ok i understand what you mean. most speakers , at least of the monople variety, try to preserve the frontal wave integrity by absorbing/redirecting/diffusing the rearwave, and reducing its influence to cause spurious cone motion not present in the signal.
absorption is one way and an accepted way, also as MJL said, (i think) wouldnt an infinitely stiff panel also exhibit infite reflection? Clearly then, it would seem that the stiffer the panel is, the more efficient the absorbent must also be for adequate performance.
Diffraction/diffusion would be a neat way of achieveing the same end, provided the wavelengths that were needed to be diffused where of manageable proportions. I have tried this with 1" deep by 3" wide serrations in a laminar style construction. It does diffuse reasonably, but i fear that only in a small BW, and perhaps not quite in the BW that wouldve been useful to me.
Taking 3" as the dimension and guessing its the half wavelength thats significant in diffusion(please correct me if im wrong) i get:
(340/2) / 0.075 = around 2.2kHz as the lowest frequency that could be diffused.
If im wrong and its the shortest dimension that is most important here, then the lowest frequency becomes 6.8k
again if im wrong and its the 1/4 wavelength thats significant in sound diffraction/diffusion, then the results are a litle more manageble at 1.1k or 3.4k depending on the most significant dimension.
Perhaps an error on my part, but unless the speakers are enormous, i would think it would be pretty hard to increase the serrations size significantly, and ultimately achieve diffraction close enough to diffusion in order to negate the rear wave influence.
Personally, as in most things, a compromise or composite of all these topologies is probably the best all rounder.
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Sound Transmission Loss
It seems that determining sound transmission losses for a panel is a bit compicated. The attached figure is from a textbook on Architectural Acoustics. It seems that different mechanisms determine how much sound is absorbed as the frequency of the sound changes. So what works for low frequencies does not work for high frequencies.
It seems that determining sound transmission losses for a panel is a bit compicated. The attached figure is from a textbook on Architectural Acoustics. It seems that different mechanisms determine how much sound is absorbed as the frequency of the sound changes. So what works for low frequencies does not work for high frequencies.
Attachments
Well, why would you use stuffing in the first place? What do ypu think stuffing does wiyh sound
Are you being vague here on purpose?
I had professors like that. Their attitude was : It took me 40 years to learn all this stuff so I'm not just going to transfer that knowledge to you!
Back to stuffing. As far as I know it has 2 effects.
1) Making an enclosure "seem" larger than it is due to changing the effective speed of sound inside the enclosure.
2) Absorb a portion of the high frequency sound energy in the enclsoure by converting the vibrations into heat.
No, it didn`t take me 40 years. About your effects; this are the common theory. My experince is that these simple solutions ruins dynamics. But I can only suggest, you`ll have to evaluate
Lets examine that. If we consider the loudspeaker + box assembly as a black box. If a surface of the black box is making sound then that surface has to be moving. If it is moving it is resonating.
dave
A good use of stuffing is to prevent sound waves from inside the cabinet to reach the backside of the speaker element. Use high density material around the edges where the pressures are highest and the sound wave velocities zero, and a lighter fill in the middle of the cabinet.
If reducing reflections back to the rear of the speaker element ruins dynamics, I'd like to hear an explanation to that.
Hi Dave,
Let's review what resonance is: "resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others" So it is a specific frequency, like 300hz for instance. If there is a significant amount of sound energy at lower frequencies, like 100hz the panel will still be excited, it will still transmit sound just not as much.
As you have demonstrated, it's not a good idea to have the resonant frequency of the panel low where the majority of the energy is in a loudspeaker. For example: If the resonant frequency of the panel is 100hz, there is plenty of energy to excite it and its output will be much greater because it is at resonance. By driving the panel resonant frequency up, it will not be as easily excited but the energy still exists at the lower frequencies and it will transmit through the panel, but at a lower level.
I think we've been through this before...
well i think were talking about resonance, in the term of an object(panel) critical resonance....rather than covering the excitation of sympathetic vibrations within said panel.
it HAS been covered before.
thus the flexibilty of material, damping losses within material, compliance of glued joints at edges of panels, density, hardness ALL play a role in the vibrations we see in loudspeaker panel. Thats my honest opinion.
the glue joint edge compliance i believe is better with MDF due to the close joining possible with the medium. PLY is definitely worse in this respect. How much worse is anyones guess.
damping is also higher in MDF: again debatable if this improves matters.
i believe that MDF is similar or slightly harder than ply, the density of which is not uniform, and neither is the hardness. higher density is MDF small dowfall. maybe ill use LDF instead and stick pins in it. poplar cored, birch face ply is far lighter than solid baltic birch. If the deficit in stiffness is not too great the lightness of this type would be better than full birch ply. much like bamboo ply would be....i would be using bamboo if i could actually get it here in the UK btw
stiffness of material is less an issue, that is if the compliance of glued joint is high than the panel compliance to loading. If this is the case, then plys benefits may be negated by the less accurate cutting and jointing abilities of the material. again debatable.
it HAS been covered before.
thus the flexibilty of material, damping losses within material, compliance of glued joints at edges of panels, density, hardness ALL play a role in the vibrations we see in loudspeaker panel. Thats my honest opinion.
the glue joint edge compliance i believe is better with MDF due to the close joining possible with the medium. PLY is definitely worse in this respect. How much worse is anyones guess.
damping is also higher in MDF: again debatable if this improves matters.
i believe that MDF is similar or slightly harder than ply, the density of which is not uniform, and neither is the hardness. higher density is MDF small dowfall. maybe ill use LDF instead and stick pins in it. poplar cored, birch face ply is far lighter than solid baltic birch. If the deficit in stiffness is not too great the lightness of this type would be better than full birch ply. much like bamboo ply would be....i would be using bamboo if i could actually get it here in the UK btw
stiffness of material is less an issue, that is if the compliance of glued joint is high than the panel compliance to loading. If this is the case, then plys benefits may be negated by the less accurate cutting and jointing abilities of the material. again debatable.
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I think this a misunderstanding of how the terms are used by different people.
Sound is the movement of compression and rarefaction waves through a medium. In air, the density of the air changes up and down ever so slightly as the sound waves travel through the air. I don't know if air has a resonant frequency. In a solid the same thing happens in that the density of the soild changes ever so slightly as sound moves (note : extreme simplification warning). All frequencies of sound will travel through a solid even though the soild has only one resonant frequency. For example a panel of drywall 16" wide, 96" tall and 0.5" thick has a resonant frequency of about 60 Hz. This panel of drywall transmit all audible frequencies to some extent and the molecules in the panel will vibrate for all frequencies while the panel remains stationary in that hte surface movement can not be measured. At resonance however the panel will have measurable deflection and will actually act a bit like a speaker.
lol are you saying that atoms in a solid are being compressed?
depending on structure i would have said that they are actually being displaced slightly, their atomic bonds thus being stretched allowing the material to bend. There may be a small space between adjacent atoms which IS compressed or rarified
in a hard solid such as diamond where there are (correct me if im wrong) 4 atomic bond to the neighbouring atoms in a tetrahedral crystalline structure, and as such diamond is almost perfectly incompressable or deformable as the bonds are very strong and take much energy to stretch them. In fact theyll break and shatter the crystal rather than allow much in the way of flexing.
From that conclusion it would seem to me that the ideal substance to make a box out of would be a cystalline solid element or compound...maybe thats why accuton use sapphire compound ceramics for the cones....thus unfortunately, wood of any type is inferior to a homogenous solid of equivalent stiffness....haha maybe ill get Rolls to make me some curved panels out of single cystal Ti-alloy
turbine blades.....
This obviously explains the greater speed of sound through a solid than that of a liquid or gas; obviously again depending on density and hardness of the substance as well.
I have a question:
does that then mean that materials with a greater solid sound transmission velocity, like titanium coned drivers verses paper ones....do they have increased opacity to soundwaves, decreased opacity, or neither?
curiousity will kill me
Im dying to know the answer guys...
depending on structure i would have said that they are actually being displaced slightly, their atomic bonds thus being stretched allowing the material to bend. There may be a small space between adjacent atoms which IS compressed or rarified
in a hard solid such as diamond where there are (correct me if im wrong) 4 atomic bond to the neighbouring atoms in a tetrahedral crystalline structure, and as such diamond is almost perfectly incompressable or deformable as the bonds are very strong and take much energy to stretch them. In fact theyll break and shatter the crystal rather than allow much in the way of flexing.
From that conclusion it would seem to me that the ideal substance to make a box out of would be a cystalline solid element or compound...maybe thats why accuton use sapphire compound ceramics for the cones....thus unfortunately, wood of any type is inferior to a homogenous solid of equivalent stiffness....haha maybe ill get Rolls to make me some curved panels out of single cystal Ti-alloy
turbine blades.....
This obviously explains the greater speed of sound through a solid than that of a liquid or gas; obviously again depending on density and hardness of the substance as well.
I have a question:
does that then mean that materials with a greater solid sound transmission velocity, like titanium coned drivers verses paper ones....do they have increased opacity to soundwaves, decreased opacity, or neither?
curiousity will kill me
Im dying to know the answer guys...
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not as much -- ie a well-damped low Q resonance.
A resoance has a Q, the resonance will have a peak at a specific frequency, but the resonance has a bandwidth.
dave
Very debatable. Based on some comments from Ron Clarke plywood should have better damping.
It would be nice to see some real data.
dave
I totally agree. There is not enough available data to be found and the reliability of some 'online' data is highly questionable. Im not sure that fricton losses are higher in ply than MDF though, but then thats assuming that it is the mechanism of damping that is most significant in either material. I still stick to my belief (until i can see data) that a homogenous material such as Nylon or a composite such as GRP SHOULD behave better than either MDF or ply, given sensible implementation.
Its a pity the nylon66 i was looking at is out of my price range
i can dream....
Dave I don't get you. A resonance is a single frequency - it's the frequency the panel "wants" to vibrate at.
You confuse "resonance" with "vibration" - you can call a resonance a vibration but you can't always call a vibration a resonance. A panel can vibrate at any frequency but will only resonate at one.
Logically you know that MDF is better damped than BB ply - it is less stiff. Deflection equals damping. Energy is dissipated.
In a panel who's ideal state is not moving, any movement can be described as a resonance, A low Q resonance can have a pretty large bandwidth,
dave
A thin sheet metal cabinet would deflect plenty and it would be much less stiff than MDF, yet I suspect you wouldn't find it damps very well...
a panel who's natural state is not moving, when excited is displaced. resonance is the peak displacement/amplitude against frequency.
vibration will occur at all frequencies, although it is a damped oscillation. eventually with increasing frequency a point where mass overcomes the acceleration of the vibration will be reached; except at resonance, i believe. sympathetic vibration. If the resonant frequency is pushed high enough, then the mass of the material, and internal losses of such, will limit the amplitude of the resonant vibration--this is where i concur with Dave
Where i dont concur, is that with its higher mass, this can just as easily be achieved with MDF, albeit with (i believe) superior damping as an added extra.
I agree that the less flexible media would be better damped, but only if the internal losses are higher also. I believe MDF is better damped as the many fibres would dissipate more energy as heat, than the plies of plywood compressing and tensioning. However, i dont think deflection = damping or we'd all be making cork boxes...
Steel has low damping internally. Also, a material with a very high tensile strength such as steel would preserve the kinetic energy of the oscillation, and not transmute it to another form such as heat. This is where i feel that ply may have a disadvantage over MDF, albeit to a much smaller degree than steel.
I would think common clay/ceramics would also be very rigid, as it is brittle and hard, but again i doubt it had good internal damping and would ring like an erm....dinner plate
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Exactly, Some materials have better internal damping.
I think that MDF, which is particles in a matrix is well damped for that reason.
Kind of like constrained layer damping.
Plywood is constrained layer, and stiffer than a solid, but the layers are very similar or identical.
I believe that the best constrained layer approach is to use layers of differing characteristics.
Often the strategy is to use a material that would doesn't have great internal damping, but is so stiff that the resonance or ringing is a high frequency that it is never excited. Various high end loudspeakers have cabinets of very thick aluminum for this reason I believe. Then you get no ringing and no damping which might "deaden" the sound..
Ply can be similar- you make the stiff plywood panels braced into small enough areas, and they don't ring and aren't overly damped either.
so I would claim that you can't just talk about a material, but but also consider the construction always..
I think that MDF, which is particles in a matrix is well damped for that reason.
Kind of like constrained layer damping.
Plywood is constrained layer, and stiffer than a solid, but the layers are very similar or identical.
I believe that the best constrained layer approach is to use layers of differing characteristics.
Often the strategy is to use a material that would doesn't have great internal damping, but is so stiff that the resonance or ringing is a high frequency that it is never excited. Various high end loudspeakers have cabinets of very thick aluminum for this reason I believe. Then you get no ringing and no damping which might "deaden" the sound..
Ply can be similar- you make the stiff plywood panels braced into small enough areas, and they don't ring and aren't overly damped either.
so I would claim that you can't just talk about a material, but but also consider the construction always..
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