The sound transmission loss of a limp barrier can be predicted with the field-incidence mass law formula. The formula in general use is, however, only valid when predicting transmission losses for materials with relatively high surface densities and/or at high frequencies. In fact, the formula will predict transmission losses that are less then 0dB for very lightweight materials (or at very low frequencies). This is clearly not possible, but is caused by unsatisfied assumptions.
In addition, the field-incidence mass law makes some assumptions about the effective angle of incidence of sound as it impinges the barrier. In fact, various authors use slightly different assumptions and this leads to differences in the mass law equations in the literature.
This paper will examine the general equation for the mass law and will look at the effect of the common assumptions when calculating the performance of very light materials and/or at low frequencies. It will also look at the assumptions involved with the angle of incidence.