The application of EN 12354-1 to lightweight construction materials requires that the resonant component of the sound reduction index be separated from the reduction index measured according to ISO 140-3 or ISO 15186-1 which also includes contributions from the non-resonant component. One method of separating the resonant component is to calculate the non-resonant sound reduction index and then subtract it from the total, measured value. In this investigation, the non-resonant component was calculated using different theories and subtracted from the total sound reduction index measured for a steel panel. However, almost all of the resulting resonant sound reduction indices were negative over much of the frequency range. The errors in the separation of the components may have been due in part to measurement uncertainty which may be larger than the difference between the non-resonant and total sound reduction index below the critical frequency. A better method of separation may be a correction factor which is applied to the measured data. However, the correction factor itself must not be prone to errors or it may underestimate the resonant component of the sound reduction index.