Polyfill for ported box-thoughts?

[TheSpicySpider]

As for how I want the system to sound, I'm looking for an SQL system. Hence the lower tuning goals. How low of tuning would still sound accurate and play decently up to maybe 60-70 hz? I like low bass (rap, edm), but also listen to metal, classical, lots of other genres. I know I won't get perfect sound for everything, but I'm going for a do-it-all kind of thing here.

[Triticum Agricolam]

46 minutes ago, TheSpicySpider said:

The polyfill would be glued or stapled to the inside of the box. There are a few types of polyfill, and the really fluffy stuff isn't very usable. The kind I would be using is called batting, it resembles carpeting in a way. Still pretty cheap and fluffy, I can just work with it how I want to.

You can do that, it probably won't lower you tuning any though and it may cost you some output.  What you are talking about doing isn't stuffing, its lining, which is done for different reasons.

I've experimented with polyfill in ported boxes before, all it seems to do is reduce output around tuning.  

[bluegmcjimmy]

6 hours ago, TheSpicySpider said:

Building a new box to match an American Bass XFL 12 I am buying. The original plan was a 2.3 ft^3 net internal vol. tuned to 32 hz (slot ported). About 50% through building it (back, bottom, both sides assembled and all cuts made). However, it is my first build and I made the beginner mistake to not subtract the internal port wall's displacement from the overall internal volume (ironically, I remembered to subtract bracing and 45's). I redid my calculations and I am now facing a net volume of 2.1 ft^3 tuned to about 34 hz. I don't want those specs but don't really have the time to start over, so I am considering polyfill. I have heard that polyfill is a great choice for sealed enclosures, and have heard mixed reviews for ported enclosures. Overall, I have heard it both increases the effective box size and decreases port tuning. Is this correct? Does adding polyfill mess with the SQ of the enclosure? Thanks for any thoughts.

i was in your boat, i just used the poly fill. at full tilt with poly fill a full tall pepsi can sits still in my cup holder, with poly it wants to jump out the cup holder, maybe it just my box.

[ShadeTreeMechanic]

2 hours ago, Triticum Agricolam said:

You can do that, it probably won't lower you tuning any though and it may cost you some output.  What you are talking about doing isn't stuffing, its lining, which is done for different reasons.

I've experimented with polyfill in ported boxes before, all it seems to do is reduce output around tuning.  

I lined mine with an open cell foam mattress pad cut and stapled on the walls inside the box. This was done to absorb the wave reflecting on the back and sides of the box to keep it from radiating through the woofer cone and also keeps the box from sounding hollow. This is also done in pro audio and PA setups. Stuffing the box will prevent the port from directly coupling with the sub cone inside the box. You don't want any obstruction or diffusion between the cone and port. Stuffing is for sealed boxes.

[06RTCharger]

2 hours ago, ShadeTreeMechanic said:

I lined mine with an open cell foam mattress pad cut and stapled on the walls inside the box. This was done to absorb the wave reflecting on the back and sides of the box to keep it from radiating through the woofer cone and also keeps the box from sounding hollow. This is also done in pro audio and PA setups. Stuffing the box will prevent the port from directly coupling with the sub cone inside the box. You don't want any obstruction or diffusion between the cone and port. Stuffing is for sealed boxes.

That sparks a noob question for me. Why doesnt a port end up causing the back wave to reach the front wave? If this would take too much to explain, nevermind lol. I just had a "wait a minute.....?" kind of moment.

[Knocker]

1 hour ago, 06RTCharger said:

That sparks a noob question for me. Why doesnt a port end up causing the back wave to reach the front wave? If this would take too much to explain, nevermind lol. I just had a "wait a minute.....?" kind of moment.

The air coming out of the port ends up being in phase (mostly) with the front wave of the driver so it actually enhances the output until you go below the tuning frequency of the enclosure. Below tuning the air coming from the port starts going more out of phase as the frequency lowers resulting in phase cancellation which causes the steeper rolloff of ported enclosures. I don't understand 100% why this happens but until I read a loudspeaker design handbook I wondered the same thing you did.

[TheSpicySpider]

7 hours ago, ShadeTreeMechanic said:

I lined mine with an open cell foam mattress pad cut and stapled on the walls inside the box. This was done to absorb the wave reflecting on the back and sides of the box to keep it from radiating through the woofer cone and also keeps the box from sounding hollow. This is also done in pro audio and PA setups. Stuffing the box will prevent the port from directly coupling with the sub cone inside the box. You don't want any obstruction or diffusion between the cone and port. Stuffing is for sealed boxes.

Would the cell foam mattress pad work better for what I am trying to do? I know I don't want to just "stuff" the material in the box and need to keep whatever I end up putting on the inside away from the port, but what you are talking about sounds like what most people assume poly-fill does. By this I mean absorbing any reflecting waves to reduce distortion that would occur from the back waves radiating through the woofer. Does the open cell foam actually absorb some waves, while poly-fill doesn't?

[ShadeTreeMechanic]

The poly fill does the same thing as lining. Over stuffing poly fill gives the added effect of making a sealed box look bigger to the sub.

Check this out. http://www.parts-express.com/acousta-stuf-polyfill-speaker-cabinet-damping-material-5-lb-bag--260-330

 

[Joe X]

Here are a few conclusions that can be gathered from modeling damping effects, the following is for a SA-12v1 in a 2 net, 35Hz, 30 square inches of port area box RED: without damping, YELLOW: with "heavy" damping.

 

1) in a ported box damping will lose you output:

 

2) With damping, the enclosure will not load the driver as much around tuning so driver excursion is increased:

 

 

Related to the above impedance at tuning goes up, and evens out a bit around tuning this arguably may help limit driver heating in some cases:

 

 

2) damping will lower port airspeed:

 

 

This is interesting, it looks like if you are experiencing port noise you could in some cases reduce it / eliminate it with damping without rebuilding the box.

 

3) Damping reduces group delay:

 

 

So if you are experiencing loss of sound quality due to group delay you may try damping as an alternative to reducing net volume. In this case a good bit of group delay reduction is observed.

 

So it looks like damping may have some applications but helping your output looks not one of them so for the most part you will want to avoid it.

 

[Triticum Agricolam]

12 hours ago, TheSpicySpider said:

Would the cell foam mattress pad work better for what I am trying to do? I know I don't want to just "stuff" the material in the box and need to keep whatever I end up putting on the inside away from the port, but what you are talking about sounds like what most people assume poly-fill does. By this I mean absorbing any reflecting waves to reduce distortion that would occur from the back waves radiating through the woofer. Does the open cell foam actually absorb some waves, while poly-fill doesn't?

Foam mattress pad would probably be a better choice for what you want to do, and its easy to get.  I use 1/2" felt carpet underlayment for the same purpose, but you usually have to buy it in a BIG roll.  Other people use recycled cotton insulation (Softtouch I think its called) and it works great, I just can't get it in my area.  

 

Honestly, in a car audio environment you will probably never hear the different between an unlined and a lined subwoofer box.  It definitely doesn't hurt though if you want make the effort.