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Changing port tuning with polyfill
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<blockquote data-quote="Moble Enclosurs" data-source="post: 7793647" data-attributes="member: 634917"><p>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.</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>What I would do is test all 4 factors:</p><p></p><p>1. WITHOUT poly</p><p></p><p>2. poly in the CC</p><p></p><p>3. poly in the port</p><p></p><p>4. poly in both</p><p></p><p>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 //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif.</p><p></p><p>Hope that helps and sorry if it sounds confusing lol</p></blockquote><p></p>
[QUOTE="Moble Enclosurs, post: 7793647, member: 634917"] 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 [IMG]//content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif[/IMG]. Hope that helps and sorry if it sounds confusing lol [/QUOTE]
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