I'm thinking of a sandwitch construction with carbon fiber/sand/carbon fiber.
Have anybody try it. What could i replace the sand with?
The inspiration came from wolson benesch speakers.
If you have any sugjestion welcome to tell me.
Carbon fibre is not as easy to use as fibreglass, having similar but different techniques for laying up etc, and requiring a level of precision and detail in workmanship that's at least a level above 'glass, especially for the design and construction of the moulds. To get anything like optimal performance out of it you would also need to use an autoclave for curing, and ones big enough for anything larger than a small bookshelf are rare and usually owned by specialist companies who often don't let outsiders use them. Plus they ain't cheap to hire. IIRC, simply laying up CF like 'glass will not give much improvement for the extra fiscal outlay.
If you can do CF, use an aluminium or nomex honeycomb as the centre of the sandwich. That way you can get a quite light, yet incredibly strong and rigid enclosure, that will cost some serious pesos.
To go light and rigid in approach, try shaping industial styrofoam or surfboard blanks, and covering it in fibregalss inside and out. I (truth, my brother made them to my design with my help) made some HUGE TL subs like this years ago, and the enclosure was very stiff and quite inert.
For mass loaded enclosures try some of the ply/sand/mdf sandwiches talked about in some recent threads, or just use concrete.
Don't mean to rain on your parade with the CF, but I tried to do it some years back with a design for a commercial (non-audio) project and found it required a lot of expertise, specialist knowledge and money to make structural CF parts. It ended up being cheaper to make the parts out of billet aircraft Al.