No references needed, you can experience it for yourself. Get a dog trainer (clicker) and stand in the middle of an empty room. CLICK... Where do you hear the echos coming from? Most often it's from inside corners.
Also ask yourself what do inside corners and megaphones have in common? Yep... 90 degree angles.
Daniel,
Any suggestions for a better one. Green glue isn't inexpensive but a couple other ones I've seen mentioned for building speakers such as Sikaflex and 2K Urethane are 2x or more $/oz. Ease of application, particularly for 4'x14' panels on the ceiling is also important.
Are you recommending” thin and stiff” materials for the sandwich? I thought low mass and flexible materials were best for maximizing the damping ratio. Much of this is new to me so I want to make sure I understand.
Thanks
Any suggestions for a better one. Green glue isn't inexpensive but a couple other ones I've seen mentioned for building speakers such as Sikaflex and 2K Urethane are 2x or more $/oz. Ease of application, particularly for 4'x14' panels on the ceiling is also important.
Are you recommending” thin and stiff” materials for the sandwich? I thought low mass and flexible materials were best for maximizing the damping ratio. Much of this is new to me so I want to make sure I understand.
Thanks
https://apps.dtic.mil/dtic/tr/fulltext/u2/a088200.pdf
^ This guy is DENSE but I was just reading the basics and it evoked memories of what's going on. Idealized, we want an infinitely thin and stiff structural material sandwiching an infinitely thin and lossy shear layer. Oh and a perfect bond between them all. The closer we can get the better, but keep in mind practicalities here without losing your mind.
Most importantly, we want to match stiffness of the structural materials surrounding the shear layer to maximize transfer.
As far as green glue vs other products, I haven't done the research. Green glue and similar viscoelastic products that bond well to drywall are effective! Personally, I know my old roommate did the 2x 1/2" drywall between a few rooms in his condo remodel and responded favorably in terms of quieting the place down. I only know what the end result is, and given the crappiness of the condo's construction elsewhere, I have to agree with my roommates assessment!
So it's quite effective but from an idealized perspective, maybe not the best in a perfect, Ivory Tower, sense. Using drywall itself is a compromise, but, again, don't go crazy with optimization in this manner. I believe Earl Geddes himself recommends a melamine glue/construction adhesive (can't remember the name), but not sure how well it works on drywall vs between sheets of wood (CLD speaker cabinets).
^ This guy is DENSE but I was just reading the basics and it evoked memories of what's going on. Idealized, we want an infinitely thin and stiff structural material sandwiching an infinitely thin and lossy shear layer. Oh and a perfect bond between them all. The closer we can get the better, but keep in mind practicalities here without losing your mind.
Most importantly, we want to match stiffness of the structural materials surrounding the shear layer to maximize transfer.
As far as green glue vs other products, I haven't done the research. Green glue and similar viscoelastic products that bond well to drywall are effective! Personally, I know my old roommate did the 2x 1/2" drywall between a few rooms in his condo remodel and responded favorably in terms of quieting the place down. I only know what the end result is, and given the crappiness of the condo's construction elsewhere, I have to agree with my roommates assessment!
So it's quite effective but from an idealized perspective, maybe not the best in a perfect, Ivory Tower, sense. Using drywall itself is a compromise, but, again, don't go crazy with optimization in this manner. I believe Earl Geddes himself recommends a melamine glue/construction adhesive (can't remember the name), but not sure how well it works on drywall vs between sheets of wood (CLD speaker cabinets).
No. If something 1" thick and as cheap and simple as this was effective at absorbing low frequency sound then it would be used everywhere. Instead effective devices tend to be 4-12" thick depending on tuning frequency.
Some of the physics that is against you. CLD damps bending waves effectively (if you get the design right) but low frequency sound exerts mainly a uniform force at 90 degrees to the face of the wall. This is effective at compressing an air cavity behind a plate/membrane as used by some effective absorbers but would require stiff mounting points in order to create some bending in your CLD wall. This conflicts with the need to use soft mounting points to keep vibration out of the structure of the building.
As mentioned earlier, the substantial impedance mismatch between air and wall means only a small amount of the energy in a low frequency sound wave will be transferred to the wall with most being reflected. To increase the amount transferred the wall must move more (become "softer") in the direction of the force which is normal to the wall. How to do this while bending the wall?
So what will it do if anything? The difficulty with answering that is there is no specific link between low frequency sound impinging on the wall and the amount of bending that will then takes place in the CLD structure on which to base an analysis/estimate. Is the air cavity behind sealed or open? Is it damped? Are the panels hung from the ceiling or attached at multiple locations? Are the attachments soft or hard. Etc... We can be confident the effect will be at best small for the area involved and fairly confident it will have little effect on the low frequencies which are the challenge.
If it is to work it needs to be one with significant damping (see membrane/plate type absorbers) rather than just a mass and a spring. The problem with the OPs setup is getting a soft spring and getting damping in the direction of the applied force.
This is the principle of a resonator, if you want to maximise your energy absorbtion you need to find an appropriate fundamental resonancy mode and therfore generate parasitic soundwaves.
Do you know any other way perform it ?
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I don't understand your first sentence.
It is usually fairly straightforward to construct lumped models for how conventional sound absorbers work by identifying the relevant mass, spring and dampers. There may be an empirical factor or two to get quantitative values but how to tune them, make them more or less effective, narrow or wide peak can be explored via a simple model and, indeed, there are a variety of such calculators on the web. How to do this for the OPs proposal does not seem clear.
Oversimplified but :
If you employ a object that have the abilty to catch some very large amplitude low frequencies soundwaves, therefore it will start to move with a large amplitude at low frequencies and the damping of this object will induce a delay in this movement until all energy is absorbed by this damping.
This object will also act as a radiating surface during the decay, and therefore produce very large amplitude low frequencies soudwaves out of phase because of the distance between the soundwave source and the sound absorber.
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This looks like a common but incorrect view of how sound absorption works. The plate of a perfect absorber will follow the particle velocity of the incoming sound wave in exactly the same way as if it were air. This results in no reflected wave from the plate and all the travelling kinetic energy in the incoming wave being transferred to the plate. The plate in turn transfers this energy to the air trapped on the other side where it is hopefully turned into heat by being squeezed through small holes in the foam or whatever.
I suppose that you are talking about an "elementary material particle" when you write "particle", could you give me a real life device that can illustrate your conception of a low ferquency "absorber" ?
I used 1/2" drywall and Liquid Nails for Subfloors. It was inexpensive, but I know that glues does harden over time, so it's not ideal. The only material that I have found that does not harden is the 2K poly. Once cured it never seems to change. That's why I used it in my speaker cabinets.
Reading back to the OP, I do disagree with this post.
If the walls vibrate on well damped supports then this is the best method of LF damping, which in most rooms is sadly lacking. Acoustical treatments are only effective at HFs and this is generally not an issue if one has good loudspeakers with a high CD DI. I use very little acoustical treatment in my rooms, it's almost entirely structural. Almost no absorption at HFs at all, just CD and LF damping.
Particle velocity is the conventional term for the motion of the medium associated with a travelling wave.
This is an example where a diaphragm closely follows the particle velocity of the incoming sound wave with the result it absorbs the incoming wave and emits no sound to a much higher degree than an equivalent passive device like a plate/membrane absorber.
If the walls are basically rigid, which at LF they will be, and lightly suspended on well damped springs, like RC-1 channel, then the whole wall will move from a LF sound wave. It will follow neither the velocity or the pressure exactly but will depend on the mass and stiffness of the wall. In general a suspended wall on springs will lower the rooms modes and its motion will be a complex interaction with the rooms air - both absorbing and reflecting depending on specific details. These things are all discussed in Room Acoustics texts.
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Thanks Earl for setting me straight, and apologies if I put words in your mouth incorrectly.
I really like to add "elementary" and "material" at this word because it really help to understand that it is the minimum quantity of air that we should mechanically consider at the human scale and will not criticize the professionnal experts solutions in term of sound "absorption" but IMO it is a is a very complex field of technicity that require outstanding competences to be done right and it seems to be even more difficult to DIY.
I'm not sure if "complex" is the right word that describe the best the intellectual nightmare that is required to understand a multidimensional diffracted soundfield enough to be accurately treated. Moreover is a space consuming art in a fixed volume environment.
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^ Idealized, we want an infinitely thin and stiff structural material sandwiching an infinitely thin and lossy shear layer. Oh and a perfect bond between them all. The closer we can get the better, but keep in mind practicalities here without losing your mind.
Most importantly, we want to match stiffness of the structural materials surrounding the shear layer to maximize transfer.
.
Daniel,
I like your approach; its always good to have a clear idea of the "ideal" and then compromise as needed. Since I want to have a good "looking" room as well as a good "sounding" room, I will want to stick with drywall as the visible layer of the sandwich. However, I have no problem using a thinner, stiffer (and lighter) material for the other layer of the sandwich i.e. plywood. When you say it's important to match stiffness of the layers, I'm assuming you mean that the product of Young's Modulus (E) x Section Modulus (S) should remain fairly constant for each of the layers. In other words if I cut thickness of one layer in half I should make sure E increases by factor of 4?
That should result in an equivalent deflection for a given pressure.
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