As stated by others, for constrained layer damping one would not want to put the layers under pressure because a fairly thick layer is desirable - one that is not compressed. I have used the CLD technique on my speakers for over ten years and tried many different materials. The best that I have found is 2k resilient polyurethane with a filler for greater internal frictional loss. Compression is not the answer.
I've read several times in articles about studio design -35 dB down in a studio is pretty much considered to be anechoic like behaviour. I'd be surprised you'd get that in a home without severe treatment/absorption.
I've seen examples of rooms that got -25 dB to 30 dB that didn't resemble a space to live in at all. Kudo's to the one hearing detail that's 40 dB down! Must be one hell of a recording too! 😉
Does a resonance in a cabinet created at 40dB below the signal remain at 40dB below the signal over it's lifetime?
Andy - that is a great question and one that would be incredibly complex to answer in general since each specific situation would be different.
A very high Q resonance that was excited with a long term signal that was terminated very rapidly just might exceed the original level differences as they decay. But in most realistic situations like music, the signals themselves have a finite decay time because musical instruments are themselves resonant devices, hence in these cases it would be very unlikely that the decaying cabinet resonance - which I would hope is never going to be a higher Q than a musical instrument - would ever exceed the original signal level differences by more than it is initially. So on the whole I would have to say that I would seriously doubt that what you suggest would ever be the case in practice.
A very high Q resonance that was excited with a long term signal that was terminated very rapidly just might exceed the original level differences as they decay. But in most realistic situations like music, the signals themselves have a finite decay time because musical instruments are themselves resonant devices, hence in these cases it would be very unlikely that the decaying cabinet resonance - which I would hope is never going to be a higher Q than a musical instrument - would ever exceed the original signal level differences by more than it is initially. So on the whole I would have to say that I would seriously doubt that what you suggest would ever be the case in practice.
The suggestion of a vacuum pump was made in response to someone who has explained his difficulty to have high pressure.
I believe that a vacuum pump can be very useful in the workshop of a diyer. And not only for the veneers...
If he uses carbon fiber, R or S fibreglass, divinycell, epoxy, etc... the vacuum pump, and the infusion technique, is certainly the best answer.
And if the diyer wants to laminate different materials, the vacuum pump offers a lot of advantages.
- It's easy to use.
- As you can laminate great panels, you win a lot of time. Laminate first, and then cut the pieces. I find that laminating piece by piece it's a bit boring.
- The pressure is homogeneous. This point is important and difficult to reach with clamps or gaskets.
- You can reduce and adjust the pressure to the precise requirements for the glue/adhesive/sealant you use. You have a total control and can offer the best at the product you have choosen. I don't know any quality glue which can be apply without a minimal pressure.
Sean Olive studied these resonance audibility questions (published long ago in the JAES), investigating resonance Q and level relative to the initial sound, and double blind tested audibility. They used noise and impulses (and so studied effect of original signal decay on audibility). From that study, get ~ 28 dB down and its inaudible under all conditions. This was with headphones, without the masking of room reverb, so can probably be considered sensitive tests
Could you provide a little more info regarding what has worked for you? An example of "2k resilient polyurethane"? How thick should the polyurethane layers be? For side panels, is plywood good, and if so how thick? Plywood/polyurethane/plywood? Or better to have thinner plywood and more PU sandwiches?
That is not a FR measure thou.
I do something very similar but in a qualitive manner using a mechnics stethescope instead of qualitative with an accelerometer (i do have one). It is quick to detect where & when panels resonate. The next iteration (if needed) will have more bracing to deal with the resonance.
A recent sealed push-push subwoofer build was very purposely built with well braced 12mm BB to test how well push-push removes cabinet vibration.
Top panel was worst, with a very small amount of vibration at loud levels, turning off the mains and cranking the levels another 10-12 dB the top really started moving. It will likely get "veneered" with a chunk of granite countertop floated on the top to eliminate anything from escaping.
dave
Very much so. What happens way down is the difference between good kit & great kit.
dave
Something that is extremely rare with music, particularily at higher frequencies.
This implies that if you can push the frequency & Q of a potential resonance up it will rarely, if ever, get excited, effectively making it non-existent.
dave
There is a cabinet resonance that takes 1 second to decay from -28dB to the threshold of inaudibility. The speaker is driven for a while at the resonant frequency and 0dB and then turned off. The cabinet resonance is heard for 1 second. This is a contrived example but hopefully it illustrates a mechanism. Like impulsive distortion and the THD measure I suspect a fixed audibility level alone is insufficient to be useful.
The Q of the resonances in a reasonable cabinet will bound things but, following Earl's suggestion, what is the worst case "real" musical signal that drives a resonance and then gets out the way to allow the resonance to be heard?
I will give "a little more info", but I won't give away all my secrets.
I use a layer of about an 1/8". I trowel it on thick and then spread it out. Lay on the second piece and press lightly until there is some ooze at the edges and then I stop and let it setup.
The rule of thumb is that both layers should be about the same stiffness and not too thin, but actual practice would take some experimentation. I don't CLD the side panels - I used to, but I could measure a difference when I stopped. The back panel and baffle are the largest panels and they are both CLD, with CLD internal cross braces. A single layer of damping fill is all I have ever seen done. This is not for "bullet proofing" mind you, that's different, this is damping for bending.
For some tests, noise is most sensitive, some impulse.
Can you find odd ball musical samples that are more sensitive than this? Maybe, but that's getting mighty picky. Take the results available from Olive and run with them, you won't find better.
Your one second ringing example is emulated with an extremely high Q test condition. They studied low to high Q. They even studied the effect of delaying the resonance onset. Check out the papers, I think you'll be surprised at their applicability
Worst case and highly unrealistic. 1 sec from -28 dB to below audibility might only be -2 to -4 dB/sec - an extremely low level of damping. Why anyone would make an enclosure with that low damping is beyond me, as well as why one would use it as an example.
I will repeat what I said before, I don't see any way that this could happen in common practice. One might contrive an example, but what point would that make?
I have been Googling "2k resilient polyurethane" and "2k polyurethane" and "2k polyurethane adhesive" and mostly coming up with surface treatment or what I would just call paint. There are some European tubs but I can't find any NAmerican brands. What are some name brands and sources for this material?
I am contemplating a cabinet with a tear drop shape and using thinner laminates would mean less kerfs to cut and make the bending much easier. I was going to use wood glue because it is inexpensive and easy to work with. But I am up for working with some new material.
I would appreciate some recommendations and sources.
Thank you
I am contemplating a cabinet with a tear drop shape and using thinner laminates would mean less kerfs to cut and make the bending much easier. I was going to use wood glue because it is inexpensive and easy to work with. But I am up for working with some new material.
I would appreciate some recommendations and sources.
Thank you