In number two above I am concerned about using the wavelength of sound in air, to correlate to the wavelength in a solid material. Maybe something was left out of the description of the formula?
And in the US mineral wool is frequently used as insulation. Sheeps wool is mostly used in clothing but can be found for use in speaker enclosures. I have never seen a test result indicating it to be significantly better than fiberglass. But consistency of packing density and indiviual fiber thickness and shape may be the largest cause of discrepencies. And I have never seen a test of high quality goose down, or thinsulate for that matter.
And in the US mineral wool is frequently used as insulation. Sheeps wool is mostly used in clothing but can be found for use in speaker enclosures. I have never seen a test result indicating it to be significantly better than fiberglass. But consistency of packing density and indiviual fiber thickness and shape may be the largest cause of discrepencies. And I have never seen a test of high quality goose down, or thinsulate for that matter.
There is actually literature that claims that test proved that wool is better than fibreglass or rockwool. It is up to you to trust that or not. I have made my own research on the topic (with my ears as well) and I only use sheep wool now.
As to the wavelength: The coincidence frequency is the frequency when the wavelength of two materials (in this case air and wall) are the same. Of course the velocity is different, but if you look at at waveray hitting a wall on an angle then you see that the wavelength changes but the frequency stays the same when a wave travels from one medium to another. This is the same with light and called refraction (prism etc). It is a law of physics and applies to all waves (Newton and Descartes found that out a couple of hundred years ago). I am teaching physics and you can read the details in any physics book so I do not want to dwell on it. Apart from this you can find these formulas in any detailed acoustic literature. Prof. Mueller, Prof. Heckl and Prof. Cremer were actually my Professors at University, so I am pretty sure that there is no flaw in the formula. Prof Cremer is actually one of the Godfathers of room acoustics and he was responsible for many concert halls in Europe.
The books I cited were: Heckl/Mueller: Taschenbuch der technischen Akustik (Berlin, Heidelberg, New York 1975) and Cremer, L: Theorie der Schalldaemmung duenner Waende bei schraegem Einfall (1942). I am afraid I do not know of the english titles.
As to the wavelength: The coincidence frequency is the frequency when the wavelength of two materials (in this case air and wall) are the same. Of course the velocity is different, but if you look at at waveray hitting a wall on an angle then you see that the wavelength changes but the frequency stays the same when a wave travels from one medium to another. This is the same with light and called refraction (prism etc). It is a law of physics and applies to all waves (Newton and Descartes found that out a couple of hundred years ago). I am teaching physics and you can read the details in any physics book so I do not want to dwell on it. Apart from this you can find these formulas in any detailed acoustic literature. Prof. Mueller, Prof. Heckl and Prof. Cremer were actually my Professors at University, so I am pretty sure that there is no flaw in the formula. Prof Cremer is actually one of the Godfathers of room acoustics and he was responsible for many concert halls in Europe.
The books I cited were: Heckl/Mueller: Taschenbuch der technischen Akustik (Berlin, Heidelberg, New York 1975) and Cremer, L: Theorie der Schalldaemmung duenner Waende bei schraegem Einfall (1942). I am afraid I do not know of the english titles.
Hi Kea,
this is most interesting. I've always been concerned in using rockwool or fiberglass for building devices a-la tube traps and so on...
Should you be willing to share or point to more theoretical details, I would be glad to follow them (hoping to remember my maths enough)
Kind regards,
Stefano
this is most interesting. I've always been concerned in using rockwool or fiberglass for building devices a-la tube traps and so on...
Should you be willing to share or point to more theoretical details, I would be glad to follow them (hoping to remember my maths enough)
Kind regards,
Stefano
"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."
This is the part that I question. And it may still be in the fact the full equation isn't posted in your example. The frequency in 34 cm of wood will not be the same as in 34 cm of air. The equation doesn't appear to be "balanced" in that regard. Regardless of the coincidence. It simply appears the standing waves are being calculated. But I admit not remembering that day of physics class long ago.
And any physical difference between long hair wool and the thousands or more other insulating materials can be tested, calculated, etc. I think the most successful tests results for thin materials to absorb sound used perforated metal to compress, in the test I remember, fiberglass. Other materials likely weren't considered because of economical and durability issues in industrial environments. The tests indicated fiberglass density / compression set, and variations in the perforated materials pore size, open area percentage, material thickness and density all had effects on the results. The net result is that the type of material, install method and amount of material will all affect the frequency at which it has the greatest results. And that one material may be superior in one install method at a particular density, etc. but not superior to another material installed with a different method or different density, etc.
This is the part that I question. And it may still be in the fact the full equation isn't posted in your example. The frequency in 34 cm of wood will not be the same as in 34 cm of air. The equation doesn't appear to be "balanced" in that regard. Regardless of the coincidence. It simply appears the standing waves are being calculated. But I admit not remembering that day of physics class long ago.
And any physical difference between long hair wool and the thousands or more other insulating materials can be tested, calculated, etc. I think the most successful tests results for thin materials to absorb sound used perforated metal to compress, in the test I remember, fiberglass. Other materials likely weren't considered because of economical and durability issues in industrial environments. The tests indicated fiberglass density / compression set, and variations in the perforated materials pore size, open area percentage, material thickness and density all had effects on the results. The net result is that the type of material, install method and amount of material will all affect the frequency at which it has the greatest results. And that one material may be superior in one install method at a particular density, etc. but not superior to another material installed with a different method or different density, etc.
It's been a long time since I read comparative stuffing tests, and I cannot recollect where; but wool was definitely different from rock wool, fibreglass and Acusta-Stuff. The reason is that wool is made up of overlapping circular "scales" with protruding edges that give it a much rougher surface than synthetic materials. As a result, it engages the air better than other materials, which accounts for its improved insulation as well as sonic properties. Acusta-stuff is finely crinkled, and it is an improvement over smooth materials, although less effective than wool.
Because it slows the speed of sound in a stuffed enclosure, the effect is as if there is a mild increase in enclosure size. Stuffing density is a factor.
I once designed a series of high quality inwall speakers, with full enclosures. For decent bass, (which turned out surprisingly well) the volume was a bit small, and the tendency to "organ pipe" had to be surpressed. Internal baffling helped with the latter, but wool made the design possible. Rock wool and Acusta-stuff were tried, but neither was as effective as wool. It also turned out that the grade of wool made a difference; the heavy wool known variously as rug, or red, or Navajo was noticeably better.
Drawbacks were that even though double washed and combed, it still had a residual smell, mats, and the occasional twig. Somewhat seasonal in availability. And of course something has to be done about insects.
(Fortunately a fine screen across the vents happened to work in our favor.) Cost was noticeable. And finally, you want to prevent settling over time.
Cabinet maker once stuffed rock wool (used in less expensive version) in place of wool; lost a third of an octave in the bass.
Because it slows the speed of sound in a stuffed enclosure, the effect is as if there is a mild increase in enclosure size. Stuffing density is a factor.
I once designed a series of high quality inwall speakers, with full enclosures. For decent bass, (which turned out surprisingly well) the volume was a bit small, and the tendency to "organ pipe" had to be surpressed. Internal baffling helped with the latter, but wool made the design possible. Rock wool and Acusta-stuff were tried, but neither was as effective as wool. It also turned out that the grade of wool made a difference; the heavy wool known variously as rug, or red, or Navajo was noticeably better.
Drawbacks were that even though double washed and combed, it still had a residual smell, mats, and the occasional twig. Somewhat seasonal in availability. And of course something has to be done about insects.
(Fortunately a fine screen across the vents happened to work in our favor.) Cost was noticeable. And finally, you want to prevent settling over time.
Cabinet maker once stuffed rock wool (used in less expensive version) in place of wool; lost a third of an octave in the bass.
I must say interesting replies concerning the wool. The book I read was from England but I cannot recall the title. I might find it if I have some time to search. The explanation with the rough surface of wool is very good imho.
I use woolbatts (from New Wool NZ) and they are already in sheets and treated for rodents and a kind of glue. So the batts are easy to attach to the walls and then you can make waves of the batts that reach into the middle of the cabinet. That way they will never sink down. I read that combed wool (untreated) should be better. I cannot confirm this but with that wool you get the problem that it sinks.
As to the coincidence frequency: There seems to be a mix up of 2 parts: the wave hits the board (on an angle) then part of the energy travels inside the board (along the board) and then part of this energy gets transmitted to the outside air, again on an angle.
I think a drawing would make this easier.
I will try to find one from physics and include it.
But if you look under Snell's Law in a physics book, you get the details.
I use woolbatts (from New Wool NZ) and they are already in sheets and treated for rodents and a kind of glue. So the batts are easy to attach to the walls and then you can make waves of the batts that reach into the middle of the cabinet. That way they will never sink down. I read that combed wool (untreated) should be better. I cannot confirm this but with that wool you get the problem that it sinks.
As to the coincidence frequency: There seems to be a mix up of 2 parts: the wave hits the board (on an angle) then part of the energy travels inside the board (along the board) and then part of this energy gets transmitted to the outside air, again on an angle.
I think a drawing would make this easier.
I will try to find one from physics and include it.
But if you look under Snell's Law in a physics book, you get the details.
An externally hosted image should be here but it was not working when we last tested it.
http://www.gcsescience.com/a/pwav35.htm
you can see when the wave travels from one medium to another with a different velocity the wavelength changes but the frequency stays the same. The wave gets refracted as well (because of the angle you get different wavelength) The same happens with sound.
Now the other part I mentioned was baffles. Now we are only looking at the wavelength of the wave in the wall (forget about all the other parts now). At this coincidence frequency there is a certain wavelength in the wall. If this wavelength is big in comparison to the dimensions of the wall, then the dip in the graph at that coincidence frequency will be less severe. Therefore the wall should be baffled.
Sorry I am tired. Maybe I can give a better explanation next time. Cheers
Now the other part I mentioned was baffles. Now we are only looking at the wavelength of the wave in the wall (forget about all the other parts now). At this coincidence frequency there is a certain wavelength in the wall. If this wavelength is big in comparison to the dimensions of the wall, then the dip in the graph at that coincidence frequency will be less severe. Therefore the wall should be baffled.
Sorry I am tired. Maybe I can give a better explanation next time. Cheers
density of materials
My little handbook of everything (Pocket Ref, Sequoia Publishing), gives specific gravity of wool as 1.31 and 82 lbs/cu ft or 1300 kgs/cu m.
Dry sand sg 1.60
epoxy 1.26
cast polyester 1.25
PVC 1.30
solid carbon 2.15
glass, window 2.58
granite 2.69
marble 2.56
basalt 3.01
douglas fir .53
oak, live,dry.95
oak, red .71
plywood .62 (I'd guess, softwood)
concrete asphaltic 2.24
concrete gravel 2.24
concrete lightweight wth expanded clay aggregate 1.09
Stranded material, (wool, glass, mineral) used as sound deadening material would depend for its effectiveness on density, on how closely packed the the strands are, how rough the surfaces are, and whether it's are actually attached to a solid wall. Wool fibres are rough, glass and mineral (basalt?) fibres seem smooth and polyester fibres seem to vary in roughness depending on how they're manufactured.
Modern building techniques depend for their strength, lightness and vibration transmission characteristics (sound deadening) on sandwiching light materials - foam, balsa, various honeycombs - between heavy ones - concrete, fibreglass, wood, metal. The basic structural model is, I believe, the I-beam, but the skinny vertical bit is replaced by lightweight material which fills up the whole area between the top and bottom of the 'I'.
If the glue attaching the sandwiched layers is flexible, then the panel will not only be rigid but will have the advantage of constrained layer damping.
My little handbook of everything (Pocket Ref, Sequoia Publishing), gives specific gravity of wool as 1.31 and 82 lbs/cu ft or 1300 kgs/cu m.
Dry sand sg 1.60
epoxy 1.26
cast polyester 1.25
PVC 1.30
solid carbon 2.15
glass, window 2.58
granite 2.69
marble 2.56
basalt 3.01
douglas fir .53
oak, live,dry.95
oak, red .71
plywood .62 (I'd guess, softwood)
concrete asphaltic 2.24
concrete gravel 2.24
concrete lightweight wth expanded clay aggregate 1.09
Stranded material, (wool, glass, mineral) used as sound deadening material would depend for its effectiveness on density, on how closely packed the the strands are, how rough the surfaces are, and whether it's are actually attached to a solid wall. Wool fibres are rough, glass and mineral (basalt?) fibres seem smooth and polyester fibres seem to vary in roughness depending on how they're manufactured.
Modern building techniques depend for their strength, lightness and vibration transmission characteristics (sound deadening) on sandwiching light materials - foam, balsa, various honeycombs - between heavy ones - concrete, fibreglass, wood, metal. The basic structural model is, I believe, the I-beam, but the skinny vertical bit is replaced by lightweight material which fills up the whole area between the top and bottom of the 'I'.
If the glue attaching the sandwiched layers is flexible, then the panel will not only be rigid but will have the advantage of constrained layer damping.
Kea,
Regardless of me thinking there is something missing from that formula I think I know what you were attempting to convey. Most of the time when discussions become technical it's best to stick to the basics instead of trying to provide complex answers. It's too easy to leave and important detail out. If more technical details are needed the person who needs them can use a search engine or a library. I don't have too many formula's memorized that I don't use regularly anymore - It's been more than a decade since I took 15 credits of calc, physics etc. I can simply go to my bookshelf and skim to the info I need, when needed.
- James
Regardless of me thinking there is something missing from that formula I think I know what you were attempting to convey. Most of the time when discussions become technical it's best to stick to the basics instead of trying to provide complex answers. It's too easy to leave and important detail out. If more technical details are needed the person who needs them can use a search engine or a library. I don't have too many formula's memorized that I don't use regularly anymore - It's been more than a decade since I took 15 credits of calc, physics etc. I can simply go to my bookshelf and skim to the info I need, when needed.
- James
Thanks a lot for the feedback. I am not quite sure what you meant by you can look up the details in books. If you do not have acoustic literature you won't find much. Apart from that a lot of people have hardly any books and do not go to the library (a bit sad really but that's how it is nowaddays), however some people search on the internet and that provides a lot of sources.
I often get critised for stating technical details and backing them up with scientific facts, that is why I got quite theoretical. (Otherwise I am sure some people would have said again "ok that is your opion but we have another...")
I must admit that my explanations were not really that clear (shame on me). So I found some websites that explain quite well the physical background of the coincidence frequency and the Weighted Sound Reduction Index (in North America often called STC) and they also have some interesting tables. (This is actually how it is called in America, not weighted sound transmission factor, sorry)
http://thediagram.com/5_6/coincidence.html
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd239_e.html
http://personal.cityu.edu.hk/~bsapplec/transmis2.htm
I think it is easier for me to find the sites because I know what I have to look for. So I hope this helps to clear things up a bit.
I often get critised for stating technical details and backing them up with scientific facts, that is why I got quite theoretical. (Otherwise I am sure some people would have said again "ok that is your opion but we have another...")
I must admit that my explanations were not really that clear (shame on me). So I found some websites that explain quite well the physical background of the coincidence frequency and the Weighted Sound Reduction Index (in North America often called STC) and they also have some interesting tables. (This is actually how it is called in America, not weighted sound transmission factor, sorry)
http://thediagram.com/5_6/coincidence.html
http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd239_e.html
http://personal.cityu.edu.hk/~bsapplec/transmis2.htm
I think it is easier for me to find the sites because I know what I have to look for. So I hope this helps to clear things up a bit.
Thanks for those links. I hope I have time to look through them. And also for the clarification of STC. I recognize STC, but not the other terminology you were using. I can understand either, but one is more familiar.
I have a fair collection of reference books. In American college, books are bought at the beginning of a semester and most are sold back at the end of the semester, or pair of semesters. But a lot of the time a book may only be used 2 - 4 semesters and the bookstores won't buy them back. If that was the case, I probably still have those books. Calc and Physics I still have. Of course I only kept interesting books - LOL.
I also have numerous audio resource books. Speaker building books, acoustics, etc.
I have a fair collection of reference books. In American college, books are bought at the beginning of a semester and most are sold back at the end of the semester, or pair of semesters. But a lot of the time a book may only be used 2 - 4 semesters and the bookstores won't buy them back. If that was the case, I probably still have those books. Calc and Physics I still have. Of course I only kept interesting books - LOL.
I also have numerous audio resource books. Speaker building books, acoustics, etc.
100% Wool felt always worked well for me. The stuff I had was made for hospital use to prevent bed sores. Not cheap.
Read somewhere that sandwiches of different materials work well to dampen/attenuate sound because the speed of sound changes at each layer of different stuff. Each speed change means a loss of energy.
True?
Read somewhere that sandwiches of different materials work well to dampen/attenuate sound because the speed of sound changes at each layer of different stuff. Each speed change means a loss of energy.
True?
Not necessarily. If you get resonances between the outer layers of a sandwich wall then you can actually worsen the whole case.
If you use sand then you are pretty much on the safe side because of the weight and the loss factor, but light materials like foam etc can backfire (ie give you resonances in a crucial frequency range which get transmitted).
My opinion is use good solid materials like MDF or Plywood (only the muliplex grade) in a appropriate thickness (like 20mm for small cabinets and thicker for large cabinets) and you are pretty much on the safe side.
Rather look at other things to improve the sound that are much more important (port, damping material like wool etc, crossover parts and most importantly the drivers!)
If you use sand then you are pretty much on the safe side because of the weight and the loss factor, but light materials like foam etc can backfire (ie give you resonances in a crucial frequency range which get transmitted).
My opinion is use good solid materials like MDF or Plywood (only the muliplex grade) in a appropriate thickness (like 20mm for small cabinets and thicker for large cabinets) and you are pretty much on the safe side.
Rather look at other things to improve the sound that are much more important (port, damping material like wool etc, crossover parts and most importantly the drivers!)
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