On Sat, 27 Dec 2003 19:03:59 +0100, Peter Larsen
wrote:

Stewart Pinkerton wrote:

Freehand a manifold, i. e. a conical horn (x), as the adapter between
the unit and the slit. Conical horns are wonderfully simple: the faster
they expand the higher their range. They also have the advantage of low
distortion due to the fast initial expansion.

This doesn't make sense to me, as there is no horn expansion if the
slit is the same area as the driver Sd, as you suggest below.

Expansion prevents too many parallel walls.

Which are completely irrelevant in a sub. In fact, a cube is the
*best* enclosure shape for a sealed-box sub, as it has maxium wall
rigidity for any given volume and material.

If you go for the suggested
model with 3" as the smallest dimension of manifold and slit then you
have to make some kind of a transition from the 13" membrane. This is
not about midrange, so doing it the crude way comes to mind, i. e.
starting the manifold with a - tja da dum - 3 by 13 inches or 3 by 10
inches initial area. Even to simply enter the room with the membrane
area will then require expansion.

You can however transit directly from a 13" circle to a 3" x 44" slit
(to maintain equal areas) without any expansion whatever, simply by
making a double-tapered box, so why bother with an intermediate
constriction? BTW, such a manifold box will have minimal resonance
problems due to the heavy taper of the box sides, so it's also a
mechanically sound design not requiring heroic wall thickness.

I'm still looking at a manifold exit area of 0.3-0.5 the Sd of the two
drivers (for improved WAF....) , expanding into the 1/8 space of the
room corner. As you say, there may need to be a back box to equalise
cone loading.

Ah, going to the room corner is a mitigating factor because it allows
you to use the corner as an extension of the conical horn in the
manifold. And with an "L" shaped slit in the ceiling in the corner it
ought to be possible to make it a lot more compact than I had first
thougth.

I currently favour a 10" square aperture, which is reasonably discreet
and gives a 3:1 area compression from a pair of 15" drivers. Might
have to be a foot square to allow for grille solidity.

OTOH, since I'm staying well below Xmax for normal operation, perhaps
this is all a little paranoid! OTGH, design time costs me nothing, so
why not do it right first time..................

Minimizing distortion and getting an optimum coupling to the room are
worthwhile aims and probably pre-requites for a system that is supposed
to be able to play well in trippel forte as well as in the way more
difficult trippel piano.

Indeed so, hence lots of thinking before firing up the jigsaw!

Try searching for hornresp ... it may be possible to express this as a
conical horn it can understand, in which case it could be helpful to try
modelling in it.

Aside from the basic expansion into 1/8 space given by corner
mounting, I have no intention of getting into the multitudinous
problems of horns!
--

Stewart Pinkerton | Music is Art - Audio is Engineering