the foam apparently does need to be acoustically absorbent, according to the inventor's description. The foam that he specifies is not really a special acoustic foam or an acoustically transparent , it is just a generic soft open-cell foam. I think that you could probably locate something similar at local upholstery or sewing shops. here is what he specifies: from the McMaster-Carr on-line catalog (http://www.mcmaster.com/#) paste the part number 85735K72 in the search bar part no. 85735K72 (Same as 8643K512) Polyurethane Foam Sheet 3/4" Thick, 12" X 12", Charcoal, Firmness Rating 4 In stock at $5.94 Each Material Type - Polyurethane Foam Shape - Sheets, Bars, and Strips Backing - No Backing Thickness - 3/4" Thickness Tolerance - ±0.063" Length - 12" Length Tolerance - ±1/8" Width - 12" Width Tolerance - ±1/8" Temperature Range - -20° to +180° F Cut With Scissors, Knife, or Saw Tensile Strength -27 psi Stretch Limit -115% Density - 2 lbs./cu. ft. Foam Structure - Open Cell Texture Type - Fine Cell Foam Firmness - Very Soft Firmness, psi - 1.09 (25% Deflection) Compression Recovery - Excellent Color -Gray Finish -Textured Tolerance - Standard Specifications Met - Underwriters Laboratories (UL) UL Specification - UL 94HF1 Properties - Oil Resistant, Abrasion Resistant, Tear Resistant, Impact Resistant, Weather Resistant, Chemical Resistant, Electrical Resistant Notes - Material does not have a skin.