Does the AR3 fiberglass stuffing age?

[Guest mazur]

Hi.

I was wondering whether the fiberglass used to stuff the cabinet of the AR-3 age and loose its original properties, therefore altering the sound?

I know it's been discussed here often, but if I wanted to replace with newer fiberglass would it make any difference? Would standard fiberglass found on Home Depot work?

Thanks.

--

Elias

[dynaco_dan]

Hello Elias

Your fiberglas should not under normal circumstances be a concern.

Now that I made that statement let me expand upon the story.

It may be slightly settled from the day it was placed in the enclosure.

This is very likely a hair splitter unless it was wet or got wet and compacted somewhat.

Under normal circumstances there won't be mould, mildue or rot inside the enclosure.

Being glass fibres they should be unchanged from day one.

If you had a foam insulation then you may have an issue.

I'm not familiar with the various foam fillers that have been used in

packing the enclosures.

I'm sure the amount and placement was probably critical.

If polyurethane type foam was used, the same material as the surrounds, then we do have an issue.

In this case we would have basicly dust inside the enclosure.

Advent also used fiberglas and/or foam as well as others I'm sure.

If you were going to change from foam to fibreglas or reverse then you would need factory advice or highly technical equipment to test the correct amount to use.

I believe this is beyond our usual simple modification.

Doing this can seriously alter the bass frequency and tonal balance.

I've read on this site where even in the same model AR3a speaker there has been ounces of difference in fiberglas insulation.

Maybe we would not be able to detect the difference and maybe AR couldn't as well.

It would be a slow process but not impossible to remove the woofer on an AR3a and change the filler.

You would then need to re-seal and tighten the woofer and then only by using your ears try to judge the correct amount.

Slow by trial and error and not accurate but not impossible.

I have an old project page from I believe, Audio magazine, about adding a sub-woofer to a pair of Quads. They used wool as the stuffing and that was interesting. Baa.

Could you imagine someone suggesting running a hair pick through the fiberglas to fluff it up. Sort of teasing it. lol

There is much more knowledgeable members that will add their commentary as they read your question I'm sure.

Excellent question and we will all learn more as others join in.

Good luck.

>Hi.

>

>I was wondering whether the fiberglass used to stuff the

>cabinet of the AR-3 age and loose its original properties,

>therefore altering the sound?

>

>I know it's been discussed here often, but if I wanted to

>replace with newer fiberglass would it make any difference?

>Would standard fiberglass found on Home Depot work?

>

>Thanks.

>--

>Elias

[RoyC]

I agree with Vern.

My unscientific observation is that the fiberglass used in the early AR cabinets is more dense...more like yellow hot water tank wrap than pink attic insulation....and not subject to deterioration.

In the past I've messed around with all kinds of damping material, including the expensive "Acousta Stuff", mainly because I hated working around the fiberglass. I'm sure many of these materials can be made to sound quite good with lots of trial and error, but even after 30 to 40 years of use, I've never found anything to sound better than the original "stuff" in an acoustic suspension speaker of AR's caliber. Determining the right amount of a new material with different properties in an old, well engineered design can be a nightmare.

Roy

[Guest lmrosenthal]

>Hi.

>

>I was wondering whether the fiberglass used to stuff the

>cabinet of the AR-3 age and loose its original properties,

>therefore altering the sound?

>

>I know it's been discussed here often, but if I wanted to

>replace with newer fiberglass would it make any difference?

>Would standard fiberglass found on Home Depot work?

>

>Thanks.

>--

>Elias

Elias

I refer you to an answer Tom Tyson gave in a thread from about a year ago to member Soundminded:

#4952, "RE: Internal volume of an AR-3"

In response to Reply # 2

Original question from Soundminded:

>Tom, have you got any data for the amount and density of the

>fiberglass? As I recally you once posted that it was about

>one pound per cubic foot. I assume that this is critical

>since it controls the aerodynamic velocity related drag

>coefficient and therefore the resonance damping. Is it

>uniformly distributed? Any data for AR9? Is it exactly twice

>the volume of an AR3a? As I recall the fiberglass is only in

>the upper part of the enclosure. Any data for the weight of

>the fiberglass there? Thanks.

Answer from Tom Tyson:

Soundminded,

I think the "one-pound-per-cubic-foot" rule is probably close to correct for the single-woofer AR box, but I am not sure about the AR-9. The density of the fiberglass is pretty much the same that is used for house insulation, but I am sure there is a spec for it. I will look to see if I can find the spec somewhere in the files.

The important thing is to control the speaker's "Q," as you alluded to earlier. The ultimate way (brute-force method) to check the efficacy of the proper "Q" is to measure the speaker's output down to and below resonance, and determine the peak, or attenuation associated with damping, at resonance. Ideally, the AR-3-type system, with a "Q" falling between 0.7 and 1.0, yields a peak of about 1.5 dB at resonance, therefore providing very uniform response to just below resonance, with the predictable rolloff below. This "Q" also results in no appreciable ringing (that is, it can't be improved by lowering the "Q"). Too much fiberglass will lower the "Q" and attenuate the response at resonance. The resultant thin-sounding bass, sometimes referred to as "tight" bass, is actually a form of coloration. Too little fiberglass would underdamp the system, and cause a peak at resonance, hence a "boominess."

--Tom Tyson

Hope this helps you.

Larry

[Guest Ken Perkins]

I agree with Tom on this. In my experience, 1 lb/ft^3 is about right. Forget polyfil and Acousta-stuf as they simply do not have the absorbant properties (density) of fiberglass.

Home Depot sells a small package of pink fiberglass that's basically a small rolled up sheet of a 2"x18"x48" pad for about $4.00. It will properly stuff a 1 ft^3 box and I used one and a half of these packs to stuff my 1976 Advents, which had those ridiculous yellow foam blocks in them. I measured the result and ended up with an Fb of 41 hz with a box Q of .9 for both speakers, which is about right for the original and New Advent's woofer/box combination. I would think that would be about right for the AR3 as well.

It's too cheap NOT to try.

[johnieo]

The small roll sold by Home Depot only contains 11 ounces of Fiberglas.

The internal volume of an AR-3a is 1.5 cu ft (not 1.7) and should contain about 1 lb 14 oz (30 oz) of Fiberglas. That amounts to three of these little rolls at $4.00-to-4.50/roll. An economically better choice is the smallest bundle of 15-in-wide R-19 Fiberglas retailing for $9.00. Can't remember how much is in it. It has enough to fill your speakers and stuff all the leaks in your house walls!

[Guest Ken Perkins]

I never actually weighed the small packs but was going by predicted vs. measured results with the Advents and others I've done in my DIY stuff. Plus, I guestimated they were about a pound or so. I actually stuff, listen, measure until I get it the way I want it. If the AR3 specifies a certain amount, I'd certainly recommend going with your suggestion.

[RoyC]

From my observations there are differences in amount and type of fiberglass used in the AR3a as it evolved. I have 4 pairs of 3a's currently in various stages of restoration. Each of the 3 pair of 1970+ vintages have a yellow dense wadded fiberglass approximately the equivalent to just under 2 bags of the Home Depot bags referenced above. I do not have the ability to weigh it, but thats how it "balances" out on the metal yardstick without exception.

The 1968 vintage pair with the old style woofer has a different type of fiberglass (cut pieces...still dense) and has a noticably greater amount, equivalent to approximately 3 bags of Home Depot pink. That would make it 11+ ounces more based on John's figures. By the way, those cabinets had the "controversial" AR#7 woofer inductors mentioned on this board in the past compared to the AR#9 in all the rest. I contend changes were made when the cloth surround woofers were switched out...so I would "stuff" according to woofer style.

It would be nice if Tom T. "weighed in" on that possibility.

None of the above speakers show any signs of having previously been tampered with. The serial #'s range from 09xx to 67xxx...

Roy

[dynaco_dan]

Hello

Just as a side note to this discussion I remember Dynaco A25's and it's particular insulation. If I remember correctly it was larger yellow pieces wedged into the cavity. A little more fitting to the enclosure rather than just tossed in.

I remember the design behind the screened vent and their 5 hz reference.

As simple as that sounds we don't have the equipment to test or prove this.

Advents had large chunks of a foam shoved in, I believe, in later models.

It never looked very scientific or effective to my old mind. But what do I know.

I find it difficult to believe that there wasn't some serious scientific thought given to the change from fiberglass to foam.

I wonder what audible sound difference there was, if any, from the earlier pink fiberglas stuffing.

I'm in a business where I see one less screw being used times 10,000 units or 100,000 units.

A car manufacturer may not add a $.50 part because they see a million or 5 million units.

A speaker company makes changes from fiberglas to foam and there was probably a down charge but also less injuries and sick time as well. This is also a consideration.

Maybe the foam was reject material.

The rumour I remember about the original Larger Advent woofer was, there was several boxcars full of an oddball speaker frame and Henry bought them for a song and designed his woofer to suit it.

AR went from a putty type sealer to a stamped out foam gasket.

Try pealing a putty strip and placing it into the speaker groove and then just grabbing a gasket and placing it in the same groove.

Do this on a production line where there may be hundreds of enclosures on an assembly line waiting to be shipped.

I am sure there was penny pinching.

I don't remember where I read years ago about thicker insulation directly behind the woofer and padding in the cabinet joining edges. To give it a smooth radius rather than a sharp 90' angle.

I think there was mention of a substance coating like car body undercoating to the inside wall of the enclosure as well to deaden the enclosure.

This is something that I only read about the once, but this was and is used to deaden car road noise somewhat.

There was a speaker, I don't remember which company, that had broken up handfuls of fiberglas padding.

The earliest Infinity 2000 used a long fibre Dacron or something similar for padding.

I just about said insulation but I have also seen different qualities of fibrerglas in my work.

Some of the old electric water heaters had a black fiberglas and some had yellow or white.

We had a local manufacturer that made a pink house insulation.

I am not knowledgeable in the manufacturing of fiberglas but I think it is made similar to making candy floss. Spun at high speed and heat. Centrifical force I believe.

Re-cycled glass is turned into slivers or threads.

The glass fibres are solid glass which cannot absorb moisture, are not porous and have a zillion slivers.

I have seen larger slivers and actually found some by accident. Ouch.

That is why it is recommended that you wear gloves, safety glasses and a mask to lessen personnal injuries.

I am sure there is a science behind the manufacturing of fiberglas and the different densities which we may see on occasion.

It would be nice if someone on board worked at a fiberglas manufacturinng plant and could add to this area of information.

We sure could learn a lot from them.

I sure enjoy some of the topics on this site and look forward to the many knowledgeable members always willing to add their wisdom and experiences and 2 cents worth to each topic.

Have a wonderful Christmas and all the very best in 2005.

>I never actually weighed the small packs but was going by

>predicted vs. measured results with the Advents and others

>I've done in my DIY stuff. Plus, I guestimated they were

>about a pound or so. I actually stuff, listen, measure until

>I get it the way I want it. If the AR3 specifies a certain

>amount, I'd certainly recommend going with your suggestion.

[johnieo]

Perhaps it would be appropriate if we retained the focus of this discussion on AR-3a and Fiberglas.

What is of interest here is whether there might possibly have been minor changes in AR-3a crossover and damping factor to compensate for minor differences in the behavior of the (reportedly identical) cloth- and foam-surround woofers. The output in the 200-400 Hz range will decrease if the woofer coil is changed from #7(1.9 mH) to #9(2.85 mH). Reducing the damping would tend to increase the output. The two would tend to have the opposite effect. Anyone have real historical manufacturing data? If there was a real change, it would be helpful information for anyone restoring AR-3as.

All Fiberglass is not the same. All glass does do not have the same density. Fibers can absorb a huge amounts of water vapor, and let us not forget that dye and binder resin have likely changed in the last 40-50 years.

[soundminded]

The exact nature, amount, and arrangement of the fiberglass is a critical element in a loudspeaker system, especially an acoustic suspension loudspeaker system for at least two reasons. First and most obviously, while the empty box of an AR3a is about 1 1/2 cu ft, when you put the fiberglass in it, it displaces a fair amount of air and so the remaining air is much less. The reduced air increases the springiness of the air restoring force on the cone as it undergoes displacement. This may explain the previously commented on nonlinearity (electrical input vs acoustical output) of the acoustic suspension speaker others have mentioned. The less air in the box, the greater percentage of compression and rarifaction it undergoes as the cone creates greater displacement. So while P1*V1=P2*V2 holds true, the more higly compressed the air is as a percentage of its total volume, the greater the relative air spring return force will be. (The obvious solution is to make the box bigger which will also increases the amount of stuffing required. The second reason has to do with why the fiberglass is in the box in the first place. Fiberglass or other stuffing increases damping. What exactly does that mean? AR3a like all acoustic suspension speakers and all vibrating systems of any kind for that matter obeys Newton's second law of motion which considers mass, damping, and springiness to determine its exact motion when forced to vibrate. It is given by the well known ordinary second order differential equation F(t)= ma + bv + kx where m=mass of the moving element, b is the velocity relatied frictional loss (damping due to viscousity of air) and k is the springiness. a=acceleration, v=velocity and x=displacement. F(t) is the driving force. The well known solution is f (resonant frequency)= 1/2pi*{sq rt (k/m + (b/2m)squared} As m and b go up, the resonant frequency goes down. As k goes up, the resonant frequency increases. A speaker in a large sealed box which is highly damped and which uses a driver with a heavy moving mass which is loosely suspended will have a low resonant frequency. The b or drag coefficient is due to the air being squeezed through the spaces between the fibers. This frictional drag force on the cone is directly related to the velocity of this air motion. If the fiberglass is tightly packed, air will have a tough time moving through it. Loosely packed and it is much easier to move. The great advantage of this is that none of the controlling elements m,b,or k is a function of frequency as it is in ported systems.

BTW, this same equation is used for virtually every mechanical vibration problem from the suspension of a car, the swaying of buildings and bridge towers in the wind, the performance of phonograph cartrides, and is also the same equation used in LCR electrical circuits where only the symbols are different. It is not surprising therefore that tuned resonant electrical circuits can be used to enhance the tuned mechanical performance of loudspeakers systems.