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-   -   expanding pass through dampening (http://www.diyaudio.com/forums/multi-way/138481-expanding-pass-through-dampening.html)

mcmahon48 13th February 2009 03:21 AM

expanding pass through dampening
 
I am wondering what type of dampening affect using fiberglass matting material fixes to a frame area around the speaker area my idea is that it will dampen the high frequencies by being slightly flexible and allow a certain amount of air to pass through and then stretch and breath for the lower frequencies and this is for a .2 a ported cubic foot area and 5 1/4 woofer/mid speakers

the box is framed all the way around
I am deciding between using acoustic foam or to try this idea:confused:

mcmahon48 13th February 2009 03:23 AM

the mattig cloth does not have fiberglass in it
 
it is that cloth that is used to make fiberglass cling to it to form

speakerdoctor 13th February 2009 12:28 PM

It seems to me you are looking for the same effect you would get with an overly tight grille cloth. Is that correct?
If so, the highs would be dampened and the bass relatively un-effected.

mcmahon48 13th February 2009 06:19 PM

not tight
 
the material would have some flexiblity to hopefully move with the bass some what, and from what I am hearing is that to dampen the bass resonance I will definitely need to use the foam for the amount of movement of the cloth will not be enough to properly dampen the bass resonance effectively enough thank you for you answer

pjpoes 13th February 2009 09:09 PM

What are you asking? Your sentence parsing is a bit unclear, so I can't really understand what it is you want help with here?

Are you asking if putting fiberglass insulation inside a speaker enclosure will have the same effect as foam or polyfill? Yes it will.

Are you asking if putting fiberglass on the front of the cabinet's baffle will reduce diffraction in the same way foam will? Again, yes it will, but fiberglass isn't something you generally want to expose like that. Cheaper alternatives to foam are industrial felt, use that if you are trying to avoid the cost of the foam.

Are you asking if placing fiberglass infront of the baffle and drivers, and if sound will pass through in the same way a foam speaker grill works? Fiberglass has a somewhat uneven porosity, as well as being somewhat denser than the foam typically used for this application. This means that at suitably high frequencies the fiberglass may not only allow sound to pass through, where yes, some of it will be absorbed and converted to heat, but some of it will be reflected back. At certain point it will all be reflected back. Foam is better in this regard since it has a known porosity, and thus a more predictable effect. While it is correct that these materials will absorb higher frequencies more than low, it will absorb some from all, with density and mass playing the biggest roll in how much low frequency absorption happens.

These answers are based solely on my speculation as to what you are asking. I apologize if I haven't answered your true question, and if so, please try to ask what it is you are trying to achieve, and where this fiberglass is going (as well as what type of fiberglass we are talking about).

mcmahon48 13th February 2009 10:48 PM

you are think about the wrong materials
 
This is a cloth like material that used with fiberglass resin I will not be using any resin or fiberglass insulation. The material stretched to form a total cube except one side . Imagine the following a speaker box that has 1/2' X 1/2' bracing along the corners of each wall, then stretched on these bracing is the cloth that forms a 5 sided cube. And each side has some give in it so that it will easily flex for about 1/4" for a total of a 1/2" of movement on each wall of cloth

pjpoes 13th February 2009 11:00 PM

I know what material you speak of now I think, if this is the stuff commonly used in car audio enclosures, it's fiberglass matte. This is fiberglass.

I don't understand why you want to do this? What are you trying to accomplish? An alternate method of loading a driver? This sounds conceptually similar to aperiodic loading, except that it will have zero loading below a pretty high range. I would imagine anything below 500hz is going to pass right through that as if it doesn't even exist. This would mean you have an open baffle enclosure with, I'm assuming, a narrow baffle. This would cause cancellation of low frequencies.

mcmahon48 13th February 2009 11:09 PM

It was thought of how to dampen cabinet resonance
 
It was a thought of how to dampen cabinet resonance and increase effiency by reducing resisitive compression and not creating a vacum affect in the low frequencies and eliminating standing waves and cost a little less money for acoustic foam is not cheap. I will be trying to load the driver a little with the cloth for the Qts is .28 and the Fs is 42.5

pjpoes 13th February 2009 11:13 PM

There wouldn't be any standing waves inside the enclosure for the low frequencies, and at higher frequencies they can be dealt with by polyfill just fine. That will surely cost less than fiberglass matte.

As for the loading, that's going to be driver dependent. That qtc can only be achieved if that is the qtc of the driver, or it's lower, and probably implies little to no enclosure loading.

Go ahead and build one as you want, and take measurements. Prove me wrong, won't be the first time, but I don't see how this is going to do what you want it to. That cloth is going to be pretty much completely transparent. In fact, while earlier I said a number, I take that back, since fiberglass matte is pretty similar to the woven solar shade material being used for acoustically transparent screens. Meaning it should be acoustically transparent, the waves will barely even see this material at all, no loading.

tsmith1315 13th February 2009 11:19 PM

edit:
(looks like there were other replies before this posted, sorry for any redunancy)

Just for clarity, the cloth IS the fiberglass. It is actually long strands of glass fibers woven into a cloth.

The resin is typically some type of plastic, and helps the fiberglass cloth hold its shape when cured.

A stretching of fiberglass cloth around a skeleton box will be approximately equal to the same skeleton with no cloth attached.
It would certainly have some effect beyond blowing itchy little fibers aloft, but not a very predictable effect.

If you want to try it, use the acoustic foam first. If you want to try a cloth, consider using something other than 'glass.

A port in either case would be somewhere between unpredictable, benign, and counterproductive. Their predictability and function (as we use it) depends on having a sealed box where the only leakage is the controlled air mass in the port.

There are (or at least were) foam baffles available to fit behind automotive speakers. Their prime function was simply to protect the speaker cone. This is very similar to the concept you have, and they had very little effect on sound in most cases IME.

By all means experiment. It's fun.

I would suggest you build the same box out of wood, and use that as a standard of comparison for whatever else you try.


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