Most of the stuff I find is over my head. Is there a brief explanation of why I need damping material in a bookshelf speaker, what happens if I have too little, and what happens if I have too much? Thanks.
Damping material absorbs the radiation from the back side of the speaker cone. Some absorption is needed, but too much starts to reduce cone excursion.
An empty box will resonate at specific frequencies (see: standing waves), and those resonances will be audible through the cone (or out the port, if not sealed). A frequency response measurement will be ragged, with a lot of peaks corresponding to those resonances. It won't be anything specific, the speaker will just sound bad.
If you put in too much, you lose bass output. There's a happy medium inbetween, that's best found by ear.
Too much is difficult to achieve. You would have to put so much stuffing in that it can't move well, This basically makes it a solid block that reduces the internal volume of the box. As long as there is some springiness to the stuffing it's probably fine. Measurements will tell you everything.
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