Damping/Absorption material in cabinets

[xyrium]

So I'm confused about how people are calculating the amount of material they put in a cabinet when doing a build. I've seen recommendations of Whispermat, Sonic Barrier, and Black Hole, but not only do these damp the cabinet and increase rigidity, but also absorb sound. Therefore, you are effectively reducing the amount of air to the port in a ported design.

This said, is there a calculation that people are doing, or is everyone just throwing material at the designs and hoping for the best? I was under the impression that it would be best to "right-size" the cabinet and only use damping mats.

Tks,
Paul

[wintermute]

I could (probably am) well be wrong here but I would have thought that sound absorbtion and port air velocity were unrelated. Basically the absorbtion is helping to stop sound reflections which is desirable. What is coming out of/going into your port should be air that is being displaced by the cone movements.

Sooooo apart from the fact that the absorbtion layer will tend to increase the volume (not intuitive I know) of the cabinet, it shouldn't have any negative effects (provided your port is a good distance away from any damping material, to allow adequate air flow.

Tony.

[xyrium]

Thanks Wintermute. I noticed the values of absorption rates on the Whispermat and they are frequency dependant just like any other. Therefore, you're getting a great reduction of SPL in the higher frequency range, but you can still see a reduction in the low frequency range (same as room acoustic absorption panels).

The pupose is to change sound into heat usually (you have to do something with the energy), and I would thus believe that the velocity and frequency that exits the port are both impacted.

In conclusion, aren't we tuning the cabinets using this stuff, and if so, very similar to a port size determination, how do we determine where and how much we want to change?

Rgds,
Paul

[JMB]

Tuning of the cabinet in a ported speaker is based upon both the volume of the cabinet as well as the port dimensions. To the effect that stuffing changes the apparent volume of the box, this may impact the tuning. The speed of the air exiting the port is estimated in Mach by: 13.7*SQRT(Reference Efficiency of the bass driver*power rating in watts of the driver)/(tuning frequency of the enclosure*radius of the port^2). Using non porous damping materials on the walls will not increase and may decrease the effective size of the enclosure while using porous damping materials may slightly increase the effective box size but only at higher frequencies (roughly where the wavelength is at least 25% of the thickness of the damping material) so this will have little impact on the frequencies coming from the port.

HTH,

Jay

[richwalters]

The formula 0.62........1........1.62 used for typical box loudpeakers isn't bad as it suppresses the 5th harmonic. Generally with well constructed cabinets (min 30mm) wall thickness I use surplus egg crate fiber boxes (which is excellent for HF absorption) and 1" glass fiber glued around whole box except front. See enclosed photo in early stages of 180 liter cab for 2x10" units. Due to oblique construction, (recommended for large cabinets) bracing can be minimised but essential for front/rear.
Oblique construction cabinet permits simple 6dB/oct crossover without upper bass colouration.

Don't get confused..... inadequate bracing can lead to a drummy panel resonance/colour sound and temptation is to add more damping material.....this can swallow critical lower midrange frequencies.

richj

[xyrium]

Very interesting concept on the egg crate routine. I guess using that to diffuse the sound prevents the signal from returning to the rear of the cone with any velocity that would interrupt the signal exiting the front of the cone. I thought about diffusion, but wasn't sure how effective it would be in such close quarters.

Now I'm thinking that egg crate open cell foam would be a good idea for the rear of the cabinet on top of a damping sheet (vinyl, asphalt, whatever you like), sort of a diffuser/absorber due to the shape.

[morbo]

We really need to stop treating damping/absorption material as the same thing, since particularly in cases like this, they have 2 very different effects. If we are talking cabinet resonance damping material, IE a rubbery or otherwise lossy material attached to cabinet walls, it will reduce box volume. If we are talking about absorption material, IE fiberglass, open cell foam lining the walls, polyfill, etc, it will increase apparent box volume. In simplest terms - if the material in question is air-permeable, it will increase volume (if anything). If it is airproof (ie any sort of putty, rubber, closed cell foam), it will reduce box volume by an amount equal to its own volume, which should be simple to calculate.

BTW, nice idea with the eggcrate material, I have used something similar in a recent project, I found some 5' x 1" strips of 'wavy' wood, which undulates from .25" to 1" in height. I think they are intended to be put on the top of decorative fences and the like. I cut these into smaller strips, then glue them to the back wall and one side of the cabinet, staggered so that the waves are not all lined up. This gives a fairly randomized surface to the walls in question, though since the maximum wave height is only about 1" it will probably not do much at the lower frequencies. I will try to post a picture of this soon. In my next box I am going to greatly oversize it, and put some angled panels and the like inside to make the inner geometry very different than the outer geometry.

[wintermute]

in the case of black hole and wispermat you can't separate the two (into damping or absorbtion) because they are both they have a mass loaded vinyl layer for damping and an open cell foam layer for absorbtion. I haven't used either (but have read about both). I have just obtained a sample of something similar (soundsorber barrier) which I am going to test.

I have tried egg crate foam and found that whilst it had some effect (in reducing rear wall reflections) it was only marginally better than without. I also tried some packing foam which is completely flat but is less dense and it was much more effective at reducing the reflections than the egg crate foam.

I'm still not convinced that the foam will reduce your port air speed I'd need to do more research but my gut feeling is no.

Tony.

[xyrium]

Thanks guys, this is good information. I think I'm going to try the packing foam behind the woofer. I have some 3 inch foam that should work well. I'll do a damping mat on the rear wall, then the square of foam to diffuse the rear wave. The rest of the walls will get whispermat or something similar as a damping/absorption layer. So, damping diffusion on the rear wall, and damping absorption on the remaining walls. Some needs to do some empirical testing on this. I guess I'll looking at incremental improvements at best, but anything is better than nothing.

Paul

[xyrium]

Now I need to figure out where I can fit the crossover with all of this material inside the box!