It is not true that manufacturers do not add polyfill to their subwoofers and speakers. Many high-end manufacturers do. I bought a 15" DBX sealed subwoofer in 1989-1990. It had a layer of polyfill inside the cabinet. In 1998, I bought two 12" Hsu cylindrical ported subwoofers. They were fully stuffed with polyfill. Even my relatively inexpensive 12" Acoustic Audio ported subwoofer had its interior covered with polyfill. Moreover, companies like Parts Express sell different types of cabinet damping material, including high-end polyfill.
http://www.parts-express.com/wizards...AT&srchCat=137
Generally speaking, adding polyfill requires large storage facility and is a relatively labor intensive activity. Manufacturers are too concerned about their bottom line and don't want to be bothered with a tweak that consumers can add with minimum hassle and expense. When I asked Elemental Designs to build a subwoofer box for my 15" Dayton Titanic driver, this is the respone I got from Brett Bell in one of his messages:
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Port Tuning with minimal to no stuffing will be just under 22 Hz, with medium to full stuffing you should be just under 20 Hz.
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BENEFITS OF POLYFILL
Polyfill can deepen bass, tone down any unwanted reverberation, improve the dynamic extension, and will give you tighter and more accurate bass. It also helps to give a flatter frequency response to the overall sound. Besides this, it also helps reduce the vibration of the box, which results in less distortion and tricks a subwoofer into thinking it is in a bigger box. Damping also increases subwoofer efficiency.
It is a good idea to use sheet dampening material to cover the entire inner surface of the cabinet. A small amount of glue may help. The loose polyfill is good for filling the cabinet. The type of subwoofer (down-firing or front-firing) does not play a role.
Polyfill can have several positive effects:
- It will dampen the cabinet.
- You will hear less noise, particularly the noise associated with the flow of air through the port.
- It will slow down the sound waves and makes the enclosure appear larger. When polyfill is added to an enclosure, it changes the behavior of the airflow in the enclosure from "adiabatic" to "isothermal". The term "adiabatic" means that there is no heat transfer occurring. An isothermal process means that as the air passes through the polyfill, the fibers wiggle and cause some of the energy created by the air to be dissipated as heat. This heats the surrounding air molecules warmer, causing the air to become less dense. Sound waves travel easier through a denser medium. As a result, the driver interacts with the enclosure as if it is larger than it actually is.
- Larger cabinets are more efficient and as a result low frequency extension of the sub will be lowered. It will also lower the resonant frequency of the port.
Adding polyfill to an enclosure is a good idea. However, too much polyfill can be a bad thing. At a certain point, the stuffing becomes too dense and the fibers no longer wiggle. At this point, not only have you taken away the size benefit of adding polyfill, you have actually decreased the effective volume as the polyfill is now taking up room inside your enclosure. You should add anywhere between 1/2 pound to one pound of polyfill to one cubic feet of enclosure.
The following diagram from
Basic Car Audio demonstrates the effect of polyfill on the frequency response of a subwoofer.
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Poly-fill can be used to make an enclosure seem slightly larger. It does this by slowing the movement of the air in the enclosure. This, in effect, smoothes the frequency response of the subwoofer's output and slightly improves its low frequency extension. This image shows, to what degree, it does both. The red line (no fill) has a slightly higher peak than the yellow line before it starts to roll off. The yellow line has a slightly improved low frequency extension when compared to the red line. The effect is not very dramatic but using poly-fill in this manner can sometimes make a subwoofer sound better. The red curve is has no poly-fill. The yellow line is 100% filled with poly-fill. 100% fill is equal to 1 pound of poly-fill per cubic foot of air space.
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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 very complicated. 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.
As was stated earlier, polyfill has these three main effects on a subwoofer box:
- It dampen the box and reduces the effect of back waves.
- It makes the enclosure appear to be larger than it actually is.
- It slightly lowers the tuning frequency of the ported subwoofer.
Pollyfill is a sound absorption dampening fiber that may deepen the sound and tone down any unwanted reverberation of any subwoofer.
Contrary to all the nonsense that is repeated in most Internet forums, we do not have standing waves in a subwoofer box, particularly in HT applications. Assuming that the vast majority of subwoofers for home audio are crossed at 80Hz to 100Hz, you can imagine the stupidity of this claim. These are the approximate wavelengths of bass frequencies:
20Hz: 56.5ft
50Hz: 22.6ft
80Hz: 14.1ft
100Hz: 11.3ft
For standing waves to occur, we need a subwoofer that is the size of a room. How many of you have a subwoofer that big?
Polyfill affects both sealed and ported subwoofer boxes. As a general rule, polyfill has more benefit for small boxes and since most sealed boxes are smaller than ported boxes, it appears that sealed boxes are affected more.
In car audio, most people play bass-heavy music at loud volumes and the subwoofers are tuned at a higher frequency for maximum SPL. They are more concerned about loudness.
In home audio, most people care about the quality of audio for HT and music applications. Most HT subwoofers are tuned lower for better low frequency extension.
This is from
Ultimate Polyfill Subwoofer Enclosure Resource
Usually about 1/2 pound to a maximum of one pound of polyfill per one cubic ft is enough. Over-stuffing a subwoofer box may have a negative effect. Some people suggest that you experiment with the amount of polyfill until you get the desired effect.
It is a good idea to staple or glue a thin layer of polyfill to all the interior panels of the subwoofer box and add sufficient amount of loose polyfill to the box. Remember that if you put too much loose polyfill inside a ported subwoofer, it may lower the sound quality and the polyfill may blow out of the port.
Although fiberglass can also be used, I do not recommend it as it can become a health hazard.
Polyfill can also have a positive effect on regular speakers. Speakers are more subject to back standing waves as the drivers generate much higher frequencies (lower wavelengths).
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