Interleaving methods and apparatus are disclosed for an in-band on-channel digital audio broadcasting system. The spacer has a flexible membrane defining an interior cavity. % of a transparent polyamide selected from the group MACM9-36, MACM9-36PACM9-36, MACMI12, MACMIMACMT12, or a mixture of at least two of said systems; between 10 and 60 wt.