Speaker Box Damping

For damping my favourite is fibreglass. Another option is to use polyeter foam (the material in mattresses), I tried this recently and was surprised how well it works. It is also nicer to handle, and I know that some people have issues with the health aspects of fibreglass.

Both affect the apparent box volume, it appears larger due to the isothermalising effects of the materials.

I have tried a few types of 'stuffing' and after trying the obligitory fibreglass I decided against it...why? Simpley put I would put the stuff up to the ear & listen...fibreglass does not dampen any frequencies....you can hear unobstructed thru it just fine...so what function, if any, is it accomplishing?
Try the same with my favorite stuffing...cotton batting...and the effect is obvious...cotton damps frequencies , fibreglass does not. Unfortunately, cotton can absorb moisture, so that is a downside....moisture from any source will ruin it.
As to the quantitiy, I stuff it 'full', but not so much as to protrude into the speaker basket cutouts to touch the cone from the backside.


2007-12-24 4:24 pm
Well, I have a 2 way system with 2-6 1/2" mid-woofers from the Zetag Hi G1s, (not the copper looking ones), and 1 Focal TC90 in each box. I finished these about a year or so ago, but haven't been entirely happy with the sound. I was kind of waiting for them to "open up" some but frankly they just aren't changing much. So I'm looking for some ways to possibly improve the overall sound without blowing them up and starting anew.
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fantfool said:
What is the best material for damping the inside of the cabinet?

Also, does the amout of damping material you use effect the box volume design?

Damping is the suppression of resonance emanating from the cabinet itself. Several of the above suggestions address backwave reduction but don't, for the most part, do much for the cabinet.
There are many ways to address the cabinet situation; constrained layer damping, added mass, bracing and cabinet configuration (spherical, etc.).

Quickly (it's Christmas after all):

1. Constrained Layer is the lamination of layers, having a visco-elastic bonding medium (liquid nail or the HiEnd "Green glue") or not. It's possible to just laminate layers of materials of different stiffness and densities. Speakerlab, here in Seattle, used to laminate 1/2 inch Particle Board to an outside layer of 3/4 inch veneered Plywood. Simple and works well.

2. Added Mass simply mass loads the walls of the cabinet. This can range from gluing a layer of vinyl flooring to the inside walls to special secret Goo's and Gunk :^).

One such is an approximation of North Creek's fabled "Glop," which was simply a mixture of Drywall Mud and Aleene's Original Tacky Glue (Aleene's is a "DIY Must" for any shop, IMHO). The actual proportions are to suit, IOW experiment for yourself to find the ratio that works best for you). When it sets up it will develop cracks and fissures but that's normal and doesn't detract from it's purpose.

Another method is to get "Oil Based Modeling Clay" (and only "Oil Based Modeling Clay," the type that never hardens) and then using your Wife's Best Rolling Pin, roll it out into sheets that can be applied to the inner surfaces of the cabinet. Anywhere from 1/4 inch to 3/8 inch will work. Just use your fingers to press it into place, leaving a slightly dimpled appearance that also has some additional benefits.
If more mass is needed, then you can roll in lead shot with the clay to increase mass (I use #9 shot myself). If you need some real mass, roll the above mix of shot and clay onto, and into, precut panels of Screen door hardware cloth, which can then be stapled onto the inside panels of the cabinet. Again, after stapling, press the clay into place to make complete contact and adherence to the entire interior panel. This method was pioneered by Ed Heath on the old "Bass List" and works well.

3. Bracing is just that, namely using dowels, wood strips cut from boards, Window panes cut from plywood, etc., are commonly used.
One method, seldom seen but very effective, is the use of Steel angle iron or Aluminum "L" extrusions (of significant section) that is installed by using pan head screws and epoxy to fasten it to the interior of your cabinet. You need to use epoxy to insure complete contact, otherwise you may actually cause additional resonance issues. It can be cross braced with more "L" iron using more screws and epoxy. This "L" iron idea is another Ed Heath idea, TTBOMK.

Another factor to remember is to break up the various panels into sections of unequal area which ensures that you won't have several panels with a common resonance frequency that can (and will) combine.

4. Cabinet Configuration is a topic in and of itself, that can cover many pages. A search of past posts will lead you in that direction.

Finally, this was intended to be a brief overview of *some* of the methods employed to Damp cabinets. Obviously, it's possible to combine several of these techniques for a more comprehensive control of those pesky Vibes.
One more thing, any of these methods does displace volume, but knowing that you allowed the 10-15% additional volume recommended by all the basic books on speaker design this shouldn't be an issue (except perhaps with very small mini-monitors), although it's easy to become too anal about all this. This is a hobby, so enjoy the journey, it's supposed to be fun.

Best Regards,
Just to complicate things further I'll put in my "two bobs worth". You original post/question was open to two interpretations.

TerryO has dealt extensively with one of them i.e. the resonance of the cabinet walls but the resonance of the air mass is also an issue. (I initially understood that was the topic you were raising.)Like TerryO says its a large topic. Most DIY designs advocate covering the walls with some sort of fibrous material. This isn't great physics as the place to damp the standing waves is in the middle of the box and not the walls. That is where the air molecules velocity is greatest and damping most efficient. So filling the internal volume is most effective.....if that's what you actually need to do.
Your question about it altering the effective volume is a good one.
I had an old handbook by Gilbert Briggs (Wharfedale fame). They (Cooke who went on to built KEF was his designer at this stage) measured the effect experimentally and found that it increased the Effective volume by a factor of 1.4. James Moir another UK guru of some years ago had done the theoretical calculations from the point or theomodynamics and came to the same conclusion. Svante is correct when he raises the isothermal properties of the material.

So a well stuffed box will have a slightly lower res frequency compared to an empty box.

But as one or two people have said its easy to get too hung up on this sort of thing. 40 years ago the UK mags that we got here in Australia were mad about 'longfibre wool' or what is also known as "roller lapping". It can get a bit silly.

Also damping of the physical structure of the box is something that B&W did some very good work on. I found their conclusions at the end of their experimental work was a good place to start thinking about what option to use from the sort of variations TerryO has suggested.

Some had previously said that all vibrations are wrong and that high mass, high density, high stiffness materials were the way to go. So concrete looked very attractive and was occasionally used! (also bricks, lead etc.)

But B&W (Bower and Wilkins) found that subjectively the ear responded most sensitively to the decay time of the resonance. (Reverberation time) If you could lower that you got cleaner sound. Now in practise that meant high stiffness, high damping and low mass. (I am doing this from memory so correct me gently if I haven't recalled this precisely 100%)
The end product was the Matrix series of enclosures and they are a design that is time cconsuming to copy but very achieveable for the home constructor.
I haven't been to the B&W website for a while but if they have put up their paper its is well worth a read.
You see, the simple addition of a large mass to a structure can sometimes move the resonance to another frequency and increase the "Q" of the resonance and the end result may not be an improvement...........aurally... worse infact. The B&W work allows more confidence in applying "tweaks" because you've got some serious theory to use.
I'll stop there as this is getting too complicated and it's Boxing Day in Australia and the India V Aust' cricket match is about to start.....


2002-01-07 6:02 pm
Terry and Jonathon have raised excellent points. The B&W Matrix is excellent, and if you plan the job properly, not all that hard to do. And I'm not a very good woodworker. I plan on knocking up 6 cabs this way in the next few months, four for MI/PA use. For hifi, I try to keep no more than 100x100 are unbraced; for MI/PA 150x150, just because of weight.

The Matrix construction also allows fibrous fill materials to be placed more easily throughout the enclosure, especially the centre of the box, where as noted previously, it's maximally effective. In MI/PA enclosures I also try to use aluminium L brackets screwed and epoxied inside and out. Adds some time, complication and weight to the build, but makes the portable enclosures damn near indestructible.

I'm not so fond of fibreglass as fill for home use. Next boxes I'm going to experiment with treated wool and cotton waste.
No one has yet covered off a very important consideration wrt damping the airspace inside a box... how you approach it depends on what kind of enclosure it is.

You deal with a sealed box differently than a bass reflex, A horn has little or no damping, and in a transmission line the damping is a very critical part of the design.

fantfool said:
That software is really cool, will give me something to play around with since I don't know the T/S specs for those drivers. Any feel for how much of a difference getting the damping/stuffing issues corrected will make on the sound quality of the boxes?

It's fortunate that I looked at this thread tonight, as the prior research on this is somewhat chaotic and ill-defined.

However, the Research Scientists working at Honest Olson's Ultra-Fi Audio Emporium and Laundromat have found when the damping/stuffing issues are corrected you will gain a 37.263% increase in clarity, a 26.74% improvement in definition and an overall 68.92% increase in Macro-detail.

Please be aware that with the new alignment parameters the drivers will require 230.2 hours before "enclosure/driver reorientation" is accomplished, although you will achieve 72.4% of the cited improvements within the first 27.65 hours.
All these calculations are based on correct assumptions, if you contemplate use of incorrect assumptions, subtract 10.54% from the above figures.

Best Regards,
Terry Olson
CEO and Chief Research Scientist of
Honest Olson's Ultra-Fi Audio Emporium and Laundromat
Getting it right will not be a great turning point in your system. There will be subtle improvements. So if the system is "boomy" and underdamped the end result should be a "dryer" bass and less fatigueing. This thread has reminded me of a "test" from much earlier in the peace. It was known as the "Click/bong test". It involved wiring a 1.5volt dry cell across the bass unit/woofer. Close the circuit. If it went "Bong" the system was underdamped and if it "clicked" it was okay. Sounds corny but it lends a vaguely scientific note to a fairly subjective field.
You can measure impedence curves and the actual SPL of the system but having done that you still have to live with it and experience and science have a complicated "Dance" going at the bass end. For instance in theory a maximally flat system with Q of 0.7 when measured anechoically seems to be ideal in theory. But in a "normal" domestic setting these parameters invariably make the sound far to bass heavy. Collums in his book on High Performance Speakers points out that a theoretically well designed 4th order reflex unit with a wider bandwithd than a 2nd order sealed box will often produce less subjective bass..........

Good luck.....Jonathan

I suggest it is often a case of try it and see. That's half the fun of it. You can get your "current life partner" involved in the auditions which will ease the Wife Approval Factor etc...

I don't think this forum will really pre-empt all issues and solve all problems for this sort of qestion....