I can't comment yet, so will provide this as an answer too.

The Magnetic Reader, by 3M, original patent #US3013206A filed in 1958 and granted in 1961, allowed the user to look at data bit integrity on tape.

It was common to find these in mainframe shops. We had one at ours, though it was only used on a few occasions. We could view standard 7 and 9 track 1/2 inch tape, used on IBM 2401 tape devices, though the Magnetic Viewer could be used on almost any sized tape.

https://patents.google.com/patent/US3013206A

Adafruit has a blog entry as well, describing how it can be used to "view" sound, however I believe the viewing of computer tape was its original purpose.

https://blog.adafruit.com/2020/03/01/the-magnetic-tape-viewer-see-the-sound-on-a-tape/

I can't comment yet, so will provide this as an answer too.

The Magnetic Reader, by 3M, original patent #US3013206A filed in 1958 and granted in 1961, allowed the user to look at data bit integrity on tape.

It was common to find these in mainframe shops. We had one at ours, though it was only used on a few occasions. We could view standard 7 and 9 track 1/2 inch tape, used on IBM 2401 tape devices, though the Magnetic Reader could be used on almost any sized tape.

https://patents.google.com/patent/US3013206A

Adafruit has a blog entry as well, describing how it can be used to "view" sound, however I believe the viewing of computer tape was its original purpose.

https://blog.adafruit.com/2020/03/01/the-magnetic-tape-viewer-see-the-sound-on-a-tape/