AR-4x Cabinet Damping

[Carlspeak]

I agree the mechanical properties of stuffing fibers (rather than thermal) are the critical variables.

Also had to look up the definition of a Rayl since your text was the first time I'd ever seen it. Wikipedia had the answer and also an indication it's units are very similar to viscosity (i.e. dyne.sec/cm cubed vs dyne.sec/cm sq. for viscosity) which I'm a bit more familiar with coming from a papermaking background. Does that mean the fibrous mass in a cabinet is acting as a viscous drag on the air movement in some three dimensional sense?

I still would love to try some carded melt blown PP in a 4x cabinet. The diameter of those fibers is in the same range as FG. I tried some in the form of sheets of MBPP stuffed in a 4x, but wasn't satisfied with the result. The sheets were the only form of it I could get my hands on.

[speaker dave]

These differences in energy storage and release in the form of vibrations of individual fibers can be viewed in a different way by looking at the vibration frequency of a beam. Each "free" part of a fiber (between the points at which they touch) represents a miniature "beam" with a resonant frequency that is determined by the fiber's length, stiffness, and diameter. FG is much stiffer and of smaller diameter than PE. The "average" length of a free FG fiber is 60 um, whereas the same for PE is ~250 um. the boundary layer for air drag at 40 Hz is about 200 um, so our stuffing situation is crudely analogous to the difference in air resistance of wind blowing through fly screen versus chicken wire -- the widely spaced PE does not vibrate (absorb and release) nearly as much energy as does the FG. Said another way, the difference in natural resonance frequency between a glass fiber and a PE fiber can be observed by suspending pieces of thread and FG in one's fingers, tweaking them and observing their natural resonance!!

My current view is that the mechanical properties of ordinary fibers - properties such as Young's Modulus (stiffness), fiber diameter, fiber density, and cabinet porosity deterimne how well it vibrates in sympathy with the acoustic signal. Therefore it would be interesting to examine other non-glass fibers of similar diameter as PE, but with increased stiffness. Hmmm, got any other fibers to test, Carl?

The natural or uncompressed fiber density which you descrbe, Carl, is important and is contained in the above terms (diameter, density, cabinet porosity).

Very interesting stuff and somewhat beyond my physics knowledge. My understanding has always been that the act of forcing air through the many minute cavities gave rise to the "adiabatic to isothermal" conversion, although that is probably just adding pretty words to a lack of understanding! I do know that if you carefully measure Thiele/Small parameters of a woofer in a box, then add fiberglass stuffing and re-measure the parameters, you will see that box compliance has increased by up to 20%. (Roughly half the theoretical) At the same time Qm will drop and calculated woofer mass will increase, so more is going on than just effective volume change.

It is also likely that the optimum density depends on the woofer, so that the best density for an AR-4x is probably not the same as for a 3a--but I know little about woofer design! Thus, I have tended to use units of "pounds per model x cabinet". :-)

It is an economic reality that the majority of systems are designed with less box volume than would be ideal, since adding either box volume or magnet strength both have a cost. So adding as much effective volume as possible is (generally) a good thing. The optimum then becomes achieving the lowest resonance before Qm really starts to drop. Qm drops when the density gets to the point where flow resistance is too great. Your initial resonance vs. stuffing plots show this nicely.

Don't forget that controlling resonance and Q are only one side of the stuffing picture. Control (reduction) of internal box standing waves is crucial, and most polyfibre materials are quite bad for that. Its hard to beat fiberglass for acoustic absorption and it would be universally used if it weren't for the "itching" problem. For most of the companies I've worked for the final decision about stuffing material came down to how vocal the production side was.

David

[johnieo]

I do know that if you carefully measure Thiele/Small parameters of a woofer in a box, then add fiberglass stuffing and re-measure the parameters, you will see that box compliance has increased by up to 20%. (Roughly half the theoretical) At the same time Qm will drop and calculated woofer mass will increase, so more is going on than just effective volume change.

Thanks, Dave> Yes we did measure parameters as a function of FG and PE and found the suffing quantity that produced the lowest resonant frequency. Adding more stuffing simply raised the resonant f, because the volume of air in the cabinet decreased too much. The issue was simply that PE did not produce as low a minimum resonance as did FG. When you say theoretical, one has to consider what theory. If adiabatic/isothermal, I don't think the percent realized is meaningful, as I doubt that this theory is the cause of our measured effect! It has been noted that "theory" says the cabinet must be stuffed uniformly, but in practice, it has more effect near the woofer. I recall Alex Barsotti telling me that the AR-3a Limited (Asian) had 18 oz of mixed diameter (17- 34-um PF) and that it was located mostly near the woofer.

Don't forget that controlling resonance and Q are only one side of the stuffing picture. Control (reduction) of internal box standing waves is crucial, and most polyfibre materials are quite bad for that. Its hard to beat fiberglass for acoustic absorption and it would be universally used if it weren't for the "itching" problem.

Absolutely! Plots of the real part of the acoustical impedance versus frequency illustrate this in spades! Z(FG) -- both real and reactive -- are very high at low frequency and decrease as f is increased (only plotted to 700 Hz), wherease Z(PE) is quite small except for a resonance at 400 Hz, indicating a standing wave as you note. As we all know, this is why FG is used in acoustical tile

Cheers, John

[johnieo]

I agree the mechanical properties of stuffing fibers (rather than thermal) are the critical variables.

Also had to look up the definition of a Rayl since your text was the first time I'd ever seen it. Wikipedia had the answer and also an indication it's units are very similar to viscosity (i.e. dyne.sec/cm cubed vs dyne.sec/cm sq. for viscosity) which I'm a bit more familiar with coming from a papermaking background. Does that mean the fibrous mass in a cabinet is acting as a viscous drag on the air movement in some three dimensional sense?

Yes, the Rayl - a unit of acoustical impedance (pressure divided by velocity) is almost like viscosity, which is pressure divided by a rate of velocity change in the perpendicular direction. It is my impression that drag induced vibration is important. The "average" free length of FG is about 60 um; 250 for PE. The boundary layer is the distance from a surface at which the air velocity goes from its max value to zero (at surface), so if the BL is of order 200 um, it means that there is a lot of drag on the FG fibers and little on the PE -- hence my crude analogy to air flowing through fly screen versus chicken wire. It's my feeling that the oscillating pressure waves set FG fibers in motion and their vibration acts like an energy storage and release mechanism. It would appear that PE is too soft to do much at all.

I still would love to try some carded melt blown PP in a 4x cabinet. The diameter of those fibers is in the same range as FG. I tried some in the form of sheets of MBPP stuffed in a 4x, but wasn't satisfied with the result. The sheets were the only form of it I could get my hands on.

I have a very large bag of about 5-um-diameter melt blown PP fiber -- the kind used to soak oil, I believe. If you would like some I'll be happy to send you lots! Have never been able to think of a way to card it. It is in the form of clumps of medum length fibers. Want some Carl? just PM me your mailing address, I lost it!

Cheers,

John

[Carlspeak]

While browsing thru the local craft store, I discovered they now stock carded bamboo fiber for use as pillow stuffing. I bought a 12 oz. bag which, BTW, was mfg. by the makers of polyester fiber-fil. The sales pitch for the bamboo is that it's a 'greener' stuffing material than PET.

I tried it in an AR4x cabinet recently while comparing it's performance to the fluffy, yellow FG used in later model AR's after they stopped using rockwool. The particular 4x I used contained 12 oz. of the yellow FG. So, I decided to use the same amount of bamboo for my comparison tests.....

The results show similar performance to polyester fiber-fil - not quite as good as FG.

In one non-repeated test, cabinet resonance rose a mere 3 hz (59.8 to 63.8) with the bamboo. QTc stayed about the same at 0.85 (FG) vs 0.88 (bamboo).

Below are some photo-micrographs of the fiber (thank you Johnieo) which appears to be quite unique in it's morphology - almost like a twisted bundle of individual fibers. Nominal diameter measured about the same as 1.5 dpf polyester fiber-fil.

Also attached are two close mic'd response curves comparing the two stuffings. Note the ever-so-slight bass extension in the test with the FG.

My conclusion from this brief study is bamboo seems like a promising candidate for loudspeaker stuffing if one doesn't want to bother with FG.

Ar4x_with_12_oz_yellog_FG_resp.pdf

AR4x_with_12_oz_bamboo_resp.pdf

[Carlspeak]

I just finished restoring an early 3a (ser.#17909). It has a #7 coil, Alnico magnet woofer with cloth surround. The coil/Alnico woofer combination are correct according to numerous posts here.

Based on my earlier post within this thread regarding the inherent density differences in stuffing media, I decided to see what would happen to the Fc if I stuffed this baby with 20 oz. of OCFG instead of the 28 oz. of rock wool I took out and disposed of. I carefully weighed out the OCFG and did the usual stuffing of the box starting at the top and working my way down to the woofer area.

When I completed the FG install, I put an open mesh cloth in place of the old crepe paper and installed the woofer and ran a WT3 test on the speaker.

To my considerable surprise, the Fc come in at 37.6 hz - considerably lower than the typical target of 40-42 hz. Qtc was a very respectable 0.77.

Attached is the output of the test. Note the impedance level stays quite low for much of the 20-20Khz range. The test was run with the midrange pot at max and the tweeter pot backed off a bit. New, Ohmite rheostats were installed as part of the resto.

One must keep in mind this is only one data point, so the trend seen here may or may not be repeatable. However, those doing restorations on older vintage 3a's with Alnico magnet woofers should think twice about installing 28 ounces of new FG and consider experimenting with lower levels.

AR3a_with_7_coil_alnico_woofer_and_20_oz_ocfg.pdf

[RoyC]

I just finished restoring an early 3a (ser.#17909). It has a #7 coil, Alnico magnet woofer with cloth surround. The coil/Alnico woofer combination are correct according to numerous posts here.

Based on my earlier post within this thread regarding the inherent density differences in stuffing media, I decided to see what would happen to the Fc if I stuffed this baby with 20 oz. of OCFG instead of the 28 oz. of rock wool I took out and disposed of. I carefully weighed out the OCFG and did the usual stuffing of the box starting at the top and working my way down to the woofer area.

When I completed the FG install, I put an open mesh cloth in place of the old crepe paper and installed the woofer and ran a WT3 test on the speaker.

To my considerable surprise, the Fc come in at 37.6 hz - considerably lower than the typical target of 40-42 hz. Qtc was a very respectable 0.77.

Attached is the output of the test. Note the impedance level stays quite low for much of the 20-20Khz range. The test was run with the midrange pot at max and the tweeter pot backed off a bit. New, Ohmite rheostats were installed as part of the resto.

One must keep in mind this is only one data point, so the trend seen here may or may not be repeatable. However, those doing restorations on older vintage 3a's with Alnico magnet woofers should think twice about installing 28 ounces of new FG and consider experimenting with lower levels.

Hi Carl,

When John O'Hanlon and I were comparing notes awhile back, I ran many WT2 measurements with various amounts (and types) of stuffing in 3a cabinets. My experience is very consistent with your results, and I agree with your suggestion.

Roy

[johnieo]

To my considerable surprise, the Fc come in at 37.6 hz - considerably lower than the typical target of 40-42 hz. Qtc was a very respectable 0.77.

Hi Carl:

Yes, agreed, but there is one added variable to watch when making these measurements--the free resonance of the woofer. If it is of order 21 Hz, then all is fine with a cabinet resonance is 37-38 Hz. However, I had a couple of Alnico woofers with Fs = 111-13 Hz. These also produced low cabinet resonance, but 6 months after putting them in to regular use, the magnet began to hit the backplate on loud passages. Problem: really old soft spider. After replacing the decayed spider, performance was restored, even though the new spider was a tad stiffer than the old. Since then I realized that 11-14 Hz woofer resonance was a measure of spider decay. That was easily verified by adding increasingly larger loads to the woofer cone and noting that it hit the backside with about 1/3 the load of a good woofer.

As we expect, materials decay is the major issue in restoring these great speakers. Let's see- a 1960 AR-3 woofer is now 50 years old!

Cheers,

John

[Carlspeak]

Hi Carl:

Yes, agreed, but there is one added variable to watch when making these measurements--the free resonance of the woofer. If it is of order 21 Hz, then all is fine with a cabinet resonance is 37-38 Hz. However, I had a couple of Alnico woofers with Fs = 111-13 Hz. These also produced low cabinet resonance, but 6 months after putting them in to regular use, the magnet began to hit the backplate on loud passages. Problem: really old soft spider. After replacing the decayed spider, performance was restored, even though the new spider was a tad stiffer than the old. Since then I realized that 11-14 Hz woofer resonance was a measure of spider decay. That was easily verified by adding increasingly larger loads to the woofer cone and noting that it hit the backside with about 1/3 the load of a good woofer.

As we expect, materials decay is the major issue in restoring these great speakers. Let's see- a 1960 AR-3 woofer is now 50 years old!

Cheers,

John

Hi John:

The second 3a I restored had ser.# 17312, #7 coil and Alnico magnet woofer with a resonance (Fs) of 21 hz. I first tried stuffing with 16 oz. OCFG. This stuffing level was arrived at based on the ratio betwee my measured densities of OCFG and rockwook noted in an earlier post. Box Fc was higher at 39 hz and Q was 1.18. I added 4 oz. and Fc dropped to 36.7 hz and Q was 0.87.

I've had some of those floppy spider woofers to repair. Either the back edge of the voice coil former was damaged or the ring of perfed hols around the VC former just above the coil windings was collapsed due to the VC slamming against the back magnet.