What Is the Main Noise Protecting Component in an Earmuff?

What is the main noise protecting component in an earmuff?

I just bought an earmuff but the inner sponge is very easily moved out of place if pushed, and now I can touch the inner plastic wall of the earmuff.

Is this earmuff compromised or is it the plastic cup that does the protecting?

 

I think it might be compromised. The inner sponge in my muffs is securely attached to the body of the muffs...