Signal, Underwater Sound (SUS) Mark 84 ~ 1970 US Navy Training Film (Patreon)

more at http://quickfound.net/

Originally a public domain film from the US Navy, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.

The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

https://www.sonobuoytechsystems.com/webfoo/wp-content/uploads/2019/08/MK84.pdf

Air to Submarine Communications Device

The SUS MK-84 Mod 1 underwater sound signal device is an expendable electro-acoustic device which provides one-way acoustic communications with submarines. It can also be used to simulate the drop of an ASW weapon during a tactical exercise. It may be dropped or deployed from fixed-wing aircraft or helicopters as well-as by over-the-side ship-launched methods. The SUS MK-84 Mod 1 is able to transmit any one of five pre-selected coded acoustic signals, each of which may convey a predetermined message to the submarine. Four of these signals consist of a continuous tone that alternates in frequency in accordance with a timing sequence defi ned in the coded table (see specifi cation). The fifth code is a continuous tone at a single frequency. Third harmonics of the fundamental frequencies are also generated at levels slightly less than those of the fundamentals. A five position switch on the side of the unit accomplishes the settings. The MK-84 is designed to provide a source level of 160 dB/uPa within 2 seconds of water entry throughout the life of the unit...

https://en.wikipedia.org/wiki/Underwater_acoustic_communication