Hi
Some advice as to if I am on the right track please.

As I understand it, carbon fibre doesn't vibrate [or asorbs vibration better] as much as the other materials, so therefore should be the 'best' or better material to construct a sub box.
{Concrete is probably the best!}

The added benefits are:
lighter weight - the box will be removed at times as my hatch area is small
greater stiffness - so the material will act less like a cone= cleaner sound
stronger so less layers needed - faster fabrication

What else should I know about?

Ignore the cost for this comparison! REPEAT THAT OK!

thanks
Nigel

carbon fibre is pointless unless you can put it in a vacuum press and bake it

ply and other materials...

Isn't that just a process where pre preg carbon is used, then vacumed to the shape of the mould and heated to set the resin?

Like what other materials please?

I thought MDF was denser than ply, therefore a better choice for SQ, but ply is better for your health when cutting!
MDF also needs more water proofing to stop swelling due to moisture asorbtion, maybe better to use the moisture resistant product used in kitchen cabinets but may be a thickness limitation.

I'll let you ponder of it, I'm off to bed...

what exactly are you after with your sub box?
light / rigid/ strength/ looks/ performace

YEP!
That just about sums it up. I figure boxes are usually made out of mdf/ply or fibre glass. Seeming that I belive carbon absorbs vibration better [look at tennis racuets/skis etc] then why not use it instead of glass? The lightness is good as my sub is very light, so why not complete the package. Also as mentioned it will need to be removed quite often so I can carry loads in the hatch.

chewing gum?? masking tape?? Hot glue??

What your after is a box that is dead. ie, it doesn't move with the music (or with your hand if you thumped it). But I understand the you want to have one that is also very light.
So, yes carbon fibre could work, but you will need to end up making the wall thickness very thick to minimise flex. Carbon can only really be devoid of flex when the strenght of a flat surface is not exceeded by the forces implied.
Tennis request etc get their strength from the fact that there is almost no long flat surfaces (look carefully nearly all surfaces are rounded), unlike a sub box which does. A surface that absorbs vibrations is not ideal as "absorbing vibrations" implies that the material has a damping affect, like rubber (maybe this is not what you were trying to explain), you want one that doesn't move at all.
One way though would be to build a aluminium skeleton of the box, with lots of small ribbing and some internal cross bracing, then carbon weave wrap the whole thing. It would take some serious work and $$ but could be done...
Another way to get around it would be to build a circular (ie round ball) sub box... as the full rounded surfaces would have almost nill give (until it broke).

But if you need your car/hatch to carry gear around, consider using your spare tyre as a craddle. That is, flip the spare to the face of it faces down, mask and fibreglass the inside space of the spare tyre with several (ie 5-8) layers, then use a top piece of MDF bond and mount driver, use that as sub box. If your good enough you can keep this all under the top surface of the hatch floor and have plenty of storage room and you wont need to remove the sub. A sub will play through the factory carpet as long as it not hard backed.
Something to think about.

You read my mind Clint, all of the above

thanks Syd.
I have dismissed the spare wheel option as I won't travel without a spare and my spare is a deflated space saver, needs a compressor, some cars in the states have followed this option. Its certainly neat and saves some space.

I question your feeling that carbon is not stiff enough as a flat surface. Yes I know a curve increases strength tremendously, but for equal weight i think carbon is 5 times stiffer and stronger than fibre glass.

I had thought about the frame idea, like a rib cage! May be the easiest way to fabricate any way, or say , use a 3mm mdf box and then carbon over it.

I am thinking of some rods going from side to side internally to limit the flex.

I guess that if you built a sub box out of 25mm mdf, what thickness of carbon would give the same result.

Its maybe time to consult some boat builders who use the product.

thanks again

why not make the box out of several thin layers of ply with a layer of glass in between the layers ( i have parked a car on one)
it works very well
i have made several glass boxes
all were light
most were removable
and all were very strong (120kg jumping on them with out flex)
so dont dismiss standard chopped mat with some weave going thru it

if you would like to learn more or talk more on this
just ask me
i have "some" fiberglass knowledge

ta, i will have a chat at a meeting one night. Can't make the next one as I have a course on that night.
I have done some glassing with boats and once joined three broken carbon bicycles into one good one using vinyl ester resins, still together after about 18 years!
Also looking at sheets with a honeyconb sandwich.

just found this info:

Click to view attachment



Table 1
Typical Property Values for Australian Producers
Property Units Thickness Class - mm
<=5 6 - 12 13 - 22 >23
Density kg/m3 800-850 775 725 650 -700
Bending Strength (MOR) MPa 44 42 38 30 - 40
Bending Stiffness (MOE) MPa 3800 3500 3300 3200
Internal Bond Strength MPa 1.15 1.0 0.75 0.6
Surface Soundness MPa 0.7 1.0 1.3 1.4
Screw Holding - Face N - - 800 850
- Edge N - - 1150 1000
Thickness Swell (24hr) % 20-30 10-20 8-12 5-8
Formaldehyde Potential
(Desiccator Method) mg/l 0.7 - 1.0 0.7 - 1.0 0.7 - 1.0 0.7 - 1.0


Sound acoustic 16mm = STC29

adding "ribs" to a flat panel only increases the stiffness of the panel by increasing the crossectional area about the axis of bending. its not very efficient.

if you halve the length of the panel, you increase the stiffness by a factor of 2. this is very efficient.

you want an efficient design, so you need to get the length of the panels as low as practical. then i would use a 'waffle slab' arrangement to further increase the stiffness of the panel.

CF is a good material to work with and i would use a timber brace. it is nice and stiff.

adding "ribs" to a flat panel only increases the stiffness of the panel by increasing the crossectional area about the axis of bending. its not very efficient.

if you halve the length of the panel, you increase the stiffness by a factor of 2. this is very efficient.

you want an efficient design, so you need to get the length of the panels as low as practical. then i would use a 'waffle slab' arrangement to further increase the stiffness of the panel.

CF is a good material to work with and i would use a timber brace. it is nice and stiff.