn't fit in most rooms! If you really wanted it to be infinite baffle, you would have to put it in the wall.

Choose a bandwidth extending down to 40 Hz and then the baffle "only" needs to be 2.9m wide.

Regarding that design, the mid driver is much too small to operate in open baffle mode. It would bottom with a moderate amount of power, and require eq to even go flat down to a point where you would cross to the woofer.

To get fequal at 200 Hz requires a 600mm wide front baffle, or 300mm wide plus 450mm deep wings. Unfortunately the wings don't help the sound, and take away the openess of an OB design. Hence you need a driver that can move more air even for a midrange OB. That tiny driver won't have a chance of keeping up with those midbasses in terms of output.

I'd say a 6.5" driver is a minimum for an OB, and an 8" is probably a better choice. I'd also consider going with OB bass as well.

Something like Behringer Ultradrive is worth considering for a project like this, or perhaps you should look at Linkwitz site as he has a cheapish prototype you can build, even a passive version.

I think for the bass drivers you are looking at not infinite baffle but acoustic suspension. If you have a certain box volume to VAS ratio you may consider a box to be infinite baffle - I think it is about 10.
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Old 25th September 2005, 12:15 PM   #16
derf is offline derf  United Kingdom
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What if there's a door/doors in the wall you're using, when they're open, the back wave can still come round, no?

Do people generally go for walls without doors when creating a infinite baffle?
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Old 25th September 2005, 01:04 PM   #17
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Derf, in that case the path difference (D) is so large that it would not likely have any dipole cancellation within the audible bandwidth. IB is probably most used with subwoofers, where drivers are ceiling mounted, and the volume of the attic space is the "infinite" air volume, or at least large enough to be considered as infinite.
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Old 25th September 2005, 04:17 PM   #18
qi is offline qi
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Quote:
Originally posted by Andy Graddon
just a thought.. (don't take seriously)

how can an infinite baffle have a size and shape ??


I thought this was hilarious
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Old 26th September 2005, 01:09 AM   #19
poptart is offline poptart  Canada
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it's not only the low frequencies that come around the baffle though, pushing the low frequency diploe rolloff down below the audio range is fine, but above that you still have higher frequency back waves bouncing around the room interacting with the front wave. Assuming it's a round baffle (as using a single number for D does) you get a comb filtered high frequency response, and still figure of 8 radiation pattern right?

Infinite baffle using a wall has no out of phase rear radiation interacting with the front wave at all, avoids baffle step compensation and box resonances, but has it's own problems I imagine. The wall is a pressure peak for some room modes and a presure node for others, so you'd get an all or nothing excitation of them I guess.
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Old 26th September 2005, 02:32 AM   #20
AJinFLA is offline AJinFLA  United States
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From Vifa's (DST) site:

Conditions for measurements


--------------------------------------------------------------------------------

This is a short description of the measuring methods used at vifa/scan-speak.

FREQUENCY RESPONSE
The frequency response curves are measured in one of vifa/scan-speaks anechoic chambers.
The chambers are 6x7x8 meters which gives a lower cut-off frequency at 100 Hz.
The curves are measured in 0, 30 and 60 with microphones in a distance of 1 meter. The input voltage is 2.83 V.
One of the walls in the anechoic chamber is without damping material and acts like an infinite baffle (2-pi room) for the driver. Behind the driver is a closed box - typically at 320 litres.



Click the image to open in full size.

cheers,

AJ
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