At times, the transmission of sound through hardwood flooring can disturb others living in levels directly beneath.  There are three methods available to lessen this sound transmission considerably.
Rubber matting: Adhered to the substrate, with the hardwood flooring adhered above it.
Cork flooring: Adhered to the substrate, with the hardwood flooring adhered above it. ¼ or ½ inch can be used, with an approx.1/16 inch bed of adhesive needed to adhere the cork.
Bostik M.V.P.: A trowel on substance that dries to form a rubbery layer between the substrate and the hardwood flooring.  This method adds less height to the finished hardwood flooring installation (approx. 3/16 inch.)