The classical form of the Waterhouse correction, the purpose of which is to take account of the higher energy density near room boundaries, has been developed to an improved form, which is a function of room modal density. The Waterhouse correction can be determined for each room by measuring or calculating its modal density. This is advantageous if the room is not a parallellepiped, in which case the accuracy of the classical form of the Waterhouse correction is worse. The improved form of the Waterhouse correction normally differs from the traditional one at third octave bands with center frequencies less than 100 Hz. There is a tendency for some measurement methods in building acoustics to be used in an extended frequency range down to a third octave band with a center frequency of 50 Hz. With that kind of extended frequency range, the refinement of the Waterhouse correction has an obvious effect. The Waterhouse correction of the receiving room should be subtracted from the result of traditional measurements of the sound reduction index. This is especially important if the results are compared with those of intensity measurements. No Waterhouse correction is needed for the source room. The measurement of sound insulation by the intensity technique needs no Waterhouse corrections.