I was going to post this earlier and got side tracked.
Found some info here at AH:
http://www.audioholics.com/loudspeaker-design/loudspeaker-cabinets/identifying-legitimately-high-fidelity-loudspeaker-the-cabinet-face-stuffing
Myth #4: Polyester fiber fill is the best thing you can use to eliminate internal reflections and improve the sound of the speaker
Most polyester fill (those white or black fibers put inside speaker cabinets) are not the best thing to use to stuff speaker boxes. For those of you too young to remember back to the 70's, we typically used fiberglass in the cabinet to accomplish two main goals. Firstly, it absorbs mid and high frequency reflections from the back of the box, preventing them from exiting the system after a strong reflection from the inside of the box, and then again through the speaker cone, causing temporal and frequency aberrations. Secondly, fiberglass actually physically makes the box behave larger than it really is, effectively letting your woofer reach down to a lower frequency cutoff than it would without the fiberglass.
Fiberglass Insulation (left pic) ; Polyester roll (right pic)
Back in the day when the USA still had a significant manufacturing sector in the audio industry, we routinely used fiberglass to accomplish these goals. The problem with fiberglass, and the primary reason it was replaced with polyester fibers, is that it is created from short straight glass fibers (normally held together with a glue) which come free from the material and stick in your skin providing a great deal of irritation when it is handled. The loudspeaker industry’s collective solution to this was to switch to polyester fiberfill, the same material used in perhaps as many as 90% of all those hypo-allergenic pillows you find in the department store. This was a great relief to those who had to stuff boxes, and a great irritation to the few of us who actually performed side by side comparisons with real fiberglass. Having measured different varieties of polyester fiberfill (including hollow fill) and comparing it to fiberglass, the results of using most fiberfill are much closer to using nothing than they are to using fiberglass or higher density sound fill. (Yes, long haired wool is still the best, and at last check it still cost about $13 per pound.) Ironically, Owens Corning solved the problem of fiberglass irritation by creating a product some years ago where the fibers were curled, which stopped them from irritating exposed skin. Sadly, because it was slightly more expensive than the straight fiber fiberglass, despite the fact it was non-irritating to the skin, the company ceased production of this material. It is a good bet, the cheapest and fastest and most dramatic fix a consumer can make on his system would involve replacing polyester fiberfill with real fiberglass in his cabinet. To keep it in place, a slightly porous cloth (we use grille cloth) can be stapled on top of the fiberglass to prevent the escape of the material through the ports, or its entry into the internal moving parts of the loudspeaker drive units. The grille cloth is acoustically transparent so its presence does little to stop the wave from being absorbed (in part or whole) by the fiberglass.
Editorial Note by Philip Bamberg
SPL inside the box can be 140dB, so even fiberglass will not reduce the level to prevent the bad sound from emanating back out of the woofer.
Inside the constrained volume of the box we have standing waves and traveling waves. Standing waves occur at low frequencies, while traveling waves occur at higher frequencies. The transition occurs at a frequency whose wavelength equals double the distance between two walls. A good example is a tower cabinet whose large side panels have an interior spacing of 7”. Standing waves then apply for frequencies below 13500/7/2~=1000 Hz. Traveling waves are above this point. Say the 7” woofer crosses over at 2 kHz. Now there is one octave of passband traveling frequency, and another octave or two of stop-band frequency. Stuffing of any kind will help to attenuate the internal (bad) sound. But making 140dB much quieter is simply not possible through stuffing quality alone. For this reason, I would never use fiberglass!
Larger volume boxes show lower internal SPL. This is because there is a longer path to provide attenuation of traveling waves.
Stuffing the entire box volume also effectively increases the box interior volume, since the speed of sound is reduced slightly. This is akin to increasing the box volume by about 15%. This will skew the bass rolloff a little, and the designer can take this into consideration.
All my closed-box monitors are fully stuffed with poly-fil, and all my vented designs are about 80% filled, because the area near the port inlet must be unblocked. I arrived at the decision to stuff ported speakers this way after empirical measuring and listening tests.
Now here is a simple test that can be performed on any speaker with the woofer removed. We all possess a small wideband sound source, which is the sound made by snapping our fingers. Snap your fingers inside the box by reaching through the woofer hole. Listen to the sound with your ear near the hole. Do this with 100% stuffing, and with no stuffing. You can also knuckle-rap a cabinet panel and listen. The sound is much more dead with the fully stuffed configuration.
Another way to listen to the inside of a speaker is to play music or test tones at a low level, and then put your ear up to the port exit. You may be surprised at how audible and colored sounding is the midrange band. Add more stuffing to this situation and the colored sound reduces in amplitude. So there is also a myth that one can adjust the detail of their monitor speaker by adding or removing stuffing from inside the box. Never remove stuffing! If the audiophile needs something to tweak, he needs to look elsewhere. Any increase in “detail” he hears with less stuffing is truly an increase in the colored interior box sound, and not more signal detail.
Editorial Note by Jeff Symanski
Loudspeaker fill serves a number of purposes, but the more surface area that is included, the better the outcome in terms of loudspeaker performance. Polyester will inherently have less surface area than most glass fiber materials. Thus, I would probably come out on the side that glass fiber is usually the better way to go. Foam material with a surface area similar to or higher than glass fiber will also perform as well as or better.
Surface area in this context is the composite surface area formed by all the interstices in any porous material. It can also be thought of in terms of density. The denser the better as long as the material is significantly porous. Polyester is generally less dense than other porous materials, such as glass fiber and open-cell polyethylene / polyurethane foams.
The one caveat would be related to the actual surface area / density of polyester. For a more thorough analysis, I would recommend obtaining information on the specific fill materials being used. A simple comparison of their (ASTM) flow resistivity values should be sufficient. The hard part would be obtaining that sort of information from loudspeaker manufacturers.
Bottom Line: Some poly fill / dacron can be a compromise and better insulation (ie. fiberglass or high density poly fill) will improve the sound quality of your speakers. If the manufacturer is willing to spend the time and money to do it right, that is a bonus. If not you as the end user should be willing to replace the stuffing of your speakers. It is neither difficult nor expensive, and can result in an audible and measurable improvement in your sound.
