Consequences of using 7/8 " (22,225 mm) felt for damping in a design that prescribes 8mm

[maxime.levesque]

I have a roll of 7/8 felt, and I was wondering what would be the consequence of using it for a build that prescribes covering the inderior surfaces with 8mm felt.

I would be doubling the mass and volume of felt.

Is felt accounted usualy for in volume calculation ?

The design also calls for filling volumes with accoustilux pads. Should I compensate by putting less accoustilux, or ommiting it ?

 

[bigjacko]

I have a roll of 7/8 felt, and I was wondering what would be the consequence of using it for a build that prescribes covering the inderior surfaces with 8mm felt.

I would be doubling the mass and volume of felt.

Is felt accounted usualy for in volume calculation ?

The design also calls for filling volumes with accoustilux pads. Should I compensate by putting less accoustilux, or ommiting it ?

For felt I assume those are very dense, those will take up internal volume. I am not sure about accoustilux, if it is fluffy stuff then putting correct amount of it can increase the effective internal volume.

The felt works best at velocity maximum, that means not directly on the wall. If possible I would put the felt a bit away from the wall, any distance will help, even 5mm. But don't need to stress this, if putting the felt way from wall is hard put it on wall still has effect.

 

[Plcamp]

From what I have seen of wool felt, it will be resistive above 1 kHz and transparent below. I don’t therefore expect it would subtract from box volume, and can be treated like any other fill.

 

[maxime.levesque]

It's getting complicated, the recommended 8mm felt has a density of 1600 g/m2 :

http://www.jantzen-audio.com/wp-content/uploads/2020/06/Damping-Materials-Catalogue.pdf

And the substitute has a density of 3.75 oz per square foot, which is 1144.319344 g/m2 in non medieval units :

http://natfelt.com/wp-content/uploads/2015/08/FLEX-XEL-S-125-en-high.pdf

So it is thicker, but less dense than the prescribed felt.

@bigjacko it would seem as if a lesser density felt would have the same effect as increasing it's distance from the walls.

It seems as if enclosure dampening is a trial and error process, a bit like tuning an instrument,

So I suppose the most important thing is to measure/detect the presence of unwanted sound artifacts.

Then, just like instrument tuning, training one's ear is probably the most important thing, and for a DiY, it means one has to build a few speakers, or at least make many experiments and listen.

I've seen a few articles where insufficient damping could be detected on the SPL graph, but I suppose there could be subtle artifacts that would only be detectable be ear. Or perhaps the objective and measurable methods will only detect extreme cases.

 

[headshake]

Newbie DIY speaker maker here....

I found this researching sound absorption and it is what @bigjacko is saying:

Nevertheless, it is common practice to mount sound absorptive
layers directly against a wall, because it is very convenient to do so.
We must, however, realize that, in such cases, only the outer one-third of the thickness of the layer is effective in absorbing sound.
The rest of the material is simply acting as a convenient support!

http://iperf.org/files/5713/9265/8816/Acoustical_Application_Tests_of_Perforated_Metals.pdf

That's from a guy who designed acoustic treatments for concert halls and the military. Air needs space to pass into if you want all of the felt to do its job.

You could prob calculate the dead space and subtract it from a simulation of the cab to see the change in power. VirtuixCAD's "enclosure tool" can do this. Just pick your driver and put in the two diff volumes. Trace the lines to compare them.

I also second what @bigjacko is saying, felt sucks for low freqs.. I made a post where I have a bunch of felt vs freq. graphs ripped from various papers.

Melamine is what NASA uses. I like melamine + accousta-stuf. Bugs won't eat it. Won't burn.

I've seen a few articles where insufficient damping could be detected on the SPL graph

Be sure to measure all around the speaker. I was able to mess up certain freqs. with too much stuffing at 90 degrees.

 

[maxime.levesque]

Newbie DIY speaker maker here....

I found this researching sound absorption and it is what @bigjacko is saying:

http://iperf.org/files/5713/9265/8816/Acoustical_Application_Tests_of_Perforated_Metals.pdf

Be sure to measure all around the speaker. I was able to mess up certain freqs. with too much stuffing at 90 degrees.

Interesting, so the challenge is finding ways to support the felt with a centimeter or so from the encosure walls. I'm surprized I havent seen any DiY (or even comercial) enclosure with this feature.

One way I could imagine doing this, would be to glue the felt to a very low density sponge/foam that would act as a support, the foam would be glued against the wall, and the felt bonded to the foam.