Sound absorbent lining for reflex enclosures

[kimbo]

I'm nearing completion of some mid sized bass reflex enclosures (about 70 litres (2.3 cub ft) with non-parallel sides to reduce standing waves etc. with a 100mm dia (4inch) port) and I'm trying to decide what material to use for lining them. I have conducted a number of searches of posts, and although there are many discussions regarding stuffing materials, there are few recommendations for lining to reduce reflected sound.

Please correct me if I'm off the mark, but my understanding of the requirements for a BR enclosure are:
1/ a minimal to moderate amount of stuffing, usually often just on sides and back, and minimal near the port as it will interfere with the port action/tuning.
2/ sound absorbent material lining the box, sometimes only the rear surface, to reduce the amount of mid-range bouncing out of the port.

There are several posts that talk about dampening the surfaces, but my box is very heavily braced and I do not feel that reducing panel resonances is going to be an issue and this is not what I am trying to do.

In the past (decades ago) I used old fashioned fiber carpet underlay. It occurs to me that I could use foam carpet underlay, closed cell mat (like you take hiking), thin closed cell mat as sold by craft shops, or even acoustic tiles. I have looked inside the port of a large off the shelf BR enclosure and it appeared to be lined with 'egg carton' soft foam.

Do any members have any recommendations?

[Salas]

Absorption in the sub LF and LF range does nothing more than adding virtual mass to the driving cone and stealing dB.
Sub LF is 20-40 Hz. LF range is 40-80 Hz. Mid Bass is 80 to 150-200.
The meaning of absorption is to reduce internal standing waves that escape through the cone as they get reflected from the inside of the box. The lighter the cone the more they escape. Also they pipe through the BR port along its own turbulance noise and pipe resonances.
The internal standing waves of even large domestic boxes are located in midbass and mid area up to 800Hz mostly (upper end of music's ''power range''). Just try to speak in an empty box...
When making crossed subs with heavy slopes up to 150 Hz better avoid any absorption.
Yes we see materials in many woofer boxes.
That is for: 1. The woofer crosses high. 2. The damping of the added mass is a part of the alignment. 3. Both 1&2 4. The designer has no more insight on the matter than the popular missunderstanding. 5. The clients will feel weird if they ever happen to remove the woofer, so a little acetate fiber damping will keep em happy.
Exceptions are TL boxes that use damping creativly and alignments by MJ King.

[Salas]

...If you are going to reproduce mid frequencies in that 70lt Box better use 1/2 inch wool felt for 50% of each surface to apply.
This has much better absorption for mid resonances than BAF wadding and the like, so keeping loose damping material masses out of your design and from moving around in your box and stealing dB from your LF alignment.
Heavy damping is good for behind of cone mids with smaller individual boxes in main box. There the paths to the back are short and the hooting from within the box strongly pronounced through the cone to you.

[kimbo]

thanks for the prompt reply Salas.
Just to clarify, it is a full range box so mid bass will be an issue.

Your reply ties in with my thoughts and thick felt is definitely on my list. Has anyone out there used other materials sucessfully?

[Lusso5]

In this speaker, I first tried a small layer of open-cell foam lining (grey) all the surfaces, except around the port, and on top of the crossovers.

I then listened to the speaker, lined as above, with and without light stuffing. The bass leaned out a bit from the stuffing, but the midrange cleared up a bit - I could hear more detail.

So I removed the stuffing, and lined the open-cell foam with a 1.5" layer of bonded dacron (white). The bass was there, and the midrange was still more clear than with just the open-cell foam.

I did not listen with the dacron lining alone, so the first layer of foam may not have any improvement over just the dacron.

In the pictures, you'll notice the shelf-brace in the upper-portion of the enclosure is also covered in material. From the angle, you can not tell there is space above the shelf-brace.

[GM]

Greets!

>1/ a minimal to moderate amount of stuffing, usually often just on sides and back, and minimal near the port as it will interfere with the port action/tuning.
====
Traditionally, end opposite the vent, one side and back with 1" acoustic fiberglass. This assumed though that mids/HF would be reproduced by a horn or other acoustically isolated driver. Much later, along comes acoustic suspension (AS) both sealed and TL, with fiberglass insulation stuffing densities at up to 1.5 lbs/ft^3 depending on its BW, cab Vb and/or desired Qtc.
====
>2/ sound absorbent material lining the box, sometimes only the rear surface, to reduce the amount of mid-range bouncing out of the port.
====
It doesn't 'bounce' out of the port. What occurs is the port has its own 1/4WL resonances and the more the cab excites them, the more likely they are to comb filter with the driver's output. Also, these same cab modes can be strong enough to modulate the driver so there should be sufficient damping to keep them in check. Then there's reflections off the back, and possibly, side wall in the driver area that can modulate the driver.

Bottom line is that as cab Vb and/or increasing port area and/or its length increases, the density requirement increases. Historically, finding the right amount was a trial and error ritual, but some recent programs make a good stab at simming a starting point if you use the material it's programmed around.

Obviously, if the speaker's BW is limited to below the cab's first axial mode (~13560"/(2*L)) then only the port's modes (if used) potentially need damping.
====
>There are several posts that talk about dampening the surfaces, but my box is very heavily braced and I do not feel that reducing panel resonances is going to be an issue and this is not what I am trying to do.
====
Hmm, this is a separate issue from damping reflections, port modes, which assumes the cab is already rigid/well damped enough to not 'sing along'.
====
>In the past (decades ago) I used old fashioned fiber carpet underlay. It occurs to me that I could use foam carpet underlay, closed cell mat (like you take hiking), thin closed cell mat as sold by craft shops, or even acoustic tiles. I have looked inside the port of a large off the shelf BR enclosure and it appeared to be lined with 'egg carton' soft foam.
====
Right, jute or felt carpet underlay can be ideal in a large cab, though will tend to overdamp a small one. For best results with foam only use open cell, no closed cell, re-bond, etc..
====
>Has anyone out there used other materials sucessfully?
====
Sure! There's numerous suitable materials for various BWs that you can use or layer to get the desired results. I use info from various books, manufacturers, but I've seen quite a few sites with this info so I imagine doing a search for acoustic absorption of various materials will get you some data lists.

GM

[kimbo]

thks for the info GM and Lusso5. I think I'll have "a bet both ways" and line it with felt if I can get it, and then a moderate layer of polyfill in a similar fashion to what Lusso5 has done. I have a feeling that felt will do a better job than foam.

As I said, I am not really aiming to change the Vb of the box or dampen panel resonances, just dampen internal reflections, so i think this will do the trick.

thanks again for the advice.