This photograph shows the sound spectrograph produced by the Kay Electric Company. The device made it possible to create visual representations of the spectrum of audio signals as they change over time. In a sound spectrogram, a two-dimensional visualization shows the distribution of energy across the acoustic spectrum. Along the horizontal time axis, higher frequencies are shown in the upper part of the spectrogram, lower frequencies in the lower part. In an image produced by the Kay Sona-Graph, the density of black corresponds to the level of the respective frequency range.

The development of the technology of sound spectrography started at Bell Laboratories in 1941. During World War II, the spectrograph was used as a tool for analyzing and decoding scrambled enemy messages. After the war, the sound spectrograph, along with many other war-related developments, entered the commercial market. In the late 1940s, it was used in attempts to teach hearing-impaired people to read spectrograms of speech. The spectrograph also played a key role in the development of phonetics and other areas of study, including the analysis of bird song. Today, spectrograms and depictions of the wave form are the two standard methods of visualizing sound.

The acoustics department of the Technische Universität Berlin acquired a Kay Sona-Graph, possibly the one seen in the picture or a comparable model, in 1963 as part of Prof. Fritz Winckel’s project “Structural Connections between Speech and Music” (“Strukturzusammenhänge von Sprache und Musik”). Around 1977, a newer model was acquired with the help of the university’s English department.