The recommendation depends on the amount you want the sound to be reduced by.The best is a 30mm three-layer laminate panel of acoustic mineral board with a sound reduction rubber membrane, which is equivalent to 135mm acoustic material. For more information, visit www.noisestopsystems.co.uk or your local suppliers are E.T.S Ltd, Berwick-upon-Tweed or Northburn Acoustics in Cramlington. But you can use other types of insulation materials.