3 way closed box damping materials

Colakurt

Member
2017-12-28 6:37 pm
N/A
Hi

I am in the middle of a speaker project intended to work as a studio monitor inspired desk speaker.
The box is a 3 way closed box design with a 8" sub, 5" mid/woofer and 1" dome tweeter.
For the prototype i have used some 30mm thick foam used for engine bay sound proofing with the heat foil removed. The midrange chamber had all walls covered while the sub chamber had most walls covered. I think the result was pretty good but i am wondering if I would benefit from using a wool like material like polyester padding instead for one or both chambers.

I have read the article by Troels Gravesen about cabinet damping, but sadly his project is for a 2 way ported box so i am not sure if the same theories can be applied to my project?
 
For a sealed driver, measure its impulse in the box. Box resonances will show up as (often tiny) wiggles. Damping can help tame those wiggles and get you a good frequency curve.
One material that I like very much for it's damping effect of these resonances is (real Sheep) wool felt. Often a combination of a few different materials might work better than using one by itself. The impedance curve is a very useful tool to show you this.
Just compare the impedance graphs with a frequency measurement and it will show it's usefulness as a tool. I always take an impedance measurement of the driver in free air (to know what that looks like), the driver in an unstuffed enclosure and start working from there while taking those impedance readings after each change in stuffing.
 

Colakurt

Member
2017-12-28 6:37 pm
N/A
My issue here is that due to the construction and layout of the box i have limited access to add more material once the box is assembled.
I think one of my concerns is over stuffing and maybe blocking air movement withing the box i cant use the full volume.
But from what i can read, then a better option would be for me to use something like a felt sheet, instead of the foam i have in my prototype?

I have also been reading this thread with a line source speaker and how the user experimented with different stuffing. It seems like fiberglass might be a way to go, but how does the density of the fiberglass affect the damping effect? i can see that it is widely available at 16kg/m3 but some is also denser.
In this regard my follow up question would be what i should go for in the sub chamber compared to the mid/woofer chamber?
 
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Galu

Member
2018-04-17 6:50 pm
The link I gave in post #3 may have been updated to this one: Using Sound-Damping and Acoustic Materials in DIY Loudspeakers

Some damping materials are only effective at high frequencies e.g. closed cell acoustic foam.

Others are more effective at low frequencies e.g. loudspeaker felt or dense fibreglass.

To reduce internal cabinet reflections from emerging out through the speaker cone, creating coloration, a thick wad of felt, say (20-25) mm, may be placed on the rear cabinet wall.
 
I would add Poly over the Foam. The Foam is denser and more to prevent sound escaping, as in sound proofing (more or less) cars. The Poly is more fluffy and more intended to absorb sound and stop reflections.

Damping a cabinet is not that easy unless you have Testing Software (Room EQ Wizard, and similar) -

REW - Room EQ Wizard Room Acoustics Software

I had some cheap Sony bookshelf speakers that just a small bit of Poly in them. I had some laying around so added more, make the speaker worse. There is an optimum amount, it is just that it is hard to find. Especially if you have to take the cabinet apart every time you need to adjust it.

On my latest build, I used 2.5" Fiberglass (standard in-wall house fiberglass) and covered that with 1/2 of Poly batting. Seems to have worked out OK, though I confess I don't have test software.

The drawback to Fiberglass is that is can particulate, that is, particles of fiberglass can escape into the room and potentially into your lungs. The Poly layer helps capture any fiberglass particles.

Some people, as has been mentioned, use Felt Sheets. Other use convoluted Acoustic Foam. Others cheat and use common mattress pad foam. I've seen others use Rockwool, though Rockwool is more for sound proofing, thought is is also used in sound absorbing panels.

When I used Fiberglass, I gave up on stapling the fiberglass as the staples just cut through it, and it didn't look like it would last long. So, I used some cheap fiberglass SCREEN (window screen) and stabled strips of this to hold the Fiberglass/Poly in place. Worked very well.

While I HAD photos, unfortunately I lost them in a hard drive crash.

Just a few thoughts.

Steve/bluewizard
 
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The geometry of the damping material also affects how it works.

If you put sheets on the walls 30mm thick then you're only adding effective damping at frequencies which are less than 30mm in wavelength (over 11.3kHz), no matter what damping material you choose.

In my experience, uniformly filling the box with a less dense material like the white poly fill is far more effective than just adding a lining to the walls as it also provides damping for low frequencies. Just keep it away from the backs of drivers to prevent stray fill from buzzing against the cone.
 
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Colakurt

Member
2017-12-28 6:37 pm
N/A
So the sub chamber is devided into 3 parts by the box bracing, that's also why I can't just adjust the amount of stuffing. It sounds like the best thing would be to leave the part with the driver empty and then fill the remaining two parts with fiberglass or forming blocks of the wool I liked to earlier. Is that correct?
 

Galu

Member
2018-04-17 6:50 pm
If filling the space inside the enclosure, be sure not to compress the absorbent material. Let it fluff out naturally before installing it.

The sound energy must be transformed into movement of the individual fibres of the material for absorption to occur. If the material is too compressed this won't happen.

I would think it desirable, if at all possible, to have absorbent in the volume immediately behind the sub driver. Make sure the absorbent can not come into contact with the driver cone.
 
Examples of working with impedance plots can be found here: https://www.diyaudio.com/forums/full-range/242171-towers-25-driver-range-line-array-62.html#post4140167 for a mid driver and here: https://www.diyaudio.com/forums/full-range/242171-towers-25-driver-range-line-array-555.html#post6017043 is an example done for a subwoofer build.
I'll have to add, I don't rely on the box + driver to shape my frequency curve. So I don't mind if I rob some output here or there. My project uses DSP to shape frequency curves (long story).

Here's what adding just one layer of 5 to 7 mm (!) thickness of wool felt on the inside walls does, nothing more added at this time:
802280d1576427738-towers-25-driver-range-line-array-wool-vs-empty-jpg
 
"I am in the middle of a speaker project intended to work as a studio monitor inspired desk speaker. The box is a 3 way closed box design with a 8" sub, 5" mid/woofer and 1" dome tweeter."

The bolded is crucial information! What are approximate dimensions of the (sub)woofer enclosure! Intended xo for W-M ?

Damping is to kill standing waves inside, within the intended passband! If lowest standing mode is well above passband, stuffing is irrelevant and not needed. Two-way speaker crossed around 2kHz is a different story! Midrage subenclosure should be asymmetrical and have heavy stuffing , it is the really difficult one!

Internal Standing Wave Calculator for Loudspeaker Enclosures and Rooms
- 800mm height means lowest mode is at 214Hz
 
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Here in the States you can buy raw furnace filter material. These are available in different densities and various materials. I've not tried using "made up" filter panels, which are available in many size configurations, but I don't see why these wouldn't work as well. Some even have a "saw toothed" profile.