We use physical models to examine the effect of scatterers in the D' region of the mantle on the decay rate of the Pdiff phase in the core shadow. Scatterers are in the size range 7.5-81 km, and measurements of the decay rate are made in the period range 3-60 s. We find that the presence of scatterers in D' can change the decay rate of Pdiff by as much as 50%. The heterogeneity does not affect the decay rate for periods greater than about 10--20 s. At shorter periods, the decay rate can be either lower or higher than the laterally homogeneous case, depending on the scatterer geometry. We find that a layer of small (7.5 km) scatterers can decrease the decay rate, while layers of larger scatterers (41-81 km) mainly increase it. We did not identify any major effect of scatterer aspect ratio on the decay rate in the one relevant test that we performed (dikes and sills on the core-mantle boundary).