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View Full Version : Changing port tuning with polyfill



RSDXzec
07-02-2012, 08:53 AM
Got a box design I'm currently working on, and due to space limitations I'm only going to get about 1.7cf per 12" sub after port displacement. Now I've done some calculations and if I set my port length to a tuning of 33hz then if I can get polyfill to increase the volume seen by the subwoofer to 2cf then with the same port length I'd get closer to my goal of 30hz and I'd get the response I'd like. It's my understanding that polyfill can increase the volume seen up to 40% but realisticly about 20-25%, for this I'd only need about 17-18%. Does this seem like something that could work? I would obviously have to staple the polyfill to the walls so it doesn't fly out the port.

RSDXzec
07-03-2012, 12:47 AM
anyone?

snyderd758
07-03-2012, 01:50 AM
Theoretically yes your gonna have to play with it though i have never had much luck with polyfill in a ported enclosure. If your gonna go to all this trouble listen to your box at 33hz and see if you like it then add polyfill to suit your needs.

RSDXzec
07-03-2012, 04:00 AM
Theoretically yes your gonna have to play with it though i have never had much luck with polyfill in a ported enclosure. If your gonna go to all this trouble listen to your box at 33hz and see if you like it then add polyfill to suit your needs.

yeah since my box will have 3 chambers I'll be filling one up with polyfill and comparing it side by side to one without. I just wanted to know if anyone here noticed it lowered their tuning.

My way of testing tuning is to use some test tones and see whereabouts I get the least excursion. My last box was tuned to 25hz and it also had the least excursion at that frequency. But I'm not testing polyfill in that enclosure because it's bigger (2.2cf) so polyfill would be less effective therefore making it harder to lower the tuning frequency of it, which isn't something I'd like to do either.

However in my current situation due to space limitations I really think it could help, when you tried it in your ported enclosure did you test to find the tuning frequency after? did you notice it sounded any different?

Moble Enclosurs
07-04-2012, 11:18 AM
Hey man! Yea, as far as polyfil in a ported box, it is completely acceptable. There will be other noticeable changes though that occur than tuning, though. You may notice output decreases in a certain bandwidth due to the lowered efficiency, but as long as you keep the poly to the walls, and not just "fill" the box, that shouldn't change much, especially with high power. There may be a more dramatic loss at ONE certain point in the frequency response due to excursion vs phase changes, but this would be tough to calculate off hand as there is no real solid density formula for polyfil for use in ported boxes that I know of yet that configures those important factors. There are very good tests that have been completed to get an average per cube, but just like port area that effects cutoff, polyfil does similar things below tuning as well, so since I do not agree with the general square area per cubic ft rule, I do not believe in using a general rule for poly either.

That being said, I would personally leave that up to testing for the minor adjustments as calculation of that due to the many differences of each variable in each vehicle/box configuration, it may be inaccurate to formulate any solution PER box as of yet. BUt, now that this is an issue, I may look into it in the near future.

Polyfil works best in the use of QW transmission line enclosures, as those who know of them have likely heard, mainly from to the way it is designed. SO, in a conventional ported box, due to the fact that it consists of a compression chamber, the general formula for polyfil use is inaccurate because of the acoustical effects within the CC before the port is different.

Usually, compression chambers operate at much higher frequency efficiencies than the port, which is why the port is there in the first place........ so polyfil WITHIN the CC is not as desirable because of the minimal effects it will have on the response at lower frequencies, even if it were to change based on harmonics of higher ones. BUT if you were to line the port, you will likely notice more of a difference just thinking about the effects that occur. AND if you WERE to just fill the whole thing, though I still recommend only lining, then distortion may also decrease a bit and you will create a LP filter in the response more than just lowering tuning. SO, that may be worth testing first..........putting it in the port area only.

The whole idea that poly slows down the sound is not necessarily correct. The speed of sound is the speed of sound. Its more so that the reverberant effects are increased from dampening factors of the polyfil, so its not that sound slows down, its that the losses that occur increase at certain frequencies due to the density of the polyfil used.

That being said, Im sure I, or others, can come up with a formula to figure what density works for each frequency band and how big that band is. That will help a lot in figuring how much to use in the future.

It is a good idea to do what you mentioned by testing the excursion limits to figure for those changes. That is a great start. SO, trial and error is the real only way to get where you want to be with it right now, but try lining the port more than the CC.
What I would do is test all 4 factors:
1. WITHOUT poly
2. poly in the CC
3. poly in the port
4. poly in both

Likely the best audible changes will occur with the port only. BUT keep in mind that again, this does not just change tuning. It creates a filter and causes absorption within the enclosure which may lower efficiency and output at certain points. Its a balancing act at this point. Hopefully not in the future though. Maybe its time to do other testings now :D.
Hope that helps and sorry if it sounds confusing lol

fasfocus00
07-04-2012, 12:43 PM
if you're gonna use poly fill, get the batt version. easier to work with and easier to get it to just line the walls.

RSDXzec
07-04-2012, 01:35 PM
Hey man! Yea, as far as polyfil in a ported box, it is completely acceptable. There will be other noticeable changes though that occur than tuning, though. You may notice output decreases in a certain bandwidth due to the lowered efficiency, but as long as you keep the poly to the walls, and not just "fill" the box, that shouldn't change much, especially with high power. There may be a more dramatic loss at ONE certain point in the frequency response due to excursion vs phase changes, but this would be tough to calculate off hand as there is no real solid density formula for polyfil for use in ported boxes that I know of yet that configures those important factors. There are very good tests that have been completed to get an average per cube, but just like port area that effects cutoff, polyfil does similar things below tuning as well, so since I do not agree with the general square area per cubic ft rule, I do not believe in using a general rule for poly either.

That being said, I would personally leave that up to testing for the minor adjustments as calculation of that due to the many differences of each variable in each vehicle/box configuration, it may be inaccurate to formulate any solution PER box as of yet. BUt, now that this is an issue, I may look into it in the near future.

Polyfil works best in the use of QW transmission line enclosures, as those who know of them have likely heard, mainly from to the way it is designed. SO, in a conventional ported box, due to the fact that it consists of a compression chamber, the general formula for polyfil use is inaccurate because of the acoustical effects within the CC before the port is different.

Usually, compression chambers operate at much higher frequency efficiencies than the port, which is why the port is there in the first place........ so polyfil WITHIN the CC is not as desirable because of the minimal effects it will have on the response at lower frequencies, even if it were to change based on harmonics of higher ones. BUT if you were to line the port, you will likely notice more of a difference just thinking about the effects that occur. AND if you WERE to just fill the whole thing, though I still recommend only lining, then distortion may also decrease a bit and you will create a LP filter in the response more than just lowering tuning. SO, that may be worth testing first..........putting it in the port area only.

The whole idea that poly slows down the sound is not necessarily correct. The speed of sound is the speed of sound. Its more so that the reverberant effects are increased from dampening factors of the polyfil, so its not that sound slows down, its that the losses that occur increase at certain frequencies due to the density of the polyfil used.

That being said, Im sure I, or others, can come up with a formula to figure what density works for each frequency band and how big that band is. That will help a lot in figuring how much to use in the future.

It is a good idea to do what you mentioned by testing the excursion limits to figure for those changes. That is a great start. SO, trial and error is the real only way to get where you want to be with it right now, but try lining the port more than the CC.
What I would do is test all 4 factors:
1. WITHOUT poly
2. poly in the CC
3. poly in the port
4. poly in both

Likely the best audible changes will occur with the port only. BUT keep in mind that again, this does not just change tuning. It creates a filter and causes absorption within the enclosure which may lower efficiency and output at certain points. Its a balancing act at this point. Hopefully not in the future though. Maybe its time to do other testings now :D.
Hope that helps and sorry if it sounds confusing lol

hmmm, interesting, most info I've found on the net says to try and keep it away from the port. I also think it would be hard to line the port with it as my port is 1.57"x11.16".

Wouldn't putting it in the port disrupt air flow?

thanks for all the info btw, I'll be reading over it a couple of times before putting the polyfill in the enclosure.

Moble Enclosurs
07-04-2012, 05:11 PM
hmmm, interesting, most info I've found on the net says to try and keep it away from the port. I also think it would be hard to line the port with it as my port is 1.57"x11.16".

Wouldn't putting it in the port disrupt air flow?

thanks for all the info btw, I'll be reading over it a couple of times before putting the polyfill in the enclosure.

Its not for every enclosure, so there is not a definite that it will or can be applicable in every case. Its a matter of trial and error. And yes, that is a pretty small port for it, not to mention the ratio is stretching the boundaries already for having resonance issues. And the point of it IS to disrupt air flow, at least in terms of propagation patterns. As far as stopping it, that depends on the frequencies and the power, but mainly the acoustical effects of LF response pass right through it due to the length of the sound waves and the timing of each cycle. This is why it stops the higher frequencies first, due to wavelength characteristics.
Also, "airflow" and sound wave propagation are two complete different concept. The connection between the two are in terms of efficiency, and that, yes, will decrease a bit to create the effect you are looking for. It is pure compromise when it comes to purposely manipulating the sound other than using gain and acoustical boundaries of the such.
Putting anything in the way is usually not good for the efficiency, but in some cases where filters are needed, this is ideal due to the density of the material.
But yes, again, that port is fairly small and may change things more than you hope for-much like a CC where the larger it is, the smaller the changes, the same applies to the port. SO, having such a small port may give you less points of one sound to another to utilize in tweaking the effects properly.
Never know till you try it though :D. I would like to hear how it works out also!