  As a fundamental physical quantity of thermal phonons, temporal coherence
participates in a broad range of thermal and phononic processes, while a clear
methodology for the measurement of phonon coherence is still lacking. In this
Lettter, we derive a theoretical model for the experimental exploration of
phonon coherence based on spectroscopy, which is then validated by comparison
with Brillouin light scattering data and direct molecular dynamic simulations
of confined modes in nanostructures. The proposed model highlights that
confined modes exhibit a pronounced wavelike behavior characterized by a higher
ratio of coherence time to lifetime. The dependence of phonon coherence on
system size is also demonstrated from spectroscopy data. The proposed theory
allows for reassessing data of conventional spectroscopy to yield coherence
times, which are essential for the understanding and the estimation of phonon
characteristics and heat transport in solids in general.
