Quote:
Originally Posted by rwojtalewicz
BD, this is exactly what I was saying. Yes it lowers the fb of the box or the resonant frequency of the port when polyfil is added. The density / cubic ft data clearly shows that is the case in the source that you got the data from.
That's fine. But what the author is overlooking are other effects of doing that. Designing a ported enclosure makes use of Qts and Qes of the driver, Xmax, and voice coil temperature or thermal limits and the desired box volume to keep cost down. Yes you can make the port turning frequency go lower by making the driver 'think' it's in a larger box by adding 0.5 to 1.0 lbs / cu. ft. However, if you don't know the driver parameters, you could end up fighting a three headed dragon.
First you need to know if the driver parameter Xmax is exceeded by trying to go deeper in the bass region. The cone needs to move more air to produce lower and lower bass frequencies. When the cone exceeds Xmax, it will likely bottom out depending on how far it tries to go.
Secondly, will the voil coil exceed it's temperature limits when being supplied with more current to keep up with the demand producing greater EM strenght to move the cone more?
I'm not saying that polyfil shouldn't be added. What I am saying is that the sub designer knows what these critical parameters are and their limits. It is the sub designer who should calculate the amount of polyfil necessary to achieve the optimum tuning frequency: that is to say, how low can I go without bottoming out?
You said earlier that I may be knocking AA when in fact I was praising them for not adding polyfil. It just may well be for their sub's driver parameters they knew exactly how much polyfil should be placed in the enclosure without causing it to bottom out with the lower bass extension. There is no such thing as a free lunch. You may go lower now, but not louder too.
My bone of contention about adding polyfil post production stage is that one ends up going about it blindly if he/she knows nothing about he enclosure and the driver parameters that the enclosure was specially built for. That's what I am saying, and that's why I am not adding polyfill to my PA120, eD, SVS, AA, Polk, Klipsch, Krell reference sub or any other sub I buy unless the manufacturer can tell me how much to add or provide me with the driver's Thiele-Small parameters, so I can calculate how much needs to be added.
Just because after someone opens the subwoofer box and is horrified after seeing a scant amount of damping material in the box doesn't necessarily mean that it needs more. How do you know for sure? All you can do is trial and error and hope for the best. That's not the way I like to do things. You could end up making a big mistake if the designer intended that the amount of damping material was optimum to begin with.
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Polyfill has three main benefits:
- It provides enclosure dampening.
- It makes the subwoofer box appear to be larger.
- It will lower the tuning frequency of the port and extends the low frequency response of the subwoofer.
I believe your conclusion that polyfill increases the risk of damage to the driver is incorrect. Let me start with an introduction and explain why polyfill will in fact lower the risk of damage to the driver.
In any type of enclosure, the woofer relies, at least partly, on the enclosure for damping. If there is insufficient damping, the woofer’s cone will move significantly more than it should. The ported box will provide virtually no damping below the port frequency (all of the control will be provided by the woofer’s suspension). Below the port tuning frequency, it is possible to cause damage to the woofer well before power levels approach the maximum power ratings of the woofer.
Port tuning is not rocket science. The builder decides what the tuning should be, based on the type of applications (HT, music, or car) that the subwoofer is made for. A subwoofer for car audio is tuned high 35Hz-50Hz because most car audio enthusiasts are SPL junkies. Music subwoofers are tuned in the 25Hz to 35Hz range, and HT subwoofers are tuned below 25Hz. The tuned frequency of the port, usually a length of PVC pipe, depends on the two port parameters of area and length for a given enclosure/woofer combination.
By adding a port, the rear wave of the cone motion is used to reinforce the front wave. This typically results in a system with a higher efficiency (it plays louder with less power). The disadvantages of this design are lower power handling and poor response below the port tuning frequency.
At frequencies above the port tuning frequency, cone excursion will be very well controlled, and will actually be lower than that of a sealed enclosure, but at frequencies below the port tuning, as there is no internal box pressure to control excursion, the driver can easily reach its excursion limit with very little power. This is not a big issue with tuning frequencies down in the 20-25Hz range, because most content do not go that low. However, if the tuning is in the 35-40Hz or higher range, you will have the potential for significant content to be below your tuning. Cone excursion below the tuning frequency in a ported enclosure is very, very high, and can easily cause damage to the driver if it is not controlled. At frequencies below the tuning frequency of the port, a woofer starts to de-couple. This means that the controlling function of the enclosure begins to disappear. The collapse is gradual rather than immediate, but at some point below the tuning of the port, the speaker behaves as if it were operating without an enclosure and suffers from potentially damaging over-excursion. This is why it is a good practice to use an subsonic filter when running a ported enclosure. Some subwoofer amplifiers come with a subsonic filter, which block these lowest frequencies.
What happens if the box is too small?
If the box is too small, the bass will be slightly boomy with strong mid-bass and less low bass. Typically, power-handling is improved if the box is not too small. You can compensate somewhat for a small box volume by adding polyfill to the box. The stuffing can make the box appear up to 40% larger to the driver and will lower the tuned frequency.
A slightly lower tuning frequency will add a little more safety due to the driver not unloading below tuning frequency as soon. Polyfill provides this additional safety.
What happens if the box is too large?
Typically the low frequency extension of the system improves a little, at the expense of power handling. If the ported subwoofer is too large, there will be a response peak around the resonance frequency of the system that can easily be corrected with an equalizer.
Ultimately, people are free to do what they please with their equipment and many people choose to modify their speakers and other electronic equipment. There are also a few upgrade companies around that offer equipment modification and upgrades and a company like Hsu Research was one of the first companies that offered variable port tuning with their VTF subwoofers.
It is important for people to understand that nobody is forcing anybody to add polyfill or do any other modification to their equipment. Furthermore, some modifications may improve the performance significantly and some modifications may not offer that much of a change or perhaps they may deteriorate the performance. That is a chance that all tweakers take.
I recently modified my vintage Polk speakers by upgrading their tweeters with silk dome tweeters and also adding super tweeters to them. I became very pleased with the results. I plan to replace their passive radiators with better quality radiators from another company and will definitely add polyfill to them. Remember that equipment manufacturers are not gods and modifying something is not blasphemy.
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