yep, that is what I mean. The top one in your pic looks OK, but the bottom does not.
Giving a bit of material a knock does not give a sensible result. I used to to do that before I measured things, and I was correct 50% of the time, meaning completely random guesswork.
Chipboard works as a damping material because of the friction between those particles. Lots of small particles appears to be better than a wide range of sizes.
Again, I would stress that there are many kinds of chipboard (and mdf and ply), but mdf is so far behind chipboard as a damping material, I would be very surprised if any mdf was better than any chipboard.
Also, not that when we knock a lump of material, there are several things to bare in mind. Firstly, how the material is being supported, secondly, what you hit the material with, and even size of material (whether a panel or a beam, for example). What you might hear is the radiation from the material above its critical frequency, although there will be some radiation, which can depend on which modes are being excited.
Giving a bit of material a knock does not give a sensible result. I used to to do that before I measured things, and I was correct 50% of the time, meaning completely random guesswork.
Chipboard works as a damping material because of the friction between those particles. Lots of small particles appears to be better than a wide range of sizes.
Again, I would stress that there are many kinds of chipboard (and mdf and ply), but mdf is so far behind chipboard as a damping material, I would be very surprised if any mdf was better than any chipboard.
Also, not that when we knock a lump of material, there are several things to bare in mind. Firstly, how the material is being supported, secondly, what you hit the material with, and even size of material (whether a panel or a beam, for example). What you might hear is the radiation from the material above its critical frequency, although there will be some radiation, which can depend on which modes are being excited.
nope, no-one is suggesting that. The original idea was predicated on the results of a sample of a single layer of acrylic stuck to a single layer of mdf, which I measured to have a damping factor of 0.315. (Sample came from 'Page Lacquer'.)
To jump from that to acrylic/chipboard/acrylic is a big leap of faith. There is no suggestion as to what the damping factor will be. Why not just use chipboard? It might be that adding materials which don't damp as well, won't add but subtract from the final figure.
The only way is to make it and measure it. There is no other way, I'm afraid.
To jump from that to acrylic/chipboard/acrylic is a big leap of faith. There is no suggestion as to what the damping factor will be. Why not just use chipboard? It might be that adding materials which don't damp as well, won't add but subtract from the final figure.
The only way is to make it and measure it. There is no other way, I'm afraid.
@cats squirrel,
sorry for late reply...
1. isn't the proposed similar to what michel has with focus one? someone mentioned this, maybe even you.
2. we are talking about cld right? in this variant, plate (elac miracord or l75 etc.) is vibrating layer together with acrylic its locked to with screws tightly (ok, maybe acrylic could be “stand-off”), mdf or chipboard should be "viscoelastic" and another acrylic as constraining layer?
3. you think maybe it makes sense putting additional damping under and on the plate itself - some double sided elastic foamy sticker thing with attached piece of metal (coin)?
sorry for late reply...
1. isn't the proposed similar to what michel has with focus one? someone mentioned this, maybe even you.
2. we are talking about cld right? in this variant, plate (elac miracord or l75 etc.) is vibrating layer together with acrylic its locked to with screws tightly (ok, maybe acrylic could be “stand-off”), mdf or chipboard should be "viscoelastic" and another acrylic as constraining layer?
3. you think maybe it makes sense putting additional damping under and on the plate itself - some double sided elastic foamy sticker thing with attached piece of metal (coin)?
Michells' use of chipboard and acrylic dates back a long way...the Transcriptors Hydaulic Reference* was constructed using it......that is from the mid 60's . I suspect that it was just a design aesthetic and budgetory decision rather than an auditory choice. Michell decks looked amazing but were cheaply made from cheap and easily workable materials.
I wouldn't necessarilly hold them up as an exemplar of best practice plinth design.
* which you can still buy new for £4000 I think.
I wouldn't necessarilly hold them up as an exemplar of best practice plinth design.
* which you can still buy new for £4000 I think.
I mentioned the acrylic/chipboard plinth as used by Michell, and asked if they knew something we (the general public) didn't. I wasn't stating they did, or as deadhead has mentioned, just an inexpensive build material/s. I would agree with that!
We are definitely NOT talking about cld (constrained layer damping). This confusion goes way back, too, probably to Jean Nantais and his Lenco's. It is not, and has never been cld. Cld works by losses due to shearing, sideways movement of the viscoelastic layer.
Cld comprises a thin panel (or beam) which is to be damped. This is attached to another thin panel (usually of the same material and thickness as the panel to be damped) by a viscoelastic adhesive, forming a thin three layer composite. The panel may have total coverage, or smaller patches. It may have stand-offs. It may have active components. BUT it is not three or more thick panels glued together, made of the same or different materials. These structures are called glued laminates, contracted to 'glulams'. They do not work as cld structures. And they act as one new material, not several separate layers (" which act to cancel out vibrations").
So acrylic/chipboard/acrylic would be a glulam, not cld. Whether it works is anyone's guess (or measurement).
Last edited:
Critical Frequency
Cats Squirrel,
Thanks very much for the PVB. I made a small batch and dabbed it on some Acrylic to see if I'm mixing it right.
Meanwhile, a question...
The attached screen grab was created by placing a microphone close to a 100mm square, 12mm Acrylic/ 14mm MDF/12mm Acrylic piece. Do the peaks and dips give an indication of what Fc might be?
Hugh
Cats Squirrel,
Thanks very much for the PVB. I made a small batch and dabbed it on some Acrylic to see if I'm mixing it right.
Meanwhile, a question...
The attached screen grab was created by placing a microphone close to a 100mm square, 12mm Acrylic/ 14mm MDF/12mm Acrylic piece. Do the peaks and dips give an indication of what Fc might be?
Hugh
Attachments
Hello Hugh,
what you have measured is the sound radiation from the sample, not the vibrations. The radiation efficiency changes dramatically with frequency, eventually reducing, asymptotically to 1.0 (100%). The picture gets very complex as each vibrational mode (frequency) behaves differently to the fundamental. Here is just the fundamental mode with the peak showing the critical frequency:
radiation efficiency acrylic mdf acrylic 100mm square by cats squirrel,
what you have measured is the sound radiation from the sample, not the vibrations. The radiation efficiency changes dramatically with frequency, eventually reducing, asymptotically to 1.0 (100%). The picture gets very complex as each vibrational mode (frequency) behaves differently to the fundamental. Here is just the fundamental mode with the peak showing the critical frequency:
radiation efficiency acrylic mdf acrylic 100mm square by cats squirrel,Hi Hugh,
are you confusing the critical frequency (Fc) with the fundamental resonance frequency (Fr)?
Fr is about 602 Hz, Fc is about 9779 Hz. These figures do not take into account any influences that the adhesive may add, or exactly the physical properties of the acrylic and mdf.
What you may be seeing is the problem associated with radiation efficiency. There is a small peak about 600 Hz, which would be about a tenth the height it should be taking into account the loss due to radiation efficiency.
You may appreciate how the 'knuckle test' can be so misleading!
are you confusing the critical frequency (Fc) with the fundamental resonance frequency (Fr)?
Fr is about 602 Hz, Fc is about 9779 Hz. These figures do not take into account any influences that the adhesive may add, or exactly the physical properties of the acrylic and mdf.
What you may be seeing is the problem associated with radiation efficiency. There is a small peak about 600 Hz, which would be about a tenth the height it should be taking into account the loss due to radiation efficiency.You may appreciate how the 'knuckle test' can be so misleading!
Last edited:
Bryan,
Thanks, that helps.
I'm now getting email notifications from DIYAudio again. Oddly, in the email, your sentence above came through as " Fr is about 2330 Hz, Fc is about 2525 Hz ". That's weird.
Hugh
JohnnoG, Cats Squirrel,
At the moment, I'm having more success in having PVB stick to Acrylic using Nail polish remover (Acetone) rather than Isopropanol. But, a question about the method of dissolving it...
If I drop a little bit of powder into quite a bit of Nail Polish remover, it very quickly forms a gummy blob. I crush it a few times and soak for about 30 seconds, then apply it to the Acrylic. I place the mdf on top and lightly clamp overnight.
The cured PVB is quite elastic.
Does it seem like I have it right?
Hugh