AVS Forum banner

1 - 17 of 17 Posts

·
Registered
Joined
·
81 Posts
Discussion Starter #1
Hi

I just realized that the MDF available in the local market is actually 18mm in thickness, not 20mm. I was planning to use these to build a full Marty sub. All panels are already cut the the standard sizes for full marty. Could this difference in thickness be an issue?


Thanks
 

·
Registered
Joined
·
1,484 Posts
yes, your internal volume may be a bit smaller affecting low end performance a bit. I don't think you'll need to brace anymore though.
 

·
Registered
Joined
·
805 Posts
Are you sure you did your conversion correctly? 18mm is less than 3/4" thickness MDF. If anything, I think MORE bracing will be needed since the panels are thinner at 18mm.
I also wouldn't worry about adjusting bracing based on 18mm vs 3/4" material.

You're talking about a 0.04" difference.
 

·
Registered
Joined
·
95 Posts
yes, your internal volume may be a bit smaller affecting low end performance a bit. I don't think you'll need to brace anymore though.
Other way around. I believe the OP was saying locally available material is THINNER than the plans call for.

I would be inclined to beef up the bracing a bit (more braces, not necessarily larger, but closer together) to compensate, in a Full Marty.
 
  • Like
Reactions: Craig S King

·
Registered
Joined
·
1,484 Posts
Are you sure you did your conversion correctly? 18mm is less than 3/4" thickness MDF. If anything, I think MORE bracing will be needed since the panels are thinner at 18mm.


What dsl1 said, you aren’t losing very much material, the box is already inert as it is.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
1,484 Posts
Other way around. I believe the OP was saying locally available material is THINNER than the plans call for.

I would be inclined to beef up the bracing a bit (more braces, not necessarily larger, but closer together) to compensate, in a Full Marty.


He is free to do that, but the difference in thickness is tiny at best.

Not like you can’t add bracing after the fact though.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
376 Posts
.75in×25.4mm/in=19.05mm

The stiffness is a function of t^3

So (19)^3/(18)^3=1.176

The 19mm material is 17.6% stiffer than the 18mm.

If it were me if throw in some extra bracing -- especially considering you got extra volume to work with now.

Sent from my Moto G (5) Plus using Tapatalk
 

·
Registered
Joined
·
81 Posts
Discussion Starter #10
ok thanks everyone, I can certainly add more bracing just in case.
I hope this difference would not have any adverse impact on sound quality? With thinner MDF (which is the case with 18mm), volume is bit larger.
 

·
Registered
Joined
·
197 Posts
I think most 3/4" is actually 23/32" which works out pretty close to 18mm. Maybe that's the issue, more accurate measurements.
 

·
Registered
Joined
·
805 Posts
I think most 3/4" is actually 23/32" which works out pretty close to 18mm. Maybe that's the issue, more accurate measurements.
For plywood yes, MDF I've gotten is actually 3/4".

ok thanks everyone, I can certainly add more bracing just in case.
I hope this difference would not have any adverse impact on sound quality? With thinner MDF (which is the case with 18mm), volume is bit larger.
Not going to change anything you can hear. Us DIY'ers tend to go way overkill for our sub boxes.

Here is a commercial sub about the size of a full marty. Essentially no bracing.



Do the best you can, if you want to throw in some extra bracing go for it, but from a performance standpoint don't stress. Just make sure it's sealed up well.
 

·
Registered
Joined
·
376 Posts
For plywood yes, MDF I've gotten is actually 3/4".







Not going to change anything you can hear. Us DIY'ers tend to go way overkill for our sub boxes.



Here is a commercial sub about the size of a full marty. Essentially no bracing.







Do the best you can, if you want to throw in some extra bracing go for it, but from a performance standpoint don't stress. Just make sure it's sealed up well.
To be fair those JBL boxes are made to transport -- nobody wants that box to weigh 150#.

That aside, I've been over-killing speaker boxes for 30 years -- why stop now!?

Sent from my Moto G (5) Plus using Tapatalk
 

·
Registered
Joined
·
805 Posts
To be fair those JBL boxes are made to transport -- nobody wants that box to weigh 150#.

That aside, I've been over-killing speaker boxes for 30 years -- why stop now!?
Those are from their theater series not touring.

I am all for over-engineering! I just get worried when I see these threads and we over-engineer someone into misery needlessly.
 

·
Registered
Joined
·
376 Posts
Those are from their theater series not touring.

I am all for over-engineering! I just get worried when I see these threads and we over-engineer someone into misery needlessly.
Some of us have 'special' requirements. My (4) Johnny-sized subs are going on top of my family room built-ins. I want/need those babies not to vibrate... I'd hate to lose any of the wife's display plates... Bruhaha haha haha!
 

·
Registered
Joined
·
1,484 Posts
ok thanks everyone, I can certainly add more bracing just in case.
I hope this difference would not have any adverse impact on sound quality? With thinner MDF (which is the case with 18mm), volume is bit larger.

With bracing you are trying to make the box inert and prevent it from flexing, the Marty design is already very inert as it is which is why I believe you don't need more bracing. We tend to get into this bracing-means-everything loop hole where builders just really want to add more just-incase. Honestly just build the thing and if it sounds off, add a cross section or two but I still doubt you'll need it. I think the Marty is already well braced.


The thing about the Marty build has really nothing to do with the science behind the bracing, its a common build model that maximizes 4x8 sheet goods to bring costs down and make it easy to build. I have two cross braces (dowels) in my sealed sub and its inert.
 
1 - 17 of 17 Posts
Top