Steel Sphere Midrange

[JerryLove]

I've been reading enough that I think I have a reasonable idea how a good MDF speaker box is put together: but what's involved in building a steel sphere?

Not "how do you make a sphere out of steel", but more "do I need lining on the inside? what of? is bracing required? where are the likely problems?".

Though I suppose the volume for a sealed midrange is reasonably small. One could build a wooden box inside the sphere (likely would need to).

 

[adwilk]

One could build a wooden box inside the sphere (likely would need to).

Thats what I was thinking before you got to that point.... You could avoid the box, but it would require a relatively thick steel enclosure with lots of attention to avoid resonance. Does it have to be steel? Sounds like an interesting project, thats for sure...

 

[JerryLove]

Thats what I was thinking before you got to that point.... You could avoid the box, but it would require a relatively thick steel enclosure with lots of attention to avoid resonance. Does it have to be steel? Sounds like an interesting project, thats for sure...

Steel, while one of the harder ones to work with, is what I think I want for the end appearance. Brushed aluminum may also work, though it's in some ways harder to make a sphere out of .

 

[highfigh]

Steel, while one of the harder ones to work with, is what I think I want for the end appearance. Brushed aluminum may also work, though it's in some ways harder to make a sphere out of .

Why would it be harder to make the sphere from aluminum? Either could be stamped (if large enough for a woofer), but making forms would be extremely expensive. How thick would you want it? Why does it need to be a sphere- attempting to make a 'point source' speaker?

Steel would be awfully heavy if the thickness is sufficient to make it non-resonant at most frequencies but either could be made using patterns and welding them together after hammering to create camber.Then, they could be turned to make them truly spherical.

What size of driver would be in this?

 

[JerryLove]

Why would it be harder to make the sphere from aluminum? Either could be stamped (if large enough for a woofer), but making forms would be extremely expensive.

I don't have a massive press for either. When manually rounding, aluminum is prone to tearing. It hardens first then breaks. Steel can be re-shaped repeatedly without this problem.

On the other hand: steel is harder. But I know a machinist with extensive experience with steel. Aluminum is certainly an option.

How thick would you want it? Why does it need to be a sphere- attempting to make a 'point source' speaker?

As thick as need be. If it's sonically unimportant, it would be rather then (thick enough to be sturdy). Steel gets heavy fast.

I originally was looking at shapes for speakers: and there was discssion of the advantages of a sphere, or tapered sphere. Then I remembered the metal-sphere speakers I've seen on audiogon and that I thought they were pretty. Then I thought: I could integrate that into a 3-way and make something very artful that sounds good).

So, in the end, "because it has the look I want without detracting from the sound".

Steel would be awfully heavy if the thickness is sufficient to make it non-resonant at most frequencies but either could be made using patterns and welding them together after hammering to create camber.Then, they could be turned to make them truly spherical.

Yes. I'll have a conversation on construction with my friend (it will be a while before I can do this anyway as I'm poor). So resonance is a problem? Is there any other way to brace besides thicker steel? Can I run braces as one would in a wooden box?

What size of driver would be in this?

I'm thinking about 5".

 

[highfigh]

I don't have a massive press for either. When manually rounding, aluminum is prone to tearing. It hardens first then breaks. Steel can be re-shaped repeatedly without this problem.

On the other hand: steel is harder. But I know a machinist with extensive experience with steel. Aluminum is certainly an option.

As thick as need be. If it's sonically unimportant, it would be rather then (thick enough to be sturdy). Steel gets heavy fast.

I originally was looking at shapes for speakers: and there was discssion of the advantages of a sphere, or tapered sphere. Then I remembered the metal-sphere speakers I've seen on audiogon and that I thought they were pretty. Then I thought: I could integrate that into a 3-way and make something very artful that sounds good).

So, in the end, "because it has the look I want without detracting from the sound".

Yes. I'll have a conversation on construction with my friend (it will be a while before I can do this anyway as I'm poor). So resonance is a problem? Is there any other way to brace besides thicker steel? Can I run braces as one would in a wooden box?

I'm thinking about 5".

Aluminum can be spun to shape- think about how mixing bowls are made. If it's spherical, it won't need much bracing and could be very thin, but that brings its own issues- how do you make a mounting surface for the speaker? If you can make it from two pieces (front and back, then weld (or epoxy) the halves together and conceal the joint), the mounting flange could be spun into the half where the speaker mounts, then reinforced. This is done by making a wooden mold and forcing the sheet metal over it until it conforms to the shape. It might be possible to make the form is several parts so it can be spun in one piece and then a center can be removed to allow removing the rest of the form's sections.

As far as resonances- you could use some kind of resin to coat the interior, or maybe you could brush on some trunk liner. A sphere is inherently very string and rigid- thick wouldn't be necessary. If a bird's egg can support the weight of the parent setting on it but breaks easily when pressed at one point, the internal forces will only be testing the tensile strength of the shell and that won't be a problem for it.

This would be pretty easy to make from wood, too. a drill, a lathe and some practice is about all it needs. Also, the wood needs to be well dried- if it's wet at all, it will deform badly when it dries. I wouldn't try to make this from a solid piece but laminating enough layers would work.

 

[highfigh]

Wait- you want it to be "pretty"? Ohhhh, noooooo!

 

[Swerd]

If you're interested in spherical enclosures for a driver, check this DIY design at the Parts Express design showcase. Maybe you can adapt the methods in the Enclosure Assembly section. They were made from a gypsum plaster material poured over spherical candle making molds. You could always paint them with a metallic finish.

 

[JerryLove]

Wait- you want it to be "pretty"? Ohhhh, noooooo!

If you've ever bothered to put a veneer on your MDF other than paint: you have made the same decision

Thank you for your many suggestions. I was thinking of making in pieces and welding together. A flange would make it possible to disassemble, but would both mar the look and cause a seam inside I'd have to deal with.

So a bonded aluminum sphere 12"-18" in diameter with a thickness of 1-2 street signs, or perhaps a steel sphere half that thickness. No braces? Lined with... something. Sound like it would work or do I need more to *really* make it sonically dead?

 

[Quickley17]

Totally Spitballing here...

Composite materials, such as biax weave fiberglass will help with stiffening to a great degree for significant weight savings. What it does for resonances though, I do not know. I am currently picturing a bracing structure of composite wrapped 1x1 boards with a composite inner "shell" on the inside wall of your sphere... you could also make the whole thing out of composite and give it a metallic finish.

With strong, multi axis weave fabrics though, you may start to run into cost issues.

 

[Swerd]

…I was thinking of making in pieces and welding together. A flange would make it possible to disassemble, but would both mar the look and cause a seam inside I'd have to deal with.

So a bonded aluminum sphere 12"-18" in diameter with a thickness of 1-2 street signs, or perhaps a steel sphere half that thickness. No braces? Lined with... something. Sound like it would work or do I need more to *really* make it sonically dead?

I really couldn't predict if it would be sonically inert. In theory a sphere should be less resonant than a box, but…

In theory there is no difference between theory and practice. In practice there always is .​

So, be prepared to line it with something soft and flexible that can stay put. How about modeling clay or plasticine?

How large a woofer are you planing to mount? With a sphere as large as you describe, it might have to be thicker to avoid flexing from the backwave of a large woofer.

Do not use steel, as it will interact with the speaker magnet and the inductor coils of any crossover. Some types of stainless steel are not magnetic, but they will cost more. Aluminum might be OK, but you should check that first to see how a large sphere of aluminum would affect a speaker's magnet and voice coil.

 

[WmAx]

I really couldn't predict if it would be sonically inert. In theory a sphere should be less resonant than a box, but…

In theory there is no difference between theory and practice. In practice there always is .​

So, be prepared to line it with something soft and flexible that can stay put. How about modeling clay or plasticine?

In reality, a sphere/cylinder, etc.; is stronger in terms of resistance to pressure. Not so much in terms or vibration or resonance response. Wind chimes as example, with cylinders, resonate just fine, despite the incredible strength/pressure resistance inherent of the structure.

I would use the same methods on a sphere or cylinder as I would on a rectangular structure to reduce vibration response.

-Chris

 

[JerryLove]

So, be prepared to line it with something soft and flexible that can stay put. How about modeling clay or plasticine?

How large a woofer are you planing to mount? With a sphere as large as you describe, it might have to be thicker to avoid flexing from the backwave of a large woofer.

I'm completely flexable on what's inside the sphere, including deforming the interior out of sphere shape as needed.

Do not use steel, as it will interact with the speaker magnet and the inductor coils of any crossover. Some types of stainless steel are not magnetic, but they will cost more. Aluminum might be OK, but you should check that first to see how a large sphere of aluminum would affect a speaker's magnet and voice coil.

I'm planning on an external crossover (or, if I was going to use a passive crossover, I'd put it in the woofer box which will be MDF).

The steel in the basket doesn't interfere, nor do the actual magnets in other drivers when drivers are placed on top of each other. Would it really be a problem as long as the sphere was at least twice the length of the driver with magnet in diameter?

On the other hand: there should eventually be a tweeter that will be mounted to the top of top-of-the-front of the outside of the sphere: that would be right next to the metal. Perhaps I do need to consider a different material that can be made to look like brushed steel (such as aluminum).

As to mounting: it would be sphere on mounting on woofer box. The woofer box will actually be my first project as soon as funding permits (a long time sadly), the mid and upper the second (assuming the first goes well). I'm hoping the bass unit will be rather inert (assuming I build correctly), and I can do some isolation in the connecting materials.

It's a bit of a thought exercise right now. My next real task is to get my speakers from Chris. I'll learn a lot more about external crossovers then when I'm staring at a pair; as well as more about how very well braced cabinets behave in reality. Can't wait !

 

[Swerd]

I'm planning on an external crossover (or, if I was going to use a passive crossover, I'd put it in the woofer box which will be MDF).

Good, that eliminates one potential source of trouble.

The steel in the basket doesn't interfere, nor do the actual magnets in other drivers when drivers are placed on top of each other. Would it really be a problem as long as the sphere was at least twice the length of the driver with magnet in diameter?

The steel in a speaker's basket is part of the speaker and its magnetic qualities are known when the speaker's motor is designed. It is also present when the speaker's Thiel/Small parameters are measured. If you mount the speaker in a steel cabinet, the electromagnetic properties will probably change. The question is by how much? Rather than guess, you should measure the T/S parameters of the driver both outside and inside the steel sphere. The change may be so small that it won't matter, but you should measure that to find out for sure.

To do that, you will need something like this.

The same would apply for a tweeter.

If it were me, I would make the sphere out of a non-magnetic material.

 

[JerryLove]

Thanks for all the suggestions there. It's something I'll definitely quantify or avoid.

Chris, if you are out there: you've often used steel bracing in your speakers. Do you have any input on the dangers of a steel enclosure for a midrange driver?

 

[jamie2112]

Good luck with that.

 

[lsiberian]

This thread gives the phrase. Balls of steel a whole new meaning.

 

[highfigh]

Thanks for all the suggestions there. It's something I'll definitely quantify or avoid.

Chris, if you are out there: you've often used steel bracing in your speakers. Do you have any input on the dangers of a steel enclosure for a midrange driver?

I think the magnet of your midrange should be minimally affected by the shell if the speaker's magnet is shielded. Aluminum or wood will obviously negate the need to worry about this but they might not be what you want. If you plan to paint the exterior, the material doesn't matter as much.

IIRC, the formula for spherical volume is V=(4/3)Πr³

 

[TLS Guy]

I've been reading enough that I think I have a reasonable idea how a good MDF speaker box is put together: but what's involved in building a steel sphere?

Not "how do you make a sphere out of steel", but more "do I need lining on the inside? what of? is bracing required? where are the likely problems?".

Though I suppose the volume for a sealed midrange is reasonably small. One could build a wooden box inside the sphere (likely would need to).

Steel has the wrong Youngs modulus for a speaker enclosure. It rings!

 

[JerryLove]

Steel has the wrong Youngs modulus for a speaker enclosure. It rings!

Is there a way to compensate for that? Doesn't ringing occur along certain frequencies? Would I do better with Bronze (1/4 the rating of steel on the Youngs modulus chart).