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Acoustic Foam blocks for NHT iW1, iW2, iW3 and iW4 speakers. The NHT iWF also works with almost any other brand in-wall speakers. For use in 2 x 4 studded wall only. Dense acoustic foam that dampens vibrations and improves frequency response. Underwriters Laboratories (UL) flame class 94 HF-1 rated. One NHT iWF kit provides enough acoustic foam blocks for 2 speakers. The difference is remarkable.