I want to build a spherical speaker with a diameter of about 50 cm. The frequency response should be relatively deep. Now my question is, which type of plastic is best for it and how thick does the housing have to be?
It is a spherical case that should have a diameter of approx. 50cm. It's an active speaker means amplifier and everything is in the sphere. The loudspeaker woofer is on the front and has a high deflection. The other speaker drivers are on the left (as seen from the woofer).
Do you still need information? :) I'll try to answer it as best I can.
By the way: It would be good if the material was very shiny
I think it is the infinite baffle type so needs to be airtight. How will the amplifier be cooled in such case? Not sure if you can have multiple speakers with an infinite baffle. Also, the woofer is non-directional, so why not put that at the side, and the higher freq units facing forward? Regarding material, usually we don't want the housing to make a musical sound when we tap it. So maybe a slightly soft plastic? With wood it is usual to use very thick and heavy material to avoid resonances.
I think this question is more about plastic sphere production than acoustics. I think you can use anything from an acoustic perspective. Of course some won't be optimal choices, but you can compensate with thickness and other parameters. I would be worried mostly about production costs, myself. How many of these things will you be making?
You'll need a clear idea of the design details, things like internal flanges, post mold machining, cosmetics, etc. in order to know what the good choices are. I would draw up a design of what you want first, and then shop it around to fabricators to get their input. Then you'll completely redo the design.
So, I'll toss out roto-molded HDPE for starters, but I really don't know.
I think eliminating resonance will be very important. For that reason a composite material should be used to give the enclosure a dead sound without resonances. I would use GRP for prototypes.
For mass production, roto-molding would be practical, but it would need to be done with two layers having different sound velocity, a bit like a Californian fuel tank.
Since the woofer should be backed by a separate cavity to the mid-range speaker, and the tweeter, it will require separate internal tapered scrolls like inside a snail shell. That will make the casting more difficult to design and it may require fabrication in three subsections that are then glued together to form the sphere.