Failed stuffing experiment

[soundminded]

A servo controlled woofer changes everything. Non servo systems are open loop. Whatever the open loop transfer function of the system, that's what you have and tuning is strictly a matter of changing the mechanical parameters (As I said elsewhere, the electrical filter is a no brainer.) In a servo controlled system, you have another story altogether. But remember what the servo system controls, the motion of the cone. It compares the input voltage to the speaker to the output voltage of a sensor whether it is a displacement sensor, a velocimeter, or an accelerometer of course connected to a suitable differentiating circuit if necessary depending on the type of sensor. Therefore the servo system when optimum will assure that the cone motion tracks the input voltage because it is driven by the difference to that of the sensor. This however does not change the fact that the mechanical load impedence changes with frequency in vented enclosures and that at some frequencies, there will be substantial contribution to the acoustic output from sound coming out of the vent or port while at others there isn't. Therefore a servo controlled system in a vented enclosure will not assure flat extended response of the system as a whole. One caution about all servo systems is the phase versus gain of the feedback loop. If the phase reaches 180 degrees before the gain falls off to less than unity, the system will go into spontaneous oscillation so you don't want to go anywhere near that condition. This can be avoided by skillful design of the loop circuit. This I think is a mistake many inexperienced designers of audio amplifiers make and a large number of weak designs may explain why so many people don't like negative feedback amplifiers, they are in some conditions only quasi stable. From my own experience, the equations describing servo system response are very complicated and you would do well to not only realize that a complete Bodie plot includes phase as well as amplitude response but analysis of the pole zero diagram and Nyquist diagram for the system is also critical. That's what I rememeber from 40 years ago.

[Guest Bret]

>Only if taken with a shot of port.<

No, no, no. With port you want Flomax. That's how you get the most, lowest-frequency output from a small spheanker-system.

Bret

[kkantor]

Thanks, Bret.

With friends like you, who needs enemas?

-k

www.kenkantor.com

http://kkantor.spaces.live.com

[speaker dave]

Hi Carl,

I have experimented with the KEF activated charcoal. It really works.

I did measurements on the KEF 9000 on-wall extrusion type system. Calculating the internal volume they had about 8 to 8.5 liters internal. The charcoal was stuffed into cloth bags that were squeezed into the cabinet and snaked around the drivers. With the charcoal removed the system resonance was 105Hz. With the charcoal in place it was dropped to 79 Hz. The charcoal volume was about 2.4 litres and with some calculation it looked as if the 8.5 litres initial volume was acting more like 15 litres total with the charcoal (1.767 times or a 77% increase). Since 2.4 liters of charcoal looked like 8.9 liters of air (2.4 + (15 - 8.5)) then the charcoal was 3.7 times as compliant.

This is pretty good going only limited by your ability to package the ACE material and fill the cabinet with it. I have also heard that the material is strongly effected by humidity absorbtion.

I would second you dissapointment with pillow stuffing. I have never found BAF (bonded acetate fiber) to be any good at either lowering resonance/increasing box compliance or absorbing internal resonances. Its hard to beat good old fiberglass!

Regards,

David Smith

psb Speakers

[Carlspeak]

>Hi Carl,

>

>I have experimented with the KEF activated charcoal. It

>really works.

>

>I did measurements on the KEF 9000 on-wall extrusion type

>system. Calculating the internal volume they had about 8 to

>8.5 liters internal. The charcoal was stuffed into cloth bags

>that were squeezed into the cabinet and snaked around the

>drivers. With the charcoal removed the system resonance was

>105Hz. With the charcoal in place it was dropped to 79 Hz.

>The charcoal volume was about 2.4 litres and with some

>calculation it looked as if the 8.5 litres initial volume was

>acting more like 15 litres total with the charcoal (1.767

>times or a 77% increase). Since 2.4 liters of charcoal looked

>like 8.9 liters of air (2.4 + (15 - 8.5)) then the charcoal

>was 3.7 times as compliant.

>

>This is pretty good going only limited by your ability to

>package the ACE material and fill the cabinet with it. I have

>also heard that the material is strongly effected by humidity

>absorbtion.

>

>I would second you dissapointment with pillow stuffing. I

>have never found BAF (bonded acetate fiber) to be any good at

>either lowering resonance/increasing box compliance or

>absorbing internal resonances. Its hard to beat good old

>fiberglass!

>

>Regards,

>David Smith

>psb Speakers

Thanks Dave for sharing your experience with us. I have some questions, however.

1) Exactly what type of AC did you use? coal based or coconut based?

2) Did you take any extra measures to dry the AC before installation?

3) What was the approximate total weight of the AC you used? I estimate it was about 2.5 lbs based on an assumed 30 lbs/cu. ft natural density of the AC. Est. stuffing density of AC was 8.6 lbs/cu. ft, about half of what I calculated from the KEF white paper.

4) What was the normal stuffing and weight used for the KEF 9000? FG, PET? and what was the speaker's resonance Hz with that original material? That is what you need to compare your results to.

It's all about the music

Carl

Carl's Custom Loudspeakers

[speaker dave]

Hi Carl,

I didn't make it clear but the KEF 9000 is a speaker that comes from the factory with the KEF's ACE material. It is one of the on-wall compact aluminum speakers. Keeping the size to a minimum is crucial for this particular market, so this is a good application for the technology. (Probably makes more sense to use it here than in a flagship speaker where people don't care about the size.)

I wasn't interested in weight at the time but your estimate is probably about right. The real issue is apparent volume increase and you have to look at this two ways: how much more effective is the material than the same volume of air, and, how much material can I get into the system.

With the 9000 all of the charcoal was sewn into a number of cloth bags, mostly in a series of round tubes about 1" diameter. These were stuffed into the free areas of the enclosure and wrapped around the woofers. As packed as it was, they could only get about 2.4 liters of material into a roughly 8.5 liter cabinet. Still, that was enough to drop the system resonance from 105 to 79.

So, 79Hz was the normal resonance of the system as sold. 105Hz would be the resonance with the material removed, i.e. no stuffing at all. Fiberglass might lower the resonance by 10% as opposed to 25% realized by the ACE material in this application.

My impression is that the KEF material is very specialized stuff. I believe it is coal based but I don't know if normally available activated charcoal has the particular qualities that their proprietary material has.

The comment on humidity was based on something told to me by someone at Peerless India (I believe they had quoted on building this model); that high humidity could degrade the material qualities. For that reason the system was well sealed from moisture.

Regards,

David

[Carlspeak]

So, basically what you did was simply remove the AC from the KEF speaker and compared the Fc empty with the Fc after the AC was installed. I misunderstood.

Suggest you find a speaker cabinet of similar volume (different manufacsturer) and put the KEF AC in that one and compare the Fc with the AC and with FG stuffed at about 1 lb/cu ft.

That would be an interesting experiment since you already know you have some AC that works like KEF claims.

It's all about the music

Carl

Carl's Custom Loudspeakers

[Pete B]

Hi David,

Interesting! Don't know if you still have the system available but why not try stuffing it with fiberglass, without the KEF material, just as a sanity check? I'd be very curious to hear the outcome. We have to keep in mind that while the compliance shift for example is theoretically 1.4 for the shift from adiabatic to isothermal, the shift in Fc would only be lowered by the square root of this value due to Fc's dependency on compliance.

I noticed that KEF wrapped the material around the magnets so that it is very close to the cone, this could very likely also cause an increase in the mass load to the driver which would also lower Fc. A full T&S measurement of the drivers in system with and without the KEF material would help determine what is actually going on.

Thanks for your input!

[kkantor]

David,

Are you the David Smith I met at Snell, KEF, etc? If so, welcome, and thanks for sharing!

(If not, welcome, and thanks for sharing!)

Some questions:

If the carbon is working as claimed, wouldn't it most likely have a highly non-linear effect? At small driver excursions, it would be asked to "adsorb" much less air volume than at high excursions. (Adsorption is highly asymptotic vs. pressure, unlike gas compression.)

- What drive level(s) did you measure Fs and Fc at?

As is well known, surrounding the rear frame of some drivers with just the right flow resistance, (with dense fiberglass, etc.), can also drop the Fc very significantly. This happens via two somewhat different mechanisms: increasing Mma and adding a resistive term to the radiation impedance. (The reason this is not commonly done is that it drops efficiency and increases Qtc, so you might as well have built a different driver in the first place.)

- What was Qts, Qtc (empty) and Qtc (w/carbon)?

If you still have the speakers, it would be fun to test them using plain old resistive damping material.

Thanks again! Your contribution of actual data is much appreciated, and is cause for optimism about the method.

-k

www.kenkantor.com

[johnieo]

>The charcoal volume was about 2.4 litres and with some

>calculation it looked as if the 8.5 litres initial volume was

>acting more like 15 litres total with the charcoal (1.767

>times or a 77% increase). Since 2.4 liters of charcoal looked

>like 8.9 liters of air (2.4 + (15 - 8.5)) then the charcoal

>was 3.7 times as compliant.

Dave:

If there are only 2.4 L of activated charcoal in the cabinet, then simple surface adsorption can't describe the effect. Given its loose density of .42 g/cc, the cabinet was filled with 1 kg of activated charcoal. This could adsorb release some gas molecules with pressure changes but could not explain the effect. The micropores inside activated charcoal are not of uniform size- there are very small pores of atomic-sized dimensions, as well as micron sized pores, and some inbetween. The quantity of atom-sized pores is dependent on the temperature and ambient in which the gas is driven from the starting material.

What may be happening here is the "inkwell effect." That is, larger cavities connected to the surrounding air via small atom-sized passageways take time to fill and empty. Let's imagine a passageway 5 Angstroms diam x 10 Angstroms long connected to a spherical cavity 1/2 micron in diameter. The volume of this micro cavity is V = 5E-13 cc. The pumping speed (filling/emptying rate) of the little passageway in molecular flow is S = 1E-11 cc/s. The time constant for filling/emptying this micro cavity is then V/S = 0.05 s. It's in the range. In real charcoal there will be a range of time constants.

Since the 2.4 liters of activated charcoal is likely 2/3 or more void, this volume of charcoal yields a lot of micro resonators that could produce the measured increase in compliance. Ken tells me that stuffing near the woofer has a great effect on system resonance.

>This is pretty good going only limited by your ability to

>package the ACE material and fill the cabinet with it. I have

>also heard that the material is strongly effected by humidity

>absorbtion.

Any activated charcoal is seriously affected by moisture; worse yet by organic compounds. Here is where water adsorption would play a major role -- it would seal the atom-sized pores eliminating any resonance effect. It sounds like your material was obtained freshly baked and sealed in the cabinet in a controlled environment. I'll bet the cone isn't made from porous paper or the surround from foam!

Regardless of your starting material Carl, moisture adsorption would have done it in by the time you receive it, if not before.

Cheers,

[soundminded]

>>The charcoal volume was about 2.4 litres and with some

>>calculation it looked as if the 8.5 litres initial volume

>was

>>acting more like 15 litres total with the charcoal (1.767

>>times or a 77% increase). Since 2.4 liters of charcoal

>looked

>>like 8.9 liters of air (2.4 + (15 - 8.5)) then the

>charcoal

>>was 3.7 times as compliant.

>

>Dave:

>

>If there are only 2.4 L of activated charcoal in the cabinet,

>then simple surface adsorption can't describe the effect.

Very true. If the reason for the acoustical effect is that charcoal adsorbed nitrogen from the air, it would create a negative air pressure inside the box if it is hermetically sealed. When the box is opened or if there were even the smallest hermetic leak, more nitrogen would enter and the charcoal wouldn't function until it adsorbed all the nitrogen again. As it did, the acoustical properties would continually change. Furthermore, it would continue to adsorb nitrogen until all available adsorption sites were filled and would cease to function altogether until it was replaced or reactivated. (Is this were true in fact, would it be acceptable in a $100,000 speaker? I don't think so.)

>Given its loose density of .42 g/cc, the cabinet was filled

>with 1 kg of activated charcoal. This could adsorb release

>some gas molecules with pressure changes but could not explain

>the effect. The micropores inside activated charcoal are not

>of uniform size- there are very small pores of atomic-sized

>dimensions, as well as micron sized pores, and some inbetween.

>The quantity of atom-sized pores is dependent on the

>temperature and ambient in which the gas is driven from the

>starting material.

>

>What may be happening here is the "inkwell effect."

>That is, larger cavities connected to the surrounding air via

>small atom-sized passageways take time to fill and empty.

>Let's imagine a passageway 5 Angstroms diam x 10 Angstroms

>long connected to a spherical cavity 1/2 micron in diameter.

>The volume of this micro cavity is V = 5E-13 cc. The pumping

>speed (filling/emptying rate) of the little passageway in

>molecular flow is S = 1E-11 cc/s. The time constant for

>filling/emptying this micro cavity is then V/S = 0.05 s. It's

>in the range. In real charcoal there will be a range of time

>constants.

>

>Since the 2.4 liters of activated charcoal is likely 2/3 or

>more void, this volume of charcoal yields a lot of micro

>resonators that could produce the measured increase in

>compliance.

I said above that I think it is due to the increased velocity related frictional loss resulting from the viscousity of air being forced through the pores but this may be a different way of saying the same thing.

>Ken tells me that stuffing near the woofer has a

>great effect on system resonance.

This may be due to the fact that air trapped between the back of the cone and the stuffing creates a second undamped resonance cavity at a higher frequency. Also the packing density and size and orientation of the fibers closest to the cone will have the greatest effect on the overall damping effect because as it becomes more densely packed, it makes the effect of fibers further away in the enclosure less relevant because it blocks off air flow to them.

>

>>This is pretty good going only limited by your ability to

>>package the ACE material and fill the cabinet with it. I

>have

>>also heard that the material is strongly effected by

>humidity

>>absorbtion.

>Any activated charcoal is seriously affected by moisture;

>worse yet by organic compounds. Here is where water adsorption

>would play a major role -- it would seal the atom-sized pores

>eliminating any resonance effect. It sounds like your material

>was obtained freshly baked and sealed in the cabinet in a

>controlled environment. I'll bet the cone isn't made from

>porous paper or the surround from foam!

>

>Regardless of your starting material Carl, moisture adsorption

>would have done it in by the time you receive it, if not

>before.

I don't know that I can agree with you there. Isn't activated charcoal used for some water purifiers such as cartridge filters you screw into a kitchen sink faucet spout? Had it been "done in" by the time you receive it, it wouldn't work for anyone unless it were re-activated by driving the mositure off before it is used. I think the charcoal cartridge filters in home water purifiers are usually expected to last about 6 months before they need to be replaced. Besides organic contaminants, what they probably adsorb most is hypochloric acid added by water companies to kill bacteria.

[Pete B]

http://www.stereophile.com/news/11624/

By the way David, I like PSB speakers very much, wish some of the AR guys here would give the Stratus Golds a listen!

[johnieo]

>Very true. If the reason for the acoustical effect is that

>charcoal adsorbed nitrogen from the air, it would create a

>negative air pressure inside the box if it is hermetically

>sealed.

You miss the point. The sorbent is loaded with its saturated quantity of dry air when installed (~8 cc/g). It is the of the sorbed quantity versus pressure (the sorption isotherm) that describes how gas is removed as the pressure increases and returned as it decreases. This slope is too small to describe the observed behavior with this carbon charge.

>I said above that I think it is due to the increased velocity

>related frictional loss resulting from the viscousity of air

>being forced through the pores but this may be a different way

>of saying the same thing.

In molecular flow (mean free path large compared to micropore diameter), "viscosity" and "air velocity" have no meaning - its all surface scattering. The inkwell effect is a filling/emptying time constant, that if in the range 10 to 100 msec, would explain the observed behavior. Both small pores and microcavities are required to have a resonance effect. As was noted to me off line, this effect would likely not be very strong six months to a year after manufacture, as water would indiffuse through joints in the same manner as in any acoustic suspension speaker.

>I don't know that I can agree with you there. Isn't activated

>charcoal used for some water purifiers such as cartridge

>filters you screw into a kitchen sink faucet spout? Had it

>been "done in" by the time you receive it, it

>wouldn't work for anyone unless it were re-activated by

>driving the mositure off before it is used.

The purpose of installing a charcoal cartridge in a water line is to remove odors and chlorine -- a "taste" filter. Organic compounds and chlorine displace the water molecules because their binding energies are larger. The carbon is saturated with water instantly, but can remove more active stuff for some months. In industrial pre-filtering applications, we usually size the tanks to last six months. In gas filtering, water adsorption ocludes the micropores.

[speaker dave]

Hi Ken,

Yes, I am the D. Smith formerly of Snell and KEF (and Mac and JBL). I remember we had a pleasant conversation at a Stereophile cocktail party once. I'd like to ask you some questions about the MGC1 at some point, a system that greatly interests me.

Unfortuantely I don't have the KEF speaker anymore. We were the distributor of KEF in Canada at one time and I took some quick measurements back in 2004.

I've attached some impedance curves that the resonance frequencies came from. They were done at a low level with LMS at several volts (open circuit) through its 600 ohm output impedance. I can't comment on the large signal behaviour of the system but it would be a bummer if it lowered resonance while dramatically increasing nonlinearity.

http://www.classicspeakerpages.net/dc/user_files/2534.txt

The impedance curve shows that the system uses a series capacitor (3rd order system), that the resonance drops and that Qm drops considerably as well. I don't know if the Qm drop is from flow resistance adjacent to the woofer or is inherent in the process. I do remember that some of the tubes of charcoal encircled the woofer but I don't think they were tightly blocking the chassis windows.

As an aside on charcoal quantities, you would think that you could stuff more than 2 liters into an 8+ liter box but once the material is placed in socks it becomes hard to pack the enclosure completely, in spite of a pretty good effort to fill every nook and cranny. This limits the total benefit of the process since only 25% of the air volume is being "expanded" by the charcoal.

Here is a near field curve of the system with and without the charcoal. I shows a pretty significant response extension and a useful drop in Q. I think the drop in Q is more than would be expected from the resonance shift: that is the Qm drop from the impedance curve is in evidence.

http://www.classicspeakerpages.net/dc/user_files/2535.txt

To John and soundminded, I can't really comment on the chemistry of what is going on. The KEF white paper that someone has linked to gives a good description of the material and the adsorption/desorption process. Clearly there is no negative pressure buildup, it is a continuous process of absorbing and releasing air pressure from the woofer. They do mention the time constant issue in that they have a curve that shows the compliance enhancement going away above 100 Hz.

Finally, Thanks Pete B. If you like the Stratus Gold you'll love the upcoming Synchrony line (shameless plug!)

Regards,

David

2534.txt

2535.txt

[kkantor]

David,

That's great! I think you should introduce yourself....

[Readers... David is a truely first-rate speaker designer and experienced industry professional. A fantastic addition to CSP!]

Yeah, I tend to agree with your take on those curves. Not magic, but something is going on, at first impression. Maybe I'll take out a ruler and calculator later, and see if I can come up with some estimates of Qtc.

-k

[kkantor]

http://panasonic.co.jp/corp/news/official....en070730-1.html

(or http://tinyurl.com/39oqgo)

-k

www.kenkantor.com

[Carlspeak]

>http://panasonic.co.jp/corp/news/official....en070730-1.html

>

>(or http://tinyurl.com/39oqgo)

>

>-k

>

>www.kenkantor.com

>

>

Thanks Ken for the laugh.

Monkey see, monkey doooo.

So much for the public domain and proprietary technology.

It's all about the music

Carl

Carl's Custom Loudspeakers

[johnieo]

It is still difficult for me to accept that surface adsorption of the gas causes this effect. Adsorption isotherm data show that cubic centimeters of nitrogen could be adsorbed/released on to the mainly saturated surface of 2 Liters charcoal at atmospheric pressure if the interior pressure of the cabinet reached 120 dB. The surface is already saturated and the change in adsorption with pressure is small.

I vote for the charcoal working because of actual "microcavity volume" rather than adsorption. The theoretical limit of surface area is over 2,000 square meters per gram, requiring monolayer distribution of the matter, but measured values can approach this in reasonably "chunky" particles.... one reason for this is that the bottle-necked "inkwell" microcavities that trap gas, which the measuring devices mis-interpret as surface adsorption.

Further, the "inkwell" filling and empting scenario would not work above a particular frequency, because there would be no time for the gas pressure inside the cavities to change--a point noted in the Panasonic release.

Some time ago, when we discussed replacements for fiberglass, Ken Kantor mentioned that Dupont Hollofill fiber was as close to a direct replacement for fiberglass as he had seen. Depending on the diameter of these fibers and their average length, they could very well be acting as micro-resonators for substantial amounts of gas in the same manner as for charcoal. That is the only explanation that I can think of that would explain why hollow polymer fibers are better than solid polymer fibers.

Panasonic may well understand exactly what is happening but not choose to disclose. Likely they are applying for a patent that describes a method of processing or particular type of stuffing material.. Can't patent theory explanations.

[Carlspeak]

As I speculated in an earlier thread (click below), going finer in fibrous stuffing diameter was the way to go. Well, it appears now the 'tortuosity path' has found it's utility via nano-size pores in AC.

http://www.classicspeakerpages.net/dc/dcbo...id=&page=#12727

It's all about the music

Carl

Carl's Custom Loudspeakers

[Pete B]

>Hi Ken,

>

>Yes, I am the D. Smith formerly of Snell and KEF (and Mac and

>JBL). I remember we had a pleasant conversation at a

>Stereophile cocktail party once. I'd like to ask you some

>questions about the MGC1 at some point, a system that greatly

>interests me.

>

>Regards,

>David

I've been meaning to comment about this.

I would enjoy reading your discussion with Ken about the MGC1

here on the board, if you and Ken don't mind. I too am interested

in the design, but don't have any specific questions at the moment.