Structural characteristics of a sphere

Disco-Pete

Paid Member
Hey all. I'm wondering what the required thickness of a given material would be going from a box to a sphere to achieve an equivalent damping ability. It's tough to break an egg putting equal pressure all around but obviously not applicable in the opposite direction. I'm thinking of building a pair of 2 way spherical speakers. TIA

weltersys

Regarding damping, Earl Geddes wrote:
"For a given internal volume all shapes will have the exact same lowest modal frequency AND will have the same modal density as frequency goes up. The exact locations of the modes will differ with shape, but on the average all shapes will have the same number. Hence there isn't much advantage at all to internal shape. They should all be damped internally."

If you are asking about the required wall thickness for a box to be as stiff as a sphere, it would depend on the material used for each, which you have not given.

A box with the same wall thickness, material and internal volume would require bracing to be as stiff as a sphere.

Disco-Pete

Paid Member
Thanks. A comparison using the same material is what I'm after. For example would a sphere made of 1.5" baltic birch be stiffer than a box of the same material? Intuitively it seems a sphere doesn't need anywhere near the same thickness but how much of a role does stress distribution play in the damping ability of a shere vs a box? And if so, could the same stiffness be achieved in a sphere with no internal bracing if the same thickness as the box, ie: 1.5" was used?

Paid Member

weltersys

Thanks. A comparison using the same material is what I'm after. For example would a sphere made of 1.5" baltic birch be stiffer than a box of the same material? And if so, could the same stiffness be achieved in a sphere with no internal bracing if the same thickness as the box, ie: 1.5" was used?
Yes, the sphere would be stiffer than a box. Baltic birch 1.5" thick is overkill unless the box is like 60 cubic feet .
Intuitively it seems a sphere doesn't need anywhere near the same thickness but how much of a role does stress distribution play in the damping ability of a shere vs a box?
Baltic birch is highly reflective, it won't dampen sound waves much at all.

Damping would require some internal damping material.

woody

Aside of structural strengths a sphere shows a far smoother frequency response just look at this article about this by Trueaudio. On a similar note Focal found the ideal shape of a speaker enclosure was an egg and for a while even offered fiber glass reinforced plaster of Paris enclosures for the DIY community about 50 years ago. https://trueaudio.com/st_diff1.htm

planet10

Paid Member
A sphere has no corners. it is incredibly stiff. Only s guess but i would think that less than a cm would still be much stiffer than (at least) 2.5 cm quality plywood in a well-braced rectangular box.

It certainly is in a cyclinder, it has “corners’ at the ends, really decent subwoofers are made/can be made from similar thickness cardboard tube.

dave

Paid Member

dave

hifijim

Are we talking about the stiffness and strength of a sphere compared to a box? rather than the structural damping?

If so, there is really no comparison. A spherical pressure vessel of thin sheet metal can safely contain many atmospheres of pressure (several hundred psi). It would be almost impossible to make a cube-shaped pressure vessel (with no internal reinforcements) that could do the same job, even with very thick walls. The stresses at the corners are very high, and we would need such a huge internal fillet, that the internal shape would start to resemble a sphere anyway.

All of this has very little to do with whether or not a sphere is a good shape for a speaker.