Depends on the screen. Micro-perf screens are designed to be as acoustically transparent as possible. But the "normal" screen isn't. Exactly what you lose depends on the screen material, but you will lose a significant amount of the high end.
If you use a perforated screen, which is fine for placing a center channel behind it, you reflect less light, which could be a problem based on your FP lumens output as well as background material; decreased ability to display a bright image can also affect your ability to obtain a decent contrast ratio. That said, folks use perforated screens successfully with good visual result.
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