# Poly fill with ported box



## 98Quicksilver (Jan 15, 2001)

Hi everyone
This may sound like a stupid question but. Should I fill a ported sub box with poly fill? if NOt, then how do you avoid standing waves in a ported enclosure.
Thanks
Mer


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## 2.ohh (Jun 21, 2002)

*Re: Poly fill with ported box (98Quicksilver)*

first off, building a rectangular box is a NO NO! make sure the back wave gets deflected so it doesn't come straight back at the sub. that is the reason for angled front or backs, or both. also you can staple insulation to the inside of the box, but do NOT stuff it with poly fil.


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## Jetta Go Boom (Dec 15, 2002)

*Re: Poly fill with ported box (2.ohh)*

Actually in a car audio enviornment the shape of the box makes no difference at all because the cabin of the car is too small for standing waves to form.


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## vedubau (Jan 17, 2001)

*Re: Poly fill with ported box (98Quicksilver)*

It may help to put some poly in a vented enclosure, but you do not want to stuff as much as a closed box system. You should try to keep the area around the port open. The best advice I could give is like 2.oh said and that is use sheet fiberglass insulation or poly sheets and coat the interior walls.
Jetta Go Boom: What? Standing waves are created whenever a speaker driver is located in an enclosure. It doesn't matter if the enclosure is in the car, home, or even outdoors. Standing waves are created by the movement of the cone and they are 180 degrees out-of-phase from the outside of the enclosure. If no fill is used and the front baffle is parallel to the enclosure's rear wall, this wave gets reflected back to the driver and causes colorations.
An ideal enclosure would have no parallel walls. 


[Modified by cchristensen, 9:17 AM 12-15-2002]


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## 2.ohh (Jun 21, 2002)

*Re: Poly fill with ported box (cchristensen)*

quote:[HR][/HR]
Jetta Go Boom: What? Standing waves are created whenever a speaker driver is located in an enclosure. It doesn't matter if the enclosure is in the car, home, or even outdoors. Standing waves are created by the movement of the cone and they are 180 degrees out-of-phase from the outside of the enclosure. If no fill is used and the front baffle is parallel to the enclosure's rear wall, this wave gets reflected back to the driver and causes colorations.
An ideal enclosure would have no parallel walls. 

[Modified by cchristensen, 9:17 AM 12-15-2002][HR][/HR]​thanks for answering that http://****************.com/smile/emthup.gif i was about to flame his ass







i built a box that had perfectly parallel front and back and top and bottom. i just did it to shoot the cone off the sub. i did it to prove to a friend that the back wave is very strong, and can cause damage and lots of distortion. but i was also talking to an accoustical engineer, and he said if you build a perfectly square box, it will give you superior sound and output. i haven't tried it, cause i go for SPL not killer SQ







and in a perfect cube, the back wave won't interfere with the cone at all, supposedly


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## vedubau (Jan 17, 2001)

*Re: Poly fill with ported box (2.ohh)*

I am not an accoustical engineer, but I don't see how a perfect cube can eliminate standing waves. Now I have heard that a perfect sphere is the perfect enclosure for standing waves.


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## 98JettaGLJoe (Jul 30, 2001)

*Re: Poly fill with ported box (cchristensen)*

I have to agree with Jetta Go Boom here...for the frequencies in the subwoofer enclosure the wavelength is about 11 feet (on the short end). Therefore to get a complete waveform out of the speaker and box to generate a standing wave you need a reflecting surface 5.5 feet away. True that the standing wave is 180 degrees out of phase with the original wave but that's only after the wave has reflected off a surface. Also true that you get the same wave out of the front of the driver as you get out of the back of the driver but you still need to reflect the whole waveform; you don't simply get one from the natural movement of the cone. If this were indeed the case, isobarik loading of subs simply would not work. You may get something similar to a standing wave if your car is big enough but you will not get a standing wave. There are not enough flat reflective surfaces in the right spots. The purpose of the polyfill in a box is to absorb the high frequencies that have a shorter wavelength and thus will form a standing wave in a shorter distance. So yes, you should put some polyfill in your box, usually about 10% of the total volume of the enclosure. But don't block the ports and round the edges if its a homemade box, it'll reduce the air friction around the port. The biggest issue here is, I think, that your enclosure has the appropriate air volume for the driver and that the box is tuned for the effect you want. Search around the web for the AVI sound subwoofer enclosure box designer program...last I found one at http://www.wssh.net/~wattsup/audio/ . For some straight understanding of box design and waves in car audio check out http://www.jlaudio.com/tutorials/boxdesign/index.html . JL's tutorial section is pretty good.


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## Jetta Go Boom (Dec 15, 2002)

*Re: Poly fill with ported box (cchristensen)*

Told you so







is a common misconception in the car audio world so I wont hold it against you....however if you had your mids in a square box you might see some problems...As far as the perfect box size goes I hear it was more of a pyramid type shape like you see in many B&W home speakers and such...but maybe a sphere is better but not very practicle. 
T=1/f ; @ 100Hz the period is 0.01s 
d=v*s ; d=(330m/s)*0.01 = 3.5m = wavelength 
(The lower the freq, the longer the wave. so, this is worst-case scenario, at 20hz the waves are 16.5m long). 
Now, even a 10ft long box is only 3m. So, you would need a very unusually big box to get any standing waves. If you use a box with separate chambers then there is no worry at all... DUH











[Modified by Jetta Go Boom, 7:41 AM 12-16-2002]


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## JLJetta (Nov 24, 2001)

*Re: Poly fill with ported box (Jetta Go Boom)*

As a general rule, stuffing a ported enclosure with polyfill ( or other like material ) is called a "resistively tuned enclosure". 
In a nutshell, it lowers the effective "q" of the port. (*) If done right, it can allow you to use a smaller than normal enclosure, yet still maintain a good low-frequency cut off. If the enclosure is already build, by stuffing it you will reduce the peak sensitivity at the "hump" in the response, and fatten up the bottom end a bit. 
The downside is that you can increase port noise, especially if the polyfill is in/near the port. 

As far as standing waves, you are correct. At the frequencies in question, it simply can not form a standing wave inside of a box. For midranges in home speakers, it is a problem, but not for subs. 
Mechanical strength of the box matters more. Many enclosures are trapazoidal because #1 makes for a shape that fits into a car better #2 generally will be structurally a bit stronger.
(*) yes, I know this is not technically accurate terminology


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## vedubau (Jan 17, 2001)

*Re: Poly fill with ported box (Jetta Go Boom)*

As I said, I'm no engineer and perhaps I am wrong. But tests documented in Vance Dickenson's Loudspeaker Design Cookbook, suggest unfilled enclosures with parallel baffles do experience colorations in the woofer's response down to Fs. I will agree that the differences are much greater as you go up in frequency, but you will still get some coloration. Is it bad enough to shy away from a particular design, perhaps not.
Also, correct me if I am wrong, but if you have a large enough enclosure for one complete wave-length or two or three etc, wouldn't you get cancellation? But any other length or portion there of, wouldn't it be coloration? And we haven't even touched on harmonics!
I realize I/we are getting off the topic originally posted, and I do agree that if an enclosure was not tuned based on poly stuffing, it shouldn't be added. But I would design an enclosure with poly or some form of damping


[Modified by cchristensen, 8:08 PM 12-16-2002]


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## Jetta Go Boom (Dec 15, 2002)

*Re: Poly fill with ported box (cchristensen)*

yea if you had a box that was about 7cuft and only had one chamber than it would be possible to have a standing wave


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## 2.ohh (Jun 21, 2002)

*Re: Poly fill with ported box (Jetta Go Boom)*

apparently you all missed the whole point that the back wave, the one in the box, causes damage to subs, if the box is not properly designed. you can create standing waves inside the box itself. have you ever worked with transmission line boxes? in the cab of the car it takes specifically designing the enclosure to do that for it to be possible. but what happens inside the box greatly affects the subwoofers performance. the rear wave can cause severe distortion and damage to a sub. there is no arguement about that. that is a fact. as far as a perfect cube goes, i was told that from an accoustical engineer. i don't know it to be true, but it intrigued me. standing waves are extremely uncommon in a car environment. i was making the point that the rear wave can be very destructive, if the enclosure is not properly designed. i made no comment about standing waves before. and the shape of the box can make a huge difference on how a sub sounds. and the distortion caused my the rear wave. if you doubt that, you are a fool. design is everything when it comes to subwoofers. any installer will tell you that


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## gtikev813 (Aug 26, 2002)

*Re: Poly fill with ported box (2.ohh)*

with all the talk of stuffing boxes and nonparallel sides, i was wondering what the effect might be if instead of using polyfill, cut sheets of egg crate foam mattress material would be used. any ideas?


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## 2.ohh (Jun 21, 2002)

*Re: Poly fill with ported box (gtikev813)*

adding insulation of any kind to a sub enclosure will change the tuning of the box. adding polyfill to a sealed enclosure is more important than any other type, in most cases. it slows down the wave, and tunes the box lower. slowing down the back wave reduces the chances of having it interefere with the cone of the sub, which causes distortion, and sometimes worse damage. in a ported enclosure it is not as necessary, but if used properly it will improve the sound of the enclosure.


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