Spectral Holography: Acoustic Imaging with Broadband Sound

Spectral holography measures shifts in the amplitude and phase of scattered waves as a function of their frequency to obtain information about the distribution and composition of the matter that scattered the waves. This approach contrasts with conventional optical holography, which encodes partial information about the amplitude and phase of light at just a single frequency. Such monochromatic holograms still provide valuable information because the spatial distribution of intensity values can be recorded at millions of locations simultaneously using standard video cameras and analyzed using the well-established theory of light scattering. Recording optical holograms over a wide range of frequencies typically are not feasible because of the cost and complexity of projecting and detecting spectrally-rich coherent light. Sound recording, therefore, offers two very substantial advantages over optical holography: (1) digital signal processing provides complete access to both the amplitude and phase of a sound wave’s pressure field, and (2) sound can be projected and recorded over a wide range of frequencies simultaneously. The additional information encoded in the spectral content of an acoustic hologram therefore can replace the spatial information recorded in optical holograms to provide volumetric imaging capabilities with just a few acoustic “pixels”. This talk introduces the methodology of spectral holography for acoustic imaging and provides a few illustrative examples of its use for imaging simple scatterers.