In November 2018, Radio Spectrum Archive curator Thomas Witherspoon announced a partnership with the Internet Archive to provide public access to his expansive collection of “spectrum recordings.” While traditional radio recordings document content transmitted across a single channel, spectrum recordings—typically made with Software Defined Radio apps and plug-in antennas—capture the entire swath of spectrum within an antenna’s receiving range at once, letting users later navigate the component signals through a visual interface by turning a virtual dial, similar to real-time terrestrial broadcasts. Heralded in press coverage as a veritable radio “time machine,” spectrum recording archives raise several pressing questions for radio history and preservation.

Highlighting the arbitrary nature of channel divisions and cultural politics of frequency allocation, spectrum recordings invite us to reimagine the very “stuff” of radio history. Agnostic towards individual programs and stations—historians’ usual objects of study—they indiscriminately mix familiar medium-wave and shortwave transmissions with lesser-studied forms of radio activity ranging from nondirectional beacons used in ship navigation to “whistlers” and other natural electromagnetic phenomena.  A spectrally-minded historiography must grapple with this intrinsic complexity of spectrum activity, historicizing its shifting forms and both human and nonhuman sources. Such work must also reflect on the role of the radio interface, with SDR’s own sound-on-screen displays and dynamic navigational mechanisms offering valuable reminders of the centrality of visual design, acts of user selection, and the heterogeneity of listening experiences throughout radio history.

Archiving spectrum recordings for future study presents daunting practical challenges. File sizes exceeding 60GB per hour for AM radio alone strain capacities of repositories scaled for conventional audio recordings, with sampling rates for higher-frequency transmission consuming even greater storage. Selection criteria for capture and preservation will likely be set in terms of times, locations, atmospheric conditions, and frequency ranges, rather than specific broadcasts. Defying work- and artifact-based approaches to archival description, resulting recordings will challenge traditional modes of discovery, but may also provide new opportunities for machine learning. Lack of coordination among SDR developers raises additional concerns about digital longevity and preferred modes of user access. However, these obstacles are by no means insurmountable, and archives such as Witherspoon’s offer exciting new possibilities for expanded histories and preservation of radio sound—all of it.