If you think about the way a (cardioid) mic works, it becomes very obvious that a
broadband absorber (eg. duvet) placed in front of the mic but behind the performer (and
ideally, extending around the sides too) will have a significantly greater effect in
reducing unwanted room reflections than a 'filter' placed behind the mic.
Both
are useful, but I'd rig the duvet before worrying about a filter.
hugh
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Technical Editor, Sound On Sound