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Converting Pressure into Heat

  1. Mar 31, 2010 #1
    I have an argument that I would like to have settled. In another forum, the original poster asked a general question as to why there was what appeared to be cotton stuffing inside his speaker box. Here are some excerpts that were posted in response by the person I'm in disagreeance with:

    “It is MOST often referenced to as Poly-fill. It is NOT cotton. Cotton would provide the exact OPPOSITE of what (and why) you are wanting to "fill" your box.”

    “As a Mater of fact the ONLY thing adding a loose fibrous material does is convert the "pressure" to heat.”

    “As far as the quantity to use... 1 to 1.5 lbs of polyfill per cubic foot volume of the enclosure. It is NOT an exact art, but the benefit is about 25-30% increase in perceived volume.”

    So here we have three statements that to me appear to be somewhat absurd; First of all that although cotton filler is also a loose fibrous material it would have properties contrary to this polyester filler. Secondly that it is able to convert air pressure into heat. And third that by reducing the air volume by filling it with a denser material that you are in fact increasing the "perceived" volume.

    Just so you understand more about the necessity for a certain amount of cubic volume for a speaker box all you have to do is understand that the combined surface area of the speaker and throw (which is the distance it is capable of moving in and out) displace a certain amount of air space and that this creates a certain amount of pressure that causes the speakers itself to have to do more work in order to function the smaller the enclosure. I believe most 12” subwoofer manufactures suggest about 1’-1.5’ cubic volume. People believe you should use polyfill in situations where the cubic volume is less that this suggested amount and that it will make up for it.

    I’ll briefly explain my perspective. It would appear that this polyfill is actually just a dampening material. She is saying that it is not and that it only converts air pressure into heat. I’m not denying that in dampening that there is some heat, although I’m quite sure that if I were to attempt to measure it that it would be undetectable.

    I hope this is the right forum, and that I have included enough information for this to be settled.
     
  2. jcsd
  3. Mar 31, 2010 #2
    I'm afraid there is not enough information in your post to answer your question.

    It really depends upon the type and purpose of the box you are stuffing and also the position of the stuffing in the box.

    A loudspeaker cone produces two waves of acoustic energy. A forward travelling one and a rearward travelling one.
    Unfortunately these two waves are out of phase and so would cancel (destructively interfere) if they met.

    Essentially there are two types of box.

    1) Those that simply try to suppress the rearward wave. Obviously this sound energy has to go somewhere, it cannot disappear. So it ends up as heat in the box and its contents (the stuffing). Here the stuffing is simply used as an absorber.

    2) Those that try to use at least some of the rearward wave to reinforce (constructively interfere) with the the forward one. Clearly the designer wants as little as possible of the rearward energy of this type to end up as heat. This is the type where the gas dynamic properties of the stuffing is important.

    There is more than one approach to this second more difficult speaker design. A leading figure in this area was Dr Baily

    http://en.wikipedia.org/wiki/Acoustic_transmission_line

    Finally the other point of the stuffing is to prevent the rearward wave bouncing (reflecting) off the rear wall of the box and passing back out through the speaker cone, which has a much less substantial construction than the box walls.
     
  4. Mar 31, 2010 #3
    That was my arguement to the other person.
     
  5. Mar 31, 2010 #4
    So what is your question?
     
  6. Mar 31, 2010 #5
    Well my opponent says it doesn't matter where you place the stuffing, and this is in any box where the cubic volume is less that the recomended cubic volume. She's saying that this makes up for the volume.

    Thanks for any help ya'll can give me, let me know if there are any more questions I need to answer so that you can settle this.
     
  7. Mar 31, 2010 #6
    I don't like playing referee in some other forum's game of ping pong.

    Try reading my post#2 again thoroughly, without just picking out something you agree with.

    Your respondent appears to have some merit but like I said, it depends upon the box.

    Since you aren't telling us about the box how can we go any further?
     
  8. Mar 31, 2010 #7
    I'll try and address all questions you present to me.

    1. Type of Box: Well, understand that the person I'm speaking with doesn't acknowledge that there are any sound waves in the box. She says because there's not enough room for them to form that they essentially don't exist. But let's say we have box A and box B, and box A is a perfect cube measuring 12"x12"x12" inside, while box B is quite simalar with the only differnce being that the depth is 9" reducing the volume by 25%. Now place 1lb of polyfill in this box increasing the volume to match box A (please understand that is what she says that it does). In box A I would assume the waves constructively interfere, so type 1. In box B I would assume that the 1lb of polyfill would make it a type 2 box.

    Edit: I wanted to include that these are sealed boxes, and let's make the subs 10" with a 1" throw, installed in the center of the face of box A and box B.
     
    Last edited: Mar 31, 2010
  9. Mar 31, 2010 #8
    jesus, ok first of all, the only way that destructive interference would happen is if the box had a dimention that would allow the returning wave to be compeletely out of phase, and since the sin thea goes from to 2 pi, the likelyhood is really not so great unless they have been made to do so. since that requires a lot of craftsmanship and lambda isnt really so large, not really great chance of that. not to mention that a box is box shaped, and a wave is circle shaped, somewhere there is not going to interference in phase.

    secondly, i can not beleive you think that pressure f/l^2 can be converted to heat (thermal energy) f*l. a basic unit analysis will leave you with the fact that this miraculous material somehow adds 5 dimentions of length to your pressure. since this event is not happening on a sub atomic level, sting theory is out and it is therefore, not your explaination is not dimemtionally homogeneous. the way that ANY fiberous material (teddy bears, hair, and bales of hay included) damneds any sound wave is by converting the sound energy (which is vibration of air molecules) into kinetic energy of the fibers. the reason polyfill is better at this is that its more ridigid and therefore requires more energy to move. seriously, someone needs a BASIC physics class.
     
  10. Mar 31, 2010 #9
    i would like to add that there is some thermal energy expressed when anything is moved, so there is a conversion to from Kinetic energy to thermal energy, however that is not the point of any fiber sound filler. if thats how things worked, if you were in a litteral box of ice, when you yell the sound would melt the ice. you can go try taking 2 peices of ice, yelling at one and not yelling at the other and see if the one you yelled at faster. im going to go ahead and say it didnt.
     
  11. Mar 31, 2010 #10
    It would need to be a box, made of ice, with 1lbs of polyfill per foot of cubic volume.
     
  12. Mar 31, 2010 #11
    I should consider the mechanism by which fibrous or other material increase the 'apparent volume', and also the mechanism by which sound energy is dissipated in absorbers.
     
  13. Mar 31, 2010 #12
    i dont know what you think polyfill is made out of that would change the sound energy into enough heat energy to increase the melting proccess of ice. I promise, its just the same as batting or wool or something. polyfill isnt made out of magical leprechan fibers. sound into fiber = vibration of fiber = increased kinetic energy of fiber. no magic in equation.
     
  14. Apr 1, 2010 #13
    Since you and your debating partner appear disinterested in addressing the comments by others I am not surprised no one else is making any in this thread.

    I repeat
    There are two fundamentally different types of enclosure (box).
    The design requirements for these are therefore quite different.
    In particular, the requirements for the wadding are diametrically opposed.

    It is not uncommon, even among manufacturers, to wrongly apply parameters suitable for one type of enclosure to the other.

    So let us examine some statements in the light of Physics.

    This is a misconception.

    Yes there is not enough space for a compete cycle of low frequency sound to fit into most boxes.
    Yes boxes are plenty large enough to accommodate several cycles of high frequency sound.

    At 100 cycles a full cycle occupies 11 feet
    At 1000 cycles a full cycle occupies 1.1 feet
    At 10,000 cycles a full cycle occupies 0.11 feet

    However it is the frequency not the wavelength that matters. Whatever the wavelength there is a pressure variation at the frequency of the sound. So there is still sound even though only part of the wave will ‘fit’ in the box.

    There are two types of enclosure

    Type 1 where the rear wave is suppressed by absorption within the box

    Type 2 the rear wave is used to reinforce the sound by some mechanism within the box

    Type 1 wadding needs to be absorptive. Type 2 wadding needs to be transmissive.

    You have specified a type 1 enclosure for discussion.

    The most efficient dissipative (absorptive) media are porous, preferably with interconnected pores.
    These pores act as tiny helmholtz resonators to absorb and dissapate the sound energy, rather in the same way as auto silencers work.
    Fluttering ( direct transfer if kinetic energy to the fibres) is a minor mechanism.
    There is no ‘apparent increase in volume’ effect in dissipative enclosures.

    There are also mechanical effects due to the nature of the speaker cone construction and the box geometry.

    I have not produced any maths to describe all this. How far are you and Bobcat in a position to understand the math?
     
  15. Apr 1, 2010 #14
    BobCat123 is not the person I'm debating with.

    I gave you specifications for both boxes. I would assume box A would be type 1 and box B would be type 2, although when designing and constructing these boxes nothing is being taken into consideration other than the manufacturer's suggested cubic volume. They are sealed boxes. Box A is within the suggested cubic volume, box B is 25% short, and this is accounted for in the shallow depth of 9". To make up for the loss in volume my opponent believes that placing 1lbs of polyfill anywhere in box B will make up for the lost volume by converting air pressure into heat.

    I know, the person I'm debating doesn't.

    I can not speak for anyone other than myself but I can understand basic concepts like pressure equals force divided by area, although I would not be able to actually do the math. The last math class I took was calculus in 1997. Again I would like to thank you for your assistance in this matter, =).

    Edit: Here's a link to the original post since it might clarify any other questions you might have http://www.preludezone.com/i-c-e/28070-cotton-sub-box.html =)

    Edit: Edit: Oh, and please do give the math involved in figuring this out. I would like to know!
     
    Last edited: Apr 1, 2010
  16. Apr 1, 2010 #15
    Can we get one thing straight.

    Sealed boxes are type 1.

    Have a good look at what I said about the characteristics of the wadding.

    The size of the volume in the sealed chamber has nothing to do with the dissipation of sound energy. The driver manufacturer will have recommended a particular volume to provide a particular mechanical stiffness (= resistance to compression) at the rear of the cone. This is known as acoustic or air suspension.

    The idea that converting air pressure to heat makes up for volume, lost or otherwise is nonsense.
     
  17. Apr 1, 2010 #16
    Right, so how do I explain this to someone who believes otherwise?
     
  18. Apr 1, 2010 #17
    Physics should be a result of observation and logical deduction, not a belief system.

    Perhaps you/they should consult a cleric?

    Can you not see for yourself what has to happen to the energy of the rear wave in a sealed system?
     
  19. Apr 1, 2010 #18
    studio.
    Ive gone through differential equations and every physics class that my college offers. this discussion deals strictly with wave mechanics, which any sophmore could handle. calculus isnt even a factor hear unless you want to use a coordinate system with i so to question my mathematical capability is purely insulting and just as uselss as trying to convince anyone that the puffy fluffy stuff in speakers is to CONVERT pressure into heat. its rediculous and obvious to anyone that has had a basic algebra based physics class that those units dont match, or that you can increase a volume by taking it away. its silly and doesnt make any physicals sense. if you were versed in interferometry or something like that then maybe your opinon would have more merrit, but if you think that lambda and frequency are not related, then you are obiviously no veteran of the sine function.
     
  20. Apr 1, 2010 #19
    Apart from insulting xiau and myself your point is exactly what?
     
    Last edited: Apr 1, 2010
  21. Apr 1, 2010 #20
    I'm pretty sure you started the mud slinging. my point is that pressure can not be converted directly into heat by a magical fluff, lamba is always inversely proportional to the frequency and constructive/destructive frequency happens only in/out of phase, not just because you bounce it against a wall in a box. you know, basic unit analysis and wave mechanics.

    also that that any fibrous material used to dampen sound uses the basic transformation of energy from kinetic energy of the air to kinetic energy of the fiber. no magic. no numerical dimensions. if you want those things they will depend strictly on lambda since the frequency does not change the position of the nodes and anti nodes, its the lambda that does that. maybe im trying to teach you physics. idk. but im sure that i am just as able to use my mathematical capabilities as you are so im not sure if your questioning mine was an attempt at superiority but i dont have a problem solving wave equation for you if that is needed.
     
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