The introduction provides an outline of the so-called acoustic turn of the late eighteenth and early nineteenth centuries, when emerging scientific constructions of sound and its movement through the material world rendered that world audible in new and exciting ways. It argues that the new acoustic culture of the nineteenth century raised questions as to what lay beyond the limits of the human ear or scientific instrument and pointed to the existence of an inaccessible, intangible space between sound and silence, whose boundaries could not be measured and were always inherently unstable. That space, beyond the limited powers of human sensitivity, was a rich source of scientific, literary, and broader cultural reflection throughout the period. I delineate the volume’s progression through a series of auditory thresholds, each of which was brought to prominent scientific or medical attention in the period while becoming the subject of literary response and experimentation.