These pictures show the measuring device Type 2304 produced by the Danish company Brüel & Kjær. Level recorders like this one made it possible to create visual representations of changes in volume and other measurable variables. A 50 dB potentiometer Type 2347, which enables the correction of the input level, is included in the device. When connected to external measuring and filtering instruments, the Type 2304 could be used in a laboratory setting to carry out specialized measurements. The roll of perforated paper at the front was used for printing the results. The photograph shows the last measurement that was produced with this instrument.

The quality and aesthetic appeal of the early devices produced by Brüel & Kjær contributed to their high reputation among sound engineers.