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Carlspeak

Speaker box stuffing study

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I've just completed an interesting day of experiments with box stuffing materials and an AR 4x speaker.

The results are summarized in the attached document. It's not extremely comprehensive however. But, results do show some trends worthy of further study.

What brought me to this is the large number of posts and views of the brown stuffing post. I thought I'd evaluate some alternatives to rock wool. I think I've found 1 or 2. For those of you cleaning or replacing those corroded pots, please consider throwing out that old rock wool and replacing it with something more friendly to those cleaned up pots and less dangerous to work with.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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Hi Carl;

A wonderful report, thank you very much.

Very professional looking.

I feel that the AR-4X is simple enough to do the test with, rather than a larger, more complex system.

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>What brought me to this is the large number of posts and views

>of the brown stuffing post. I thought I'd evaluate some

>alternatives to rock wool. I think I've found 1 or 2. For

>those of you cleaning or replacing those corroded pots, please

>consider throwing out that old rock wool and replacing it with

>something more friendly to those cleaned up pots and less

>dangerous to work with.

>

Both fiberglass and rock wool are non-flammable and won't melt at any temperature likely to be found inside a speaker enclosure. Previous posts here indicate that the pots used in AR speakers can get quite hot. Replacing the fiberglass with poly-fill would require evaluating the risks associated with replacing non-flammable fiberglass with a flammable stuffing.

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>>What brought me to this is the large number of posts and

>views

>>of the brown stuffing post. I thought I'd evaluate some

>>alternatives to rock wool. I think I've found 1 or 2. For

>>those of you cleaning or replacing those corroded pots,

>please

>>consider throwing out that old rock wool and replacing it

>with

>>something more friendly to those cleaned up pots and less

>>dangerous to work with.

>>

>

>Both fiberglass and rock wool are non-flammable and won't melt

>at any temperature likely to be found inside a speaker

>enclosure.

.

.

>>>>Previous posts here indicate that the pots used in AR speakers >>>>can get quite hot.

.

.

From my personnal experience, I have seen many incidences of charred pots, melted pot shafts and toasted woofer voice coils.

.

.

>>> Replacing the fiberglass with

>>>poly-fill would require evaluating the risks associated with

>>>replacing non-flammable fiberglass with a flammable stuffing.

.

.

A very good point regarding safety, considering that what we are writing about is, out of sight, and out of mind.

.

.

I, along with invaluable help from James, have been working towards finding a heat resistant and non-combustible pot (rheostat) heat shield, that is readily and reasonably available.

.

.

I have more than a few samples, perhaps 3 dozen that I have bought and that I continue to carry with me, to show samples of what I am searching for.

.

.

I also carry two samples of pots with me to trial fit them.

.

.

If a Chinese tea cup looks familiar, I've got a collection of them now.

.

.

I have a Chinese sample and an Ohmire sample, both different sizes.

.

.

When I find something that I feel is suitable, I will announce it here.

.

.

Another good member has been working on a rotary switch with small resistors, offering a stepped controller.

.

.

His commentary is that there is actually very low wattage and that the small resistors will not be overloaded.

.

.

I have not read any new information from him, but it is a step in a different and it appears, a positive direction.

.

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It would give everyone another option in tweeter and mid driver level controlling, rather than just using pots.

.

.

I've contacted Ohmite with a suggestion and they didn't even reply.

.

.

A Chinese manufacturers contact said that unless 50,000 or more is in mind, not interested.

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>>What brought me to this is the large number of posts and

>views

>>of the brown stuffing post. I thought I'd evaluate some

>>alternatives to rock wool. I think I've found 1 or 2. For

>>those of you cleaning or replacing those corroded pots,

>please

>>consider throwing out that old rock wool and replacing it

>with

>>something more friendly to those cleaned up pots and less

>>dangerous to work with.

>>

>

>Both fiberglass and rock wool are non-flammable and won't melt

>at any temperature likely to be found inside a speaker

>enclosure. Previous posts here indicate that the pots used in

>AR speakers can get quite hot. Replacing the fiberglass with

>poly-fill would require evaluating the risks associated with

>replacing non-flammable fiberglass with a flammable stuffing.

>

Thanks Joe. Your point is well taken.

If polyfill is what someone wants to use, perhaps a simple cover could be fabricated to cover the pots and keep most of the fiber away from them. If ohmites are used to replace the Aetna-Pollak pots, their wipers are completely exposed and a cover is necessary anyway to avoid shorting.

BTW, the melt point of polyester is about 450 deg. F. That's pretty darn hot. It's hard to imagine a 25 watt rated pot could get that hot. But, then again, Vern's fusing recommendations make all this discussion moot should they be followed.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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>But, then again, Vern's fusing

>recommendations make all this discussion moot should they be

>followed.

>

>

>Remember, it's all about the music

>

>Carl

>Carl's Custom Loudspeakers

Carl,

I agree that a speaker protected by a reasonable fuse is not going to cause a problem, no matter what it's filled with. A problem could only occur with an unfused speaker and extreme abuse.

Joe

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Just a thought...

Having just recently jumped back into vintage AR (after a 30+ year absence), I've noticed a couple of thing: some AR speakers have pots shot to hell, others simply need a cleaning; AND some grills look fine, while others are badly stained (darkened) over the cut-outs.

In my LIMITED experience there is a correlating factor: rock wool.

The ARs with fiberglass stuffing have pots in better condition (as has been discussed), and the grills don't have as much (or any) darkening of the cloth over the grill cutouts.

Isolated, or is the sulfur leeching out of the rock wool also darkening the cloth?

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>Just a thought...

>

>Having just recently jumped back into vintage AR (after a 30+

>year absence), I've noticed a couple of thing: some AR

>speakers have pots shot to hell, others simply need a

>cleaning; AND some grills look fine, while others are badly

>stained (darkened) over the cut-outs.

>

>In my LIMITED experience there is a correlating factor: rock

>wool.

>

>The ARs with fiberglass stuffing have pots in better condition

>(as has been discussed), and the grills don't have as much (or

>any) darkening of the cloth over the grill cutouts.

>

>Isolated, or is the sulfur leeching out of the rock wool also

>darkening the cloth?

>

>

Is there a chemist in the house that might be able to answer Mr. Summer's question?

Having posed that question, I wonder how the sulfur can get past the sealed drivers. Could fumes leak thru the "porous" cellulose speaker cones or screen dust caps?

The grill cloth is made mostly of linen but I suspect the original stuff also had strands of burlap in it.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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>Isolated, or is the sulfur leeching out of the rock wool also

>darkening the cloth?

Once outside the cabinet, IMO, too many variables to single out one - temperature, smoking, crud in air, sunlight, storage conditions, and so on; fabric absorbs. Were diffusion through the cone a problem, might the area directly in front differ in color?

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It's the pattern that intrigues me... the darkening only occurs over the cut-outs, indicating the cause of the fabric darkening comes from BEHIND the grill covers, not in front of it (sunlight, smoke, fried foods).

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Hi there;

I will make a guess that it is the vibrations caused by the woofers and there is that border.

The cloth is fastened to the frames and the cutouts are acting like a mini vacuum cleaner.

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I would tend to agree with Dynaco Dan's theory. The pumping action of the woofer pulls and pushes air back and forth thru the grill cloth. Any 'stuff' in the air - and those of us who clean house know, will get captured in the microfigers that stick out of the strands and build up over many years of use.

I'll stick with this theory until a degreed chemist can convince me otherwise.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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Okay posters, I have to eat some crow here. In the first paragraph of my report, I mentioned that little had been published regarding this subject.

Today, I was browsing thru my copies of the first and fourth editions of Vance Dickason's Loudspeaker Design Cookbooks. In the first edition he devoted a couple of columns to AS speaker stuffing. He targets a starting point of 1.5 lbs/cu.ft. which is midway in the range I explored. He also referernces Chase's 1974 IEEE paper which, I believe, has been discussed here already.

However, when I opened up the forth edition to the same subject area I discovered Vance had expanded his discussion to almost 9 pages and included a new detailed emperical study somewhat similar to mine, but much more comprehensive in the measurements taken. Numerous impedence and SPL charts are presented for the 17 stuffing variables studied. He doesn't reveal who did the study or when. A larger box (52% larger) was used than what I used. However, his study used the same size woofer. This seems to me to be a rather large box for the woofer used. It's outside David Weem's recommended box volume vs woofer size chart depicted in his 1984 Tab publication: "Building Speaker Enclusures".

It's not clear to me what effect the difference in woofer/box size relationship might have on the outcome of such a study. I guess that's another varible worthy of further study?

I won't go into any further detail here regarding the study results published in the 4th Edition due to potention copyright infringement issues. However, I do recommend those who seek further information might want to pick up a copy. The Cookbook now is in a much later edition. The 1st ed. 1987. 4th ed. 1991. Who knows what may have been published beyond the 4th ed.?

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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Hi Carl;

No need for you to eat crow, Carl.

You can only do the best you can do, with what you have available at any given time.

I am certain there is more information out there, but we are here and now.

It can be added to our pool of knowledge, if and when it arises.

I really appreciate all of your efforts to do what you have done, both in the stuffing and grille cloth studies.

I certainly couldn't have done what you did.

It makes for good and enjoyable reading, Carl, thank you.

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FYI all. New flyer from Old Colony Sound Labs shows the Loudspeaker Cookbook is in its 7th Edition and the price is reduced from $40 to about $28 until Dec. 29th.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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Two new data points to share with you all.

I just finished opening up 2 AR 3a speakers. I'm preparing to replace the pots and upgrade the tweeter caps.

One had no serial #. The other was marked #68861. Each contained the more recent yellow fiberglass. It was completely removed from each cabinet and weighed separately. The unmarked cab. was 590 grams. The marked cab. was 515 grams. The inside dimensions were measured and some allowance for bracing was subtracted from the total internal volume. The stuffing density calculated to about 1 lb/cu. ft and 0.81 lb/cu. ft respectively.

The median of these densities falls in the range I found was quite satisfactory (0.9 lb/cu. ft.) for OC fiberglass in my study using AR 4x speakers.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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RE: stuffing density

Stuffing *density* is not the appropriate parameter by which early AR speakers should be compared. A major issue for the designer was setting the cabinet Q correctly. If too little stuffing, the cabinet would be underdamped, if too much it would be overdamped. AR's designers determined the appropriate stuffing weight to meet each speaker's design parameters; the final weight in a given speaker did not yield a common density. Examples:

Early AR-4x: AR-#4 woofer coil-- 18 oz. (511 gr.) fiberglass or 1.7 lb./cu-ft.

Later AR-4x: AR-#5 woofer coil-- 12 oz. (340 gr.) fiberglass or 1.15 lb./cu-ft

Early AR-3a (sn < ~38,500): AR #7 woofer coil, Alnico magnet woofer-- 28 oz. (795 gr.) fiberglass or 1.18 lb./cu-ft.

Late AR-3a (sn > ~39,500): AR #9 woofer coil, ceramic magnet woofer-- 20 oz. (567 gr.) fiberglass or 0.844 lb./cu-ft.

All AR-4x and AR-3a cabinets have interior volumes of, respectively, 0.65 and 1.48 cu-ft. The stuffing weights many of us have measured were within ~10% of the design value.

These numbers are only for fiberglass. When using an alternate fiber, a different quantity will be required to obtain the correct damping; but other parameters will not be the same.

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>RE: stuffing density

>

>Stuffing *density* is not the appropriate parameter by which

>early AR speakers should be compared. A major issue for the

>designer was setting the cabinet Q correctly. If too little

>stuffing, the cabinet would be underdamped, if too much it

>would be overdamped. AR's designers determined the appropriate

>stuffing weight to meet each speaker's design parameters; the

>final weight in a given speaker did not yield a common

>density. Examples:

>

>Early AR-4x: AR-#4 woofer coil-- 18 oz. (511 gr.) fiberglass

>or 1.7 lb./cu-ft.

>

>Later AR-4x: AR-#5 woofer coil-- 12 oz. (340 gr.) fiberglass

>or 1.15 lb./cu-ft

>

>Early AR-3a (sn

>magnet woofer-- 28 oz. (795 gr.) fiberglass or 1.18

>lb./cu-ft.

>

>Late AR-3a (sn > ~39,500): AR #9 woofer coil, ceramic

>magnet woofer-- 20 oz. (567 gr.) fiberglass or 0.844

>lb./cu-ft.

>

>All AR-4x and AR-3a cabinets have interior volumes of,

>respectively, 0.65 and 1.48 cu-ft. The stuffing weights many

>of us have measured were within ~10% of the design value.

>

>These numbers are only for fiberglass. When using an alternate

>fiber, a different quantity will be required to obtain the

>correct damping; but other parameters will not be the same.

Thanks for all the details John. I understand the designers needed to target a box Q in developing a new speaker that didn't exist. I was simply trying to establish some historical emperical data on some of the more popular AR speakers and the effects various stuffing materials. Your response is most welcome in filling in most of the blanks. You have also verified the initial data point in my results table. The AR 4x speaker I used did indeed have #4 coil and my results closely match the stuffing density you listed above.

But, I also have to wonder why my 0.9 lb/cu. ft Owens Corning glass trial matched my initial data point so closely.

Your apparent wealth of AR facts and details has raised some questions in my mind:

1) Do you know exactly what Q were they targeting for for various models? something in the range of 0.5-1.2 perhaps? My BassBox Pro Qtc results from my Qts tests on the 8" woofer in a heavily stuffed .625 cu. ft box resulted in a Qtc of slightly over 1. Is this anywhere near the designer's target for that speaker?

2) How did the engineers measure Q back in those early days? Was it 'seat of the pants' trial and error or was some science used?

3) Villcur's AS patent mentioned Q briefly relative to the use of fiberglass stuffing but didn't mention any target in his claims. When did the AR engineers adopt the use of T/S parameters in loudspeaker design?

4) Did the AR engineers go back and measure the Qts of some of the prior woofers following publication of the Theil/Small AES papers? Surely spares were still around in the early 70's. The engineers would have gained mutch knowledge from such data.

6) If that data was collected, could someone like yourslelf share it with us mere AR mortals? I find posts from time to time that give glimpses of AR facts on the 'classic' speakers from the 60's and 70's. Your box stuffing info is a glaring example. Surely, there must be some people around that worked at AR in its heyday who could share some memories and/or facts from those days. AR posts are far and away the most active of any speaker discussed on these pages. That kind of information would be of great value to many of us around the world who visit regularly that are trying their best to be as accurate as possible with their salvaging efforts of those classics.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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PM,

It appears that the earlier version AR-2ax equipped with the cloth surround woofer contains 28+/-oz of rockwool, and the later version equipped with the foam surround woofer contains 18+/-oz of yellow shredded fiberglass. This is based on measurements I have taken of a number of AR-2ax specimens in the last two years.

Of four early '70's AR-5's (same cabinet as 2ax with a slightly different foam surround woofer) I have weighed only yellow shredded fiberglass ranging from 20oz to 21oz.

Roy

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>PM,

>

>It appears that the earlier version AR-2ax equipped with the

>cloth surround woofer contains 28+/-oz of rockwool, and the

>later version equipped with the foam surround woofer contains

>18+/-oz of yellow shredded fiberglass. This is based on

>measurements I have taken of a number of AR-2ax specimens in

>the last two years.

Roy - I have to wonder what really drove the reduction in stuffing weight between the two woofers. Was it the foam surround? A different woofer altogether? different coil? or, simply the yellow fiberglass accomplished the same target Q as the rockwool did? Here again, we need some factual AR history to answer these questions.

>

>Of four early '70's AR-5's (same cabinet as 2ax with a

>slightly different foam surround woofer) I have weighed only

>yellow shredded fiberglass ranging from 20oz to 21oz.

>

>Roy

Thank you Roy for some additional data on another classic AR speaker. In an effort to help answer P.M.'s question, I tested a pair of original AR 2a woofers I salvaged and did some calculations on box Q and stuffing quantity. The speakers I tested were 11" od and had original cloth surrounds and + & - terminals on opposite sides of the magnet. Results for both were very closely matched. Below is a summary with nominal values

Fs = 28 Hz (seems high relative to Villcur's AS patent stating 18 Hz as ideal. Perhaps the spider has stiffened over the past 30-40 years?)

Qts = 0.51

Vas = 6.35 Cu ft.

Mms = 29 gms

Eff = 89 dB.

An AR 2a box internal volume is slightly less than the AR 3a. I estimate it to be net 1.3 cu. ft. with bracing taken into consideration.

With that box volume and the above T/S parameters, I calc. a box Q or Qtc of 1.24 for a non-stuffed box using the formula:

Qtc=Qts*sq.rt.(Vas/Vb)+1).

It is generally accepted that 100% fiberglass stuffing material will add about 18% to the effective volume of an AS speaker. Recalculating Q with a new Vb of 1.53 cu. ft yields an expected reduced Q that computes to 1.15. At this Q, there will be a theoretical response ripple at box Fs of about 2 dB.

P.M. Summer: If you use standard 3 inch thick OC R-19 fiberglass as stuffing material, it's uncompressed density is 1 lb/cu ft. Thus for a 1.3 cu. ft box, 20.8 oz. should be used. This is very close to what Roy measured with yellow stuffing.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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On 11/1/2006 at 8:59 PM, Carlspeak said:

I've just completed an interesting day of experiments with box stuffing materials and an AR 4x speaker.

The results are summarized in the attached document.

Hi, new here and just stumbled on this...I don't see an attached document. ?!?!?  Possible to post in-line? I'm always interested in information like this, for advising others...

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