OB, where to place drivers?

[crazyhub]

Hi all,

I project to build a 3-way OB (planar) approx. 24"W and 62"T; I post in the subwoofer forum because I wish to know where I have to place the eight 5" subwoofers (fs 45hz fx 300hz) I'll use in order to get the max spl in the low range...? i.e. randomly on the full area, or more on a broken vertical line?
I thought something like this:


Does it exist any rule?

Thanks!

[GM]

Assuming an 8 ft (96") ceiling and (8) drivers equates to (9) 10.66" spacings or ~1/4 WL apart at 317.8 Hz, so a 300 Hz XO is theoretically marginal, but being an 'infinite' array where you'll be in the near-field, it shouldn't be an issue.

Considering you're using a single point source mid and tweeter though, clustering the woofers in a circle around them to create a very large virtual point source is what me and some others have done with excellent results.

If for space reasons this isn't viable, then mounting the woofers butted up against each other and mounted on a concave curved baffle with it focused at the listening position is acoustically similar.

Anyway, some options to mull over and if not too much cost/hassle for you, best to try one of each to see what works best in your room as each interacts very differently.

GM

[LineArray]

For maximum LF output of the subwoofers a narrow mounted
cluster near the bottom and a foot/stand for the baffle that
is tight (foot of same width as the baffle) against the bottom
is the best solution.

(Strong) asymetrical mounting is preferable.

You can use the program "edge" from Tolvan data for
rough simulation. The bottom effect can be simulated
by using a mirrored baffle (with same drivers) where
the bottom is the mirror.

Seamless integration of the midranger is a different task ...
The source made up by the woofers should not be too wide
to avoid beaming in the horizontal plane.

My first try would be 2 vertical lines of 4 Woofers each,
which are "crossed" as tight as possible.

O_
_O
O_
_O
O_
_O
O_
_O

Maybe the upper 2 woofers with a larger distance to
get the Midranger in between.

Other possibility is to get the Midranger ontop
of the (double)woofer Line:
This would it make it easier to get tweeter and midrange
close together and the tweeter on the right (ear) height.


WW
.T
.M
WW
WW
WW

.T
.M
WW
WW
WW
WW

Tweeter too high?

.W
.T
WW
WW
WW
WW

A long vertical woofer line array does not fit into the
concept IMO, because your multiway
single midranger/single tweeter
approach goes more towards a point spource.

Kind Regards

[crazyhub]

Thanks guys for your thoughts!

1) I find the "edge" to be difficult to use, sorry...Even Jeff Bagby's Baffle Edge Diffraction Simulator...
2) Regarding that I want to keep a point-source concept, I understand that I have to surround the mid-driver and tweeter, at least to a certain degree. This will also give me a certain latitude in the fr point and slopes...
GM, as you have experienced such "suroundings", what about cavity effects du to so many cones placed arround the mid driver and tweeter?

So what do you think about this one (tweeter at ears height):

[LineArray]

Hi crazyhub,

i would prefer midrange and tweeter in a vertical line,
because summation of both will usually be angle dependent.

Normally one should try to avoid those angle dependencies
in the horizontal plane since you may not want to sit
in perfect symmetry to the speakers on your sofa all the time
but want to move little bit. Even then both speakers should
sound as similar as possible.

Concerning the woofer configuration i told my opinion before.
That circular arrangement may work well, but the
geometric extension of the bass source is unecessarily enlarged
compared to wavelength at XO. So the "point source approximation"
is hampered in making the bass source wider than has to be by nearly
a factor of 2 ... directivity of (summed) bass source and midranger
will differ more than necessary in the crossover region.

The woofers could be mounted closer for less beaming.
To have some of the woofers close to the bottom will
positively affect the lower frequency limit which can be reached.

But everyone will build the system of his interest in the
end ... so its just an opinion.

edit: You could also think about lowpass filtering some of the
woofers at lower XO depending on room situation and Qts of
the drivers. That could be interesting to fatten up the deep bass
and shrink the size of the bass source at XO to the midranger...

Cheers

[crazyhub]

Originally Posted by LineArray

Hi crazyhub,

i would prefer midrange and tweeter in a vertical line,
because summation of both will usually be angle dependent. OK, even if some off-axis isn't a big issue when listening at some distance...

Concerning the woofer configuration i told my opinion before.
That circular arrangement may work well, but the
geometric extension of the bass source is unecessarily enlarged
compared to wavelength at XO. So the "point source approximation"
is hampered in making the bass source wider than has to be by nearly
a factor of 2 ... directivity of (summed) bass source and midranger
will differ more than necessary in the crossover region. I don't understand your point of view... Assuming the ears are in-axis with mid + tweeter, placing the woofers surrounding these drivers is the best way to limit directivity deviation patterns at Fx, no?

The woofers could be mounted closer for less beaming.
To have some of the woofers close to the bottom will
positively affect the lower frequency limit which can be reached.

But everyone will build the system of his interest in the
end ... so its just an opinion.

edit: You could also think about lowpass filtering some of the
woofers at lower XO depending on room situation and Qts of
the drivers. That could be interesting to fatten up the deep bass
and shrink the size of the bass source at XO to the midranger...That's the tricky part of my hoped design: if I make a vertical line (even ww,ww,ww,ww), directivity patterns don't mate this of the mid-driver, exept if I make two different low-pass on two groups of woofers as you did advise, which I wished not to do at first view.
At this point, I really think to place all the woofers near the bottom in a
ww
www
www
configuration, with a low Fx (200hz) and use a mid-driver that could go low enough...finaly a subwoofer + wide band + tweeter design; in such a way I avoid directivity problems and get a better spl in the bass range...seems to be what I have to do...


Cheers

[LineArray]

Originally Posted by crazyhub

I don't understand your point of view... Assuming the ears are in-axis with mid + tweeter, placing the woofers surrounding these drivers is the best way to limit directivity deviation patterns at Fx, no?

No. Best way to avoid deviation in directivity is to have the both
sources which shall be XOed small compared to wavelength each in itself.
Or at least to have them both in about the same relation to wavelength.
You cannot avoid the bass source to be larger than the midranger,
but cou can keep it as compact as possible.

The bass source in the "circular" approach is not small against
wavelength: It is larger than lambda/2 in both height and width while
the midranger is smaller than lambda/4. That is a big difference in
relation to wavelength.

Addditionally the distance of the drivers to be crossed over has to
be as small as possible compared to wavelength.

To me your last design is a bit extreme in that respect:
Too much distance to the midranger IMO.

The configurations i proposed in my former post are already
compromises between deep bass efficiency (uses the bottom) /
compactness of bass source and small distance from bass to
midranger.

They are not a perfect solution, there are always tradeoffs.

To put cuttoff lower for some of the woofers (near bottom)
might be a way to "harmonize" things.

If you do not want to make simulations you will have to experiment
with the real object. Not the worst way to go ...

Only thing you have to loose is some wood and some hours of
your time ...


edit: Sorry !!!!

I haven't read carefully. With significantly lower XO (90-150 Hz ?)
your last proposal seems very viable to me.
(Needs big coils an caps when implemented as a passive XO)
Have you googled for PLLXO ... ?

And i think there are some "Fullrange And Subwoofer" designs out
there which go a similar way. So you are in good company !


Kind Regards

[LineArray]

Hi,

just a rough simulation to get tendencies ...

red curve is your last proposal (works good)

green curve is my modification (see pic)


Please Note:

- edge works with idealized drivers, only the pure baffle
behaviour is calculated ! (for a driver with Qts=1) the curve
may fit approximately above fs of driver.

- the "mirror" is the technique i personally use to
take into account floor reflections.

I think both configurations work, my mod is a little more
compact and asymetric, which results in a litte advantage
in rolloff behaviour in the XO range ... bass is about the same.

Have fun

[crazyhub]

Originally Posted by LineArray

No. Best way to avoid deviation in directivity is to have the both
sources which shall be XOed small compared to wavelength each in itself.
Or at least to have them both in about the same relation to wavelength.
You cannot avoid the bass source to be larger than the midranger,
but cou can keep it as compact as possible.

The bass source in the "circular" approach is not small against
wavelength: It is larger than lambda/2 in both height and width while
the midranger is smaller than lambda/4. That is a big difference in
relation to wavelength.

Addditionally the distance of the drivers to be crossed over has to
be as small as possible compared to wavelength.Hi Oliver, I understand this matter: I have to consider the entire area of my 8 drivers i.e. the diameter/area of the entire footprint, including the dead spaces inbetween each of them, not only 8 x Sd...OK, OK...But are you absolutely sure I have to regard this way EVEN if the drivers are circularly surrounding the mid-driver? Because at the end, it's no less than a sort of co-axial building...

[crazyhub]

Oliver, thanks for your simulation! However I don't understand the first pic, I understand it's a mirror configuration because of the 16 drivers but where in the virtual separation line? at point "1"? so point "1" is the bottom of the baffle? also, is the black point left the mid driver place?
Thanks!