Converting Pressure Into Heat

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 subwoofer enclosure. Here are some excerpts that were posted in response by the person I'm in disagreeance with (Let’s call her Jane):

“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 by Jane that to me appear to be incorrect; First of all Jane says that although cotton filler is also a loose fibrous material it would have properties contrary to this polyester filler, secondly that polyester fill is able to convert air pressure into heat, and thirdly that by reducing the air volume of an enclosure by filling it with that you are in fact increasing the "perceived" volume.

To explain the necessity for a certain amount of cubic volume for a subwoofer enclosure, the surface area of the speaker combined with the throw (which is the distance it is capable of moving in and out) displace a certain amount of air space and this creates a certain amount of pressure that causes the speakers itself to have to do more work in order to function. So the smaller the enclosure, the greater the pressure, and the more work that the driver has to do in order to function. I believe most 12” subwoofer manufactures suggest about 1’-1.5’ cubic volume. Jane believes you should use polyester fill in situations where the cubic volume is less that this suggested amount and that it will make up for it. Jane says it doesn't matter where you place the polyester fill, and that in any box where the cubic volume is less that the recommended cubic volume that filling the enclosure with polyester fill makes up for the volume.

Here are two hypothetical enclosures box A and box B:

Box A is a perfect cube measuring 12"x12"x12" inside, while box B is quite similar with the only difference being that the depth is 9" reducing the volume by 25%. Now place 1lb of polyester fill in box B. Jane believes that by placing 1lbs of polyester fill in box B increases the volume to match box A. In box A I would assume the waves constructively interfere, but that’s not really relevant to the air pressure caused by displacement which is what we’re discussing. In box B I would assume that the 1lb of polyester fill would act as dampener. 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.

I’ll briefly explain my perspective. It would appear that this polyester fill is actually just a dampening material. Jane says 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 generated, 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. I would like to thank everyone ahead of time for helping me out.

Hi, I'll take a poor stab at this


My guess is that your both somewhat correct. It is reducing unwanted vibrations by absorbing the sound waves which also heats up the material whether that be cotton or some kind of synthetic.

Can you link to the original thread?

The rest of the post is a bit difficult to follow.

Thanks for your post. I've considered that it's possible that heat is generated since she mentioned and was tempted to test materials and see what the results would be but the more I think about it the more I think that it would be an imperceptible amount.

The original post can be found at preludezone dot com/i-c-e/28070-cotton-sub-box.html

(sorry for the format, it won't let me link since I'm new here)

http://www.preludezone.com/i-c-e/28070-cotton-sub-box.html


fixed. VTEC forum


So apparently 'fill' increases the woofer box 'volume' making it perform like a larger one as the thread goes.

My explenation would be that the 'fill' stops unnecessary vibrations, therefore allowing the woofer to be played at a higher volume before distortions would be apparent, and so theoreticalloy be doing the same job as a 'larger' box woofer.

Stuffing 'fill' into a small box will also increase the total operating temperature of the woofer and so consequently will reduce the lifespan of your speaker.

That seems like a logical deduction!

See:

http://www.polkaudio.com/forums/archive/in...hp/t-32824.html

For more discussion on this subject.

Arthur

This is just a guess, but would the fill material reduce the amount of air-flow in and out of the box as the volume changes?

It seems like the fill would compress and decompress and also act as a semi-permeable barrier, which would cause the air to "ooze" instead of slamming against the sides of the box in waves. Am I just talking about "damping" of soundwaves and making it more complex than it needs to be?

Interesting

i've learned a lot on this forum, its a cool forum.

Anti spammer post only - Forum members should support a topic in each area of the index to take commercial spam posts off the index page top spots. Please assist.

OK, it's winter.
Is there any material that could be used inside the shoe soles to transform pressure into heat?

I would guess the stuffing material inside a woofer enclosure is to absorb higher frequencies than base. If you place a cloth over a tweeter, it will sound more basey.

The crossover network will usually chop out higher frequency so the woofer is only broadcasting base. But some times, the case, itself, will get a mid-range hum or buzz at certain frequencies. The stuffing can soften that.

As far as turning pressure into heat; as you walk, your weight and steps are generating work/heat to warm your feet.

Any gas will do. One practical problem is you need to enclose the gas in a flexible balloon or ball thingy, that is strong enough to last many cycles. If you take a tennis ball and bounce it many timer it will warm up. One problem that arises is the walls of the ball, and the rest of the shoe, may act as an insulator, so the heat never reaches the feet.

The balloon/ball full of gas will need be strong but also be a good conductor of heat, but only on the side facing the feet. This present technical challenges. It would good practice for ingenuity.

An alternate design would use friction instead of pressure. Picture, as you walk, two things rub together; two sticks start a fire.

This is also a third way. There are things called bi-metallic strips where two metals are sandwiched together, where each metal expands at a different rate. They bi-strips wants to curl. If you walk and it, to make it straighten out, it can generate heat or electricity.