Before the widespread use of radar,these instruments were used to listen for enemy aircraft during WW1 and WW2. The ability to hear approaching craft from 20 miles away was now possible. Blind men, who had developed an acute sense of hearing, volunteered for this service in order to play their part in patriotism.

The designs are very impressive and somewhat vulgarly industrial Britain, with their cast metal and awkward angles. You can learn more here.