Report: is closed cell foam the right material to decouple mass loaded vinyl?

[Justin Zazzi]

This was originally posted on DIYMA forum on December 16, 2018:



Back in the summer of 2017 I wanted to test a common rule of thumb: mass loaded vinyl should always be decoupled from nearby surfaces, and closed cell foam is the right material to do it.

Many of you kindly donated materials for this experiment and I am very, very thankful for it. I am thrilled to finally finish and post the results. This took a very long time to write because I did not have a solid method to interpret the results until just recently when we covered the transmissibility ratio in an acoustics class I'm taking. I apologize for the delay, but I hope the wait is worthwhile.

the report is at my dropbox here --> MLV decoupling report v1.3

 

[spwath]

Very interesting report, I am going into my senior year majoring in mechanical engineering with an acoustics concentration so this stuff is very interesting to me.

I wonder though, how much does the size of the mlv squares used affect results? It seems like scaling up, a lot more factors could come into play.

 

[Justin Zazzi]

Cool! Mechanical engineering is my degree as well and it has been very useful. I'm happy to answer any questions you have about the acoustics industry or how to pursue a career in sound. Actually, a great place to start would be ALMA's annual event called AISE. It is normally a few hundred bucks and you'd have to travel to Las Vegas and rent a hotel and everything, but this year it is free to anyone who registers and it'll be online.

As for the size of the MLV squares, it should scale nicely since the stiffness of the decoupling layer depends on the surface area of the material, and so does the mass of the vinyl. The common surface area should cancel out the change in mass and the change in stiffness of the spring. I'm pretty sure the scaled-down samples I tested were a good representation. Do you see something different?

 

[Pb82 Ronin]

Can we have a cliff's notes of the results? You know, for those of us that don't read engineering jargon well.

 

[Justin Zazzi]

Can we have a cliff's notes of the results? You know, for those of us that don't read engineering jargon well.

Have you tried the conclusion on the last page?
Sorry it feels full of jargon. I tried to make it less jargony but it's hard.
Is there something specific I can clarify?

 

[spwath]

Cool! Mechanical engineering is my degree as well and it has been very useful. I'm happy to answer any questions you have about the acoustics industry or how to pursue a career in sound. Actually, a great place to start would be ALMA's annual event called AISE. It is normally a few hundred bucks and you'd have to travel to Las Vegas and rent a hotel and everything, but this year it is free to anyone who registers and it'll be online.

As for the size of the MLV squares, it should scale nicely since the stiffness of the decoupling layer depends on the surface area of the material, and so does the mass of the vinyl. The common surface area should cancel out the change in mass and the change in stiffness of the spring. I'm pretty sure the scaled-down samples I tested were a good representation. Do you see something different?

Not too sure if it would make a difference, but I was just thinking, as right now the 1.5" square is somewhat rigid, but a bigger sheet of vinyl would have a lot more flex to it.

Also, while these results do show a lot at what is the most effective, it doesn't really show how big of a change it makes. For that the experiment would need to be done by mounting the system on a car doo/metal piece and measure the transmition loss. That actually sounds pretty interesting to do, I will have to see if I can do that at all next year, as my college has an anechoic chamber and reverberation room, with a removable panel between them to specifically measure transmition loss.

 

[rton20s]

Have you tried the conclusion on the last page?
Sorry it feels full of jargon. I tried to make it less jargony but it's hard.
Is there something specific I can clarify?

I think you did a great job of keeping the "jargon" down. I thought the document was well written and straight forward enough for a laymen like myself to understand. And before I saw your reply, I was going to point Ronin to your conclusion, as well.

Not too sure if it would make a difference, but I was just thinking, as right now the 1.5" square is somewhat rigid, but a bigger sheet of vinyl would have a lot more flex to it.

Also, while these results do show a lot at what is the most effective, it doesn't really show how big of a change it makes. For that the experiment would need to be done by mounting the system on a car doo/metal piece and measure the transmition loss. That actually sounds pretty interesting to do, I will have to see if I can do that at all next year, as my college has an anechoic chamber and reverberation room, with a removable panel between them to specifically measure transmition loss.

You might be interested in taking a look at the test rig that Chris Purdue (aka TooStubborn2Fail) is currently building. It is designed to test CLD, but I believe he plans to expand the testing beyond that to materials like MLV and lead for transmission loss.

 

[preston]

I think the conclusion is there is no point to "decoupling" MLV except to sell stick-on CCF. Just install the MLV and skip the whole decoupling mantra.

That being said I do like to have some stick-on CCF around, its great for isolating wire bundles or trim panels that touch but one shouldn't expect it to "absorb" sound waves or reduce vibration in MLV installs.

 

[Justin Zazzi]

I think the conclusion is there is no point to "decoupling" MLV except to sell stick-on CCF. Just install the MLV and skip the whole decoupling mantra.

That being said I do like to have some stick-on CCF around, its great for isolating wire bundles or trim panels that touch but one shouldn't expect it to "absorb" sound waves or reduce vibration in MLV installs.

Essentially, yes that is what the report says.

I should have highlighted some of the benefits though like having a stick-on material like you describe, or the benefit of having something between the MLV and the sheet metal to reduce the chances of rattles/buzzing. There can certainly be benefits to having intermediate materials, but not for the sake of "decoupling" like is commonly claimed.

 

[opekone]

Very great work! I love the idea of mounting to the driver to create a simple and repeatable process where the harmonic resonance of the mounting surface is continuously variable.

I'm interested in the effect of the overall size of the MLV or the decoupling layer since they both gain their spring and dampening curves from the interaction of neighboring material. Replicating results with a much smaller piece, say, 5mm x 5mm, would go a long way to demonstrating the efficacy of this methodology.

In vivo, as it were, the speaker's output is significantly dampened and modified by the air and when it hits an object (like your inner door skin) it excites that object at it's natural harmonic resonance. While mounting directly to the speaker does provide a great view into the variability across difference resonant frequencies, I wonder if it's an accurate recreation of our use case. Here the only interaction we are seeing is that of the decoupler + MLV interacting with the harmonic resonance of the material. We don't assess it's ability to mitigate the airborne sound nor do we assess the combination of effects. This is particularly curious to me because in our application we typically use CLD rather than decoupled MLV to deal with harmonic resonance.

Is Jazzy even still here?

Sent from my SM-G965U1 using Tapatalk

 

[Justin Zazzi]

Very great work! I love the idea of mounting to the driver to create a simple and repeatable process where the harmonic resonance of the mounting surface is continuously variable.

I'm interested in the effect of the overall size of the MLV or the decoupling layer since they both gain their spring and dampening curves from the interaction of neighboring material. Replicating results with a much smaller piece, say, 5mm x 5mm, would go a long way to demonstrating the efficacy of this methodology.

I very much doubt it. Spring stiffness from these kinds of materials is proportional to surface area. A bigger piece can support a heavier load. Similarly, a bigger piece of vinyl is heavier. The two things cancel out so the performance should scale nicely with size.

Is Jazzy even still here?

Yes I am. I changed my screen name a little bit.

 

[opekone]

I very much doubt it. Spring stiffness from these kinds of materials is proportional to surface area. A bigger piece can support a heavier load. Similarly, a bigger piece of vinyl is heavier. The two things cancel out so the performance should scale nicely with size.

Unfortunately I'm a few years too late, it wouldn't have taken much time to verify that when you had this set up. The total surface has a very significant impact on the springrate only when the piece is very small as in this test. Once you are far enough away form the place of measurement the natural damping effect overrides this. This is obvious in the case of a large mat that we can generously consider 3" thick, but is 10 feet x 10 feet. When measuring the springrate or damping curve of the material at the center I expect there is going to be no change if you increase the size of the material considerably to 20'x20'. What seems like an edge case is, in our application, the main use case. I'm not convinced that these results extrapolate to that case, but it could certainly be tested.

 

[Justin Zazzi]

What experience do you have that makes you so certain? Can you share a paper that explains how what I did was an extreme edge case?

 

[opekone]

What experience do you have that makes you so certain? Can you share a paper that explains how what I did was an extreme edge case?

I offer criticism and direction to stimulate more testing and of course to improve my understanding of these systems. Acoustical engineering is not my areas of expertise and I unfortunately don't have access to such academic journals online, or better yet - paper copies delivered to me directly. But we still must move forward!

You strongly imply the translation from a 1.5" square to, say a 3' square is trivial. I assume this is because of your background. Can you shoot me a recent review paper, preferably such a paper on methods, or perhaps a section from an elementary engineering text, or even a suggestion of such a text.. or if you have a specific paper in mind I'd be interested in that also!

 

[Justin Zazzi]

I don't have any papers off the top of my head, but it's all about pressure. You know the saying "many hands make light work" ? This is what it's all about.

If one person can carry one heavy box, then a couple people can lift a litter to carry a person.

View attachment 10990

If you have an object that is ten times as heavy, you need ten times as many people to lift it. Many many people can lift a house and move it.

View attachment 10991
(video in this link here)


If you'd like to find some reading, try searching for "pressure" and learn about force per area. You can also look for "spring constant" to learn about how far a spring will compress based on how much force is pressed against it (or in this case how far foam will compress based on how much weight is above it).

This falls under the physics branch of school, specifically the "mechanics" branch of physics which is usually the first class in the physics series. The other classes cover electricity, magnetism, light and optics, heat transfer, and so on. My favorite physics teacher is Walter Lewin. He is a treasure and has posted dozens of amazing lectures and smaller videos on his channel. The one blow is all about springs:

Here is another on simple harmonic oscillators which is the fundamental theory of the report I made in the first post above.

Finally, a practical experiment you can try that is really easy:
Take something squishy like a yoga mat (or a bed mattress) and stand on it with your feet. Notice how far your feet will sink into the material. Then place a piece of wood on top and stand on the piece of wood. Notice how far the wood sinks into the material.

Your feet will sink in much deeper than the wood. This is because the same amount of weight is being applied (you) however the weight is being spread out over a larger area when you use the piece of wood. If you were to get a couple friends to stand on a small piece of wood with you, then it would sink in deeper because you're increasing the pressure by adding more weight while keeping the surface area the same.

This is the same effect as growing or shrinking the size of the vinyl in my experiment because I would have to grow or shrink the size of the foam underneath it by the same amount. The added weight of the vinyl would be cancelled about by the added surface area of the additional foam and the performance of the system as a whole would be the same.

 

[Alain]

Did you try a vertical test, rotate 90deg your speaker. Do you think the softer spring material would work as well?

Sent from my SM-T530NU using Tapatalk

 

[Justin Zazzi]

Did you try a vertical test, rotate 90deg your speaker. Do you think the softer spring material would work as well?

Sent from my SM-T530NU using Tapatalk

No I did not try a vertical orientation and I doubt the results would be any different. The softest materials would have a hard time supporting the weight of the vinyl though. The softest material had the consistency of hair clippings in the floor of a barbershop so it wouldn't be able to hold the vinyl, and this is one of the reasons I think that material is impractica even though it performs well in this test.

 

[opekone]

I don't have any papers off the top of my head, but it's all about pressure. You know the saying "many hands make light work" ? This is what it's all about.

If one person can carry one heavy box, then a couple people can lift a litter to carry a person.

View attachment 10990

If you have an object that is ten times as heavy, you need ten times as many people to lift it. Many many people can lift a house and move it.

View attachment 10991
(video in this link here)


If you'd like to find some reading, try searching for "pressure" and learn about force per area. You can also look for "spring constant" to learn about how far a spring will compress based on how much force is pressed against it (or in this case how far foam will compress based on how much weight is above it).

This falls under the physics branch of school, specifically the "mechanics" branch of physics which is usually the first class in the physics series. The other classes cover electricity, magnetism, light and optics, heat transfer, and so on. My favorite physics teacher is Walter Lewin. He is a treasure and has posted dozens of amazing lectures and smaller videos on his channel. The one blow is all about springs:

Here is another on simple harmonic oscillators which is the fundamental theory of the report I made in the first post above.

Finally, a practical experiment you can try that is really easy:
Take something squishy like a yoga mat (or a bed mattress) and stand on it with your feet. Notice how far your feet will sink into the material. Then place a piece of wood on top and stand on the piece of wood. Notice how far the wood sinks into the material.

Your feet will sink in much deeper than the wood. This is because the same amount of weight is being applied (you) however the weight is being spread out over a larger area when you use the piece of wood. If you were to get a couple friends to stand on a small piece of wood with you, then it would sink in deeper because you're increasing the pressure by adding more weight while keeping the surface area the same.

This is the same effect as growing or shrinking the size of the vinyl in my experiment because I would have to grow or shrink the size of the foam underneath it by the same amount. The added weight of the vinyl would be cancelled about by the added surface area of the additional foam and the performance of the system as a whole would be the same.

Thanks for taking the time to offer such a thorough and comprehensive reply. I appreciate it and wanted to make sure I understood your supplement before replying, thank you for your patience. I'm not sure if the basic maths underlying SHO's are applicable in this case, where the springrate and damping rate are seemingly interrelated. A SHO is defined as something which has zero damping, but these materials are specifically being chosen for their damping effect - and that is exactly what we are trying to assess.

In the case of a linear coil spring, or a much simpler torsion spring - both made from spring steel or music wire - the springrate is simple to calculate and the assumption of no damping is reasonable. In both cases if you were to sample the springrate it's well understood where you apply the force and in what direction. Now if instead, you test at an arbitrary place along the spring you would find an increased springrate. The section of the spring beyond where you are applying your metered/measured force will be inactive. You can easily predict the springrate by simply calculating the spring as if it's length were the length from the testing location. I'm sure this all trivial for you. But now lets extend beyond high school mechanics.

In the case of a material such a closed cell foam how does one calculate the springrate relative to the testing location? How about the damping curve? For example, if you are measuring the amount of force required to compress the foam at location A you may get some value. If you again measure the springrate of the foam at location B I presume you'll get a lower value. If you test along line C moving from location B towards location A, I presume you will see the springrate rise and the damping curve change. My guess is that as you get farther from point B, testing along the line C towards A, you will reach a point where the springrate reaches asymptote and the damping curve stabilizes. Is this reasonable? Am I just inventing things?

View attachment 11357

Thanks

 

[Justin Zazzi]

I think you understand that right. The material in the center of your drawing, point A, will have the expected firmness/stiffness/performance. The material on the edge of your drawing, point B, will appear to have less firmness/stiffness/rigidity.

This can easily be seen when you sit on the very extreme edge of the mattress on your bed. The edge will tend to wrap itself downward towards the floor when you sit on the edge, which makes it appear to be softer. However, what is happening is you are placing your full weight onto the corner of your butt which is increasing the *pressure* on the mattress which will make it move more, appearing softer. If you were to put a piece of plywood along the edge of the mattress then you could sit on the edge just fine since the wood would distribute your weight over a larger area and it would decrease the pressure on the mattress, making it appear firmer.

Going beyond high school mechanics like you ask, this behavior is like a boundary condition. If you place a weight into the middle of a "large" piece of foam then the boundary condition near that weight is uniform foam in every direction and the stiffness you measure will be uniform and predictable. This will be true so long as you place the weight or you make your measurement "far" away from the edge of the foam. If you investigate what happens near the edge of the foam, especially with a "small" contact point near the edge, then the boundary condition is no longer uniform and you will get a different result.

To measure the spring constant or other properties of a material like this is it helpful to use a "large" piece with rigid surfaces on either side so that the squishy boundaries do not distort your data. In my experiment I did this by using a rigid stage (the flat dust cap of the speaker that was vibrating as a base) and also by using a uniform piece of vinyl which was rigid enough relative to the softness of the decoupling layers.

Another way to think about is those giant parachutes that, if you're lucky, you got to play with at school when you were younger. If everyone grabs hold of the edge and randomly shakes up and down then the parachute will turn into a wavy random shape. But if everyone grabs hold and moves up and down at the same time, then the parachute will hold a flat shape. This is because the material is being excited uniformly, just like the rigid vibrating base in my experiment was uniformly exciting the decoupling layers which were uniformly exciting the vinyl layer.

It all comes back to surface area. Stiffness per square inch vs mass per square inch. They cancel out.

View attachment 11397

View attachment 11398

 

[Justin Zazzi]

Thanks for taking the time to offer such a thorough and comprehensive reply. I appreciate it and wanted to make sure I understood your supplement before replying, thank you for your patience. I'm not sure if the basic maths underlying SHO's are applicable in this case, where the springrate and damping rate are seemingly interrelated. A SHO is defined as something which has zero damping, but these materials are specifically being chosen for their damping effect - and that is exactly what we are trying to assess.

The paper I wrote explains the experiment as a simple harmonic oscillator and does not consider damping because I was trying really hard to keep the experiment accessible to the enthusiast. The same ideas and math apply though so it wound up translating really well.

In a damped harmonic oscillator, there are a few different damping terms: the damping force is the amount of damping in the system and it is the force that acts to slow the motion of the oscillator. The damping ratio ζ (zeta) refers to the combination of the mass, spring stiffness, and the damping in the system this they are interrelated like you think.
A damping ratio of zero (ζ=0) is not-damped and will oscillate forever, but this is not possible in reality.
A low damping ratio (ζ<1) is under-damped and tends to have ringing.
A high damping ratio (ζ>1) is over-damped and returns to rest very slowly.
A damping ratio equal to one (ζ=1) is critically-damped and returns to rest quickly but doesn't oscillate.

I was not trying to assess exclusively the damping force of the materials, I was more interested in the resonant frequency of the system which is driven by the stiffness of the decoupling materials. The damping force did have an influence, but not as much as the stiffness of the decoupling materials did. You can see this in the summary of results where I built the table of transition frequencies for the materials tested. The lower transition frequencies performed better in this test, which is directly related to the stiffness of the decoupling materials.