I was wondering if I could get some input on this: I’m thinking about building a PA speaker cabinet made out of 1/8" birch play skin sandwiching a 1/2" phenolic-Kevlar Nomex honeycomb core (glued with high end epoxy. The cabinet weighs around 90lbs when made out 100% 3/4" baltic birch and it has 2x12" drivers (rated 400W RMS each) and a 1.4". To my understanding, thicker (and thus heavier) paneling is traditionally being used to make the box stiffer, increase resonance frequency while reducing its amplitude (and standing waves for that matter). But then again, making the cabinet too light might be problematic for dispersing energy coming from the 12"ers. So while being able to making the box stiffer with the kevlar honeycomb core material, the reduced weight would then increase the higher resonance amplitudes (??). I would of course still use a lot of bracing, I just want to make sure I won’t run into problems, even just buying this stuff is a commitment…
The drivers are going to be mounted on 3/4 ply, while I intend to only make horn-sections out of the honeycomb, being able to reduce the total weight by 30lbs down to 60lbs! But what could be the issues here?
Have you considered an open baffle design?
I already have a fixed design, a dipolar upper bass horn. It’s more of a general question of whether substituting 3/4" baltic birch with the suggested material could have any negative effects regarding box resonances. And for that matter if additional bracing is necessary or not. Fundamentally it’s swapping a stiff and dense material with a stiffer but much lighter one. I know it’s being done, but I wonder if somebody is out here who has done it for their specific project and noticed/measured a difference.
I can only speculate as to the speakers, but this is one of the most interesting first posts we have ever enjoyed.
Ah gotcha, seems that lighter and stiffer would not be bad. Bamboo works quite well though it is very difficult to cut/machine as it’s so hard. It’s both very light and very stiff.
It appears for your use case that the weight savings would ‘out weigh’ any acoustic downsides. You know the adage, ‘fast, light, cheap’ pick two or something like this.
Stiffness=accuracy. If your product is as stiff as the product you are replacing, then all is good
You could always use two thinner double walls and fill the space with sand. Make some sort of sealable opening so you can empty the sand if/when needed for weight reduction. And in general you want thick walls AND internal bracing.
bamboo would be even more pricey, but I heard good things about its properties, I’ve never used it though.
@bassaholic I agree, that’s true if the panel weight increases with stiffness. More weight means it also absorbs sound better, but I’m going lighter. Within the equation for damped motion, stiffness is a parameter separate from damping. They are two independent variables to be optimized. I need to make sure that the stiff honeycomb panels are also damped properly.
And then questions arise such as:
a) is bracing going to be sufficient or
b) maybe use a rubber paint or a bimodal sound-absorbing material attached to the panel, such as this https://iopscience.iop.org/article/10.7567/1882-0786/ab009e or
c) any other avenues I haven’t thought of?
Care has be taken to also not overdampen it, because sound is supposed to travel out after all, see bimodal absorption values.
@Jedi I really like that idea! I think it’s not feasible for me though - can’t use sand, it’s going to be a ‘portable’ system and is going to be used in environments where sand can neither be retrieved nor may be disposed of.
Carry the sand separately in some empty milk jugs?
There is no such thing as over-damping. Well not for audiophile speakers. Optimally the cabinet should have zero movement. For something like a PA system though, you’re not going to be listening for the decay of a cymbal - it’s not a tragedy if there’s a smidge of liveliness in the cabinet.
Oh and reduction of standing waves is controlled by either the shape of the enclosure or (not as effective but easier to do) by using the golden ratios. Just google “speaker box golden ratio”.
I recommend No-Rez damping material. Contact GR Research in Texas- great product
For a stationary system YES, give me sand. For portable, that would be hardcore…and almost rebellious. But I’m really trying to loose weight here so I don’t have to schlepp so much overall and that’s ultimately going to cost more in materials and time I think, and that’s ok. And I already have trouble finding 1/8" thick plyskins locally…
I meant over-damping more in a sense of absorbing sound which is winding through the horn channels, not the vibration of speaker enclosure, I should have clarified.
And we might listen to a lot of decaying cymbals a lot - the system is for electronic music, mainly Techno, I’ll take the most detail I can get
The box is in golden ratio, not traplike though.
@bassaholic thank you, that looks good! I’ll do some more research and might order some of this, definitely need some inside the box, but not much - yay!
Has anyone tried AcryTechs Acoust-X paint inside a cabinet? I’m finding mixed reviews on this…
You’re really fighting physics here and physics always wins. If a lighter alternative existed, everybody would be using it. Some more expensive speakers use aluminum but I’m guessing you don’t have a foundry at your disposal. I love the challenge of what you’re trying to do but I just don’t see a solution
Heavier is generally viewed as better, because the speaker enclosure will move less in response to low frequencies, i.e. mass damping of the enclosure reduces non-musical resonances, which occur regardless of the stiffness of the enclosure material.
I’ve built several sets of home speakers. My building material of choice is 3/4 inch oak veneer plywood. To see my latest set, go to UTube and do a search for JP Stacks. For the woofer cabinet, I just built plywood boxes to the size I wanted, and glued and screwed support braces on all of the places that the panels met. All the screwing was done on the inside. Screwing from the outside in, looks like hell. Screws went 3 inches apart into both panels. You have to stagger the 2 directions, so the screws don’t hit each other. Then I covered to inside of the cabinets with silicon chalk, and cut rubber shelf liner to cover all interior surfaces, then a bead of chalk in all of the corners, to make the cabinet as air tight as I could make it. The braces on all facing points, screwed in 2 directions, make the cabinet pretty acoustically inert, and the rubber coatings seem to do a pretty good job of killing the back wave and not reflect. I didn’t use any other interior bracing. The speakers in the cabinets are 8" poly woofers. I built these with using a powered 12" subwoofer in mind.
This is not an answer to the project you have in mind, but may give you a few construction hints.