Enclosure Stuffing

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#81
gedlee said:
This is not entirely true. The two paths - inside and outside - are in parallel, but they are distinctly different impedances so the net flow through each leg will be substantially different - not the same at all.
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I did say not to get too techy. Always has to be someone.

Earl, let's try basic, very basic physics. Let's also stick to a sealed loudspeaker. The cone of a speaker moves in and out. The amount of energy put into the room is equal in energy terms to the amount of energy expended into the enclosure. Now if that's wrong in basic terms then I'll eat my hat.

Why do you need to complicate things when we are helping DIY's to explain basic concepts.

Back to you.

Terry
 
#82
gedlee said:
A common falicy. For a given closed shaped volume, not too much different in any direction, the number of modes in a given band is fixed no matter what the shape or how many baffles you have. The mode shap will differ, but there won;t be any more or less due to a shape change.
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It is not a common fallacy. See the very good work done by Vance Dickason in his work Loudspeaker Cookbook. The results speak for them selves. Earl, I know what you are trying to say, but, when the project is finished the shape does impinge on the final freq. response.

Terry
 
#83
CLS said:
How effective is it? Is there any 3rd party testing?

If an effective bass absorbing material do exit, then we may build a small sealed box which behave like an IB (WOW!), or a PERFECT labyrinth. That'd be fascinating!

Is it THAT effective?
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dont know about any data. but will venture an educated guess:

IB - NO

Labyrinth - YES

if you plug the whole cabinet with that stuff there will not be any reflections. the problem is you will have pretty strong attenuation in the bass which may be accompanied by increased distortion.

so i would shoot for a compromise and leave some space open. you can experiment with the actual rations but i have no doubt that perfection is easy to achieve with this route.

i have never seen a single speaker use enough of the stuff.

also keep in mind that if the foam is very dense it may subtract from effective volume a little. so you may need a bigger box. obviously all this increases cost so there is little wonder why nobody does it but they should.
 
#84
speakerdoctor said:
We're both right. In a closed box, system stuffing does both functions (e.g. damp standing waves and increase the effective box volume by increasing the compliance). Thirdly, the system Q can be controlled by the amount of stuffing. In a given closed box system, system Q's in the range of <0.5 to >>1.0 are possible with varying amounts of stuffing. The 'tightness' or 'looseness' of the bass sound is a function of system Q. Tightness is associated with lower Q. Looseness or more 'boom box' sound can be had with Q's well over 1 - All with the same volume box and driver. Attached is a NF response test of an Acoustic Research model 4x acoustic suspension speaker done as part of a larger stuffing study I conducted. The blue line is with the original rock wool stuffing. The red line is with the stuffing removed. The benefit of the stuffing is clearly evident in the flatter hump at system resonance (lowered Q as well) with is lowered appaox. 10 hz and, also the attenuation of the just over 1 khz hump in the response. This speaker certainly wouldn't be considered 'critically damped', I suspect, by design to yeild a warm, overall tone many AR enthusiasts love.

Granted, stuffing mitigates stanting waves, particularly at the upper range of a woofers response curve. However, most woofer's response are normally attenuated in the upper range with series inductors designed to mesh the response of the woofer's smooth pass band with either a tweeter or midrange element. OTOH, if you are designing a speaker with a woofer having no series wired coil, then certainly it would be judicious to address the higher frequency standing waves that could indeed be audibly noticeable.

BTW, the 4x used in the study did have a series coil with the woofer in the crossover.
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your premise seems to be that you must start with a misaligned speaker and stuff it until it is properly aligned. to me that is a waste of time and money.

what i am saying is the amount of stuffing is to be chosen based on the amount of standing wave attenuation that is required and the alignment should be dialed in by box volume and driver parameters.
 
#86
gedlee said:
Foam is foam and none of it works very well at LF. Basically the material is secondary to the situation (volume and location) especially at LFs.
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the stuff is very dense. i used a saw to cut it. foams vary in density considerably. the denser the foam the more expensive it is most of the time.

cheap furniture will use the lightest foam. sports car seats will be made out of dense foam. Auralex is probably denser than the stuff going into sports seats.

logic suggests that the more dense the stuff is the more it will absorb bass. i personally think Auralex is probably TOO dense to have 100% of speaker volume in it. i think it would attenuate bass TOO MUCH to the point where you would lose usable bass output, not just reflections.

i would preferably use, just like you said, 100% of box volume in foam but of two different grades. Auralex for most of the box and a lighter foam closer to the driver so it can actually move.

but i don't know if there is any light foam specifically designed for sound absorption ( because logically you would want heavy foam to absorb sound ) so i said use stuffing.

i agree with you for the most part but not with your claim that foam is foam. that is just simply not the case. try laying down on a tempurpedic mattress and you will see that not all foam is created equal.
 
#88
Ed LaFontaine said:
...and YOU base this on what???

I think it would work better with NO stuffing than with 4 pounds/cf

Y'all smell sumthin'?
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i base this on my bulletproof logic that states the more the merrier.

now that doesn't mean that there will not be some terrible distortion associated with it that will be absolutely intolerable - all i am saying is that more damping material will provide more damping.

however at some point further increasing stuffing density becomes a waste because you can simply use foam instead. foam's resistivity can be increased at relatively less extra cost than that of stuffing. on the other hand stuffing doesn't have any real limit to how low it can go in terms of cost per cubic foot.
 
#89
badman said:
Way to cherry pick. The obvious point of contention is that "foam is foam", which is in direct conflict with both your choice of foam for your waveguides, and reality.

Open cell acoustic foam is open cell acoustic foam, and none of it works very well at LF.
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yes. once again i think Geddes is WAY off base with his foam is foam claim.

foam is probably the most variable material out there.

there is the foam that is used to build DJ speaker enclosures out of and there is the foam placed into woofer vents ( ex: Morel, Skaaning ). one passes zero bass and one passes 100%. i would venture a guess that 0% and 100% are not the same.
 
#91
gedlee said:
A common falicy. For a given closed shaped volume, not too much different in any direction, the number of modes in a given band is fixed no matter what the shape or how many baffles you have. The mode shap will differ, but there won;t be any more or less due to a shape change.
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i agree. conservation of energy. without an energy sink ( Foam ) the energy ain't going nowhere but out the speaker.
 
#95
Borat said:
yes. once again i think Geddes is WAY off base with his foam is foam claim.

foam is probably the most variable material out there.

there is the foam that is used to build DJ speaker enclosures out of and there is the foam placed into woofer vents ( ex: Morel, Skaaning ). one passes zero bass and one passes 100%. i would venture a guess that 0% and 100% are not the same.
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Every porous absorber (foam, fibreglass, polyester, etc.) works by the same principle (dissipate sound energy into heat). I think Earl knows that and was never talking about any other type of foam. So why do you guys throw in closed cell foam? It has absolutely nothing to do with sound absorption.

Best, Markus
 
#100
doug20 said:
Isnt box dampinging and room sound absorption two different topics?

I use this great link for finding materials for building a HT room

http://www.bobgolds.com/AbsorptionCoefficients.htm

but speakers are a completely different beast and sound dampening in a speaker has less to do with absorption coeffecience stuff, no??
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Dampening and absorption are two different things. The former tries to maximize transmission loss (e.g. room walls), the latter is about removing sound energy by energy dissipation (e.g. into heat). But certain dampening methods can also lead to absorption. Green Glue provides some interesting readings: Green Glue Applications

Best, Markus
 
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