better sound

In the simplest terms, a "standing wave" (which can be produced in any loudspeaker design, by the way, not just in sealed systems) is air pressure that "cancels" a portion of the sound by restricting the driver cone's movement. or by simply being our t of phase with the direct radiated sound.  Think of it as puhing on the back of the speaker cone the opposite direction it is trying to move; or maybe like water waves crashing into each other from opposite directions.  This will create perceived "dead" spots in the loudspeaker frequency response.

Standing waves can be produced by speaker placement as well as by cabinet design, and can be a big problem in SQ car installs.  They are much more a problem at low frequencies than at higher ones.

Pink insulation will work the same as polyfill FOR A WHILE, but it is NOT recomended as the fiberglass particales can seperate and find their way into the speaker voice coils and spyders, eventually destroying them through friction.  Don't use it.  If you can't find the audio grade stuff near your location and you don't want to mail order from Parts Express or somewhere, good polyfill can be purchased at a fabric store, by the way.  It is used to stuff pillows and quilts.