Cab Resonance Theory?

[moldyoldy]

Q. Are there approximate frequency bandwidths to which the various methods employed to minimize cabinet resonance apply, in particular, bracing and damping? Assumption; theoretical vented, full-range enclosure.

Simplified; At what point (F) should we give up trying to maximize energy transmission by minimizing energy absobtion (bracing/rigid materials), and begin to apply energy absorbtion means to minimize resonant distortion, and accept the loss of energy (damping)?

Q # 2. In a theoretically non-resonant, sealed enclosure, say of concrete, how does one calculate the amount (and type) of material needed to convert the absorbed energy to heat. Traditional sealed, stuffed enclosures seem to have been designed with the assumption that the heat of absorbtion was negligible/of no concern, as they included no efficient means of removing it.

Oh, BTW, I'm math-challenged. E=MC2 gobbles up my RAM in a hurry....don't know if it was the lack of a college education, or what I did instead......:hmm:

[Mr. Widget]

While this IS a DIY topic, I suppose it isn't specifically a Lansing DIY topic... that said, I'll let the Moderator of this Forum determine if it should be moved...

So cabinet resonance?

I did a fair amount of reading on this subject a few months back when I built a pair of speakers that had a resonance problem. The most basic concepts on the subject are that ALL materials resonate. They may require more energy than your system can provide to produce a resonance... a system carved into Half Dome in Yosemite for example, but simply building a concrete loudspeaker will not guarantee a nonresonant design.

Another concept is that of driving the resonant frequency out of the bandpass of the operating system... say for a subwoofer, design it so that the resonant frequency is in the many hundreds of Hz or even higher.

Another concept has to do with damping cabinet resonance within each panel. To properly damp the resonance you need a high ratio of damping material mass relative to the panel's mass. You can go the route of Westlake and build extremely heavy cabinets with many inches of goop inside, or you can follow the lead of the many BBC inspired designs where they used very lightweight panels with relatively little damping material required to really tame them. In both of these cases the damping materials of choice are rubber or rubber like materials that may be mineral filled or even lead sheeting.


I hope this helps.


Widget

[Ken Pachkowsky]

You can go the route of Westlake and build extremely heavy cabinets with many inches of goop inside

Widget

By the way, it's a polyester based goop.

Ken

[Hoerninger]

Hi Moldyoldy,

when you are talking about cab resonances there are several aspects:

Concerning the walls there are two aspects: The wall always has resonances and it has an acoustical throughput.
As Widget pointed out, a wall may be constructed with inner damping so there are no sharp resonances, or it may be done as stiff as possible - with sharper and normally higher resonances. Rubber with its compliance is not such a good choice, doubled walls filled with sand of a birds cage may help (and is awful heavy). If you choose a stiff wall then it should be light so that the resonances are high.
Acoustical throughput can be suppressed with inner damping (t.e. glassfiber, sand) or by a stiff construction which reflects the sound. A heavy wall reflects better than a light one.
But finally said for home used enclosures MDF is a good choice.

Concerning the enclosed air you must differ ceiled enclosures and bass reflex boxes. In both cabs there are standing waves between the walls which produce resonances. The resonances depend on the dimensions. The lowest resonance frequencies are discretly hearable, the upper ones can't be distinguished. These frequencies can best be damped when the cab is completely(!) filled. This can only be done in a ceiled enclosure.
In a bassreflex box this is not a good idea. It is intended that the inner volume of the enclosure and the mass of air in the tunnel (tube?) are resonating. This frequency is much deeper than the above mentioned cab resonances. This resonance may not be strongly damped.
So in consequence in a bassreflex box you may only attach damping material to the walls which gives some damping to standing waves. Many boxes show that it works.

Inner cab damping has another aspect which has nothing to do with resonances. The inner sound is reflected by the walls and may be hearable through the membran, especially when they are light. So the way from the membran to the reflecting wall and back should be damped. Damping is much better when the way is long. This can be maximized if you insert the speaker in a damped tube. This works fine for midrange cone speakers. It's not good for an enclosured bass box because the lowest resonance comes to the bass region. And for a bassreflex box a long enclose is worthless, it is even worth.

Now it must be known that damping material reflects higher frequencies to some extend. This behaviour depends on material and density. The optimum filling for deeper frequencies is not the same as for higher frequencies. Many constructers have good experience with glass fiber. For midrange cone drivers I prefer sheep wool or acetat fiber wool (much cheaper).
The optimum amount of filling is found by experience. A good start is 20g/l with rockwool, 10g/l with sheep or acetat wool for an enclosed box. The amount of damping is most often determent by the damping of the resonance frequency of the woofer, although this is not the right starting point. A bass reflex enclosure should be developed more carefully.

Inner damping converts the acoustic energie into thermo energy, o.k. It is not very much - a sinfony orchestra has about 10 Watts peek level between 250 and 500 Hz. This can easily be dissipated. Another problem is the electric energie which converts into thermo energie and heatens voicecoil and magnet. Modern JBL speakers care for this. (Once I saw a DIY speaker with cooling air guided to the magnet, something like processor cooling.)

Nevertheless it is joyful to build speakers and listen to them.
____________
Regards
Peter

[hapy._.face]

'...I prefer sheep wool or acetat fiber wool (much cheaper). The optimum amount of filling is found by experience. A good start is 20g/l with rockwool, 10g/l with sheep or acetat wool for an enclosed box.'

Hi Hoerninger,

I was just reading an article on the use of rockwool as a damping material- especially for midrange/midbass drivers. What I did not get is the technique by which you apply it to the walls of the cabinet (especially a tube).

I remember gutting the plaster walls in my home and discovering mineral soaked wool (rockwool?), and it seemed like a material that would be difficult to work with in a speaker enclosure type application.

Can you please elaborate on the technique you use for applying rockwool to the enclosure walls? It would be greatly appreciated! Thank you.


MoldyOldy,

In response to your first question- is it ever enough? :) Widget's idea of carving a system out of natural features on the planet sounds like a viable option to me!

In all seriousness (as I admit my gross lack of knowledge on the subject) I wonder what other effects cabinet resonance has on a system. Other than effecting the direct, audible portion of the performance- can it not also effect the mechanical ?? A minor earthquake may not make my turntable skip (I could hear that) - but you can bet my cart is not picking up like it should...make sense?

[moldyoldy]

Thanks to all for the replies!

I understand all solids have a resonant frequency (I even found my own the first time I heard a Hammond/Leslie many moons ago). Guess I asked too many questions at once to expect the answer I'm looking for, so I'll stick with Q#1, vented cabs.

Being one that hates to give up any degree of system efficiency, it seems that one should try to maximize the conversion of mechanical energy into acoustical by means of cab bracing/rigid materials, up to a point of diminishing return, before "throwing in the towel" and absorbing the remainder via damping.

An example; Altec produced many fine HE drivers, yet installed many of them in fairly flexible cabs with minimal bracing. Jim Dickinson's subsequent articles on rebracing those cabs weren't just theoretical speculation, the methods really work, offering very signifigant improvement.

Q#1 rephrased; Given today's available materials, is there an an approximately-specific point where attempts to increase a cabinets rigidity should be abandoned for damping methods, assuming maximum efficiency is desired?

Sidebar;
Being in the commercial HVAC construction industry, I've seen major advancements over the years in materials and systems used to minimze system noise levels via damping and absorbtion. A recently completed University housing project included extensive onsite testing of noise levels from infrasonic thru ultrasonic, and anything exceeding specified parameters was rejected for rework. What appeared to be a royal PITA at project onset, turned into a very valid means of acheiving desired results. It also seems that certain methods/materials used here could have applications in the audio industry as well. Perhaps in a future thread, I'll try to share some ideas along these lines that might make the home environment more listener-freindly.

[tom1356]

This diagram shows the materials applied to the inside of the BBC LSU/10 monitor speaker of the 1950's. This is the first speaker to take cabinet resonances into consideration.
The LSU/10 was designed by D.E.L. Shorter of the BBC. He wrote the book on the subject, so to speak, in 1958 in a paper for the AES titled: "A survey of performance criteria and design considerations for high-quality monitoring loudspeakers."
It is available on the AES web site and is a must read for anyone interested in building loudsoeakers.
Good Luck
Tom

[Mr. Widget]

Guess I asked too many questions at once to expect the answer I'm looking for, so I'll stick with Q#1, vented cabs.Yes... yes you did...:D

If you can handle the mass, the technique I used in building Scott Fitlin's subs is exactly what you are describing. The boxes are extremely well controlled and due to extensive bracing and the choice of materials they are also extremely rigid.

I won't waste time describing them again, but if you look over the first couple of posts you'll get the idea.


http://audioheritage.org/vbulletin/showthread.php?t=9812


Widget

[hapy._.face]

Moldy',

The advancements used in the HVAC industry can already help the home enviorment:

I have heard of people picking up the sound of HVAC systems turning on/off in a recording studio session. These artifacts made it to final mix and ultimately were released to the public. :(

[moldyoldy]

Thanks Widget!

I assumed the possibility that my question could only be answered depending on the tradeoffs one finds acceptable. Your methods on Scott's cabs mirror my thoughts on the subject, so I'll continue bracing and stiffening, knowing there's contemporary, competent support for the concept.

Wish I had (another) reason to travel to the right coast, I'd love to hear your cabs and Scott's setup. At 48, I think I'd even still enjoy doing the bump thing, especially when there's some great bumpin' accompaniment!

(Now there's a cool idea for a new monicker........The Bumpin' Bumpkin!);)

Happy. .Face,

I agree. I'll try to work up some ideas that can be easily implemented for a future thread. BTW, your current HappyFace is great!

[Hoerninger]

Hi hapy._.face,

glasfiber (or mineral & stone wool= rockwool) are often delivered in plates which can be cut into pieces. They can be attached by nales or even be glued. (Strings may work as well.)
Here you can see what I mean: http://www.agsn.de/rockwool/rockwool_produkte.htm

In this forum I a saw a JBL bassreflex speaker opened. There was mineral wool for damping applied to the walls. Indeed I do not know their technique for applying it :-(

Because I do not build bassreflex boxes I can always fill the whole enclosure, the density of the materials is small enough. Glasfiber plates will be cut into pieces and layed close by close into the box. That's the way I have seen it in a commercial closed box which had a very good reputation and which i had opened. The plates are stiff enough so they will not be pressed. But in a taller box you can give some support with strips of wood.

I don't like to use glasfiber/ rockwool. Health should be considered aswell (cancer).

The closed boxes I have dealt with used one speaker for low and midrange. So I prefered sheepwool or acetat fiber. The sheepwool I know (in Germany by Visaton) are delivered in rolled plates which are fairly stiff so that they can be handled like glasfiber plates. Acetatfiber wool (cottonwool -?- for bodycare) I get in little 200 gram bags in a drugstore. This material must be drawn in lots of little pieces. It is best to weigh the right amount first and then tear it into pieces. I take thump and a finger from each hand. It's an annoying job but fills up a huge volume. (When you scream into this hill of wool your voice seems to be off.) Then I put it into the box. Normally it only needs to be pressed a little bit. Up to a height of about 40 cm this stuffing is longtime stable. Nets may be used to fix the height in a taller box..

When you are using a tube behind a midrange speaker then it will probably not be used as a bassreflex enclosure. So you will fill the whole tube without problems.

Hope this will help.
____________
Regards
Peter

[Zilch]

Thanks again to Steve Schell for this link to absorption coefficients of various damping materials:

http://www.bobgolds.com/AbsorptionCoefficients.htm

[hapy._.face]

Thanks again to Steve Schell for this link to absorption coefficients of various damping materials:

http://www.bobgolds.com/AbsorptionCoefficients.htm


Yeah, I'll second that! That's an excellent link with even better reading. Thank you!




Hoerninger,

As far as rockwool is concerned- I see that the "blown in" loose type I found in the walls of my circa 1940's home is not the standard anymore. It appears that rockwool is now more commonly found as a semi rigid panel. That clears things up. :)