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Posts: 6516
Location: America's Heartland
Joined: May 27, 2001
Contributor
Since: July 17, 2003
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If you'll excuse me for it, I'll present the long form response.
Since the Op was posting about an Acoustic Suspension speaker, I'll start there. In and AS design, the fiberglass filler is there primarily for thermodynamic reasons. It changes the compression and expansion of the air from Adiabatic (no heat transfer taking place) to Isothermal (constsnt temperature. This changes the rules for the pressure changes from power law to linear and reduces both the amount of pressure rise and lowers the harmonic distortion. Thus, the filled cabinet looks both larger and more linear to the speaker. For this to work all of the interior space needs to be filled, so that all of the air in the box is in close contact with the filler. Further, the filler needs to have significant thermal capacity relative to the air mass, and it has to have good conductivity and thermal diffusivity.
The original Acoustic Suspension experiments carried out by Edgar Villchur used brass wool. It was heavy, and it had high conductivity and diffusivity. However, it's also electrically conductive and markedly complicated building speakers, so fiberglass was tried. There was a measureable difference in harmonic distortion; but the difference was inaudible. Something like polyester fiber has low mass, low conductivity and low diffusivity and is worthless for the bass performance of an AS design.
The filler also has a second effect on the air - it increases the so-called Acoustic Compliance. That is, it reduces the speed of sound in the enclosure because some of the mass of the fiber is added to the mass of the air moving in the enclosure. A third effect is to dampen or reduce the Q of the system. So the speaker's Q must be figured into the design in order to get the right system Q for proper bass response. removing filler will raise the system Q and adding will lower it. You really want the cabinet 100% filled (leaving a small space immediately behind the woofer) and changing the Q by changing the density of the filler.
A fourth effect of the filler is to prevent internal standing waves and reflections at higher frequencies where the cabinet dimensions and the wave lengths of the sound coincide. At low frequencies, the filler is acoustically transparent; but becomes a better absorber at higher frequencies. Villchur, in his Acoustic suspension patent remarks on the filler's effect on internal reflections in the midrange and claims that it makes for a cleaner midrange. Similar claims are also made for the Transmission Line design with its wool filler.
As to higher frequencies, fiberglass is pretty good at higher frequencies; but a pretty thick layer is needed to have much effect. The problem at middle and higher frequencies is one of impedance. If the sound pressure wave in the air strikes a surface of significantly higher mechanical impedance, it will not penetrate; but will be reflected. In order for absorbtion to occur, the wave must enter the media and move the fibers, which rub against each other and change the acoustical energy to heat. Fiberglass can have a semi-permiable surface if it's made for home insulation, as the surface of the batts have a bonding agent on the surface. Acoustical fiberglass has an open surface. If you use insulation grade, be sure to orient it so that the sound waves strike the end or edge of the batts. In this respect, loose fiber materials such as Acousti-stuf can have an advantage in lower mechanical impedance to prevent reflections; but considerable depth is needed to be effective at middle frequencies.
This is already too long, so I'll leave out the consideration for ported enclosures.
Jerry
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