I've a couple of 13" woofers that I'm planning on building into a sealed box. I've done the TH measurements and calculated that a box of around 180 litres is the right ball park for a Qtc of 0.707. The EPB suggests a sealed box only. Large, I know...
Given that stuffing has an effect on Qtc would it be correct to build a smaller box of say 150 litres and progressively add stuffing and keep measuring until 0.707 is reached?
Box construction I'm thinking of thin-wall with bitumen pads, and acoustic foam (egg-crate) to reduce standing waves.
Stuffing I'm thinking old fabric based carpet underlay (not the dimpled rubber stuff) as I've got lots - or alternatively wool - except I'll have to wash and card the fleeces and that'll take many, many hours.
Am I on the right lines here?
Given that stuffing has an effect on Qtc would it be correct to build a smaller box of say 150 litres and progressively add stuffing and keep measuring until 0.707 is reached?
Box construction I'm thinking of thin-wall with bitumen pads, and acoustic foam (egg-crate) to reduce standing waves.
Stuffing I'm thinking old fabric based carpet underlay (not the dimpled rubber stuff) as I've got lots - or alternatively wool - except I'll have to wash and card the fleeces and that'll take many, many hours.
Am I on the right lines here?
I think many people are comfortable with using PolyFil, Rockwool, or Fiberglass insulation. Nothing else is likely needed outside proper bracing.
As far as what amount of damping is possible, I would recommend to simulate in HornResp.
Somewhere in my 2way thread is info on the HornResp FR settings one might use for Rockwool insulation, the board probably knows as well...
members put stuff in their boxes but hardly ever evaluate it with tests - i am refering to impedance curves - hence the age old question what to use and how much....
Adding stuffing does cause the Qtc to decrease.
The extent of the decrease will depend on the kind of stuffing and the stuffing density.
I'm with your bitumen pads which will help damp panel vibrations, but the acoustic foam is practically transparent at bass frequencies.
Instead, I would line your internal enclosure walls with your fibrous carpet underlay. Glue it on top of the bitumen to add to the enclosure wall damping.
Then experiment on the quantity and density of packing of the carpet underlay throughout the volume of the enclosure. You may have to use a lot of densely packed fibre to achieve he required reduction in Qtc.
Of course, wool has excellent absorbent properties and could give superior results. You could let the 'Woolly Shepherd' do the hard work for you!
https://www.woollyshepherd.co.uk/loudspeaker-wool/
First I need to determine box size - so I was working on the rule of thumb that wadding can decrease the needed size of a large box by 20% - so I guessedtimated the calculated 180 down to 150 litres and then add stuffing 'to taste' - or measurement? Being new to this I'd prefer to get the dimensions right before cutting any wood!
I mentioned wool as our last shearing produced 10 odd fleeces that I still have... it is a lot of work however to turn into stuffing. Regarding the woolly shepherd I contracted them to acoustically treat a hall/theatre. Not only did their designs work incredibly well (our community windband could at least hear themselves without all the time-smearing) the suspended clouds and wall absorbers really added to the decor. I can't recommend them enough!
wouldn't be needed with proper bracing
Wool is great but expensive while Rock Wool and Fiberglass insulation is basically equal and much cheaper.
I humbly beg to disagree! 🙂
Synthetic fibres are smooth, whereas natural fibres are rough - and that randomness is what makes wool such a good sound absorber.
Synthetic fibreglass fibres:
Wool and other natural fibres (and polyester for comparison):
My experiments with HR show that after a certain damping factor you reach a point of diminishing returns
12,000 mks rayls/m is plenty... after I experimented with this in HornResp I stopped being so concerned about using whatever they had at the Store because the RockWool/FibreGlass/NaturalWool are all pretty much a superior product for the task and then it became a matter of whats cheapest and safest. Otherwise, Density and Depth are more potent factors.
12,000 mks rayls/m is plenty... after I experimented with this in HornResp I stopped being so concerned about using whatever they had at the Store because the RockWool/FibreGlass/NaturalWool are all pretty much a superior product for the task and then it became a matter of whats cheapest and safest. Otherwise, Density and Depth are more potent factors.
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Wouldn't a restriction affect mass, rather than resistance, except where turbulence becomes an issue? The usual damping action involves movement of the fibres to produce heat.
Visaton Germany measured stuffing of closed boxes
https://www.visaton.de/de/service/technische-grundlagen/bedaempfung-von-gehaeusen
https://www.visaton.de/de/service/technische-grundlagen/bedaempfung-von-gehaeusen
IN other words Damping isn't effective in moving Q in a sealed enclosure. This is likely true with 2 exceptions.... Vented enclosures and possibly Aperiodic Bichambers
Careful though - harmonic distortion could excite box resonances if there's no attention paid to damping. Subwoofer drivers will generate appreciable levels of 5th harmonics, and these could be problematic, particularly for (typical) cubic boxes.
Nah, the driver itself will have distortion levels considerably above the effects that might be reduced by stuffing.
A sub-woofer will be asked to do two octaves (at most) and will most likely have some serious low-passing above that.
A good driver and good construction techniques are obviously required. But stuffing is not.......for subwoofers.
Dave.
However, those figures in post #14 do not include Qtc (or system Q).
I found the following figures, obtained using BassBox Pro, in this link: https://www.bestcaraudio.com/car-au...ng-a-subwoofer-enclosure-make-it-seem-larger/ The simulations include these degrees of stuffing: Minimal, Normal and Heavy.
"In the un-stuffed enclosure, we see a predicted Qtc of 0.924. In the enclosure with the minimal stuffing, the Qtc drops to 0.906. In the normal scenario, it’s predicted to be 0.837. Finally, in the heavily stuffed enclosure, the Qtc is a well-controlled 0.755."