-

Jump to content



Sign up for a free account!

Signing up for an account is fast and free. As a member you can join in the conversation, enter contests and you won't get the popup ads that guests get. Click here to create your free account.

Photo

Subwoofer internal bracing question


This topic has been archived. This means that you cannot reply to this topic.
7 replies to this topic

#1 of 8 Jones_Rush

Jones_Rush

    Stunt Coordinator

  • 202 posts
  • Join Date: Jan 19 2001

Posted February 22 2001 - 11:21 AM

I'm building a 90 liter sealed subwoofer.
I'm using 3/4" MDF.
Is internal bracing imprtant ?
If so, what is the easiest way of doing it?
I've seen too many designs, what is the
best bracing design ?

Another question,
My sub's height will be 22", where is the
best place to mount the driver, closest
to the floor ? or should it be at the top ?
a friend of mine told me that the driver should
be placed as far from the floor as possible since
too close to the floor can create a more boomy bass
which will rebound too much from the floor,
on the other hand he told me that I will get less
SPL's if choosing to mount at the top,
is he right ? what should I do ?

#2 of 8 Greg Monfort

Greg Monfort

    Supporting Actor

  • 884 posts
  • Join Date: May 30 2000

Posted February 22 2001 - 12:31 PM

Yes.

Build it out of 3/4" no-void plywood for starters. Posted Image

There is no 'best' per se. In your case, considering the modest size, I would just double up all the panels. IOW build the box, then use regular old wood glue to bond another layer of MDF to it.

This will sufficiently stiffen it.

GM

------------------
Loud is beautiful, if it's clean
Loud Is Beautiful, If It's Clean

#3 of 8 Greg Monfort

Greg Monfort

    Supporting Actor

  • 884 posts
  • Join Date: May 30 2000

Posted February 22 2001 - 01:19 PM

whoops! Hit send too quick. Posted Image

WL coming from a sub below the XO point are so long they don't 'bounce' off the floor, but spread out in a spherical wavefront that is contained by all the boundaries in all but the largest typical home rooms. For a floor 'bounce' at a particular frequency to occur, the distance from the driver to the floor must be 1/2WL, so for a driver sitting up 4ft would have a deep null at (1130/4)/2 = ~141Hz, or in the attenuated portion of the sub's BW. IOW, more of an issue for the mains/CC/surrounds than the sub.

It is true that the closer the driver is, and perpendicular to, a boundary, the greater its reinforcement. Up to +3dB, so it can cause a 'hump' in the FR depending on where the sub is positioned in-room, and if a high enough Q, then it will indeed sound 'boomy'. I normally just put it on one of the sides, wherever it's most convenient.

GM

------------------
Loud is beautiful, if it's clean
Loud Is Beautiful, If It's Clean

#4 of 8 Jones_Rush

Jones_Rush

    Stunt Coordinator

  • 202 posts
  • Join Date: Jan 19 2001

Posted February 23 2001 - 05:36 AM

"Build it out of 3/4" no-void plywood for starters."

Why?
I thought that MDF is the best material for a sub's
enclosure...



#5 of 8 Greg_R

Greg_R

    Screenwriter

  • 1,997 posts
  • Join Date: Apr 09 2000
  • Real Name:Greg
  • LocationPortland, OR

Posted February 23 2001 - 07:25 AM

'No-void' plywood flexes significantly less than MDF (a good thing). However, you want to get good plywood (which is expensive). Plywood comes in many flavors and is made by layering various veneers. The more layers and the quality of layers is what differentiates their respective prices and qualities. Marine grade is usually made up of N, A, or B quality layers of hardwood (and is pricey). Baltic, Finnish, or 'cabinet grade' ply typically uses alternating layers of softwood and hardwood or lesser quality interior hardwood layers. Cheap (Home Depot) ply is usually made from B, C, & D layers. Below is the government's explanation of the letter ratings:

Veneers for plywood are visually graded according to the size, number, and location of natural and processing defects that affect their strength and appearance. Knots, decay, splits, insect holes, surface roughness, number of surface repairs, and other defects are considered. More surface repairs, such as elliptical (boat-shaped) wood patches and bigger knots are allowed in the lower veneer grades. Veneers are graded as N, A, B, C, C-Plugged, and D. N-grade or natural finish veneers are virtually blemish-free, and they contain only a few minor surface repairs. A and B veneers have solid surfaces with neatly made repairs and small, tight knots. Knotholes up to 25 mm in diameter are allowed in C veneers, whereas D veneers may have knotholes as large as 51 mm (2 in.) across. Because their appearance is usually of secondary importance, panels meant for sheathing and other structural uses are made mostly from C and D veneers. The N, A, and B veneers are reserved for panels where appearance is the primary consideration in such uses as exterior trim and soffits, interior paneling, doors, and cabinets.

Hope this helped...

Greg

#6 of 8 Aaron Plaza

Aaron Plaza

    Auditioning

  • 11 posts
  • Join Date: Feb 12 2001

Posted February 23 2001 - 10:23 AM

Any speaker that i have built has been built with two laminated panels of 3/4" mdf. I cut 2 of each panel and then glue them together with wood glue. Using a drywall knife aids in spreading the glue.
When building and bracing subs, I build the shell and then add bracing roughly in the center of each panel. Then using the knock test, listen for areas that need extra bracing. You can never over brace. Take a look at the inside of a pair of B&W speakers, they are braced almost every 2 inches.

#7 of 8 Greg Monfort

Greg Monfort

    Supporting Actor

  • 884 posts
  • Join Date: May 30 2000

Posted February 23 2001 - 11:18 AM

Good post GR! Posted Image One clarification though, Baltic isn't made from any soft woods based on my info. Baltic Birch, Appleply, and marine are considered the best structural grade plys, and the only ones suitable for speaker building IMO.

AP: I agree about bracing, but there's a point of diminishing returns WRT the intended application. Two layers of 3/4" MDF is quite a bit stiffer than the above plys, which pushes the Fs of a modest sized box like JR's well beyond a sub's BW, so bracing is wasted effort IMO.

For wide BW use such as the B&W though, you're right, brace city! Posted Image

GM

------------------
Loud is beautiful, if it's clean
Loud Is Beautiful, If It's Clean

#8 of 8 Pete Mazz

Pete Mazz

    Supporting Actor

  • 761 posts
  • Join Date: May 17 2000

Posted February 23 2001 - 12:10 PM

Typical 3/4" ply has 7 laminations (plys). Baltic birch, apple ply have 13. Baltic birch usually comes in 60" x 60" panels while apple ply is typical 4' x 8'. Current pricing wholesale as of yesterday was $1.27/sq.ft. for BB and $1.67/sq.ft. for apple ply. Both are usually very high quality panels (void free).
Pete

------------------