Blocking bass ports on speakers

[Dolby]

I have rear facing bass ports on my speakers and was wondering what happens if - for example - I block them?
Would this allow me to move the speakers closer to the wall?

Lastly - is the point to totally block them with rubber or a solid substance?
I've seen some speakers use a rubber plug but others use a sponge/porous type plug - what is preferred?

 

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