cotton fill

Don't use cotton. You need to use polyfil which is just polyester fibers. Raw cotton batting retains moisture and has oils that can damage speaker parts. Polyfil is pretty inert and dosn't do much but sit there like it should. A viable alternative is fiberglas insulation.

Originally posted by Manuj
how does adding cotton to an enclosure affect the sound quality and spl and the sound itself to 2 12" polk momos?

Polyfill is used for tuning the enclosure.

I thought that it was used for slowing down the air inside to reduce port noise and internal resonance...

Polyfil works in a ported box by slowing air movement. This reduction in air speed reduces port noise because it slows the air exiting the port. However, the audible port noise is caused by turbulence at the mouth of the port. High pressure air exiting the port runs past eddys of low pressure air around the edge of the port. It creates a whoosing and sometimes a low frequency whistle like if you were blowing across the mouth of an open soda bottle. The size of those eddys is proportional to the speed of the air movement. The slower the air exiting the port, the smaller the eddys at the mouth of the port, the less noise there is. To get smaller eddys, you have two options. One, slow the air speed in the port. The problem with that is that SPL drops too. Number two, create a wide, sweeping port mouth with a very smooth 90 degree transition. The eddys can't form on the leading edge of the port because there isn't one. Check out Polk Audio's Power Port technology to see what kind of port I am referring to.

Polyfill will not tune a box. Polyfill will merely make the speaker behave like it is in a larger box than it is. You get a better frequency response because the polyfil slows the air down. This means it takes more time for the backwaves to hit the sides of the box and reflect inside the enclosure. It has more time to expand the full wavelength before being reflected which is important to the audible output from the enclosure. In this manner, it makes the box seem bigger than it really is because a box loaded with polyfil will make an enclosure that is too small seem like the right size. The problems with polyfil are that it reduces efficiency of the enclosure which in turn kills your performance. It is better to build the enclosure correctly to begin with.

Polyfil does not reduce internal resonance. It can help break up standing waves inside the enclosure but resonance is a physical property of the enclosure. Everything resonates whether we like it or not. To stop internal resonaces, you need to change the physical characteristics of the materials used in the enclosure. To do that is not easy. Coating the enclosure in a sound deadening material is one way and changing the shape of the enclosure is another. Beyond that, you need to re-engineer the material. MDF is dense and has space between the particles so it acts the same way that sand or lead shot in a speaker stand would to absorb vibrations. High grade plywoods usually have a few different types of wood and maybe a plastic or foam in the center. They have glue and other treatments too. Also, to increase strength, the layers of plywood are often laid with the grain of the woods used being arranged perpendicular to each other. Not only does that increase strength but it also reduces vibrations and resonances. It reduces resonances because it behaves the same way the Polk Audio Dynamic Balance technology works. It uses multiple materials whose physical properties will dampen the resonaces of the other materials in the structure. In this manner they compliment each other. Plywood isn't necessarily made to reduce resonances but the way it is constructed, it can greatly reduce the effects of the resonances of the material used. To really understand it, you'd have to get into some heavy science about how sound travels through things and across surfaces and such. I don't understand it completely so I won't try to explain it either. However, reducing internal resonance will not be accomplished by fooling around with teddy bear stuffing.

Another reason not to use cotton batting is that it breaks apart and can get lodged in the motor structure of the speaker, especially if it has a vented pole piece. That causes not only damage but with the heat that can build up it can start a fire. Since it can retain moisture easily, it is capable of shorting out the voice coil which will destroy the speaker and may cause a fire also. Cotton is a bad idea. Polyfill is spun polyester fibers. Entirely synthetic and it doesn't break apart or degrade. It is alot less likely to cause any problems.

I just realized I left out something important.

Two ways to slow the air speed in the port are polyfil and port size.

Already discussed polyfil.

If the air is moving too fast from the port, you can make the tube wider. The wider tube means more crossectional area for a higher flow rate. A higher flow rate means more air gets through that hole. If more air gets through that hole, there is less pressure built up in the enclosure so the air velocity in the port drops because the pressure level in the enclosure is not pushing as hard on the air in the port tube.

Problem though.

Changing port size changes the tuned frequency of the enclosure. That changes how it sounds, it's sensitivy, efficiency and response level to the changes in the music. A port's width and length changes the tone of the music. If I remember correctly, as an example, if you have a 2 inch wide port that is 12 inches long and you want to drop port velocity without polyfil, you make the port 3 inches wide. But, having the tube 12 inches long changes its tuning point. I think it needs to be shortened but I can't remember. I'd have to consult a book I have at home to be sure. Also, lower air velocity in the port makes for a lower SPL just like polyfil. However, different ports have different characteristics. While you make get a loud whistling on a 3 inch port that is 8 inches long at 800 watts, you may get no sound at all at 700 watts. There are alot of factors involved and it gets pretty tough to discuss on an internet forum.

Anyhoo, changing port size is a good way to reduce port noise and often times is the reason you are getting port noise, i.e. improper port sizing for your enclosure. Like, you wouldn't put a two inch port into an enclosure with dual 12 inch subs.

However, no matter what port size, polyfil will have the same effect and it's really more of a bandaid to cover a problem rather than really solve it. Although, in sealed enclosures, polyfil can help smooth response and kill boominess. Then again, if you have peaky response and boominess, your enclosure might be too big to begin with or leaking. Usually, a box that is too small will have a truncated frequency response while having very tight and accurate bass and not very deep low end extension. Polyfil can help in a case like that. It can be beneficial in a properly sized box also because it can, again, smooth out response and help elminate standing waves inside the enclosure which can contribute distortion and reduce efficiency.

A good book for learnin' this stuf is the Loudspeaker Design Cookbook by Vance Dickason. It was pretty much required reading if you wanted to get into car audio back in the 80's and early 90's. Everyone should have a copy if they want to play around with enclosures.

Amazon has it listed as special order:

http://www.amazon.com/exec/obidos/tg/detail/-/1882580109/qid=1119988355/sr=8-2/ref=sr_8_xs_ap_i2_xgl14/102-8128121-1660960?v=glance&s=books&n=507846

But I know that you can walk into a Barnes and Noble or Borders and they will have it on the shelf in most cases.

Great posts people. I should really pick up that book since I'm really starting to get into all this fancy diy stuff.

crikey, mac's saying this thing's the bible and i've never read it... this is bad, how am i supposed to be the theoretical guy on the forum now?

I'm surprised no one has poked fun at Mac yet for this.

Originally posted by neomagus00
i dunno, cody's sig does enough poking for pretty much any thread

:eek:uch

i have an 8'' in a sealed box and i hear backwave when i really listen to it. if i stick some spectrum from second skin, maybe two decent layers, will that help? or should i just go for the polyfill?