Cabinet damping material

I will post some pictures after I get my Jensen/Altec speakers running again, but that won’t be for a couple weeks since I need to get the woofers back.
 
Ocellia speakers take the “less is more” philosophy to an extreme. Their cabinets are made from very thin wood and the interior surface of each panel has thin strips of solid wood glued on end to provide stiffening. The placement of the strips and the dimensions of each strip are important factors in the sound quality. There are some Ocellia reviews in 6Moons with photos that show the interior construction.

The Ocellia design is Too complicated for me but I do like the lively sound of thin-wall cabinets. My homemade cabinets all use Baltic Birch plywood with the metric equivalent of a little over 5/8” thick. I tried 1” Baltic Birch several times and it was too dead sounding.

Thin-wall cabinets can lose their liveliness if there is too much bracing or if the interior damping material is resting directly against a cabinet wall. At least that was my experience with the memory foam damping. Separating the foam from the panel with a couple thin strips of wood worked for me. The sound was still lively but with minimal boxiness.

When I get my Jensen/Altec speakers running again, I want to try some alternate materials for damping, and first up will be the kimpak recommended in this thread.

Hi Salectric,

I liked your post, thanks for sharing. The 825 VOTT ( early ) boxes I run are also only 5/8 ths thick. Dennis advised me NOT to re-make them using one inch Baltic Birch.

He specifically told me to do a single 2 by 2 inch pine internal brace, on each left and right side wall, covered with see-through Batting , NO cross bracing ( across - L. to R .) wanted in the speaker, and leave the thin stock ALTEC woofer horn flares totally stock.

He tells me that this mass loading I did carefully, incrementally by ear ( 400 pounds of patio pavers, on each 825 enclosure) will be bettered..... by just doing the minimal bracing of the stock 825 enclosure. Dennis says, all the excess mass I added " takes the FUN " out of the presentation, deadens it too much.

He also was very specific, for me to get the 825 / VOTT box off the floor, either on industrial casters, or a speaker stand, or cheapo bricks, .... as this also sounds best to him. He said, I had to place the box on the floor, when I was mass loading, but once I remove the mass-loading weight, having the A7s " off the floor" will be the most lively and fun to hear.

There are similarities between what you report, and where I plan to go !! Interesting for to me to hear this, and not have to do all the experimental work.

These posts may help others.

Have a lovely day !!

Dowto1000
 
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Funny coincidence. I just read Herb Reichert’s review of a KEF speaker in the latest Stereophile and Herb talks about cabinet walls and damping. He says “To my ears, dense, highly damped boxes and/or steep sloped crossovers can sound subliminally thick and restrained. In contrast lightly damped boxes with first-order crossovers add some loose, not-so-subliminal noise to the midrange—-but they can also jump, sound expansive, and dance like Fred and Ginger.”

While I wouldn’t have used the same words, Herb is describing basically the same things described in this thread.
 
There’s a lot of useful information on this forum but one issue that I haven’t seen addressed concerns internal damping in high efficiency woofer cabinets. Over the years I have tried 1” memory foam, fiber fill batting, and long fiber wool. Each time I ended up with the memory foam as the least damaging to the music but I don’t recall any details of the differences between them. The memory foam is also easy to work with since it can be cut to size and then stapled to the cabinet walls.

At one point I experimented with removing the damping from some of the surfaces. This was interesting because I concluded the sound improved each time I removed a section of damping. This made the cabinet walls more lively, which seemed to be a good thing, but if I removed it all the sound was too boxy. I ended up with a single foam section on the back panel only. Then I tried separating the foam from the wood by spacers. I concluded the foam sounded best when it wasn’t actually touching the back panel but rather was stapled to some 1” wide cleats on the back of the cabinet. That’s how the cabinets were configured the last time I listened to them.

So that’s my (limited) experience. What’s yours?

I have been using low density polyurethane foam since building my first Fried model C speaker kit which are an aperiodic design.
When it comes to attaching it inside of the enclosure of an aperiodic or transmission line design I use liquid nails.

I am sure that many know that is does help suppress standing waves from the rear of the enclosure.

Once I tried fiberglass in a transmission line enclosure and the sound was hollow.
I tried increasing the density and very little sound came out of the line terminus [the vent] of the enclosure.

This sound can't be heard but it can be felt by placing one's hand in front of the vent which is at the rear of this enclosure.
This is the anti resonant sound from the back wave of the woofer cone.

I have enclosed a diagram of the speaker to illustrate where the vent is.
[BTW I hope my ammunition in photo doesn't offend anyone.]

Below is an example, the foam has bent a little over time but still serving it's purpose in absorbing the higher sound frequencies.DSC_0121.JPG DSC_0056.JPG
 
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When it comes to using foam for a damping material there are many densities available.

Take note of the weight per cubic foot when comparing it to other types of damping material used in loudspeaker enclosures.

The type I have used is the first one on the list.

Take note of all the specifications comparing the different types of foam.
types.php
The longevity is referring to the ability of the foam being able to return to it's shape.
 
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Anyone have any insight into the best way to attach fiberglass batts to the inside of the boxes? I am thinking about using 3M Super-77 spray adhesive, but I'm not totally sure yet. I could use 2.5" screws... but I am not sure I want to screw into the boxes.

- Woody

IMG_8601.JPG
 
I was glad to finally find this on the internet,
The Acoustical Properties of Fibrous Materials.
relay.htm
The results of these tests were published in the former Speaker Builder publication 3-1986 pages 26-27.
The author is Greg Szekeres from the University of Pittsburgh.

I remembering reading this article but I couldn't remember which year it was published in my collection of Speaker Builder magazines.

The first link shows how different damping material results in a different Fs in a closed box.
resonant.txt Resonance vs Stuffing Materials

The second link shows the attenuation of certain frequencies when comparing the different damping materials.
atten.txt Attenuation vs Frequency

Take note of the third link.
delay.txt Delay Line Length vs Stuffing
The speed of sound is changed when it passes through different damping materials in a transmission line enclosure.
One can see how each damping material gives different results.
 

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