  Frequency combs (FCs) -- spectra containing equidistant coherent peaks --
have enabled researchers and engineers to measure the frequencies of complex
signals with high precision thereby revolutionising the areas of sensing,
metrology and communications and also benefiting the fundamental science.
Although mostly optical FCs have found widespread applications thus far, in
general FCs can be generated using waves other than light. Here, we review and
summarise recent achievements in the emergent field of acoustic frequency combs
(AFCs) including phononic FCs and relevant acousto-optical, Brillouin light
scattering and Faraday wave-based techniques that have enabled the development
of phonon lasers, quantum computers and advanced vibration sensors. In
particular, our discussion is centred around potential applications of AFCs in
precision measurements in various physical, chemical and biological systems in
conditions, where using light, and hence optical FCs, faces technical and
fundamental limitations, which is, for example, the case in underwater distance
measurements and biomedical imaging applications. This review article will also
be of interest to readers seeking a discussion of specific theoretical aspects
of different classes of AFCs. To that end, we support the mainstream discussion
by the results of our original analysis and numerical simulations that can be
used to design the spectra of AFCs generated using oscillations of gas bubbles
in liquids, vibrations of liquid drops and plasmonic enhancement of Brillouin
light scattering in metal nanostructures. We also discuss the application of
non-toxic room-temperature liquid-metal alloys in the field of AFC generation.
