Wisdom Audio Series S55i - Manual (Page 25)

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One important characteristic of any replacement HVAC grille or grate you may
elect to use for aesthetic reasons: many such grille have adjustable louvres to
direct the flow of air. These should be either removed or frozen in place with
liberal amounts of glue in order to avoid rattles. A lot of air will be flowing
through these grilles, in both directions. Anything that can rattle, will rattle.
The grate shown above was purchased from
http://www.signaturehardware.com/hardware/registers.html
It is made of cast iron, has ample open area to avoid turbulence and noise, and
has no air flow control louvres to rattle.