AndrewT said:
A big difference. If a panel is stiff, it will resist deflection. If a panel is elastic it will return to its original state after deflection.
The two don't necessarily go together.
AndrewT said:
Plate glass is stiff, how elastic is it? They are related qualities, but now exclusively linked.
Hi Andrew hope this helps
I design snap-fits for the plastic enclosures my PCB's are put in and have just made the
connection between these and the phsics of panels, I use force deflection curves and stress strain curves
to picture a snap fit and how it will feel to the end user, so to help my simple brain
cope I use the following definitions. It also helps to look at some FDC and SS curves
as it helps picture what is happening and is often easier to understand than verbal descriptions.
Elasticity is the ability of a material to return to its origional shape after deflection,
it is the linear region of the stress/strain curve, after the linear region the material
enters the plastic region where it deforms and stays (yield piont) there due to changes in the material,
then it breaks.
Stifness is a materials ability to resist the deflection force, and is proportional to the
linear part of the force deflection curve (the steeper the FDC the stiffer the material).
So all materials have both properties (IMO) even plate glass, it just got a very small elastic region,
so there is some repricial relationship between the two.
I design snap-fits for the plastic enclosures my PCB's are put in and have just made the
connection between these and the phsics of panels, I use force deflection curves and stress strain curves
to picture a snap fit and how it will feel to the end user, so to help my simple brain
cope I use the following definitions. It also helps to look at some FDC and SS curves
as it helps picture what is happening and is often easier to understand than verbal descriptions.
Elasticity is the ability of a material to return to its origional shape after deflection,
it is the linear region of the stress/strain curve, after the linear region the material
enters the plastic region where it deforms and stays (yield piont) there due to changes in the material,
then it breaks.
Stifness is a materials ability to resist the deflection force, and is proportional to the
linear part of the force deflection curve (the steeper the FDC the stiffer the material).
So all materials have both properties (IMO) even plate glass, it just got a very small elastic region,
so there is some repricial relationship between the two.
I agree with Marce, but I still can’t figure out what is energy absorption related to wood. I believe that sound pressure can, and it does, bend the enclosure, but the enclosure bends back as the wave passes. This will be very unpleasant if this happens at the resonant frequency because it add distortion either in the form of a kink in the reproduction curve or because of the nonlinearity of the curve Marce was telling. But this resonance must be held out of the audio range by using braces or damping material. And MDF is less elastic than ply so why MDF should absorb more energy in the same conditions? I still feel that this is not the energy absorption we are talking. 
I think I'm getting my head slowly round the energey storage,
the kids have a ball filled with gel, when you press it it deforms almost instantly, but slowly goes back to its origional shape,
I think that is what is happening in MDF on a different scale, the problem arises then with repeated forces applied to the panel, the excess energy build up and releases slowly colouring the next bit of music, where as in ply it is dissipated more quickly (sharper peak), and is less audiable.
That is how I am starting to picture it, but I may be wrong.
the kids have a ball filled with gel, when you press it it deforms almost instantly, but slowly goes back to its origional shape,
I think that is what is happening in MDF on a different scale, the problem arises then with repeated forces applied to the panel, the excess energy build up and releases slowly colouring the next bit of music, where as in ply it is dissipated more quickly (sharper peak), and is less audiable.
That is how I am starting to picture it, but I may be wrong.
Baltic Birch is NOT that cheap in Canada.
Dave,
No Baltic Birch is NOT that cheap in Canada. I don't know what century you last bought a 5' x 5' sheet of Baltic Birch ply but you might be able to get a sheet of 3mm(1/8") for the $24 you mentioned at the beginning of this thread. 12mm is about $50 and that is not enough thickness for a cabinet wall. Don't you live out in the woods somewhere? How would you know?
Greg
Hi BTW. Back after a long hiatus. 🙂
preiter said:
Dave,
No Baltic Birch is NOT that cheap in Canada. I don't know what century you last bought a 5' x 5' sheet of Baltic Birch ply but you might be able to get a sheet of 3mm(1/8") for the $24 you mentioned at the beginning of this thread. 12mm is about $50 and that is not enough thickness for a cabinet wall. Don't you live out in the woods somewhere? How would you know?
Greg
Hi BTW. Back after a long hiatus. 🙂
marce said:
That's a big part of it -how quickly a panel releases the energy that has been transmitted into it, i.e. the decay time. The longer a panel 'stores' the energy / the longer it takes to release it, the more likely it is you'll run into problems (smearing in the time domain etc). Worse luck, although steady-state signals and / or tones are useful for measuring it in a limited sense, we don't generally listen to such things. Music by nature is constantly varying in pitch, amplitude, and just about every other way you can think of, and as such is particularly suspeptable to corruption.
I can vouch for the 'breathability' of MDF. When I worked at Community (Pro loudspeakers), MDF was used as the backing material on the big dual-table CNC routers. The panel itself was held down by a vacuum drawn by a 10HP continuous-duty vacuum pump -- through the MDF. However, I can't see how this slight breathability is a bad thing - a leaky joint in the box will cause far worse anomalies.
Where does the stored energy in MDF go? It gets turned into low-level heat, or is re-radiated. The randomly-oriented fibers of MDF, however, would seem to turn much more of the movement into heat than re-radiation.
I'm personally a proponent of MDF - it's what I've used to build everything (with the exception of my first two boxes, which were fir ply), and it's very machineable - my Porter Cable 890 router goes through it like butter.
Where does the stored energy in MDF go? It gets turned into low-level heat, or is re-radiated. The randomly-oriented fibers of MDF, however, would seem to turn much more of the movement into heat than re-radiation.
I'm personally a proponent of MDF - it's what I've used to build everything (with the exception of my first two boxes, which were fir ply), and it's very machineable - my Porter Cable 890 router goes through it like butter.
Uhm… we are talking of “sound waves”. Sound waves propagates through the air at 330m/sec (1.092 Feet per second) and through solid matter faster. Accordingly with this site, the speed of sound propagation is from 2 to 5 times faster in wood than in the air. So, may I ask where the delay comes from?
http://www.uk-piano.org/sound.html
I fell that we muddle with energy, forces, potential energy, kinetic energy, sound waves which incorporates all that forms of energy… I have this unpleasant feeling… do you?
http://www.uk-piano.org/sound.html
I fell that we muddle with energy, forces, potential energy, kinetic energy, sound waves which incorporates all that forms of energy… I have this unpleasant feeling… do you?
Not really. What I was refering to (having re-read it, I wasn't very clear -my apologies) wasn't the travelling of the energy through the panels, but how this energy excites the panels own natural resonant modes, and how long it takes for these to decay / be damped.
Unprimed fibreboard, such as MDF, is always permeable, that in itself is in most cases either not a problem, and in some cases a desired effect.
For example, if you use unprimed MDF for a midrange closed box design you will see an effect mimicking that of an acoustic vent, ie a lowering of the impedance peak at resonance frequency, and in most cases a better dynamics performance, especially in the higher mid range.
For bass and horn cabinets, permeability is not a desired effect as you do not want energy to disperse through the cabinet because at those wave lengths sound waves cancels eachother out resulting in a decreased dynamics performance. And especially in horn cabinets you do not want any interaction of the sound energy in different adjacent folds as this decreases both the effeciency and the accuracy of the horn design.
For example, if you use unprimed MDF for a midrange closed box design you will see an effect mimicking that of an acoustic vent, ie a lowering of the impedance peak at resonance frequency, and in most cases a better dynamics performance, especially in the higher mid range.
For bass and horn cabinets, permeability is not a desired effect as you do not want energy to disperse through the cabinet because at those wave lengths sound waves cancels eachother out resulting in a decreased dynamics performance. And especially in horn cabinets you do not want any interaction of the sound energy in different adjacent folds as this decreases both the effeciency and the accuracy of the horn design.
Won't the application of some hard lacquer on MDF get better characteristics regarding to energy absorption and flexibility ?
It will act as a sealant for very low frequencies and will reflect HFs. If done correctly, with one diluted layer and then full strength ones, we can get a gradual impedance change from the surface and in, and an abrupt impedance change at the lacquer-wood boundary.
Changing the lacquer characteristics (even using other sealants instead) could allow good control of surface reflectivity.
Other than that... has anyone listened to CNC layertoned BLHs, so as to give a different view of the "sound of MDF" ?
Where I live, cr*ppy ply (literal translation of local name: phenolic compensated panel) costs about double of MDF. Cabinet grade is non-existant... 🙁
Gastón
EDIT: Question mark in first sentence added...
It will act as a sealant for very low frequencies and will reflect HFs. If done correctly, with one diluted layer and then full strength ones, we can get a gradual impedance change from the surface and in, and an abrupt impedance change at the lacquer-wood boundary.
Changing the lacquer characteristics (even using other sealants instead) could allow good control of surface reflectivity.
Other than that... has anyone listened to CNC layertoned BLHs, so as to give a different view of the "sound of MDF" ?
Where I live, cr*ppy ply (literal translation of local name: phenolic compensated panel) costs about double of MDF. Cabinet grade is non-existant... 🙁
Gastón
EDIT: Question mark in first sentence added...
Re: Baltic Birch is NOT that cheap in Canada.
Hi Marc,
As plywood is more elastic than MDF, this could be a factor. I still say that in order to differentiate this audibly, you would need bionic hearing or a prime imagination.
Hi,
I could be wrong, but I think Dave buys enough of that stuff to know the price.
marce said:
Hi Marc,
As plywood is more elastic than MDF, this could be a factor. I still say that in order to differentiate this audibly, you would need bionic hearing or a prime imagination.
GringoAudio2 said:
Hi,
I could be wrong, but I think Dave buys enough of that stuff to know the price.
A 5x5 of BB costs us $20-26 per sheet depending on how thick (we use 12, 15, & 18). I think John was able to source it even cheaper. We are buying close to wholesale thou and buy enuff to make a difference. Our preferred thickness for most parts of a cabinet is the 12mm, but we have to be flexible,
dave
dave
I love Baltic Birch
You just know you are working with the best possible material when you work with BB. I love laminating 3 or 4 layers of the thin stuff but bending it into a curve and clamping it. I used to use it in the furniture industry to make curved seat pans and backrests. They then would have cushions and fabric applied. Voila, the basic offfice chair. I now use it to make skateboards for kids and want to make some big curved open baffles soon.
You just know you are working with the best possible material when you work with BB. I love laminating 3 or 4 layers of the thin stuff but bending it into a curve and clamping it. I used to use it in the furniture industry to make curved seat pans and backrests. They then would have cushions and fabric applied. Voila, the basic offfice chair. I now use it to make skateboards for kids and want to make some big curved open baffles soon.
On the topic of energy storage/transmission in speaker cabinets, the energy is transmitted to the cabinet in two main ways - sound pressure waves through the air and mechanical vibrations where the driver attaches to the cabinet. If the cabinet is not well damped it can also transfer that energy back to the driver. As was mentioned earlier, this all sounds much like a mass-spring problem and there are three types of mass-spring systems: underdamped, critically damped, and overdamped. For the case of loudspeakers it seems like we would want our drivers and cabinets to form a critically damped system so that any energy transmitted to the enclosure is dissipated as quickly as possible and without any ringing.
I'm with the folks that say cabinet design is somewhat more important than the materials used. I also feel that using more than one type of material (a lamination of mdf and plywood for example) is probably the best compromise in terms price/performance. One of these days I will get around to building some curved enclosures out of laminated layers of mdf and thin plywood...
I'm with the folks that say cabinet design is somewhat more important than the materials used. I also feel that using more than one type of material (a lamination of mdf and plywood for example) is probably the best compromise in terms price/performance. One of these days I will get around to building some curved enclosures out of laminated layers of mdf and thin plywood...
I am begining to think along the lines of BWRX, that a composite laminate of materials seems to be the way forward for future investigation. The only difference is possible the use of a plastic as part of the lamination. Modern plastic have a wide range of properties that we desire for a cabinet and are readily available in sheet form, can be machined relatively easily etc.
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