  Squeezed vacuum states are now employed in gravitational-wave interferometric
detectors, enhancing their sensitivity and thus enabling richer astrophysical
observations. In future observing runs, the detectors will incorporate a filter
cavity to suppress quantum radiation pressure noise using frequency-dependent
squeezing. Interferometers employing internal and external cavities decohere
and degrade squeezing in complex new ways, which must be studied to achieve
increasingly ambitious noise goals. This paper introduces an audio diagnostic
field (ADF) to quickly and accurately characterize the frequency-dependent
response and the transient perturbations of resonant optical systems to
squeezed states. This analysis enables audio field injections to become a
powerful tool to witness and optimize interactions such as inter-cavity mode
matching within gravitational-wave instruments. To demonstrate, we present
experimental results from using the audio field to characterize a 16 m
prototype filter cavity.
