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And the rear:
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The foam rings direct sound out through the grilles, keeping it from being lost inside the dash. The difference was so noticeable that I had to adjust the fader control a click rearward. The holes, front and rear, allow the speakers to breathe - they're not tuned, but they keep the speaker from alternately compressing and rarefying the air inside the enclosure. There *are* ways to have a completely sealed enclosure, but I don't know enough, etc, etc.. An A/B comparison with the rears (using my thumb to block the port) showed much better sound - when the port was blocked, the sound energy was transferred to the enclosure itself making a rather bad rumbley rattle. YMMV and all that.