Speaker Grilles: Acoustically Transparent Fabric?

[user-name]

I'm wondering if speaker grilles alter the performance of a pair of speakers at all? While I'm certain manufacturers go to great lengths to prevent the grilles altering the sound (acoustically transparent fabric etc), I just wonder what is actually physically possible... My bookshelves have front firing bass ports that the grille covers and I wondered if it would interfere with their operation at all?

Is the purpose of these grilles to keep dust off the speakers or is it just an aesthetic thing?

 

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