DRAFT: FAQ - Dust Extraction (Practical Aspects)
Dust Extractor Enclosures
Enclosures for dust extractors are increasingly being used by wood workers to remove dust and noise from their sheds.This post is not about the "why", but rather the “how" , to reduce noise aspects of these enclosures.Apart from weatherproofing external enclosures, and making internal enclosures air tight (otherwise dust leaks back into the shed) the principles of construction are the same for both types of enclosures.
On cyclones most of the noise comes out of the impeller outlet so if this is muffled using a free flowing Muffler most of the noise can be reduced to the point where the enclosure only needs to be a weather proof structure of some kind.
Further noise reduction may be needed in high density living situations. if this is you then just follow the same pathway as shown below.
On a DC which uses filters the noise also comes out of the impeller outlet but the filters get in the way so this is where a noise reducing enclosure is required.
The Enclosure Frame.
The frame and roof of the enclosure needs to be stiff.
To assist with this decreasing the spacing between the framing components and cladding the frame with a sheet of wood or metal will help reduce the amount by which the walls and roof can flex.
A standard timber stud wall covered with MDF should be sufficient but if you are at all concerned, then further reducing the spacing between the studs and adding extra noggins will help.
To reduce the sound escaping the enclosure at least two layers of materials are needed.
One layer needs to be made of a thick/dense/heavy material that reflects most of the sound back into the enclosure.
The other layer needs to be a thick layer of air trapping material that will absorb some of the sound as it passes through it.
Remember no single layer will reflect or absorb ALL the sound that strikes it or tries to pass through it - Egg cartons alone will not work
Dense Layer (DL)
The material for the DL can be, Lead, concrete, brick, wood, or plaster,
The denser it is, the thinner it can be, so a lead sheet 5mm thick will be equivalent to wood around 60 mm thick.
However it cannot be too thin or it may act like a drum and transmit the sound out of the enclosure.
This means the thicker this layer is the better.
As a guide, using something like 16 mm chipboard will produce a useful effect.
Air trapping Layer (ATL)
The air trapping layer ATL can be,
- multiple laters of cardboard
- foam rubber (old mattresses are excellent),
- acoustic insulation material (expensive)
- even plain old thermal insulation will do something.
The thicker this this the better.
As a guide using something like 50 mm of thermal Rockwool insulation or foam rubber will produce a useful effect.
Carpet also has to be quite thick - enough to make a 50 mm thick layer to be effective.
The order of layers is not that important in this situation and ease of construction and type of material used usually determines what is done.
For example, if the wall structure itself is not thick enough to contain the ATL then sticking it to the inside wall works for foam rubber, but no so for fibreglass.
The more layers of both the better they work and they don’t have to be sandwiched between each other.
External cladding
Flat Sheet metal is suitable as an external weatherproof cladding but not as a sound reflector so if you use use sheet metal as an external cladding the framing structure will need additional stiffening to reduce any drumming effect.
Ribbed sheet metal and corrugated iron are suitable for external cladding as would be marine ply. Less desirable would be weatherboards.
Some examples of wall/roof structures
Basic
35 x 45 mm pine stud wall clad in Colorbond, in between the pine studs fill with 50 mm of insulation fibre and clad with 16 mm chipboard or MDF or plaster
Better
90 x 45 mm pine stud wall stiffened on the outside with 12mm chipboard covered with Colorbond.
Infill with 90 or 100 mm of insulation and cover with 16 mm chipboard or MDF
Better still
Use 2 layers of the 16 mm MDF or plaster, and add 50 foam rubber on top of that.
Top of the wazza
If you want to get really fancy you can use a concrete wall, or a double stud wall setup with a 100 mm gap between the walls full of acoustic insulation.
The limit to all this is the sound escaping from the air escape path whereby it’s a waste of time making the walls more sound absorbing if the sound coming out of the air escape path is dominating the noise.
Location of DC inside enclosure
While it may be tempting to hang the DC on the walls of the enclosure this can lead to problems with the sound from the DC transmitted direct to the walls. It better to place the DC direct onto the ground or floor of the enclosure and if you enclosure has a floor consider separating that from the walls. Carpet on the floor will also help
Doors
These represent a significant pathway for noise to escape. They need to be covered in the same material as the walls and be well sealed.
Foam rubber edge seals and positive closing mechanisms will help.
Remember to make the doors big enough to get the DC in and the sawdust out
Air escape path.
The air has to be able to escape freely from the enclosure otherwise it will add resistance and reduce the extent of to the air/dust collection.
If you have a lot of spare height space then a muffler or straight tunnel is one option.
Tunnels can be
- collapsible insulated air con ducting plus some addition foam or rock wool type insulation if needed
- An MFD box lined with rockwool that is held in place with some chicken wire. This is what we're using at the mens shed for the ClearVue cyclone.
The box is 400 x 400 x 1.8m x 12 mm thick MDF and lined with 90 mm of rockwool which is held in pace by an 8" tube of chickenwire.
For a smaller DC a 350 x 350 x 2m MDF box lined with 50 mm of insulation would be OK
Baffle boxes are the same as tunnels except the air does multiple U turns to escape.
More U turns - better sound absorbance, there should be at least 2 U turns in the box, air entry and exit to the box counts as 1/2 a U turn.
Some insulation inside the turns helps.
Because multiple U turns generate slightly more resistance a large cross sectional area is needed. I recommend more than 2X the cross sectional area e.g. I use 4 for my setup at home
See BobL's shed fit.
Straight tunnels have to be longer and baffle boxes have to be fatter (Wider - taller)
The longer the travel path the more sound will be absorbed but the larger the cross sectional area of the escape path needs to be to prevent too much resistance building up.
I would recommend at least 1 and preferably 2m long for a muffler. A baffle box can be shorter but in practice you end up using the same path length because the party od more convoluted.
Here are some schematic cross section examples.
1: This is a basic muffler and insulted pipe of some kind
2: same as 1 only thicker insulation means better sound absorbance
3: Is a baffle box. This one has 3 U turns 1/2 for each of the exit and entrance, and two internal turns.
4) is a sort of combo of a muffler and baffle box. This is like a car muffler. It works great at reducing noise but requires D be larger than the other examples because it generate significant turbulence.
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