Sound Ranging in short, is a method employed to locate enemy guns by means of calculating the distance sound (the firing of a gun or mortar) travels from four microphones located in known positions and then plotting cross bearings on a map to establish from where the enemy fire came. And so enable counter bombardment fire at it. There was a Command Centre and an Advance Post.
The command centres function was to use the information derived from microphones and surveyors situated forward to determine and plot enemy artillery positions. Close in targets would be attended to with 61st Light Regiment 4.2” mortars. More distant targets would be passed to the 25 pounders or US 155mm “Persuaders” howitzers. Very occasionally, the duty officer would be contacted to see whether we had a target suitable for aerial attack!