Sound locators were the primary (non-visual) method of locating aircraft during the inter-war years. The theory is pretty straightforward: by listening to the difference between the left and right horns you point the apparatus in the direction of an aircraft, and the upper and lower horns get the elevation. Once the aircraft could be seen, visual tracking and range finders enabled a director unit, often an analogue computer, to control anti-aircraft guns. At night the sound detector could be coupled with searchlights. I believe that image is a still from a post-WW1 US Army Signal Corps film, "Anti-Aircraft Material", available [on YouTube from the US National Archive] (https://www.youtube.com/watch?v=YvcIy9kDV6g), and there's an [accompanying blog] (http://unwritten-record.blogs.archives.gov/2014/11/10/mechanical-computers-and-sound-collectors-world-war-i-anti-aircraft-technology/) with some more information.

Sound locators ranged from personally worn devices to massive concrete acoustic mirrors. Douglas Self has assembled an [impressive collection of pictures] (http://www.douglas-self.com/MUSEUM/COMMS/ear/ear.htm) spanning the late 19th century to the Second World War. Some of the most impressive mobile units were the Japanese Type 90 detectors that have become known as "war tubas", as e.g. [mentioned on the panel show QI] (https://www.youtube.com/watch?v=lf1rDZiknww).

A serious drawback of sound locators was the short range. The effective range of the M2 sound locator is quoted on Self's page as 2,000 to 8,000 yards, highly dependent on atmospheric conditions; as aeroplanes got faster, this gave less and less warning time. Range, and thus altitude, were a lot more difficult to detect than direction, as was raid strength. Radar was clearly superior, and thus rapidly supplanted sound locators, but sound location was still used extensively in the Second World War, especially the early years. For example the British Chain Home radar system initially faced out from the coast, tracking of aircraft over land was done by the Observer Corps who used sound locators when weather prevented visual tracking. US searchlight units [trained with sound detectors] (http://skylighters.org/howalightworks/index.html), though radar guided searchlights were in widespread use by the time they deployed. Richard Overy also mentions sound detectors in Soviet and Italian air defence systems in The Bombing War: Europe 1939 - 1945. Acoustic location and sound ranging was also used in both wars to pinpoint enemy artillery, though I'm afraid I'm not so familiar with that side of things.