Anechoic versus Bass ....

A friend of mine was recently working on speaker box design. The insides of the enclosure were a little large, so the first natural idea that comes to mind is such so as to cover or otherwise alter the internal walls of the enclosure with some sound absorbing materials. The general idea was to control internal resonances.  Initially, he was considering hybrid materials, such as "Dynamat", or a mixture of "pressed fluff" with some heavy bitumen materials (the black gooey stuff), or similar absorbing materials, as a means to control interior resonances. He asked for suggestions.

Not excluding the difficult, or crazy ones. He wanted science behind it.

He was considering using multiple layers, or zones, that work at different ranges if possible.

OK, so here is what I proposed to him. An idea that I came up with and actually implemented something like 25 years ago ... 

How about plain stupid polyurethane foam, such as the ones that are used in plain stupid mattresses? Check your local auctioning services and seek for materials such as or similar to this stuff: "Polyurethane foam", "Upholstery sponge", "Foam mattress" ... like the ones depicted here:
(This is just an example, by the way. The guy seems to be reluctant to ship abroad).

Find some SIMILAR on your LOCAL auctioning service. No sense to purchase it from the other end of the world. You can have stuff like this for the price of peanuts. The whole darned mattress, full size, costs something like 18 EUR or 23 USD.

As for the sonic effects, you may find that the effects will be very pleasing.

A lot of "hard manual work" - but I would suggest that the effects are worth it.

Whilst "choosing" the type of sponge material - take special care to ensure that it is POROUS and that it LETS AIR THROUGH. Conduct a simple test. Simply put a piece to the sponge tight to your mouth, to your lips and try to blow air through it. If it is possible to vent your lungs through the sponge, if it lets air to pass through its insides - then it is the RIGHT stuff. On the other hand - do NOT purchase those types of "plastic derived" sponges, those with sealed pores, i.e. those by which it is NOT possible to blow air through them.

The "thickness" of the foam mattress of choice - pick it wisely and carefully, as per dimensions of your speaker enclosure. The "thickness" of the foam mattress defines the "height" of the triangles. You need to use the "even" surface edges of the foam (i.e. the surfaces originating from the "factory" - as opposed to those originating from your hobby knife) are going to be used as the "gluing" surfaces later on.Cut the mattress into strips. The width of the strip will define the width of the triangles.

There are special considerations for this width that I write about down below.

Having such a strip of sponge/foam, put it sideways, and cut it, with a sharp upholstery hobby knife (equipped with a fresh blade!), into even armed "triangles".

Mind you, the angle of the tips of the triangles is important. I tried to make them similar to the ones as depicted in some of those professional anechoic chambers.

Google out "anechoic" and get a feel of the geometry of the triangles.

When you have a batch of triangles cut out of the strip, prepared and ready - start gluing them onto the inner surfaces of your speaker enclosure. Take note that they are being glued in a "checkerboard" , or "CHESS board" fashion. Chess boards have REGULAR SQUARES as fields comprising the board. This implies that your grouped triangles, at their base, should also add up to a "regular square", consisting of N triangles. I think that any N between 1 and 5 would be OK. When the appropriate amount (of your gut feeling and choosing) of triangles are grouped together, the length of their glueing bases should be of the same size as the "width" of the triangle. This way, the next consecutive "checkerboard field", which you shall create by turning the next group of N triangles by 90 degrees, will fit in nicely into an orderly row. Take this into consideration whilst establishing the "width" of the foam mattress strips that you cut out from the mattress in preparation to cut out and prepare the individual triangles.

Example 1:  I like to have my triangles with a very "sharp tip", i.e. with a sharp angle, of maybe 15 do 20 angular degrees. Just a gut feeling. Therefore, If I were to use a mattress of 8 cm thickness, which implies triangle height of 8cm, I would hence pick the base of such a triangle with a size of probably 2 centimeters. I would (arbitrarily) choose an N=4. Therefore, 2cm x 4 = 8cm.  As four consecutive bases of my triangles occupy 8cm, and I am willing to target for a "chess board" arrangement, I would therefore cut my "mattress strips" of a width of 8cm too.

Example 2:  On some specific wall of the speaker enclosure, I like to have my triangles with a slightly different geometry, so that they "operate" at some different frequency ranges. Here, I wish to go for relaxed triangle tips, for examples sake of maybe 25 do 30 angular degrees. Again, just a gut feeling. Therefore, If I were to use the same mattress of 8 cm thickness, which implies triangle height of 8cm, I would hence pick the base of such a triangle with a somewhat bigger size, of probably 3 centimeters. I would (arbitrarily) choose an N=3. Therefore, 3cm x 3 = 9cm. As three consecutive bases of my triangles occupy 9cm, and I am willing to target for a "chess board" arrangement on that particular inner speaker enclosure wall, I would therefore cut my "mattress strips" of a width of 9cm.

As for the glue, in my projects I have successfully used stuff similar to that used by shoemakers - the glue for gluing shoe soles. But the trick is ... Purchase a wholesale 1 liter, or even 5 liter can of the stuff. You shall find that when purchased in a BIG CAN, the stuff turns out to be "a lot cheaper" than when summed up in all those small "toothpaste format tubes" with almost nothing inside them. Mind you, you shall use up quite an extensive amount of the glue, so stock up and cut down on wholesale purchase prices. :)  The shoemakers glue has passed the test of time. In my case, I have made my "triangle" gluing experiment more than 25 years ago. They all still hold strong and are all sitting intact in their respective positions.

Ah, one more important thing ... at the very direct "vicinity" of the back side of the membrane of your speaker -  ABANDON, or locally waive, the checkerboard scheme. This is the ONLY place where you DO NOT WANT that chess board setup.

The thing is ... in the direct "vicinity" of the back of the membrane, the triangles MUST NOT be oriented in such a way, or direction, so that that they "obstruct" the air flow. The "sonic wind" MUST have to freely pass between them, unobstructed.
So, to achieve such effect, treat the area just "adjacent" to the back of the membrane, i.e. the first 20 cm away from the membrane - as a very special zone. Within this ZONE, glue the triangles on in such a manner so as to maximally "EASE" the flow of air BETWEEN them. Glue them on in the "elongated" fashion.

A SIMILAR principle APPLIES to any vicinity of the vented holes, or bass reflex tube endings or similar openings within your enclosure, if your enclosure is indeed vented.

That was a dire error of mine - my contraption seemed not to work properly ... but then I noticed that there is too much congestion at the verge of the bass reflex tube. The triangles literally have been obstructing, or even blocking the outlets of the bass tube, and the back sides of the bass membranes. Therefore, I did some scissors work, trimmed down the triangles down in that special zone area - and WHAM !    What a SLAM came out of it.

One more thing to consider before you embark on such a project.  The foam triangles that you are gluing in actually take up some volume.  But putting them in, you are actually CHANGING the working volume of your bass enclosure box.  I reckon that as the foam is porous and air can travel through it,  there is no "obvious" method of assessing the percentage, by which the volume of the foam actually "diminishes" the working volume of your enclosure.   My rough guess is something like maybe 60% of effective loss of volume.  But this is just a wild gut estimate.  But it is a thing that needs to be considered as a possible factor that shall change the behavior of your enclosure and it's  "Slam". 

Speaking of slam, ... a few words about the sonic, or "musical" effects, that you may come to expect as a result of this whole exercise:

Upon completion of the project, the bass will seem to be a bit "leaner". But at the same time, it will be much "quicker". Tighter. "Faster". More Slam. More natural. Less "strained".
Simply better.

More authoritative and authentic. Now, this indeed may sound a bit contradictory and "funny", because I just said at the outset that you may actually get the perception that there is just a tad less of the bass.  Less, but of a higher quality and much more realistic and pleasing.
None of that ill-devised bass-boom-box sound, the "one-note" ... "boom-boom-boom" effect. 

The bass is actually converted into a nice "whole range" of distinguishable notes and tones. 
Not just a "single BOOM BOOM BOOM note". 

And here you have some example photos as found with Uncle Google (search term: "anechoic chamber") - some food for thoughts and maybe some new inspiration ....
The rights to these "Google derived photos" are held by their respective owners.

Best Regards,