Acoustic quadrupole
= Acoustic quadrupole
An acoustic quadrupole has forcing $\partial_i\partial_jT_{ij}$. The <acoustic compact-source approximation> reduces its leading far-field density to $n_in_j\ddot S_{ij}/(4\pi c_0^4r)$ in three dimensions, with $S_{ij}=\int T_{ij}d^3y$ evaluated at retarded time.