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
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."
https://www.diyaudio.com/community/threads/spherical-speakers-and-all-that-jazz.138169/
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.
"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."
https://www.diyaudio.com/community/threads/spherical-speakers-and-all-that-jazz.138169/
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.
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?
Yes, the sphere would be stiffer than a box. Baltic birch 1.5" thick is overkill unless the box is like 60 cubic feet 😵.
Baltic birch is highly reflective, it won't dampen sound waves much at all.
Damping would require some internal damping material.
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
How about those foam core boxes which some people tried in different shapes, like a solid prism, which were quite stiff and sounded better? Is this that thread?
https://www.diyaudio.com/community/threads/foam-core-board-speaker-enclosures.223313/
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
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
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.
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.
I have built two pairs of spherical speakers, one a 2-way (with separate sub) and one a 3-way with integral sub (so 4 spheres). I used Ikea bowls which are made of bamboo (very rigid) and are 3/8" thick. Then added a 3-part damping system using speaker cabinet paint on the inside, thin felt glued in, and fiberglass inserted. And mounted with a center pole. No standing waves and no resonance. No need for other internal bracing. Even with a 6 1/2" sub, the larger (11") sphere does not audibly resonate. In order for a sphere to resonate it actually has to pulse in and out, almost impossible. Meanwhile a box cabinet can have vibrations both in the panels and at the joints. Plus you get the benefit of little to no external diffraction.
Attachments
Cute speakers!
Enclosed volumes of any shape resonate.
Tap on an empty sphere, easy to hear it resonate (ring) for a long time.
Your 3-part damping system reduced the sphere's resonance time.
https://www.linkedin.com/pulse/cavity-resonator-resonant-frequency-shiv-prasad-tripathy
A spherical cavity structure only needs the radial dimension to specify its geometry. The resonant frequency in a sphere is given by two equations – one each for TE and TM mode – as shown below.
where a is the radius of the sphere, unp and u’np are the p-th roots of n-th order spherical Bessel function and its derivative respectively. The symmetry of the object entails use of a spherical coordinate system.
"Out of any shapes, sphere has smallest surface area and shortest internal dimensions with given volume. Small internal dimensions mean all resonances inside are higher up in frequency and possibly outside of pass band than with any other shape. Small surface area means that even if the surface vibrates, the area is smaller than with equivalent volume cubicle, thus resulting SPL would be less. Well, perhaps that's too much simplification, but since it's one surface I'd imagine it cannot balloon without stretching, and if it vibrates there will be both positive and negative displacement likely averaging out each other. Also, no edges outside of the structure which means no secondary sound sources with diffraction. All of these combined ought to result in 'less box sound' ". (source from Audio Science Forum, tmuikku (2022). As I said, you can use a pole internally to mount spheres, and the round internal dimensions break up any standing waves. My point is the spheres do not audibly resonate, supported by the points quoted. You can attempt to use mathematics, or simply build and use your ears. 🙂
The inner pressure would have been higher so I don't agree with this.
This can be said about any enclosure shape.
The entire surface of a sphere is an edge pulling away from a wave travelling along it, continually diffracting.
Olson's data (and others since) suggests otherwise relative to audible or measurable diffraction.
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I think you have misinterpreted my statement. If one is going to take the baffle out, it is preferred to round it so that it is spread in time and space. This is why a sphere is good... but it does create diffraction, not always audible but nonetheless. An infinite baffle such as a wall would not produce diffraction if the source was suited to half space.
Spherical shapes seems to measure really well:
https://www.audiosciencereview.com/...with-chn110-drivers.52086/page-3#post-1924248
and:
https://www.audiosciencereview.com/forum/index.php?threads/sun-audio-purified-4.52297/
https://www.audiosciencereview.com/...with-chn110-drivers.52086/page-3#post-1924248
and:
https://www.audiosciencereview.com/forum/index.php?threads/sun-audio-purified-4.52297/
An ellispoid / droplet shape is even better, (ref Fujitsu 10, B&&W) something Olson did not measure.
dave
dave
I recently made a pair of sphere enclosures for the FatalPro 3” fullrangers.
The plan was to build out of plaster and fiber, but I couldn't get good plaster here so had to use grout. Yeah, they are heavy. They are also very stiff. But vibration can certainly be felt by hand. Do the walls vibrate more or less than a cube of the same volume would vibrate? I don’t know.
The plan was to build out of plaster and fiber, but I couldn't get good plaster here so had to use grout. Yeah, they are heavy. They are also very stiff. But vibration can certainly be felt by hand. Do the walls vibrate more or less than a cube of the same volume would vibrate? I don’t know.
Problem is DIYers are hard pressed to make these shapes without say using a 3D printer.