Help! How does make "dead" cabinet?

[navin]

Hi all,

At the risk of hijacking other threads I have started a new thread just o deal with the issue of how best to build a "dead" box givne the tools available to a DIY.

the box in question is a 48", 16" deep with a 10" wide baffle and 3" wide at the rear. The side panels are to be curved like Wilson Benesch's Act 2 and Sonus Faber's Amati - see links below

http://www.hifi-notes.com/sonusfaberamatifabrikage-nl.htm
http://www.wilson-benesch.com/acttwols.htm

for my cabinets (with curved sides) I was thinking of using lead sheet between 4 mm sheets of MDF. Andy suggested I should loose the lead. someone else suggested using fiberglass to build the box instead of MDF.

The fiberglass I know is either available in sheets (matting) or loose (roving). Fiberglass is then bonded together using resin. Mating has more directional strength than roving and is easier to apply and use so for all practical purposes I am thinking of using mating.

The "sandwich" then would be a three material sandwich of MDF lead and fiberglass.

1. why should I loose the lead? I know it is soft and offers not rigidity whatsoever but it is heavy and can be an excelent damper.

2. If I use fiberglass should I put it on the outside (it is known to be carcenogenic)? Should I out it between sheets of MDF?

3. The inside of the cabinet will be damped using one or a combination of 3 materials - egg crate foam, polyfil, and glass wool. What do you guys recomend? I am thinking if applying 2" sheet of egg crate foam to the walls and then filling the rest of the cabinet with glass wool.

Thanks in advance. And Andy thanks for everything. Where do you and guys like Dave find the time?

Regards
Navin

 

[lieven]

standing wave absortion

try this : build a cage inside the box at 1/3 of the distance of all walls. This cage of wich all cornerpieces could be combined also as reinforcement brackets for outside walls, is filled with longhair wool. Sound absortion is more effective in the middle of the box than on the outside walls where air velocity is less.

Imagine it as a cage hanging in the centre of the box.
Wool stuffing should be semi-transparant to sound.

btw fill the walls of the outside box with bitumen (asfalt) as used on the road.
Could borrow a few pounds if works are going on in the neighbourhood.
Makes it dead silent.

 

[navin]

good idea. i was thinking of doing something similar.

using egg crate foam on the walls and then hanging glass wool from the bracing in the center of the box.

bitumen is dead but you have to apply it hot. very hot. how does one do that.

regards
navin

 

[swirv]

You could also use sheets of Dynamat or other damping material thats easy to get at any store and sandwich it between MDF. Ive seen it done and it works quite well. Plus, Dynamat is heavy..but not as heavy and harmful as lead. Imagine having to move lead-lined speakers.

 

[lieven]

bitumen is dead but you have to apply it hot. very hot. how does one do that.

[/B]

outside walls can be flat, even if inside is bent shape.

deposite the outside (flat wall) on the ground, get the bitumen on it, press with a wacker if you want, after wacking deposite some bitumen without wacking afterwards, bolt the sandwich you just made together eventually with aid of thicker pieces of wood to even the pressure of the bolts.

It's a bit messy but it works.

 

[lieven]

oh yes, afterwards remove the bolts of course or the sandwich effect is gone. fill up with silicone or similar.

One could also make sheets of appx 1,5 cm thick let it cool down and bent in shape (and before complete cooldown. fix it between walls to make a sandwich by application of extrudable foam used in building industry (polyprop).

This way you can make any shape 3-layer sandwich.

 

[planet10]

I'm kinda busy right now, but a couple quick points.

A "dead" box is usually just a "store-energy" release it later box.

I'm in the make the box stiff, with asymetrical, non-integer, non-rectangular panels (or bracing such that the panels look like this).

The curved walls will be really stiff. I bet that two layers (i'd use plywood) of 4 mm bonded together with a west-style epoxy and additional layers of epoxy inside and out bent over an irregualr plywood skeleton will outperform the thick lead damped box you are proposing (and you might actually be able to lift it).

Furthermore, whenever i can, i load the drivers push-push so that much of the energy that would excite the box are cancelled in the driver assembly.

dave

 

[lieven]

Re: Re: Help! How does make "dead" cabinet?

dave

I agree with you if you mean 'lost my energy in total chaos', rather than in a 'harmonic' way.

Indeed you can make the rear panel out of concrete, 1 side panel out of birch plywood, the oposite side sandwich filled with sand and the loudspeaker-panel out of marble, all side in different angles.
BTW all this if you make the famous 'shoe-box-design'.

If you make an open reflex-tuned box you try to transform as much energy to the outside world in a well controlled manner, so in that case the baffle have to absorb as less as possible, only standing waves should be traced and killed.

 

[cocolino]

bitumen is dead but you have to apply it hot. very hot. how does one do that.

Indeed bitumen works excellent to achieve almost dead enclosures.
Bitumen can be bought cheap in stores supplying articles for construction (ussually it comes in 25kg bags).
The best for get a unifrom and thick layer is to melt it and to pour it into the enclosure (it works only for one panel at a time as it has to harden to some extend first before proceeding with the next panel).
For melting I use a simple portable gas cooker as used in road construction (this thing is intended exactly for that purpose).
Pouring has two other beneficial side effects:
You get really sealed enclosures.
Damping pads can be absolutely fixed while putting them into when still liquid.
disadvantages:
Bitumen produce uncomfortable gases (for people as well for the drivers) and one has to wait about a week or two until they disapppeared.
Bitumen makes nasty black marks on everything (for example on ready veneered enclosures ). Take care to cover things which have to kept clean (with packpaper etc.)!
Melted bitumen burns on skin like hell (protect Yourself and wear gloves at work!!) .

 

[navin]

I am tending to agree with Dave.

makig a dead stiff panel was only half my story.

The cabinet (48"Hx16"Dx10"W) would be braced with 6 H braces at irregular intervals. The edges of teh cabinet (where the baffle and back meet the sides) would be braces wiht 2x2 wood. After all I need is a net volume of about 1.5 cu. ft for a 8" and 6".

yes a "store energy release later box" is not what I want. But I want to be sure the box is stiff enough.

Hence what I plan to do is:
1. bend 4mm.
2. add another 4mm
check the box by building baffle (the baffle will be wider than the box at this stage)
3. add additional 4mm sheets till I feel the box is stiff enough.

I expect 6 layers will be enough.

Dave if you are sure than I can get away with thinner panels I could use 3mm as it would be easier to bend.

Then 6 layers would be 18mm.

Also why should I avoid the lead? Should I just lay a few layers of lead to the bottom of the box.

About moving the boxes I am not worried. these speakers are staying put. I am only 65kg but I have moved 120kg boxes before and lift 30kg boxes (my MTMs and my power amp are 30kg+ each) often enough. read as "lots of energy".

suggestions?

 

[navin]

Oh BTW since we are on the topic of bracing....

I also intend to use what I call "torsion braces".

These braces are effectively hollow steel or PVC pipes sealed at both ends.

the pipe are filled with lead shot, sand or thermacol pioece like the found in bean bags.

the pipes are then mounted to the read of the driver (on the magnet) using blu tak and are bolted to the back of the speaker cabinet. In this case I would 1 for each 8" and 1 for each 6".

The drivers are mounted using t nuts.

suggestions?

 

[planet10]

what I call "torsion braces".

These braces are effectively hollow steel or PVC pipes sealed at both ends.

the pipe are filled with lead shot, sand or thermacol pioece like the found in bean bags.

the pipes are then mounted to the read of the driver (on the magnet) using blu tak and are bolted to the back of the speaker cabinet. In this case I would 1 for each 8" and 1 for each 6".

The drivers are mounted using t nuts.

suggestions?

You want to be able to adjust the brace -- ie tighten it to make sure you have a good coupling to the back of the cabinet.

Push-push takes this concept a bit further, by having an equal & opposite device at the end of the rod/brace. The best rod is no rod (ie magnet to magnet), but that usually makes the box impractically thin.

dave

 

[navin]

i would use 2 8" / 6" per side if I had them.

yes i intend to tighten it so that is is well mated with the back of the woofer magnet (actually this is the pole piece isnt it?).

BTW forgot to thank you for ALL your suggestions.

the filler will dissipate kinetic energy. seen this is a tennis racket once. they had some lead balls in side the graphite tubing it think. the racket was called the Kinetic.

thanks again!

 

[planet10]

the filler will dissipate kinetic energy. seen this is a tennis racket once. they had some lead balls in side the graphite tubing it think. the racket was called the Kinetic.

Forgot to mention, a layer of shrink-wrap on the outside of the bar is helpful too.

dave

 

[navin]

what is the shrink wrap for?

 

[planet10]

what is the shrink wrap for?

An external layer of damping to resuce resonances in the pipe (i'm assumming metal pipe -- PVC isn't really rigid enuff).

dave

 

[navin]

shrink wrap? the kind they use to store food in the freezer? that is awful thin and flexible isnt it?

what about something more substantial?

 

[planet10]

shrink wrap? the kind they use to store food in the freezer? that is awful thin and flexible isnt it?

I should have maybe said shrink tubing -- the stuff that comes in cylinders for putting around wire for insulation & strength. Quite substantial. I think it's bedtime.

dave

 

[capslock]

... The best for get a unifrom and thick layer is to melt it and to pour it into the enclosure (it works only for one panel at a time as it has to harden to some extend first before proceeding with the next panel).
For melting I use a simple portable gas cooker as used in road construction (this thing is intended exactly for that purpose).

Hi Cocolino,

I am not sure how you do the coating one panel at a time. If you have a rectangular box, do you simply cover the side that is at the bottom, wait for this side to cool, turn the box and continue with the next side?

How thick can the coat be before you get problems with shrinkage etc.?

What is the melting temperature of bitumen?

As for the fumes, I wonder if they continue to escape even after a few weeks. And they are not something I would feel comfortable with in my house. You could use plastic spray to coat the inside of the box before you apply the bitumen and then you could apply another layer once everything has cooled well enough. Plastic spray ceases to outgas after about 48 h and should work well enough as a sealant.


Just two off-topic notes:
- Thanks for the article on coils. I will summarize the most important points soon. Unfortunately, some of the most interesting things did not get measured. Maybe I will do my own experiments at some point.

- Isn't that a Manger driver in your atavar image? Do you own Mangers? Which?


Cheers,

Eric

 

[capslock]

Push-push takes this concept a bit further, by having an equal & opposite device at the end of the rod/brace. The best rod is no rod (ie magnet to magnet), but that usually makes the box impractically thin.
dave

Using push-push with a magnet-to-cone rather than magnet-to-magnet configuration would rob you of the point to attach the rod, but it would provide the benefit that assymetries in the drivers cancel while you still have overall impulse compensation.

Greetings,

Eric