Clive Kidd gets in touch to say that sound mirrors get several mentions in The Baby Killers – German Air Raids on Britain in the First World War by Thomas Fegan, (Leo Cooper, 2003 ISBN 0-85052-893-3). pp153-4 Boulby and Redcar, pp176-7 Sunderland and p116 Kilnsea, briefly outlining a system of quadrants for plotting.
EW Sockett in Tyne Tees Defended:1 in World War 1 1914-18 RNAS RFC RAAF (ISBN 0952 8217 02) gives maps and further details of three north-east coast mirrors and WW1 observer lines. Also gets a write up in in T/T Defended II 1914-1941 ISBN 0-952-8217-1-0. He also gives lots of references to other articles he has written on Tyne Tees Defences, mainly in the Yorkshire Antiquites Journal.
Prof R W F Burns gives them a small bit in his IEE Hist of Tech paper on the detection and loction of aircraft and refers to a paper in Kew PRO “report 87 Preliminary report on Joss Gap Station of the acoustical section SEE W S Tucker 22 Sep 1920 AVIA 23/84 PRO Kew. Also has a ref to “Location of a/c by sound Journal of the American physical Society Vol XiV N02 1919 PP166-7.