Ground Plane BBT Array - Thinking aloud - diyAudio
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Old 30th October 2010, 07:10 AM   #1
jamikl is offline jamikl  Australia
diyAudio Member
Join Date: May 2005
Location: Melbourne
Default Ground Plane BBT Array - Thinking aloud

I have been reading some of the literature available on these
arrays, especially Don Keele Jr's papers.

What I am wondering is do you really need all those little tweeters
alongside the mid - upper base speakers. If using the ground plane
of appropriate height for your listening room and seating area could
the highs be handled by a distance aligned CD waveguide mounted
at an appropriate height able the ground plane unit or even suspended
from the wall or ceiling and angled to cover the listening area the waveguide
should be above the upper angle of the main speaker. Would this only
work, if at all, in one specific seating position or could it give broader coverage to the room.

I also wondered about suspending a full curved CBT, with the highs only,
above the ground plane CBT. Acting much like the waveguide. I don't mind cutting all the holes necessary to cover up to 1500HZ - 3KHz ( 6" or 3-4"
speakers) but not all those tiny tweeter holes.

Feel free to comment either way.
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Old 30th October 2010, 07:31 AM   #2
diyAudio Member
Join Date: Mar 2008
I think your idea is valid. Even if the match in
vertical coverage might not be perfect at crossover,
also the horizontal plane is the one to optimize for.

With a well designed array - possibly using a WG for the
highs - you will have a more uniform result in the whole
room than with a conventional 2-way.

Even a reduced length tweeter array - positioned at
appropriate height - is a way to go.

With the WG i see some problems, not only with radiation
pattern matching but also with group delay, if you want the
WG mouth and midrangers to radiate from the same plane.

Kind Regards
Oliver, RFZ believer (?)

Last edited by LineArray; 30th October 2010 at 07:35 AM.
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Old 30th October 2010, 08:36 AM   #3
jamikl is offline jamikl  Australia
diyAudio Member
Join Date: May 2005
Location: Melbourne
Thanks for the input LineArray. I've realised since posting that the tweeters are so small the mounting holes can probably be made with a spade drill.
I still like the idea of the horn or separate panel for the highs as it just
does not look right unless the units are all vertically mounted IE above each other - mid - tweeter. There may be no good reason for me feeling this way I suppose. Certainly if Mr. Keele does it who am I to feel it is not right!!!
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Old 30th October 2010, 09:21 AM   #4
diyAudio Member
Join Date: Mar 2008
HF waveguide ontop of the bass-midrange line array,
or reduced length tweeter array ontop is not a good idea.

But two close lines in parallel having the tweeter's
line array shorter and at appropriate listening height is
an approach that is frequently chosen.

Of course with "constant vertical beamwith" taken
seriously, lines of different length may not 100% conform.

In my opinion conformity to the constant beamwith
approach even for large vertical angles has it's main
purpose for public adress ...

In a living room i would assume slightly different
criteria due to dispersion.

Kind Regards
Oliver, RFZ believer (?)
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Old 1st November 2010, 05:55 AM   #5
jamikl is offline jamikl  Australia
diyAudio Member
Join Date: May 2005
Location: Melbourne
Default CBT + Waveguide

Thanks for your thoughts LineArray I have used the spreadsheets for
line arrays that have been around for few years now. I don't know
enough of the maths to do my own. I modeled 20 x 3" cone dia.
drivers with a gap between them to allow for surround/frames. I
also modeled 20 X 0.5", 0.75" and 1" tweeters. All models were
done with power tapering over the last 3-4 speakers each side of centre.
The 3" looked good up to 2800 - 3000Hz. All the tweeters looked like a
pencil point. Reasonably broad at low end but coming almost to a sharp
point at 20KHZ. I can't hear this anyway as I am 64.

It looked from that as though a 60/40 guide would be a better match if it
could be fitted in. I know that the tweeter array I modeled was quite
short but it did not look good. The spreadsheet models with the centre of
the array at listening height
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