  We experimentally determine the lattice dynamics of black phosphorus layered
crystals through a combination of x-ray diffuse scattering and inelastic x-ray
scattering, and we rationalize our experimental findings using $\textit{ab
initio}$ calculations. From the phonon dispersions at terahertz frequencies, we
derive the full single-crystal elastic tensor and relate it to the macroscopic
elastic response of black phosphorus, described by the elastic moduli. The
elastic stiffness coefficients obtained here provide an important benchmark for
models of black phosphorus and related materials, such as phosphorene and black
phosphorus nanotubes, recently emerged quantum materials, disclosing a huge
potential for nanophotonics, optoelectronics, and quantum sensing.
