A cabinet out of thin material

[ctrevith]

I'm curious how much energy is being transmitted to the cabinet via the mechanical coupling to the frame of the driver versus acoustic coupling from the back-wave and how either issue would be remedied?

 

[Bob Brines]

I'm curious also, but I'll never know. I can't think of any way to demonstrate this. My answer is a window brace above and below the driver. Dave/P10's answer is the "holey brace". Does either work? I don't know.

My gut tells me that with a reasonably narrow baffle, the baffle/sidewall structure will contain any energy transmitted from the frame.

Bob

 

[gafhenderson]

I'm curious also, but I'll never know. I can't think of any way to demonstrate this. My answer is a window brace above and below the driver. Dave/P10's answer is the "holey brace". Does either work? I don't know.

My gut tells me that with a reasonably narrow baffle, the baffle/sidewall structure will contain any energy transmitted from the frame.

Bob

I'm curious how much energy is being transmitted to the cabinet via the mechanical coupling to the frame of the driver versus acoustic coupling from the back-wave and how either issue would be remedied?

could some kind of dipole/isobaric type configuration be used, where one driver essentially cancels the inertia of the other driver, but both are loading the box? the control method for the mechanical loading would then be to run them both out of phase so they cancel the air displacement within, but double the inertia along the coherent plane of motion? compare the 2 and if there's a significant contribution then it will show up fairly obviously as you're only really changing one variable?

 

[Enochrome]

gafhenderson ,

Not cheeky a all, but that product may be a little hard to source on this sind of the pond. I'll look at its ingredients, as far a one can, and see what I can get here.

I product that I am very interested in is a underlayment for laminate floors. It really deadens the sound of anything dropped on the floor. It looks like a poly ethylene or poly polypropylene foam. Trouble is, those materials are very difficult to bond to anything else. We'll see.

Bob

I have visited Home Depot many times and wondered if that floor insulation is usable? It is certainly cheap and similar to felt that other have used. What about adhering it with "Green Glue" which is the acoustic dampening glue?

 

[DrBoar]

M Colloms High Performance Loudspeakers second issue (1982), do buy the current issue and get a really good book to read!

Matching of mechnical impedance and foam thickness for maximum decay rate

 

[Colin]

Note that with the thin-wall technique it is important not to add any additional bracing, otherwise you push the main resonant peak up in frequency once again.

I know the technique was used in reflex enclosures (and continues to be used by Harbeth). I don't know if it is applicable to the longer, thinner MLTL alignments, where GM and others have stated that stiffness is the key.

FWIW, I built an enclosure using a composite of 12mm ply and 12mm insulation board (the sort available in timber yards and often used as notice boards). It made for a stiff, lightweight enclosure wall and worked well in a version of GM's 48" MLTL for the Jordan JX92S. But no measurements to back it up and I no longer have the enclosures.

I'm intrigued by the thin-wall technique and will have a go on my next small enclosures.

 

[DrBoar]

Colin
Have you seen my homage to Spendor BC1 using 11mm OSB and 12mm bitumen loaded soft board topped of with 10mm felt. Each layer glued by latex glue. I am thinking about building a Decca corner horn using a similar way of building.

http://www.diyaudio.com/forums/multi-way/228670-one-bc-homage-classic-vintage-design.html

 

[mondogenerator]

ive used 6mm MDF and 12mm lining of plasterboard, lined with carpet and a piece of 25mm PU foam.

It worked rather well, but looked rough. (it was a testbox and i literally used mastic to glue the box together, and no nails. Painted with leftover exterior woodstain.

I wonder what ply / plasterboard / soft fibreboard would be like?

Or even just birch ply outer, with spruce or other cheaper, and weaker, lighter and softer ply?
Perhaps the thickness would become too great?

 

[Bob Brines]

I'll not be doing any experiments until after LSAF because of my current preparation. After that, I'll spend 1/2 my time on the golf course and 1/2 my time in the shop.

After seeing the Collom charts, I see the panel resonances in my speaker. Whether they are audible in the farfield, I don't know. The next test build will be a constrained layer between 6mm plywood. The first attempt will probably be cork because it is easy to get and easy to laminate. I would really like to try the foam sheet floor underlayment. I am sure it will outperform cork, but I will have to find the right glue to laminate the panels.

Additionally, I will have to abandon butt-joint joinery.It will have to be dado'd edges in solid material, probably poplar.

Bob

 

[DrBoar]

There are some measurements regarding damping and bracing to be found at
dämmung-und-versteifen
The left graph is sound transmission through a board compared to 19mm MDF, the far right graph is the same board but with a contact microphone measuring relative vibration levels.

The softboard board is "Weichfaserplatte"

 

[Colin]

Hi Bob - A lot of this work has been done before, so it is worth having a good look round the web. In particular, the KEF LS50 white paper describes their more recent research to update the cabinet design which went into the BBC speakers. They ended up with a much thicker cabinet (possibly for commercial manufacturing reasons - I don't take it all on face value). The cabinet was then cross-braced but with a mastic compound damping the points where the bracing contacted the driver and walls.

Here's a link to the PDF

http://www.kefamerica.com/july12/LS50 White Paper.pdf

The BBC papers (there are a couple of others, btw) stressed that the material of the battens made a difference to their approach, beech being the best sounding.

Here is the current BBC R&D page. It covers a wide field so digging out the loudspeaker material can be hard work

Publications - BBC RD

Some of them are directly accessible via this BBC LS3/5a site

LS3/5A Library


Dr Boar - Hi, yes I did see your BC1 build. It seemed to vary from the BBC approach. I also know that Spendor went through a period when the speakers were less well-regarded to changes in the manufacturing process in the 80s. I don't know if this related to the cabinet construction, drivers or crossover. It's worth having a Google around some of the fan sites.

 

[Bob Brines]

What I gathered from the study above is that different stiffness inner and outer boards with a soft glue is the best answer.

Bob

 

[IG81]

You guys are giving me ideas for an upcoming build here. It will be a medium-sized (43L internal) Onken-ish enclosure for an 8" driver and CD waveguide. My panel sizes would be a bit more of a hassle to cut in the 5'x5' 18mm BB-ply easily available to me, but a breeze (not to mention easy transport) in the 2'x4' sheets of 11mm BB-ply. It's just that I'll be somewhat limited to when and where I can get my woodworking done this time around. Witn Onken-venting, the sides are inherently well-braced, which would leave the front, back, top and bottom to treat with something like bitumen sheets or some kind of CLD approach. I'll have to figure if this is a good way to go.

IG

 

[xrk971]

What I gathered from the study above is that different stiffness inner and outer boards with a soft glue is the best answer.

Bob

Would a thin 1/4-in plywood glued with construction adhesive to 3/4-in isocyanurate foam sheathing work well as a speaker cabinet? Or do you need another layer of plywood on inside as well?

Several people in the foam core thread have made cabinets with 1/4-in ply glued to 3/16-in foam core board to make a light and sturdy enclosure.

 

[Bob Brines]

Would a thin 1/4-in plywood glued with construction adhesive to 3/4-in isocyanurate foam sheathing work well as a speaker cabinet? Or do you need another layer of plywood on inside as well?

Is "isocyanurate" the same as "Blue Dow"? I can get that in 1/2" and 3/4". I would thing the either glued to 6mm plywood will be stiff enough for the drivers we use here. One of the Alpairs will be my test driver.

Bob

 

[DYNABLASTERTUNERS]

This is thin wall! 😀
6moons audio reviews: Kiso Acoustic HB-1
This guy did everything "wrong", also look at the crossover coils placement lol 😀

 

[thoriated]

That sure is. It looks built almost identically to a $40.00 acoustic guitar I've seen recently.

 

[gafhenderson]

You guys are giving me ideas for an upcoming build here. It will be a medium-sized (43L internal) Onken-ish enclosure for an 8" driver and CD waveguide. My panel sizes would be a bit more of a hassle to cut in the 5'x5' 18mm BB-ply easily available to me, but a breeze (not to mention easy transport) in the 2'x4' sheets of 11mm BB-ply. It's just that I'll be somewhat limited to when and where I can get my woodworking done this time around. Witn Onken-venting, the sides are inherently well-braced, which would leave the front, back, top and bottom to treat with something like bitumen sheets or some kind of CLD approach. I'll have to figure if this is a good way to go.

IG

CLD is the way to go in my opinion, from everything i've read. due to WAF constraints, anything that i'm allowed to litter the (my own :S) house with has to be attractive. the rounded curved cabinets usually get the thumbs up. using composite style, stacked layer construction of these things gives me an ideal opportunity to use a 'sound deadening' adhesive like i listed earlier, using layers of 1-2mm aeronautical ply vaneers to make my own thicker ply with greater internal dampening than standard ply. lot of extra work and cost but i've found drivers i actually like now, so won't be changing them any time soon.

 

[Cal Weldon]

Is "isocyanurate" the same as "Blue Dow"?

Hi Bob,

Blue Dow is Type IV extruded polystyrene.

Polyisocyanurate is a thermoset plastic foam, similar to Polyurethane and Phenolic insulation.

For this use I would consider them very similar in nature. There are both rigid board insulations but are used for different purposes.

 

[xrk971]

Hi Bob,

Blue Dow is Type IV extruded polystyrene.

Polyisocyanurate is a thermoset plastic foam, similar to Polyurethane and Phenolic insulation.

For this use I would consider them very similar in nature. There are both rigid board insulations but are used for different purposes.

I think it might actually be "Pink" color foam? Does anyone know if it has paper facing or is it bare foam?

Shop Dow 3/4-in x 4-ft x 8-ft Polyisocyanurate Insulated Sheathing at Lowes.com

4x8 ft sheet is $17 at Lowes. More pricey than polystyrene but I think is less "crunchy" and less "brittle".