Author Topic: "Best" Foam for Lining Bass Reflex Box?  (Read 2357 times)

Offline afroaudio

Re: "Best" Foam for Lining Bass Reflex Box?
« Reply #15 on: May 19, 2016, 11:31:45 AM »
Reposting from another forum... this subject does get rather involved! And if you have the time, this is a massive and fascinating thread:

Interesting that there is a whole school of thought against bracing, which assumes that bracing pushes the panel resonances up in the freq range where they are more likely to be audible. Whereas mass loading (bitimen sheets etc) a less rigid cabinet in theory can shift the resonances down the range to a point where they are less perceptable.  I am now sitting somewhere in the middle with my cabinet as it is. Time to go cut some bitumen pads!

taken from above linked thread:

"It is really hard to give rules in terms of stiffness or mass. The whole notion of stiffness and mass and resultant resonance is a simplified look at a single resonance system. Cabinets clearly have lots of resonances (see the previous curves by Harwood) and so we are really into modal behavior where dimensions and speed of sound give rise to multiple modes of rising complexity.

No matter, damping can deal with all the modes so the real question is what ratio of damping to other mechanical impedances are needed. If we ask "how thick should the damping be?" we run into "depends on how thick your cabinet walls are". Harwood found, as you would expect, that the thicker walls had generally higher mechanical impedances and required even greater damping to get a certain amount of acoustic breakthrough.

I would rather think of it as starting with a target for cabinet weight or cabinet wall thickness. Lets say we want our walls to be 1" thick in some combination of MDF and bitumen padding. We can make the wood thinner and the damping thicker, or vice versa. Which is best? Under that scenario I would shoot for the thinnest practical walls and the greatest damping thickness. We have to maintain some cabinet rigidity for integrity and to support our drivers but certainly 1/2" walls with 1/2" damping would be reasonable and practical.

At this point someone will say, "Then wouldn't 1" thick with that 1/2" damping be even better? Thicker, you know." The answer is decidedly no. Growing the wall thickness may increase stiffness but it just leads to a higher mechanical impedance with less resultant benefit from the damping.

Practical experience supports all this. We have all seen the wooden cabinet with incredibly thick walls. I used to give them the knuckle rap and admire the high pitch ring. "Solid!" This is just a sign that their thickness made the modes undampable. The one cabinet I have owned that really sounded different was a BBC LS5/1ac. It was 1/2" or 5/8" plywood with heavy felt and tar pads lining the interior. No solid Rock Maple sound to it, just a dull thump.

Is all this the most important matter in diy Audio? No, not really. It is still more important that we design our crossovers correctly and achieve smooth and flat response. It isn't that frequent that I listen to a system and say: "Pity about the cabinet resonances spoiling the sound". (Infrequent but "not never".)

Harwood has it right in showing that there is a threshold of audibility to resonances. If we can push the level below the threshold or lower the resonance frequencies to a region where we are less perceptive, then we will reach a point where the problem is effectively solved and we should focus our attention elsewhere. "

Offline chipwelder

Re: "Best" Foam for Lining Bass Reflex Box?
« Reply #16 on: May 19, 2016, 12:01:37 PM »
Damping if indeed correct is not achieved by stiffening... stiffening moves the frequency and amplitude elsewhere and changes decay times to an extent... damping will be much like dampening something... it will not move as much as it did before... not just differently... unless it is a labrador... then it moves more with dampening...
OK! I've had it! I don't give anymore Kharma. Kharma should work in mysterious ways...

Offline afroaudio

Re: "Best" Foam for Lining Bass Reflex Box?
« Reply #17 on: May 19, 2016, 06:16:51 PM »
So my bracing is done and they look like this now:

Bracing by Bregtje Cals, on Flickr


Front Baffle by Bregtje Cals, on Flickr

Back to the original topic, have got some nice heavy underfelt and some of the thickest densest polyester batting I could get for cabinet lining

Offline Francois

Re: "Best" Foam for Lining Bass Reflex Box?
« Reply #18 on: June 06, 2016, 07:12:52 AM »
This might be a stupid question. Say you calculation suggest 40l box. When you damp, brace and fill it with felt and wool you obviously change that parameter ?  How much do you subtract (or build cabinet bigger) if you damp heavily ? 10% ?
I was born analog, not digit by digit....

Online bbe22

Re: "Best" Foam for Lining Bass Reflex Box?
« Reply #19 on: June 06, 2016, 10:54:36 AM »
Happy birthday and its actually a very good question.......varies from driver to driver
At the hand of an example
this is a 12 inch in a large reflex box    see pic
from little fill to heavy fill the F3 drops about 10%    and to get the f3 the same one has to increase box size 20%
However if you overlay the graphs it really DOESN'T MATTER..................

Online bbe22

Re: "Best" Foam for Lining Bass Reflex Box?
« Reply #20 on: June 06, 2016, 11:00:03 AM »
............and for a sealed box    the Qtc is slightly higher and F3  lower in the lesser damped box , again the graph overlay diff is minimal

Also see pp 39-42 in the Loudspeaker design cookbook for more details   (.pdf online)
« Last Edit: June 06, 2016, 11:07:03 AM by bbe22 »

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Re: "Best" Foam for Lining Bass Reflex Box?
« Reply #21 on: June 06, 2016, 07:15:34 PM »
I agree that the question is a good one and the answer is not straight forward Francois. Bracing is the easy part, as I try to be very precise in my designs and CAD can calculate every component in the driver enclosure. Damping in the form of Bitumen, if you go that route, I also try to factor in in the calculation of the working volume. As for filling it with absorbent lambswool(my preference for mid enclosures), rockwool/fibreglass/dacron or undefelt I try to use as little as needed, but not less. It is important to kill the reflection of the back wave behind mid and midbass drivers and prevent as well as the pipe organ standing waves in tower loudspeakers.  These kinds of fill actually increase the volume that the driver sees. Rule of thumb is often that what you loose with bracing, you might gain with filling the enclosure 
The amount of fill also changes the impedance of the driver in the enclosure and need to be taken into account with filters in the lower ranges of the driver. Furthermore, if you use too much fill a vented system starts to look like a sealed system in its performance and the fill could also impair flow around the port and sacrifice the performance of the port.
2-ways are particularly tricky I think. In a well designed 3 way, you could design the working conditions for the mids and bass frequencies differently. Sealed mid enclosures with good absorption of back-wave with heavy fill and deal with the bass with less fill and less loss of energy in the bass. Fillings like the ones mentioned are also only effective in midrange and upper frequencies.
I was told by someone with lots of experience that this question in conjunction with the ideal operating volume is where all the simulations we have access to fall short. Experience and experimentation are the only way to really get to where you want to be.. and even that is a moving target because some prefer a tight bass, almost sealed like, others a more supple and rounded presentation etc.

Offline Rhino

Re: "Best" Foam for Lining Bass Reflex Box?
« Reply #22 on: June 08, 2016, 07:25:14 PM »
Very interesting build you are busy with.
The cabs look great and thanks for sharing the results of your tests.
Will keep an close eye on this.
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