Does anyone have even the vaguest idea of where I might be able to locate the mechanical properties of MDF? Or fiberglass or carbon fiber? I am being REALLY retentive (perhaps to retentive?) about shaving weight of the box for my 10' sub, so i figured I would put my engineering degree to good use and run an a stress analysis on my potential box design to see exactly how much thickness I really need. I usually use 3/4 thick (which I know works) and just be done with it but I figured what the he!!.

Tell me if this sounds right. You take the cone area and multiply it by the excursion (will most likely use a number between liner excursion and mechanical excursion) and you get the volume change. The way I figure it is with that delta in volume I can use the ideal gas law and estimate the pressure change when the sub is in motion. The pressure will allow me to do the analysis on the panels and joints (and possibility of bracing needed). Once I get the stresses (maybe employ FEA if I am feeling lazy) I can manipulate the box design to obtain the required strength without adding any weight. Hence the need for MDF's (or either fiber's) mechanical properties.

Hmmm..is the excursion # applicable to both directions? Meaning do I have a linear excursion of .343 inches both in extension of the sub (creating a vacuum) and .343 when it moves inward (creating pressure)? Or is it a measure of total possible excursion (meaning the actual possible excursion in any one direction is half the number Polk supplied)? Guess I will have to call tech services to know for sure.

WOW! http://www.i-club.com/ubb-files/eek.gif That's deep, man. Real deep. http://www.i-club.com/ubb-files/wink.gif If you could afford a box out of carbon fiber that would be cool.

Regarding the pressure calculations... I doubt excursion alone will do you much good. But then again, I never made it that far in physics so have at it.

I can tell you what the excursion means. Xmax is supposed to be standardized as a single direction from rest, but sometimes (bad) mfgs quote peak-to-peak values to make their excursion look better. The generally accepted value for the displacement of a cone at its mechanical limits is its measured xmax + 15%.

ColinR, is there any particular reason that you think that the excursion (along with the cone area giving me volume change) will not give me a decently accurate pressure #? I myself am a little leery considering that the sub is essentially vibrating at anywhere from 10 Hz to 100 Hz, and the pressure will be quite dynamic. I am considering mounting a pressure sensor in the old box for that 10 and bringing it into work to see if I can convince someone to let me play with their data acquisition system. Going to be difficult to do that considering that it is a rather unusual "G"-job requiring a little more resources than I like to ask for. Also the recorded pressure will be dependant on the existing box. I might have to ask around.
John 2.5 RS carbon fiber sheet isn't really that expensive (I was wrong it is expensive, but it is the real stuff) (I forget the name of the place that sells it, that bookmark is at home I remembered www.selectproducts.com (http://www.selectproducts.com) ) and it works alot like fiberglass. Might be a interesting way to proceed, given that I could have a aluminium skeleton made up and lay the fiber over that. That would be the *****.

I will see what transpires
Later
Bob O

[This message has been edited by Bob O (edited June 01, 2000).]

Found the info I needed. I calculate the change in pressure as .291 psi. I know this sounds very small but if you apply it to a 1 ft square area you get about 40 lbs of force. Generally no enough to explode a box but perhaps enough to cause unwanted deflection in a thin sectioned panel.
For those that care, the modulus of elasticity for MDF is around 348,000 psi. I belive the other 3 I have is the approiate strength # (Rupture Modulus) but having not engineered any wood structures before I am not sure if that is the number I can use. I will have to ask around. Also for those that care the info came from www.pbmdf.com (http://www.pbmdf.com)

I will have to find similar info one fiberglass and carbon fiber. Being that they currently make rotationally molded sub enclosures I think that a braced fiberglass box would work well if I could damp it enough to keep it from buzzing. Dunno yet.



[This message has been edited by Bob O (edited June 01, 2000).]

Just to note, the more solid the box, the tighter the sub sounds. I've known people to use 1" MDF before on boxes (overkill, but damn).

I've also seen a box that this guy in competition made one time out of plexiglass. He had it built using 3/4" plexi and I think it housed 4 12's. Well he said he turned it on and cranked it and the box shattered (or rather cracked all over the place, spider-webbed if you will). He didn't want to waste it and decided to build another box around that one to double the thickness of the walls. Gave it a really cool effect, but I can't speak for it's solidity. http://www.i-club.com/ubb-files/smile.gif

I have built hundreds of speaker boxes and my favorite combination for weight vs. strength vs. resonance is 3/8 or 1/2 MDF (depending upon speaker size) reinforced internally with a decent layer of fiberglass.

It is super strong and the bass is tight and accurate with little to no unwanted colorations. I have also found deflex acoustic panels will help eliminate colorations when used properly.