Hi ... FYI:
damping factor values - audio qualia
Earl Geddes uses (maybe used) Renshape middle density boards, and in the link above Panzerholz or a custom bentonite resin mixture has a high damping.
Cheers,
Jesper
I do use Renshape medium density for the panels, but glass loaded polyurethane for the waveguide.
The numbers in that chart don't see right to me, and I would question the testing technique. I would make a cantilever out of the material and measure the decay, not a block of material. The block will only give you the bulk compressional damping, while the catelever will give the bending damping, which IMO is the more important. The two will not be the same.
The numbers in that chart don't see right to me, and I would question the testing technique. I would make a cantilever out of the material and measure the decay, not a block of material. The block will only give you the bulk compressional damping, while the catelever will give the bending damping, which IMO is the more important. The two will not be the same.
The standard test does in fact use a cantilever as per your suggestion.
Also, the test, as described in the audio qualia link, applies a very different force (and energy) to each material depending on its weight, mainly, and also surface properties. This force variation in the test is so great that I would say the results are -- at best -- questionable for applications like ours where the applied force does not vary with the material used.
And the relative values in the table don't even pass a cursory common-sense test. Chipboard 15x higher damping than MDF; olive wood 27x higher than oak; steel twice as high as aluminium.
Hi Earl & tnargs,
Interesting to read your comments both about the cantilever as an alternative material measurement shape, and also that the values may actually not be correct. To be honest I have mainly glanced over most of the values so I have not critically considered whether they could not be correct.
I actually did assume that since the person who did all these measurements likely has put a lot of effort into this he would have clarified that the basic methodology was OK before beginning the measurement series ...
Hmmm ... Since the cantilever may be a better measurement choice: Might you suggest a practical size for this? Or maybe there is a link on the internet where the standard measurement setup is described?
Cheers from Denmark,
Jesper
Interesting to read your comments both about the cantilever as an alternative material measurement shape, and also that the values may actually not be correct. To be honest I have mainly glanced over most of the values so I have not critically considered whether they could not be correct.
I actually did assume that since the person who did all these measurements likely has put a lot of effort into this he would have clarified that the basic methodology was OK before beginning the measurement series ...
Hmmm ... Since the cantilever may be a better measurement choice: Might you suggest a practical size for this? Or maybe there is a link on the internet where the standard measurement setup is described?
Cheers from Denmark,
Jesper
Here is one: ASTM E756-98.
But I have no intention of going there. We already have the basic knowledge. For our purposes it needs to be light, stiff and highly damped, and it is hard to beat a CLD sandwich of PUR core and GRF or CRF epoxy skins, with mid-panel stiffeners of similar material.
It is possible that one might want to optimize for price and ease, answering questions like 'the best wood', but IMO it is going about it from the wrong end of the problem if we focus on the material this way, because the problem is complex and includes implementation as a dominant factor, i.e. one needs to measure the resultant built structure more than one needs to measure the various input materials and 'presume' that will be reflected in the behaviour of the final structure.
But I have no intention of going there. We already have the basic knowledge. For our purposes it needs to be light, stiff and highly damped, and it is hard to beat a CLD sandwich of PUR core and GRF or CRF epoxy skins, with mid-panel stiffeners of similar material.
It is possible that one might want to optimize for price and ease, answering questions like 'the best wood', but IMO it is going about it from the wrong end of the problem if we focus on the material this way, because the problem is complex and includes implementation as a dominant factor, i.e. one needs to measure the resultant built structure more than one needs to measure the various input materials and 'presume' that will be reflected in the behaviour of the final structure.
@tnargs: Hi & thanks for your comments and the ASTM suggestion ;-)
Although in no way an expert it is also my impression that this field is quite complex not least if one aims for a very good mechanical structure ...
BTW any chance you can say what GRF and CRF are acronyms for?
Cheers,
Jesper
Although in no way an expert it is also my impression that this field is quite complex not least if one aims for a very good mechanical structure ...
BTW any chance you can say what GRF and CRF are acronyms for?
Cheers,
Jesper
It's normally GRP & CRP for glass-fibre reinforced plastic & carbon-fibre reinforced plastic. The plastic will tend to be polyester with glass and epoxy with carbon though either type of plastic can be used with either type of fibre. The glass or carbon can come in many forms, chop strand mat where the fibres are randomly orientated, woven, or unidirectional. The best type of material will depend on the intended use.
Niffy
Niffy
Someone a long time back mentioned homasote. I’ve been wondering about using it to line the inside of cabinets. It is not particularly stiff, easily machined/cut (though messy!), and easy to glue to wood. Thinking about either applied to Baltic birch or mdf, or sandwiched in between two layers, but I don’t have enough knowledge on this topic to know if this is particularly useful.
as this is DIY, I imagine that common and easy to use materials might be welcome. Would love to hear the experiences of others before I reinvent the wheel.
as this is DIY, I imagine that common and easy to use materials might be welcome. Would love to hear the experiences of others before I reinvent the wheel.
A cheap fix to box resonances is to use a smaller box.
Short distances inside the box reduce time the sound bounces back.
Short distances also stiffen the box better.
I have a dual sealed 500 WRMS box and it sounds fine despite being on the smaller side.
I suspect I am losing a little bass but at least I can shift it around easier.
Short distances inside the box reduce time the sound bounces back.
Short distances also stiffen the box better.
I have a dual sealed 500 WRMS box and it sounds fine despite being on the smaller side.
I suspect I am losing a little bass but at least I can shift it around easier.
2 kinds of box resonances. The internal air space having resonances within the box, and those of the box walls being pushed to resonance. The 1st is caused by the back eave from the driver cone, the 2nd mostly by the reactive force on the basket (connected to the baffle) from the cone moving in and out.
Nigel mostly talks of the 1st but the general purpose of this thread in the 2nd.
If one reduces box size, the wavelength of stuf that builds up inside the box goes up in frequency and the damping inside has a better chance to quell it.
In the 2nd smaller dimensions have a higher frequency resonance so harder to excite. This is why bracing is used so as to convert large spans into smaller spans that have a higher resonance frequency.
dave
Nigel mostly talks of the 1st but the general purpose of this thread in the 2nd.
If one reduces box size, the wavelength of stuf that builds up inside the box goes up in frequency and the damping inside has a better chance to quell it.
In the 2nd smaller dimensions have a higher frequency resonance so harder to excite. This is why bracing is used so as to convert large spans into smaller spans that have a higher resonance frequency.
dave
Hi anchorman. As per Dave’s post above, bracing is the first-choice treatment if your chosen panel material is prone to resonating. Bracing not only reduces the amplitude of panel movement, but raises the fundamental frequency, which results in less resonant modes. Adding mass to the panels themselves lowers the fundamental frequency and results in more resonant modes, even though the resultant panels themselves are a bit better behaved if the extra mass helps to dampen them, so it is very much a mixed blessing.
The most successful way to go about a layered panel approach, is to start from scratch with appropriate materials. But if the base material (the cabinet itself) is a given, then the order of desired treatment is bracing #1, CLD #2 (with proper engineering of layer materials and thicknesses), damping layer #3 (one with high internal viscoelastic damping factor, like bituminous pads). Without evidence to the contrary, I would not rate a fibrous panel very highly as a panel damping medium.
Cheers
I don't understand "reduced time the sound bounces back."
I don't know anything about this topic so I assumed it had to do with wavelengths. Wavelengths longer than the box are modal/standing. Wavelengths shorter than the box reflect off the walls and strike the back of the speaker cone.
My situation shouldn’t require much in the way of bracing, as I’m eventually building a line array à la Roger Russell, and it will have bracing most of the way up. I will dig up my old thread once I start making test enclosures. Waiting on Black Friday sales to see if I can get a better deal on drivers, and have a few already on hand to chose from.
Curious if the homasote does anything useful, and whether it could eliminate/ minimize the need to add fiberglass or other material inside. I would love to build it all from panzerholz and unicorn smiles, but I’m limited to what I’ve got available, which is BB, MDF, particle/chipboard, OSB, Masonite, homasote, or combinations of the above.
Curious if the homasote does anything useful, and whether it could eliminate/ minimize the need to add fiberglass or other material inside. I would love to build it all from panzerholz and unicorn smiles, but I’m limited to what I’ve got available, which is BB, MDF, particle/chipboard, OSB, Masonite, homasote, or combinations of the above.
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Anchorman, if you have access to scrap materials you can make tests. U will see that Ply is quite resonant if you excite it with an impulse (just hit it with your hand, hammer, stick). Depending on size it will resonate rather high (there will be a sustain and a long decay at some frecuencies after you hit it). When you fix the sides in the final box the resonance will go even further up.
So, you can make composites of different components and different thickness, even different glues, make them stand free, hanging them somehow, even have it suspended with your hand in the air. Then excite them with an impulse to compare wich ones resonate the most and wich ones the less. In my case using a material like greenglue between two layers of ply was far less resonant than the ply itself.
Then, it depends on your aplicattion weather a solution may be better than other. Example: if you make a CLD for a tweeter it may be an overkill if you ask me, if you brace it, you will make some resonances potentially get into your bandwidth, but it prob wouldnt be a problem if the tweeter excitation force isnt enough to excite panel resonances. This is the hard part i believe, knowing wich implementation bennneffits the most from a solution or the other.
If you add damping (viscoelastic materials, rubbers, polymers) you make the resonance produce less movement, but the panel will move more above that freq. If you add stiffness you make the natural resonance frequencie of the structure go up, so you control better the part thats below panel resonance that went up. If you add mass you make the natural res freq go down but you get more attenuation above that point.
In the case of a speaker driver one must bear in mind that ussually the force that gets to the panel is determined by the mass of that driver and its acceleration. So the worst case is a high wattage, high excursion, high mass bass driver. I doubt a 10W dome tweeter may need some especial treatmnet, at least you are doing it for fun, knowledge or experience.
So, you can make composites of different components and different thickness, even different glues, make them stand free, hanging them somehow, even have it suspended with your hand in the air. Then excite them with an impulse to compare wich ones resonate the most and wich ones the less. In my case using a material like greenglue between two layers of ply was far less resonant than the ply itself.
Then, it depends on your aplicattion weather a solution may be better than other. Example: if you make a CLD for a tweeter it may be an overkill if you ask me, if you brace it, you will make some resonances potentially get into your bandwidth, but it prob wouldnt be a problem if the tweeter excitation force isnt enough to excite panel resonances. This is the hard part i believe, knowing wich implementation bennneffits the most from a solution or the other.
If you add damping (viscoelastic materials, rubbers, polymers) you make the resonance produce less movement, but the panel will move more above that freq. If you add stiffness you make the natural resonance frequencie of the structure go up, so you control better the part thats below panel resonance that went up. If you add mass you make the natural res freq go down but you get more attenuation above that point.
In the case of a speaker driver one must bear in mind that ussually the force that gets to the panel is determined by the mass of that driver and its acceleration. So the worst case is a high wattage, high excursion, high mass bass driver. I doubt a 10W dome tweeter may need some especial treatmnet, at least you are doing it for fun, knowledge or experience.
The quicker the wave bounces back & forth is higher frequency. The speaker has to excite it.
dave
All good information. Wondering where sound absorbing materials like homasote play into this??? Not sure what physical mechanism to call what it does, since it seems like it’s not damping the sound waves the way that aluminum/rubber CLD type composites do. My understanding is that it would be more like sand with lots of different intersections off the material where the sound waves are dispersed/lessened as they are inefficiently passed between adjoining materials. Homasote seems to be very low resonant frequency, very low Q material. I imagine it would take a lot of energy to get it to vibrate at any particular frequency.