Polyfill.. acoustic theory behind it?

[ShiningBuick 10+ year member]

Well, most people have heard that adding polyfill to the right sealed enclosure gives you a little more sound (at least that's what I've heard), but honestly I don't get it..

Why does adding some fluffy stuff into your box add sound? Does it dampen the box from flexing or what?

I'm curious!

Click to read more...

 

[n2audio 5,000+ posts]

Surprisingly, I found this on yahoo answers -- but it's right.

Stuffing a box with polyfill makes it seem larger and it all relates to thermodynamics. When polyfill is added to an enclosure, it changes the behaviour of the airspring in the enclosure from "adiabatic" to "isothermal". The term "adiabatic" implies that there is no heat transfer occurring. An isothermal process occurs once the polyfill has been added. As the air passes through the polyfill, the fibers wiggle and cause some of the energy created by the airspring to be dissipated as heat. This heats the surrounding air molecules warmer, causing the air to become less dense. Being that sound passes easier through a denser medium, the speaker interacts with your enclosure as if it is larger than it actually is. The effective increase in enclosure size can be as much as 40%!

There are some additional worthy considerations. Adding polyfill to an enclosure can be a great choice. However, too much polyfill can be a bad thing. At a certain point, the stuffing becomes too dense and the fibers no longer wiggle. At this point, not only have you taken away the size benefit of adding polyfill, you have actually decreased the effective volume as the polyfill is now taking up room inside your enclosure.

 

[Haunz 10+ year member]

Surprisingly, I found this on yahoo answers -- but it's right.

Stuffing a box with polyfill makes it seem larger and it all relates to thermodynamics. When polyfill is added to an enclosure, it changes the behaviour of the airspring in the enclosure from "adiabatic" to "isothermal". The term "adiabatic" implies that there is no heat transfer occurring. An isothermal process occurs once the polyfill has been added. As the air passes through the polyfill, the fibers wiggle and cause some of the energy created by the airspring to be dissipated as heat. This heats the surrounding air molecules warmer, causing the air to become less dense. Being that sound passes easier through a denser medium, the speaker interacts with your enclosure as if it is larger than it actually is. The effective increase in enclosure size can be as much as 40%!

That's not accurate.. it has nothing to do with the density of the air inside of the box..

Energy from the backwave is absorbed by the material and is dissapaited as heat, which results in lower sound pressure levels inside of the box..

 

[jco1385 5,000+ posts]

3rd

 

[n2audio 5,000+ posts]

PV=T

or

P=V/T

So when T increases P decreases causing the sub to perform like it's in a larger box.

That's not accurate.. it has nothing to do with the density of the air inside of the box..

It has SOMETHING to do with density, as that relates directly to pressure, but I'm not sure if the response of the sub is due to the lower density or the lower pressure, I would imagine it's both.

 

[Jeremy M 10+ year member]

i think it was correct. in the sense that the polyfill slows down the air wave, making the speaker see the box as bigger.

 

[Haunz 10+ year member]

PV=Tor

P=V/T

So when T increases P decreases causing the sub to perform like it's in a larger box.

It has SOMETHING to do with density, as that relates directly to pressure, but I'm not sure if the response of the sub is due to the lower density or the lower pressure, I would imagine it's both.

Actually, PV = nRT.. and no, when T increases P also increases...

Anyway that is neither here nor there because the effect of 'boxfill' has nothing to do with the temperature of the air inside of the box or it's density..

It's really quite simple... Sound pressure energy is absorbed which gives rise to lower sound pressure levels inside of the box... and lower sound pressure levels inside the box with a given amount of cone displacement is akin to having a larger box...

(ie. if you take a 1cube box and compress the air inside to .5 cubes you are going to have 2x the pressure.. but if you take a 10 cube box and compress it to 9.5 cubes you will get 1.05x the pressure)

Make sense ?

 

[Immacomputer 10+ year member]

i think it was correct. in the sense that the polyfill slows down the air wave, making the speaker see the box as bigger.

That part about polyfil isn't really correct. It does dampen the response but it doesn't slow down the speed of sound. Martin J. King found in his tests that adding polyfil to a transmission line enclosure didn't change the effective 1/4 wave frequency of the line but did dampen the response. The 1/4 wave frequency is directly related to the speed of sound.

Polyfil isn't exactly like making the box bigger but it does change the effective Qtc of the enclosure/sub system just like changing box size does. It will also not affect the mechanical power handling of the subwoofer like changing the box volume will. It will lower the Qtc which will usually result in slightly more low frequency extension and it will help reduce peaks due to resonance. It will pretty much never increase output and in ported enclosures, it will reduce output around tuning, which is not what happens when you make an enclosure larger. It can also dampen upper order harmonics and improve sound quality, which increasing enclosure volume will not do.

 

[Haunz 10+ year member]

Adding fill will increase the density of matter inside of a box or port, and would actually Increase the propagation of sound... (but not to any degree where port tuning actually changes)

Immacomputer basically nailed it.. adding fill lowers the Q factor and makes the air inside less efficient as a spring.. and interestingly enough adding fill to a port will reduce output at tuning as he says, but it will also broaden the tuning and give a wider bandwidth...

Not sure I agree that adding fill won't affect cone movement though..

 

[Tehgregzor 10+ year member]

anyone have a formula for when just enough polyfill becomes TOO MUCH?

 

[n2audio 5,000+ posts]

Actually, PV = nRT.. and no, when T increases P also increases...

N is fixed in this case and R's a constant -- I was just simplifying.

I was looking at the formula wrong though (as if I didn't know the T-P relationship //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif)

I'll defer to an old RC quote...

here's the theory and the facts--------in an acoustic suspension system the cone motion is primarily controlled by the air spring behind the cone------this spring is the trapped air in the box and since air is a compressible gas it actually makes a fairly linear spring compared to most mechanical springs-------hence the inherent low distortion of most sealed box designs----------we all should know that when a gas is compressed it heats up------we should also know that when it is rarified (expanded) it cools---------as a speaker compresses the air in a box there is a temperature rise in the gas-------when the speaker moves outward the heat is removed from the gas to the same degree (boyles law tells us that there is a direct relationship between pressure' date=' temperature and volume of a gas)--------this constant rise and falling of temperature occurs whenever thhe speaker moves--------since this system constantly rises and falls in temperature strictly due to gas volume no real energy is lost------when energy is contained in a system and only converted it is considered to be adiabatic---------this is a mechanical pure reactance----ie a perfect spring (no such thing exists but for simplicity we will assume it does)-------remember that if we had a perfect spring and exerted 10 lbs of force to compress it the spring would exert 10 lbs of force back till it was relaxed----------since temp, volume and pressure of a gas are directly related, the spring action controlling the speaker movement is clearly defined----------now if we introduce a way for energy to be removed from the system things change--------if we introduce stuffing we are actually adding resistive losses--------as air moves through the stuffing it suffers resistive losses that produce heat-------unlike the heat that results from compressing air this heat is not recovered when the speaker reverses direction-------this heat is dissipated in the stuffing--------since some of the energy is lost this means that there is less spring power to move against the speaker when it moves---------this means that the speaker "thinks" the box is bigger since the air seems to be a weaker spring----------a system that loses energy permanently to resistive elements is know as an isothermal one-----------now the real issue is not if the theory is correct since it is clearly scientific and beyond question----------i see it as more like the amp sq issue--------we know we can measure differences but can we really hear them???---------it is easy to demonstrate that stuffing a box will lower the resonant frequency a few hertz----------but like the amp issue it is really thin ice to claim to be able to hear it in most cases--------is that a reason not to do it????-------not at all------any real improvement that can be supported by scientific evidence that is as cheap as a few oz of fabric just make a lot of sense--------but don't expect miracles..........RC[/quote']

 

[n2audio 5,000+ posts]

anyone have a formula for when just enough polyfill becomes TOO MUCH?

This came from an article by Tom Nousaine

http://www.diysubwoofers.org/talkshop/messages/41415.htm

SEALED ENCLOSURE

1.4-ft³ Box

Stuffing Density (lb/ft³ ) ------- System Resonance (Fsb) ----------- Effective Size---------Percentage Gain

------------0-------------------------- 56.6 ---------------------------1.4

----------0.70 -------------------------53.0--------------------------- 1.6-------------------- 14%

----------0.75 -------------------------52.7--------------------------- 1.7-------------------- 21%

----------1.50 -------------------------51.7--------------------------- 1.8-------------------- 29%

----------1.75 -------------------------50.8 -------------------------- 1.9 ------------------- 36%

----------2.60 -------------------------50.4 -------------------------- 1.6-------------------- 14%

----------3.10 -------------------------52.6 ---------------------------1.2 ------------------ -14%

SEALED ENCLOSURE

5.1-ft³ Box

Stuffing Density (lb/ft³ ) ------- System Resonance (Fsb) ---------- Effective Size -------- Percentage Gain

-------------------0-------------------- 42.0---------------------------- 5.1

-----------------0.25 -------------------42.0 ----------------------------5.1 ------------------0%

-----------------0.50------------------- 41.2 ----------------------------5.8 -----------------14%

-----------------0.75 -------------------40.3 ----------------------------6.2 -----------------22%

-----------------1.00 -------------------39.4 ----------------------------6.5 -----------------27%

-----------------1.25 -------------------38.6 ----------------------------6.5 -----------------27%

-----------------1.50 -------------------40.2 ----------------------------5.6 ------------------9%

PORTED ENCLOSURE

1.4-ft³ Box

Stuffing Density (lb/ft³ ) ------- System Resonance (Fsb) -------- Effective Size ------ Percentage Gain

----------0 ------------------------------------42.0 ------------------1.4

--------0.40 -----------------------------------39.1 ------------------1.6 ------------------14%

--------0.85 -----------------------------------37.2 ------------------1.8 ------------------29%

--------1.25 -----------------------------------35.2 ------------------1.9 ------------------36%

--------1.40 -----------------------------------34.2 ------------------2.0 ------------------43%

--------1.75 -----------------------------------35.2 ------------------1.9 ------------------36%

 

[FoxPro5 5,000+ posts]

In order for any material to absorb, it must be able to convert sound energy to heat energy. Absorption is velocity-dependent, meaning more velocity, more absorption.

Polyfill isn't exactly a matrix for sound to be absorbed in, but it is kind of a net. I could see some diffusion happening, though depending on the density you pack it in.

Keep in mind the wavelength of sub frequencies, too. They will simply ignore polyfill, your back seat, you, your engine, etc.

 

[Haunz 10+ year member]

N is fixed in this case and R's a constant -- I was just simplifying.
I was looking at the formula wrong though (as if I didn't know the T-P relationship //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif)

I'll defer to an old RC quote...

LOL, really not a difficult forumla that needs to be simplified.. anyway the point was that it has nothing to do with the temperature or density of the air inside the box..

That's a good quote from RC...