Anthony Lucas is the head of Technical Support at Eminence speakers. I sent him over some questions about the dust cap you see in the center of a speaker. I had an idea that they helped shape the sound to a certain degree but did not realize how much of affect they had.
Anthony talks about the practical and mechanical function of the dust cap as well as how the size and each material affects the sound and response of a guitar amp speaker.
This is all really great info and will help you understand more about speaker construction. It can also help you to make a better choice when purchasing a replacement speaker.