By all means, the light, stiff, damped idea works too. It was what the Celestions did. It's not normally used, as good ol' MDF works so well, is relatively inexpensive, and there are so many cabinet shops that can work it. There are many ways to the desired ends, as is often the case in this hobby of ours. The light stiff material, Hexcel for example, will need a separate dampening material of some sort, well attached, and this increases the cost and needed expertise; it seems that this is where the DIY'ers resources of time and patience might be used to come up with some interesting new approaches. (See many possibilities above. 
)
)Stiffness and mass both affect resonance differently. Stiffer raises the frequency, havier lowers the frequency. MDF's porosity is known to store energy, releasing it later. I think the most successful combo's have been with MDF and BB with a decoupling layer in the middle. I have a bit of something in stock for just this purpose. Without getting into specifics it is a double sided rubber adhesive tape. It is significantly softer than vinyl dampening sheets. But the vinyl sheets work well too. They can be found at Parts Express, and dozens of other places. I've not yet found one with a good enough PSA. Single thin layers adhere well, but heavier and thicker layers do not. But the application of multiple layers is normally for car audio.
- James
- James
I have heard the same thing actually.
According to someone I know who took a stethoscope to a PA bass horn, one made of MDF and another made of Ply. The MDF was not as loud but it droned on for long after the stimulation had stopped. The Ply was louder but stopped when the stimulation did. The Ply also had a nicer harmonic where as MDF was a 'drone' sound. Ply was preferable.
According to someone I know who took a stethoscope to a PA bass horn, one made of MDF and another made of Ply. The MDF was not as loud but it droned on for long after the stimulation had stopped. The Ply was louder but stopped when the stimulation did. The Ply also had a nicer harmonic where as MDF was a 'drone' sound. Ply was preferable.
The pores are a part of the MDF's solid microstructure as you put it.
But regardless it does store energy. The analogy, metaphor, or whatever the correct grammatical term that I was searching for is that it is similar to a sponge.
http://www.audioxpress.com/magsdirx/voxcoil/index.htm
But regardless it does store energy. The analogy, metaphor, or whatever the correct grammatical term that I was searching for is that it is similar to a sponge.
http://www.audioxpress.com/magsdirx/voxcoil/index.htm
So you believe the pores store energy like a sponge soaks up water?
Not likely. The only way I can think of that they could store energy is in compression energy of the tiny volumes of air, which would be nothing compared to the vibrational energy of the much more massive solids.
If you have anything to support your idea besides speculation, please say what it is.
Thanks
Not likely. The only way I can think of that they could store energy is in compression energy of the tiny volumes of air, which would be nothing compared to the vibrational energy of the much more massive solids.
If you have anything to support your idea besides speculation, please say what it is.
Thanks
Yeah, it's like a bass sponge..... It's capillary action, see.... It absorbs those bass waves and sends them back at you when you squeeze it.
Honestly, arguing the finer points of physics with non-scientists or non-engineers is going to be really frustrating. One must always consider the source.
Honestly, arguing the finer points of physics with non-scientists or non-engineers is going to be really frustrating. One must always consider the source.
http://search.lycos.co.uk/searchFra...ng%20with%20hexlite&cat=loc&lyca=AJ&enc=utf-8
I used to have a book written by a guy who made hill climbing racing cars out of aerolam (which is now known as its branded name of hexlite; hexlite being a generic for all manner of honeycomb composites). As I recall, he used to make folds in it by cutting a "V" section, then bonding with large quantities of epoxy. Other than that, you can mitre it but the honeycomb will need a fair amount of resin to fill the (dare I say it??) pores to make a rigid bond. Perhaps the addition of an angled cover either inside or/and out of the join would be of benefit.
The above link should take you to the hexlite spec-sheet which includes aluminium and synthetics. Hope it helps someone.
I used to have a book written by a guy who made hill climbing racing cars out of aerolam (which is now known as its branded name of hexlite; hexlite being a generic for all manner of honeycomb composites). As I recall, he used to make folds in it by cutting a "V" section, then bonding with large quantities of epoxy. Other than that, you can mitre it but the honeycomb will need a fair amount of resin to fill the (dare I say it??) pores to make a rigid bond. Perhaps the addition of an angled cover either inside or/and out of the join would be of benefit.
The above link should take you to the hexlite spec-sheet which includes aluminium and synthetics. Hope it helps someone.
It seems to me the perfect all around speaker panel would be the aluminum honeycomb glass/carbon fiber epoxy skin.
At least for the bass and midrange. For the tweeter high mass and heavy/dense may work better.
If you could manufacture the honeycomb panel with dry sand filling the inner holes the panel would have a high level of passive dampening combined with extreme stiffness. This would be very good indeed.
In fact variations of this panel would work well for different purposes.
Say for the tweeter the honeycomb holes are filled with a composite of crushed rock and epoxy resin to create a heavy and dense yet very rigid panel.
For the midrange the honeycomb holes are filled with loose sand to passively damp the panel resonances.
For the bass module were stiffness is the most important you could leave the honeycomb holes empty or filled with a lightweight structural foam that is mostly air and would stiffen the panel even more by reinforcing the honeycomb walls without adding much mass.
At least for the bass and midrange. For the tweeter high mass and heavy/dense may work better.
If you could manufacture the honeycomb panel with dry sand filling the inner holes the panel would have a high level of passive dampening combined with extreme stiffness. This would be very good indeed.
In fact variations of this panel would work well for different purposes.
Say for the tweeter the honeycomb holes are filled with a composite of crushed rock and epoxy resin to create a heavy and dense yet very rigid panel.
For the midrange the honeycomb holes are filled with loose sand to passively damp the panel resonances.
For the bass module were stiffness is the most important you could leave the honeycomb holes empty or filled with a lightweight structural foam that is mostly air and would stiffen the panel even more by reinforcing the honeycomb walls without adding much mass.
Jezz, I think that seems extremely interesting. I am currently designing a no-compromise speaker system and the idea of a bass enclosure (rest is OB) that has a fundamental resonance above the pass-band of the driver seems so... well... sensible!
Any idea how you work with this stuff? Cut it and fix it etc..? Do you have to buy in large amounts?
Any idea how you work with this stuff? Cut it and fix it etc..? Do you have to buy in large amounts?
Honeycomb composite is a neat material. 10+ years ago I thought about building speakers with it. Turns out Celestion did it before that. The stuff is very stiff and very light, but difficult to join. Using glue as a structural element even in a low stress application is a mistake, you need a better mechanical join. Reinforce the joints with fiberglass tape and epoxy, and/or use some sort of extrusion or gusset to glue or fasten them to.
You could build up the cabinet yourself by making an internal mold out of carvable foam to represent the empty space of the internal speaker cabinet.
Then skin it with fiberglass and epoxy resin about a quarter inch thick. Then cut up hundreds of small aluminum tubing segments into whatever length you want for the wall thickness you want.
Then bond them to the first inner skin with a fresh coat of epoxy resin. Lining them up in rows to make a kind of honeycome like inner structure. You could use different sized tubes for differing characteristics. You could also have control of the honeycome like wall thickness and could very the wall thickness in different areas of the cabinet.
Then cut out the inner foam core. You will have to leave the back baffle open or speaker cutout's large enough to break out the foam.
Then you could pour firm expanding foam onto the panel and after it dries cut it down flat to put on the outer skin.
You could also use other fill materials like sand and sand filled epoxy for high mass depending on what part of the cabinet it is.
This would be an awesome cabinet better than a Rockport cabinet but very time consuming to make.
The joints would be very strong because they would be part of the wall structure and could be reinforced in any manner that you wanted with fiberglass or carbon fiber Aluminum struts or hardwood.
Then skin it with fiberglass and epoxy resin about a quarter inch thick. Then cut up hundreds of small aluminum tubing segments into whatever length you want for the wall thickness you want.
Then bond them to the first inner skin with a fresh coat of epoxy resin. Lining them up in rows to make a kind of honeycome like inner structure. You could use different sized tubes for differing characteristics. You could also have control of the honeycome like wall thickness and could very the wall thickness in different areas of the cabinet.
Then cut out the inner foam core. You will have to leave the back baffle open or speaker cutout's large enough to break out the foam.
Then you could pour firm expanding foam onto the panel and after it dries cut it down flat to put on the outer skin.
You could also use other fill materials like sand and sand filled epoxy for high mass depending on what part of the cabinet it is.
This would be an awesome cabinet better than a Rockport cabinet but very time consuming to make.
The joints would be very strong because they would be part of the wall structure and could be reinforced in any manner that you wanted with fiberglass or carbon fiber Aluminum struts or hardwood.
Materials suppliers that may be of interest:
Aerospace Composites
and The Composite Store .
I've dealt with both in the past, and both are fine.
At the ACP site download the core materials pdf. At CST it's on page 4 of the cores section.
Aerospace Composites
and The Composite Store .
I've dealt with both in the past, and both are fine.
At the ACP site download the core materials pdf. At CST it's on page 4 of the cores section.
Sorry I have been away for a while and could not reply.
So now (a bit late but anyway) some more explanations to the questions that were asked about my submission:
The weight per square metre [m“] of a wall with 10kg/m2 for example has around 30dB weighed sound-transmission factor for most materials (with few exceptions). Interestingly a wall with 50 kg/m2 has insignificantly more weighed sound-transmission factor and a wall with 100 kg/m2 has around 36dB. So by increasing the weight per area by a factor of 10 the weighed sound-transmission factor increases only by 6dB. Whereas a wall with 1 kg/m2 in contrast has only 20dB weighed sound-transmission factor, which is 10dB less than 30dB of the first example.
That means that when increasing the mass over 10kg/m2 the gain is relatively small.
This is when sandwich walls come in as a good alternative.
It is important to state that the weighed sound-transmission factor is dependent on the frequency and therefore differs significantly over the frequencyband.
There are 3 main frequency ranges of importance:
1.) low frequencies (bass) where the weighed sound-transmission factor is mainly depending on the weight per area [m“] (see examples above). The formula to calculate the weighed sound-transmission factor is according to Heckl/Mueller (P.436):
R = (20*lg{pi*f*m''/ (p* c)} - 3) dB
with p(roh) = densitiy of the air and c = speed of sound in air
2.) A second frequency range where the velocity of sound in the wall is similar to the speed of sound in air and the weighed sound-transmission factor is dramatically reduced (resonance). This frequency can be calculated according to L.Cremer (p.81-104):
fg= c2/ (2pi) *sqrt(m''/B)
or fg= 640000* sqr(P/E) / h
with P (roh) = density of the wall, E = E-module of the wall and h the thickness and B the stiffness of the wall.
This frequency would be around 1000Hz for a plywood with 20mm thickness (if you look at the formula above you see that doubling the wall thickness will half this frequency to 500Hz) This is a critical frequency band since the ear is very sensitive around this frequency. Using 20mm of hard fibreboard will bring the frequency to 2000Hz (still critical frequencyband) and 20mm of heavy concrete will produce a frequency of 700Hz. Using 10cm of concrete would result in a frequency of 150Hz. This is an area where the ear is not so sensitive but where most of the fundamental frequencies in music are and therefore a lot of energy. Making the concrete wall even thicker brings the coincidence frequency into the bass range and can results in a ringing and booming bass if the cabinet walls are rather large! (Have you noticed in concrete buildings with thick walls how well the bass of the neighbours music can be heard in the flat next door?)
The “dip” in the weighed sound-transmission factor is more or less dramatic depending on the ratio of the dimensions of the wall to the wavelength of the coincidence frequency,
Or length/lamda(g) should be small. Example for a coincidence frequency of 1000Hz the wavelength would be 0.34m, therefore the wall dimensions (in any direction) should be less than 340mm. To be on the save side this should be 170mm and less. This can be achieved by bracing, which effectively decouples on part of the wall from another. So the bracing should be in distances of 170mm in this case of 20mm thick plywood.
3.) The third (and highest) frequencyband is above the coincidence frequency and can be calculated according to Heckl/Mueller (p.438):
R= 20*log (pi*f*m''/{p*c}) - 10 lg {1/(2*n) *sqrt(fg/f)}
With n = loss factor
This means that the weighed sound-transmission factor will increase by 8dB per octave (25dB per decade); i.e. the higher the frequency the less sound transmission.
By the way it has to be stressed that the less sound transmission through the wall the more reflection inside and therefore more internal resonances! These resonances however can be effectively dampened by absorption material such as wool in the centre of the speaker cabinet (not the walls!)
It is also important to note that these calculations apply to the sound in air inside the cabinet that are converted into vibrations in the wall and then converted back into sound on the outer side of the cabinet into the room. There is another form of transmission however where the vibrations of the chassis frame transmit into the cabinet and then into the air. This is a different condition and has to be calculated differently. It is important therefore to acoustically disconnect the driver from the cabinet by soft rubber like materials.
Conclusion: Cabinet walls should have a minimum weight per area of 10kg/m2 to increase the weighed sound-transmission factor but at the same time be stiff.
The material should also have a reasonably high loss factor (not the case in glass for example).
Large walls can be decoupled by bracing to decrease the dip at the coincidence frequency.
As to the sandwhich cabinets: sand as a filler is a good idea because it has a high loss factor and a high weight. As an easier (lighter) alternative you could use rubber granulate or if you want even lighter: wool. Sheep wool is much better (on the inside of the cabinet too) than polyester wool or glassfibre (pink batts) and easily available in New Zealand (as heat-insulation material). Should be available in other countries too though.
All in all please keep in mind that the most important part is still the driver (mainly the cone)! A crappy driver is still no use in a perfect cabinet, but an excellent driver in a mediocre cabinet can sound extremely well! So I would not spend a fortune of money (or time) on a 100kg cabinet with $20 drivers from east asia. This is of course no news but some people seem to forget this nevertheless. (when I was 16 I did that as well and filled cabinet walls with tons of sand!)
If still not clear I am happy to explain in more detail.
So now (a bit late but anyway) some more explanations to the questions that were asked about my submission:
The weight per square metre [m“] of a wall with 10kg/m2 for example has around 30dB weighed sound-transmission factor for most materials (with few exceptions). Interestingly a wall with 50 kg/m2 has insignificantly more weighed sound-transmission factor and a wall with 100 kg/m2 has around 36dB. So by increasing the weight per area by a factor of 10 the weighed sound-transmission factor increases only by 6dB. Whereas a wall with 1 kg/m2 in contrast has only 20dB weighed sound-transmission factor, which is 10dB less than 30dB of the first example.
That means that when increasing the mass over 10kg/m2 the gain is relatively small.
This is when sandwich walls come in as a good alternative.
It is important to state that the weighed sound-transmission factor is dependent on the frequency and therefore differs significantly over the frequencyband.
There are 3 main frequency ranges of importance:
1.) low frequencies (bass) where the weighed sound-transmission factor is mainly depending on the weight per area [m“] (see examples above). The formula to calculate the weighed sound-transmission factor is according to Heckl/Mueller (P.436):
R = (20*lg{pi*f*m''/ (p* c)} - 3) dB
with p(roh) = densitiy of the air and c = speed of sound in air
2.) A second frequency range where the velocity of sound in the wall is similar to the speed of sound in air and the weighed sound-transmission factor is dramatically reduced (resonance). This frequency can be calculated according to L.Cremer (p.81-104):
fg= c2/ (2pi) *sqrt(m''/B)
or fg= 640000* sqr(P/E) / h
with P (roh) = density of the wall, E = E-module of the wall and h the thickness and B the stiffness of the wall.
This frequency would be around 1000Hz for a plywood with 20mm thickness (if you look at the formula above you see that doubling the wall thickness will half this frequency to 500Hz) This is a critical frequency band since the ear is very sensitive around this frequency. Using 20mm of hard fibreboard will bring the frequency to 2000Hz (still critical frequencyband) and 20mm of heavy concrete will produce a frequency of 700Hz. Using 10cm of concrete would result in a frequency of 150Hz. This is an area where the ear is not so sensitive but where most of the fundamental frequencies in music are and therefore a lot of energy. Making the concrete wall even thicker brings the coincidence frequency into the bass range and can results in a ringing and booming bass if the cabinet walls are rather large! (Have you noticed in concrete buildings with thick walls how well the bass of the neighbours music can be heard in the flat next door?)
The “dip” in the weighed sound-transmission factor is more or less dramatic depending on the ratio of the dimensions of the wall to the wavelength of the coincidence frequency,
Or length/lamda(g) should be small. Example for a coincidence frequency of 1000Hz the wavelength would be 0.34m, therefore the wall dimensions (in any direction) should be less than 340mm. To be on the save side this should be 170mm and less. This can be achieved by bracing, which effectively decouples on part of the wall from another. So the bracing should be in distances of 170mm in this case of 20mm thick plywood.
3.) The third (and highest) frequencyband is above the coincidence frequency and can be calculated according to Heckl/Mueller (p.438):
R= 20*log (pi*f*m''/{p*c}) - 10 lg {1/(2*n) *sqrt(fg/f)}
With n = loss factor
This means that the weighed sound-transmission factor will increase by 8dB per octave (25dB per decade); i.e. the higher the frequency the less sound transmission.
By the way it has to be stressed that the less sound transmission through the wall the more reflection inside and therefore more internal resonances! These resonances however can be effectively dampened by absorption material such as wool in the centre of the speaker cabinet (not the walls!)
It is also important to note that these calculations apply to the sound in air inside the cabinet that are converted into vibrations in the wall and then converted back into sound on the outer side of the cabinet into the room. There is another form of transmission however where the vibrations of the chassis frame transmit into the cabinet and then into the air. This is a different condition and has to be calculated differently. It is important therefore to acoustically disconnect the driver from the cabinet by soft rubber like materials.
Conclusion: Cabinet walls should have a minimum weight per area of 10kg/m2 to increase the weighed sound-transmission factor but at the same time be stiff.
The material should also have a reasonably high loss factor (not the case in glass for example).
Large walls can be decoupled by bracing to decrease the dip at the coincidence frequency.
As to the sandwhich cabinets: sand as a filler is a good idea because it has a high loss factor and a high weight. As an easier (lighter) alternative you could use rubber granulate or if you want even lighter: wool. Sheep wool is much better (on the inside of the cabinet too) than polyester wool or glassfibre (pink batts) and easily available in New Zealand (as heat-insulation material). Should be available in other countries too though.
All in all please keep in mind that the most important part is still the driver (mainly the cone)! A crappy driver is still no use in a perfect cabinet, but an excellent driver in a mediocre cabinet can sound extremely well! So I would not spend a fortune of money (or time) on a 100kg cabinet with $20 drivers from east asia. This is of course no news but some people seem to forget this nevertheless. (when I was 16 I did that as well and filled cabinet walls with tons of sand!)
If still not clear I am happy to explain in more detail.
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