This chapter describes the basic principles of sound insulation with simplified assumptions. The methods for measuring airborne and impact sound insulation are briefly described. A sound wave incident on a wall or any other surface separating two adjacent rooms is partly reflected back to the source room, partly dissipated as heat within the material of the wall, propagates partly to other connecting structures and is partly transmitted into the receiving room. Sound insulation is measured in one-third octave bands covering the frequency range from 100 Hz to 3150 Hz. The idea of using a shifted reference curve for evaluating airborne and impact sound insulation comes from Germany and was adapted in ISO/R 717 in 1968. It was called the airborne sound insulation index I_a and was similar to R_w with the exception, that maximum unfavourable deviation in any one-third octave band was limited to 8 dB.