Questions about cabinet dampening (as opposed to stuffing)

[BobM]

Hi all! I'm hoping to draw on all the expertise on these pages by asking some questions are about the various products available that are supposed to dampen a speaker cabinet.

One is “Black Hole 5” recommended by Jeff Glowacki, at SoniCraft.

http://www.soniccraf.../blackhole5.htm

Another is “Sonic Barrier Acoustic Foam” from PartsExpress.

http://www.parts-exp...tnumber=260-520

Perhaps the foam from PartsExpress is supposed to be a dual-function substance, but I would like to understand “dampening” (or “damping”), specifically. On the face of it, these materials seem quite different in design and description from stuffing materials like fiberglass and AcoustaStuf.

First, how will adding such a material affect how a speaker sounds, in general (if all else stays the same)?

Second, do they affect box Q-factor?

I’m currently refurbishing an old pair of speakers—AR-5’s, which is why I’m posting here. Setting aside questions of preserving original sound as much as possible, and (basically) vandalizing my speakers, I’m wondering what would happen if I put some of this stuff in the cabinet, in addition to the amount of stuffing originally in the cabinet. Does anyone have any experience with dampening materials (as opposed to stuffing materials)?

Thanks!

[ironlake]

I'm at a loss as to what you mean damping vs stuffing. They sound like the same to me, but I am sure you mean something else.

[BobM]

Hi Ironlake--thanks for your reply! I think I do, but I guess that's part of the question. There are (at least) two types of material, physically speaking. One is fluffy stuff--like the fiberglass insulation that is used in buildings--which is used to fill cabinets. The other comes in relatively thin (1/2" to 1" thick, or so), flat foam-like material, that is much denser than the fiberglass. Apparently, one does not fill cabinets with this flat stuff, but you line a cabinet with it, by sticking it to the interior walls of the cabinet. So, at least they're different in concept. (By the way, I don't mean "egg-crate" foam, which is used in PartsExpress kits--I think egg-crate foam does more or less what the fluffy stuff does. I have experimented with that stuff.)

In my (very limited) experience, the fluffy stuff seems to significantly affect how a speaker sounds--it reduces "boominess" in the bass and mid-bass, makes low bass sound "cleaner," it affects box Q-factor, and so on.

I'm not 100% clear on what the other stuff does to the sound of a speaker. Physically, BlackHole 5 and Sonic Barrier are very different substances from fluffy fiberglass/AcoustaStuf/etc. Products like BlackHole5 and Sonic Barrier are also marketed differently; they are advertised as replacing bracing in cabinets, providing sonic "isolation," and absorption. (Whereas fluffy stuff is supposed to effectively increase box volume, among other things.)

Does that mean these non-fluffy products affect box Q-factor? Would they make a speaker like my AR 5's sound "dead," or "precise"? What would happen if I used both them and fiberglass?

Does that make sense?

[Carlspeak]

The non-fluffy stuff is used primarily in vented designs to dampen the backwave of woofers where cabinet back and front walls are parallel to one another (picture a monkey coffin type box).

IMO, at the density at which acoustic suspension cabinets are stuffed, the stuffing acts much like the blackhole 5 and other products. My take on your question is you're comtemplating using both a Blackhole 5 type product along with stuffing in an acoustic suspension speaker. I don't think the addition of the blackhole 5 stuff will make much difference and depending on how thick it is how it affects box Q by significantly changing box internal volume.

However, you're welcome to try both in one speaker and compare it with another, control AS speaker, that has the same amount of stuffing but no Blackhole 5 type stuff attached to the back wall. Let us know what you find.

[ironlake]

Thanks for the great answer. I am wondering what would happen if you stuffed a ar box as full as possible with the yellow type stuff. I know when I did the pot change on my ar 4x I carefully took all the stuffing out and put it back in the same way but if I wanted to I could have stuffed a little more under the woofer. What would have happened if the cabinet was really stuffed full so the speaker would push the foam down when inserted.

Also the dust cab could be seen through with light behind it,. does that leak air and does it still count as an acoustic seal?

[Carlspeak]

More stuffing generally reduces the box Q. IOW, the boomy bass effect is reduced. I believe the designers of the AR4 purposely had the box Q in the high side (>1) so the bass sounded more rich for such a small speaker. Your DC probably was a screen type which allowed the voice coild to vent heat.

You might want to try experimenting yourself to get answers to some of the myriad of questions you ask. It's a hobby. Exeriment and enjoy it. The rest of us would love to hear about your AR audio escapades.

[HarryM]

From years of experience in construction I can tell you compressed or packed insulation is useless.I've even thought of replacing the insulation in all my speakers because most of it is tight and balled up.It should be fluffy.

[BobM]

Ah! That's good to know HarryM, thanks! As it turns out, right before Roy Champagne told *me* not to throw out the old stuffing, I (had already) chucked it. Alas, my inexperience--it looked really gross to me--it appeared not only dirty, but it seemed to me as if it was compacted-- wadded up, I'd say. If that means it wouldn't have worked as well, I feel better. And, I'll be sure to put the new stuff in without mushing it up.

Unless anyone talks me out of it, I'll install both the Sonic Barrier and fiberglass (20-22 oz, as Roy C pointed out on another post--thanks!) in one AR-5, and just the fiberglass in the other. By the way, I'm still waiting on replacement caps, and my woofers are getting re-foamed, so that'll probably be a week from now, at the earliest.

Cheers!

[BobM]

Oops, I forgot to say...Carl--thanks for the redux! Very helpful!

[ironlake]

I will try and not ask so many questions. Sorry, just trying to catch up on ar since I last purchased it in 1965.

[BobM]

Ironlake--I think that was me! At least, I have been accused before of thinking that I'd asked one question when I'd really asked 5...

Carl--I just noticed that the midrange-parallel inductor is #6 in my pair of AR-5s. What value is that? (Please forgive me if there's a table I have access to, but I haven't found one.)

[Carlspeak]

#6 coil is 1.37 mH.

Here is a link to a thread on the subject.

http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=1170&st=0&p=55110&hl=inductors&fromsearch=1entry55110

I have a complete table of AR inductors 1-17. However, doing a search didn't turn up a link to the table itself. Maybe TT could post it again and/or add it to the AR library. I couldn't find it there.

[JKent]

Hi all! I'm hoping to draw on all the expertise on these pages by asking some questions are about the various products available that are supposed to dampen a speaker cabinet.

One is “Black Hole 5” recommended by Jeff Glowacki, at SoniCraft.

http://www.soniccraf.../blackhole5.htm

Another is “Sonic Barrier Acoustic Foam” from PartsExpress.

http://www.parts-exp...tnumber=260-520

I seem to recall reading about some sort of adhesive asphalt sheeting sold in home centers that's really cheap and effective, but I was never able to find it. Anyone know about that?

Kent

PS: Just did a little search and it reminded me the product is "Karnak". Never was able to find any though. Think it's this stuff:

http://www.karnakcor...-Patch-N-Go.pdf

[michiganpat]

Ironlake- the see through dust cap shouldn't cause a significant air leak, as carl mentioned, it's for voice coil venting/cooling. IIRC there isn't any sort of rear vent on the driver into the speaker, so you're still sealed

HarryM, I'm not 100% sure how applicable the home wall insulating is to speaker stuffing as far as a comparison. with insulation, you are looking for "fluffiness" and airspace between the fibers, to slow the conduction of heat from the hot side to the cold side. in a speaker, the insulation is used to give resistance to sound waves...think of taking a piece of cloth and blowing through it, and then double it up and blow through it-- more resistance...if the alignment is a high Q due to an undersized cabinet, more stuffing, even if it compressed a bit vs. fully fluffed will be effective in q reduction until it's compacted so much that air cannot move through it and it ends up reducing volume and reflecting the sound waves.

Jkent- I think you're looking for "ice and water shield". It's a lot like dynamat, only cheaper. I still have half a roll after buying a big roll to use as sound deadening in my car.

I think this is what I have:

http://www.homedepot...1&storeId=10051

it's effectiveness is in adding mass to a cabinet wall side to change the resonant frequency of the enclosure or whatever you put it on....

the stuff I have doesn't stay put real well on sharp corners (say if you're trying to wrap a woofer basket, or the backside of a horn), the ice and water shield is more of a rubber, and more flexible. it's more expensive though

http://www.homedepot...1&storeId=10051

this would be a cheaper alternative if you don't need 225 square feet

http://www.homedepot...tryId=203057402

http://www.homedepot...632&R=202288632

[JKent]

thanks Pat, but I think this is it:

http://www.wimsattdi..._IC_KAPTD2.html

It comes in various widths from 2" to 12" and is self-adhesive.

Kent

[HarryM]

Michiganpat,you may be right.The best way to find out would be a side by side test.One speaker restuffed with new fluffy insulation and the other original.I may try this when I'm ready to put the AR-8's back together.

[michiganpat]

thanks Pat, but I think this is it:

http://www.wimsattdi..._IC_KAPTD2.html

It comes in varios widths from 2" to 12" and is self-adhesive.

Kent

I don't think so...from the PDF you posted above, it's 35 mils thick, which is .035" or .9mm. how dynamat and other products like it (or for that matter, rope caulk stuck to driver baskets) primarily work is by adding mass to the component to lower the resonant frequency, hopefully out of the audible range. most products like this are 2-3x the thickness of that double sided tape you linked to.

[JKent]

You're probably right Pat--I missed the spec and .035" is really thin (about 1/32" for those of us totally stuck inthe English system). I never used the stuff but I'm sure the brand name was Karnak (reminded me of the old Johnny Carson bit) and it was self-adhesive. Apparently there are several products, including mats and glop that go under the Karnak name. My understanding is the stuff would be cut to size and applied to the inside walls of the cabinet, but damping woofer baskets makes sense too. If I understand this correctly, the whole damping issue is moot with acoustic suspension speakers. If they pass the knuckle-rap test they are dead enough.

Kent

[Gerry S]

You're probably right Pat--I missed the spec and .035" is really thin (about 1/32" for those of us totally stuck inthe English system). I never used the stuff but I'm sure the brand name was Karnak (reminded me of the old Johnny Carson bit) and it was self-adhesive. Apparently there are several products, including mats and glop that go under the Karnak name. My understanding is the stuff would be cut to size and applied to the inside walls of the cabinet, but damping woofer baskets makes sense too. If I understand this correctly, the whole damping issue is moot with acoustic suspension speakers. If they pass the knuckle-rap test they are dead enough.

Kent

I found the "knuckke-rap" test to be a quick & reasonably accurate indicator of how well the cabinet does not "ring" . A precise way is to use an accelerometer (which I've never done myself) to indicate "cabinet inertness". The "ideal" cabinet would not be "excited" at all ,contributing NO undesirable vibrations that will interfere with the woofer's output.

it's my experience that employing very "dead" cabinets by themselves is NOT a reliable indicator of sound "quality" as a whole. I've heard "high end kilobuck speakers" that fared poorly against "budget systems". If a cabinet is reasonably inert using common "audio grade" MDF properly proportioned (to minimize standing waves inside the cabinet) and srategically braced, the cabinet is no longer the " weak point".

[dynaco_dan]

Hi there

Just my 2 cents worth for today.

Many years ago I read 2 different articles.

One was the operation of removing any insulation from an enclosure and protecting the crossover and rear of the drivers cones from accidental coverage.

A compound was applied to the inner surfaces, it was car under body goop or tar like substance.

Re-install insulation when dry.

Second was applying plastercene or modeling clay in a 1/4"+ layer to the inner walls and over the rear frames of the drivers as well.

This was to reduce cabinet resonances.

I never read any further comments and never experimented with these myself.

I have thought about the first one recently and wonder if the vapours may have affected the foam surrrounds.

[stan461]

About 5 years ago, I replaced the stuffing in one of my sets of AR3a's with "Acousta Stuf".

To this day, I would have to take off a tweeter to determine which set has the "Acousta Stuf".

The point here being, if it is that unnoticeable, does it really matter?

Just my personal observation and something to consider.

[Glenn E]

Hi all, many of the classic BBC style British speakers had thin birch or plywood walls. To reduce box resonances, thin bitumous pads (wood fibre imprgnated with lightweight bitumen in some) were stuck to the inner walls but typically not the back and front. They were of course all bass reflex or vented designs. I have a pair of Spendor SP1s of this design dating back to the 1990s. They also contain a sheet of soft pliable foam about 3 cm thick which acts to provide extra dampening. This provides coverage of all inner walls except the front. The SP1s are very nice sounding speakers but do not have the depth of bass that my AR 93s do. But then they have other virtues.

Glenn

[Gerry S]

I'm at a loss as to what you mean damping vs stuffing. They sound like the same to me, but I am sure you mean something else.

There IS a difference. A simplified expanation from my experience and point of view.

1. Common "stuffing" is usually of two types; fiberglass and "pollyfill" (white fluffy cotton-candy textured stuff like that found in pillows). It's purpose is to control the interaction between the woofer and the cabinet that houses it. How much one uses IS important depending on what one is tring to accomplish. Too much or too little can adversely affect system performance @ bass and even midrange frequencies.

Also important is WHERE the "stuffing" is placed. This depends on what one is trying to achieve (usually "damp" or "tame" standing waves inside the cabinet). Fiberglass is considered to be more "acoustically effective" and less costly than polyfill. Despite the added cost of polyfill, it's more "friendly" to work with compaired to fiberglass from a manufacturing standpoint. Fiberglass can make you itch and small particles can seperate into the inviroment; not very pleasant to work with if it's your job on the production line inserting this stuff!

2. Cabinet Dampening: The "perfect" cabinet would be 100 percent "inert" when "excited" by any force internally (bass from the woofer) or externally (knuckle -test). There are numerous methods to "adequately" damp a cabinet, ranging from "moderate" to "extreme". "Moderate" is usually more than adequate IF the system as a whole is well thought out; the woofers will be the limiting factor as far as performance goes. To add anymore dampening would just add weight and cost to the finished product without audible benefits.

[speaker dave]

Hi all, many of the classic BBC style British speakers had thin birch or plywood walls. To reduce box resonances, thin bitumous pads (wood fibre imprgnated with lightweight bitumen in some) were stuck to the inner walls but typically not the back and front. They were of course all bass reflex or vented designs. I have a pair of Spendor SP1s of this design dating back to the 1990s. They also contain a sheet of soft pliable foam about 3 cm thick which acts to provide extra dampening. This provides coverage of all inner walls except the front. The SP1s are very nice sounding speakers but do not have the depth of bass that my AR 93s do. But then they have other virtues.

Glenn

We've debated this over on DIY audio quite a bit.

Cabinet wall damping is, of course, a very different thing than internal damping of the air cavity. One deals with resonances of the cabinet panels and the other with the internal standing waves related to dimensions.

For cabinet damping and the origins of the BBC approach I refer people to a great paper by Harwood:

http://www.bbc.co.uk...rts/1977-03.pdf

He looks into typical cabinet materials and measures their stiffness and Q but the more interesting info is when he looks at typical cabinet construction and how the cabinet radiates at each panel resonance. He is a big advocate of panel damping (bitumen pads being the British favorite) but shows that damping is most effective when the cabinet walls aren't overly stiff or massive. Pushing up stiffness or mass will increase mechanical impedance and it becomes more difficult to damp the resonances. His general recommendation is thinner walls but greater damping mass.

This falls in line with what Architectural Acoustics professionals know: that wall transmission loss is improved with "limp mass". That is, a very heavy curtain can give good issolation. Turn it into a stiff wall of the same mass and the issolation will be good except at the inevitable resonance frequencies where the wall becomes essentially transparent.

As to stuffing, I have never been impressed with any of the polyfil or BAF materials. My measurements of their absorption properties have shown they don't compare to fiberglass or rock wool. They may up the apparent volume of a cabinet a little but they don't absorb internal standing waves. Most open celled foams are so-so but there are some closed cell foams that are good.

Any good material will have published data on alpha vs. frequency (absorption). If you can't find certified measurements then it probably isn't a real acoustic absorber.

Regards,

David S.