For a short time period before the development of radar, militaries around the world experimented with devices that amplified listening called ‘acoustic locators’. Here are a few images of these amazing yet obsolete devices from this link (which has provided the caption text as well):

The height-locating half of the Czech four-horn acoustic locator. This picture is believed to show the testing at Waalsdorp.

Acoustic locators in Japan: 1930s.
To the right, one of the figures is the Japanese emperor Horohito. Behind him are the AA guns intended to be used in conjunction with the locators.

Acoustic locator on trial in France: 1930s.
This remarkable machine is an acoustic locator based on hexagons. Each of the four assemblies carries 36 small hexagonal horns, arranged in six groups of six.

Jean Auscher’s maritime acoustic locator: 1960.
This remarkable headgear was invented by Frenchman Jean Auscher as an acoustic navigation device in case of radar failure on small vessels. Shown at the 1960 Brussels Inventor’s Fair, and, one suspects, nowhere else ever again.