What is the ideal enclosure shape?

Bill F.

Member
2001-11-15 5:25 pm
SW MI
You want to avoid two things: air mass resonances within the enclosure and reflections back out through the cone.

To cut resonance, it's a good idea to make sure no length dimension is equal to (or a multiple of) another. Also resonant standing waves can be attenuated by making interior surfaces non-parallel. This also helps disperse midrange/treble reflections.

Have you ever seen pictures of those anechoic chambers with foam wedges everywhere? That kind of treatment with random acute angles would probably work well to diffuse the mid/treble backwave.

I've thought of taking the scraps left over after making a cabinet, cutting them into wedges, and gluing them in alternating patterns to the inside back wall of the enclosure. Then you could coat them with a good layer of rubberizing spray, like vehicle undercoating from the local Auto Zone. They would also add mass to the enclosure wall--not a bad thing.

Bill
 
Enclosure shape

Basil says: amphibious landing craft shape
 

Attachments

  • fawlty.jpg
    fawlty.jpg
    6.4 KB · Views: 1,418

Nisbeth

Member
2001-01-29 9:58 pm
Denmark
The optimum enclosure shape is a sphere. (tough to do in mdf). Vance Dickason discusses this subject in "The loudspeaker design cookbook". If you have access to this book, look it up.
I don´t have it handy at the moment, but I believe the best shape possible in a wooden enclosure would be something like the "christmas tree" shape (i.e. no parallel surfaces).

HTH
/Uffe
 
Dual tapered Octagon

Well obviously I’m biased a bit but I believe that my latest project (SST8) has the perfect (well almost) shape, I did over a year of research and testing to come to this conclusion, I tested equilateral triangles, boxes, spheres, eggs (ovoid) and three different octagons until I finally came up with a dual tapered octagon. After living with these a few months and finally getting the crossover point right I couldn’t be happier with these. Also a shape like this needs no internal bracing and they are as dead as any cabinet I have ever heard or should I say not heard.

SST8 MTM’s
http://64.33.91.28/kingdaddysst8/index.html
 
Didn't realy intend to go for the drum skin look, or drums at all in fact, just turned out all the research and expermints led to this design, sound was first and formost looks came second, or some might say not at all. the finish is bleached stone textured paint and oak inlays. interesting thing about baffle step is that with these I dont need any compensation at all.
 

woody

Member
Paid Member
2002-01-15 12:57 am
Tyrone Ga. U.S.A.
I vote for the egg

About 20 years ago I believe someone like Madisound had focal
speaker kits with what they clamed was the best enclosure shape
namley the egg. They had fiber glass reinforsed plaster of parris
enclosures for sale. I don't recall the name but there is a speaker
enclosure company that has a pattent on spherical enclosures. They sell them in many sizes. Besides hard to set a drink down
on the top of either these shapes!


Keep up the posting


bob
 
Re: Optimum Enclosure shape

There are 2 issues involved with enclosure shape, the inside shape & the outside shape.

The inside needs to minimize standing resonances and time-smear caused by re-transmission of the signal back thru the cone. If one has the space and infinite (or approximation of) baffle satisfies this. Since this is not practical in most cases, other schemes are required. Irregular or non-parallel sides are the 1st step in elimainating standing waves. The simple expediant of even two non-parallel sides can make a significant improvement -- here is an example. A sphere is actually not a good shape wrt this as it supports a single resonance (an ovoid is better here).

None of this really addresses the time-smear issue. One of the ways to deal with this is a long-tapered pipe -- ie a transmission line. This can be open or closed. The idea is to reduce the magnitude of the rear radiation and time-delay any that is left (Haas effect). Here is an graphic example of a line for LF.

On the outside, a shape with low edge diffraction -- ideal is a surface with no sharp discontinuities for a wave travelling along the surface of the baffle to be reradiated from.

John Murphy has reproduced some of Olsen. What he doesn't show is the one that shows how close to the sphere a rectangular box with edges 45ed comes.

In a situation where the outside is related to the inside, a shape similar to kingdaddy's is one of the shapes that turns out to be a good one -- essentially the same shape B&W came up with from their nautilus program.

A half-sphere on the front and a taped cylinder on the back. The length of the cylinder is related to how low you want to go.

... kinda long winded?

:^) movie time

dave
 
Re: regarding the baffle difraction loss

jmiyake said:
Planet x,
Is baffle loss avoided in In-Wall type of mounts?
Would it be reduced in a speaker that fit snugly into a corner?

No baffle loss in either of these situations -- a different set of issues thou...

Baffle loss happens when the LF transitions from radiating into 2 pi space to 4 pi space. Mounting on a wall (or a very large baffle) there is only 2 pi, in a corner you are radiating into 1 pi.

dave
 
Volume an issue?

When searching for the perfect speaker enclosure, are you looking for the best sound reproduction, or the smallest enclosure with good sound reproduction, or something that just fits in you mansion's parlour???

There are some shapes which have an advantage over others as far as interior box volume goes.... I think a sphere has more interior volume than a cube of roughly the same dimensions, but i won't stake my life on it.

I don't even know if i should post this.... Is size a consideration? Is it enough of a consideration to discard a particular shape over another?

And what's this about interior and exterior shape? Does anything have to do with what it looks like on the outside? I thought it was all about the interior shape and volume!
 
Very interesting

This has been a very interesting discussion so far.
So it seems the goals are:

Exterior:
1) Gradual curves on edges to eliminate edge diffraction.
2) Offsetting drivers to stagger the frequency of edge effects.

Interior:
1) Eliminate standing waves
a) Non parallel walls
b) lining
c) non-resonant enclosure
d) use of wedges to breakup reflection

2) Providing a path away from the front baffle to prevent the backwave from coming back towards the driver.

All of the above seems to point to a terminated transmission line, or at least a tuned transmission line.

It also seems that most vented and closed enclosures seem to cause problems by actually creating resonances in the cabinet.

Althought it looks like Kingdaddy's (ported?) enclosure is a great compromise.

If a transmission line is so much closer to the ideal enclosure, why are they not more common?

James