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  1. #1

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    Question foam and poly in a sub

    does putting foam and poly ( the cotten looking stuff ) in a sub really help or should it be used at all.

  2. #2

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    I like foam over polyfill.

    It does the same thing polyfill does + more - it adds volume to the enclosure and cancels the backwave... all while DAMPENING the enclosure from resonances...

    It helps make the enclosure larger if it needs to be done that way...
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    Polyfill does all that stuff too.
    madmax
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    but is it always needed

  5. #5

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    No. I have seen subwoofer designs which specifically call for no fill. Of course other designs call for it and some even call for overstuffed.
    madmax
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    Polyfil is a bandaid for most sealed enclosures. Properly built sealed enclosures will rarely benefit from polyfil. They will help cancel the backwave and they will help make the box seem bigger than it is but, that does not mean that it is beneficial for all enclosures. If the sealed enclosure is built to proper specs for the driver, it has been my experience that polyfil/foam is not needed at all.

    However, in a ported or band-pass enclosure, even if the box is built to perfect specs, polyfil can be a great help. It helps cancel the backwave which can lead to port resonances. That makes the port whistle and vibrate which colors to music. It can also slow down the air movement in the cabinet which can reduce port velocity which also hels to stop port noise like whistling. Lower port velocity also reduces turbulence at the mouth of the port. If the velocity is too high and there is too much turbulence, you develop a high pressure area around the mouth of the port. What this does is two things. First off, it can have the same effect as changing the length of your port tube which changes the tuning frequency of your port. The other thing it does is kind of complicated. Since the high pressure air is there and making the tube seem longer, it also moves with the wave of the air coming out of the port. The port doesn't move because it is a solid, not a gas. As that gas oscilates, it will pinch that high pressure area around the mouth of the port. Just like if you were to squeeze teh mouth of a deflating balloon. This causes a low frequency chattering that sounds like distortion. The biggest problem is that the low frequency chattering creates it's own backwave the travels back down the port to mess around in the cabinet. Not good and it can cause uneven pressure behind the driver cone which can lead to cone break up and more distortion.

    However, because of the dampening ability of the polyfil, it will smooth out the response of the driver in a ported cabinet. If the box is too small, it will also help the box behave like a larger box. It also smooths out the response in a sealed box. A driver that is in a box that is too small will have a very peaky frequency response. The dampening effect of the polyfill slows down the air flow so that the pressure waves slow down too and the air pocket behind the driver doesn't pressurize as quickly. It allows the driver to move further in its moved before the pressure wave inside the cabinet tries to push the driver cone back out. That pressure wave prematurly stopping the movement of the cone and pushing it in the opposit direction is what truncates the frequency response.

    However, one place that polyfil does nothing for you at all is if the enclosure is too big. The polyfil might help tone down the boomyness of the enclosure but too big is too big and you really should build or find a more suitable enclosure for your driver.

    Also, it should be noted that polyfil will not fix all problems. If it is used to compensate for a box that is too small, it is only effective if the box is 15% smaller or less than it has to be. You can probably get away with it at around a 20% smaller but that is really pushing it. Then again, if you are building this for home use, you really should build a properly sized enclosure.

    For full range speakers though, polyfil is used differently. It is usually glued or stapled to the inside of the cabinet walls and extend less than 2 inches into the cabinet air space off of the walls. Sometimes all the walls of the cabinet are covered, sometimes only the sides and sometimes only the back panel is covered. The benefits are the same as if you were building a sub enclosure like what i discussed above. However, I have seen many alternatives to polyfil ranging from felt batting to fiberglas insulation.
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    I've never used polyfil or foam in any of my car sub boxes. I always use big sealed enclosures.

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    Max...

    Polyfill can not dampen the walls of an enclosure... considering polyfill just "floats" around in the box and is not tied in directly with the walls.

    Where as foam mounts directly on the walls and dampens them greatly... :)
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  9. #9

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    Quote Originally Posted by Vr3MxStyler2k3
    Max...

    Polyfill can not dampen the walls of an enclosure... considering polyfill just "floats" around in the box and is not tied in directly with the walls.

    Where as foam mounts directly on the walls and dampens them greatly... :)
    I have to say that you are incorrect, sir. I have had plans for speaker enclosures in my hands that call for polyfil batting to be glued to the interior walls of the enclosure. This batting comes in sheets at your local fabric shop and is anywhere from .5 inches thick to 6 inches thick. It is the same "floating" polyester fibers that you find in the bags of polyfil but woven into sheets of varying thickness as mentioned. The stuff is used by upholsterers for making things like pillows, seat cushions and such as a replacement for raw cotton batting which has a tendency to breakdown overtime and can grow mold and other nasties if gotten wet. It is also hypo-allergenic which makes it ideal for upholstery work. It is a delicate, random weave but if using 3M Universal Trim Adhesive, one can easily glue this stuff to the interior walls of an enclosure and it will pretty much stay stuck there for a good long while if not permanently. I know this because I have done it myself with much success.
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    That is not what I am talking about...

    I am talking about the stuff you buy from Parts Express...

    (I knew I should of added in a disclaimer) - I am talking about LOOSE polyfill.

    Putting Polyfill into a sheet and applying it to the walls would basically be doing with foam does... ;)
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  11. #11

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    Say what you mean, mean what you say and you won't have a problem with people missing your point.

    Bottom line, you said "Polyfill can not dampen the walls of an enclosure... considering polyfill just "floats" around in the box and is not tied in directly with the walls. Where as foam mounts directly on the walls and dampens them greatly.."

    which is an inaccurate statement. I read nothing in to it and took it for what it stated.

    Also, you have neglected to meantion that the heavy foam is more dense and can reduce the effective volume of the cabinet to a much greater degree than polyfil in any form.
    You're just jealous 'cause the voices don't talk to you!

  12. #12

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    Quote Originally Posted by Jstas
    Polyfil is a bandaid for most sealed enclosures. Properly built sealed enclosures will rarely benefit from polyfil. ......

    Jstas my SDA2A's as Im sure most everyone elses originally came with polyfill inside the enclosure. Im sure you dont mean to say they were designed poorly do you:p


    By the way I am in need of replacing the polyfill in one of my speakers as I have somehow taken it out of one of my enclosures over the past few years and "misplaced" it while awaiting new drivers.

    Does anyone know the original preffered amount of fill or batting that should be used in this application? I dont want to rely soley on what was in the other enclosure as Im not positive that one has enough either.

  13. #13

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    Old thread, but here goes.

    Polyfill and foam won't do jack squat for cancelling the rear wave of a subwoofer. The only reason polyfill is used is to increase the size of the enclosure. It accomplishes this because polyfill will vibrate extremely fast inside a box when the sub is moving, causing it to create heat which makes the air in the enclosure less dense, which in turn, makes it larger to the driver. I have no idea why people use foam in their sub enclosures because all that does is make the enclosure smaller. If you don't believe me, check out any reputable speaker building forum (diyaudio.com, htguide.com) and those guys will tell you the same thing.

    Now, if you're stuffing an enclosure that houses a woofer that plays midrange as well, yes, using foam will cancel some of the rear wave and it is very useful, but it will, at the same time, decrease the cabinet's volume. Polyfill will make the mid's enclosure larger and barely cancel any of the rear wave.

    I need to go to bed :p

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    Quote Originally Posted by Mazeroth
    Old thread, but here goes.

    Polyfill and foam won't do jack squat for cancelling the rear wave of a subwoofer. The only reason polyfill is used is to increase the size of the enclosure.

    I dissagree. I have both my Dayton sub and Polk psw10 overstuffed with poly and the results are amazing. I have them both bypassing the plate amp and directly hooked to my Hafler DH-200. Now I can reach higher dbs because of the simple fact that the poly slowed down the waves in the box and in result gave much less to zero port noise, which in turn allows me to drive my subs alittle harder without port noise distortion.

    Now I dont know how good that is for the driver itself, but it sounds damn good.

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  15. #15

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    Quote Originally Posted by jakelm
    I dissagree. I have both my Dayton sub and Polk psw10 overstuffed with poly and the results are amazing. I have them both bypassing the plate amp and directly hooked to my Hafler DH-200. Now I can reach higher dbs because of the simple fact that the poly slowed down the waves in the box and in result gave much less to zero port noise, which in turn allows me to drive my subs alittle harder without port noise distortion.

    Now I dont know how good that is for the driver itself, but it sounds damn good.

    Jake
    What you just said doesn't correlate with anything I said. Nice. In fact, what you just said doesn't make any sense. The poly is "slowing down" the waves in the box? This, in turn, is reducing port noise? How so? If what you're saying is true, you've just destroyed the tuning of your subwoofer. Anywho, if you're happy, I'm happy for you :D

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    Your right, I should have reread what I wrote. It doesnt slow down the waves it decreases the volume of waves. Same speed just less air, seems the same as having a smaller port with a bigger box. But poly does have an impact on the waves inside the box. As the driver moves back and forth, the waves leave from the driver to the back of the box then back at the driver. Poly forces the waves through, what I see it as , like a filter. The waves have to pass through the poly before reaching the back of the box wall. Then passing through a second time after return from the wall towords the driver. Almost like a delay. I found that if your port leaves from the same side of the driver and reaches all the way to the back of the box the waves that have passed through the poly or in fact different than the waves that are on the same side of the driver, which gives a softer preasure coming out of the port. Same Fr just not as much turbulance.

    But I could be wrong.

    Jake
    Last edited by jakelm; 05-20-2006 at 10:51 AM.
    Monitor 7b's front
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    Frankinpolk Center (2 mw6503's with peerless tweeter)
    M10's back surround
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    Dayton 12" DVC w/ Rythmik 350a plate amp
    Harman/Kardon AVR-635
    Oppo 981hd
    Denon upconvert DVD player
    Jennings Research (vintage and rare)
    Mit RPTV WS-55513
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  17. #17

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    Quote Originally Posted by millerman 3732 View Post
    does putting foam and poly ( the cotten looking stuff ) in a sub really help or should it be used at all.
    1 lb. of pollyfil to 1 cu. ft. box size.
    Polyfil tricks your sub into thinking it's in a bigger box by about 30%.
    What this process really does is higher your Q.

    Qtc - value for the damping provided for a driver in a sealed enclosure. Denotes the enclosures ability to
    control the driver response at resonance. Qtc = 0.707 is the optimum value for sealed enclosures, providing
    flattest response and highest SPL for deep bass extension. Enclosures for this value are often rather large.
    Lower Qtc can give even better transient response, down to a Qtc of 0.577 for the best damping and
    transients, but the enclosure is usually huge and SPL's are down. A Qtc of 1.0 is a compromise between deep
    bass and transient response vs. smaller sized enclosure. Larger subs can go with an even higher Qtc, as their
    resonant frequency is often very low, but Qtc's above 1.5 can begin to sound very muddled and boomy, and
    sacrifice deep bass extension and transient response for enhanced mid-bass peaks (louder).

    I have a 3 cu. ft. sealed box with 2 12 in. subs in it. The exact size these subs call for. The inner volume of the box is slightly smaller because of the space the magnet of the subs take up. I added about 2.7 lbs. of pollyfil on all sides of the box. I cut 2 pillows and used the pollyfil from inside them. It was like sheets instead of a messy fluff (easy to work with). A pillow is about 2 ft. by 18 in. and 4 in. thick. I ripped the 4 in. thick part in half which gave me about 4 ft. by 18 in. and 2 in. thick. Put it in as loose as possible with staples.

    I notice a tighter bass sound. I don't know why, but this process caused my subs to hit way harder at a much lower amp gain. Before the pollyfil my amp gain was turned up 3/4's the way, almost full power from the amp. Now that the pollyfil is in I have to put my gain at less than half. I guess the positive side of this, is my amp only pushes half the wattage to slam the subs at their best. I may have gained a db. or 2 louder. It gave the subs more of a tight mid bass sound that is clean, rather than boomy (very punchy). It made a noticeable difference that I'm very happy with.

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