Passive.

This is coolbert:

Here you can listen to the very recordings as were used to train American passive sonar operators during the Second World War [WW2] and thereafter.

"Historic Naval Sound and Video"

"Below are some real (nothing from Hollywood) underwater sounds and some video. These are in MP3 format Icon for MP3 audio which should work on most modern browsers."

1. "Expendable Radio Sonobuoy Training Records, 15P3: U.S. Navy, Bureau of Aeronautics - Special Devices Division"

"These are 78 RPM records prepared by Columbia University, Division of War Research at the U.S. Navy Underwater Sound Laboratory"

2. "Sonar Training Record Series D16"

"These are declassified training recordings of USN submarines on an early radio sonobuoy system."

3. "JP Sonar Training Records"
"The JP was the most important and most frequently used submarine passive sonar used during WW II."

4. "Sound In the Sea"
"These declassified sounds from the USN were collected together by sonar development company in the 1960s."

You too can pretend you are a passive sonar operator on an attack submarine. Endlessly fascinating being able to listen to and distinguish those sounds associated with:

* A submarine snorkeling.

* A submarine cavitating on the surface.

* A submarine cavitating at a depth of 100 feet [33 meters].

* A cruiser passing overhead.

* A cruiser at 15,000 yards [meters].

* A grey whale.

* A sperm whale.

Knowing and being able the sounds of the ocean deemed as "biologics" quite important.

"In 1992 Swedish Navy commissioned a new hydroacoustic system and it has been found that sounds similar to submarine noise were made by sea otters…"

And there are sea otters in the Baltic?

coolbert.