INSUL is a program developed by Marshall Day Acoustics for predicting the sound insulation of walls, floors, ceilings and windows. The program can estimate the Transmission Loss (TL) in 1/3 octave bands and Weighted Sound Reduction Index (STC or Rw) for use in sound transfer calculations. The prediction theory, prediction comparisons to actual constructions and a demonstration version, are available for download at the INSUL website. Development on INSUL started more than 10 years ago, based on research in the USA by Ben Sharp and others. It has gradually developed into a unique tool that incorporates the best of many research papers published over the last 15 years. INSUL can be customised for particular materials.