October 2ist, 1919.] PROCEEDINGS. V. 



Professor W. L. Bragg, M.A., read a paper entitled "Sound 

 Ranging." 



A sound spreads from the point where it originates as a 

 spherical wave moving with constant velocity. If it is intercepted 

 by three or more stations whose positions are accurately known, 

 and if the time intervals elapsing between its arrival at the stations 

 are measured, a simple construction gives the position of the 

 source of the sound. 



Soon after the commencement of hostilities it became clear 

 that the struggle was going to take the form of trench-warfare. 

 This gave rise to the idea of locating the enemy guns by sound in 

 the way described above. The French made experiments with 

 "Sound Ranging" in October, 1914, and showed that it was 

 feasible, and the British Army was encouraged by their success 

 to send an experimental Sound Ranging Section to the front. 

 This section started operations in October, 191 5, taking up 

 its position opposite Wytschaete. At first the* results ob- 

 tained were poor, but they improved with experience and 

 better apparatus. The original section became a training 

 school for officers and men, and sufficient sections were 

 formed to cover the whole of our front. 



Each section had six microphones, spaced along a base 

 opposite the German front line. The microphones were connected 

 to a chronographic instrument at a central headquarters, and 

 when the sound reached the microphone it sent an electric signal 

 recorded by the instrument. In front of the base there were two 

 observation posts so placed that the sound reached them a few 

 seconds before it reached the microphones. This gave time for 

 an observer at the post to press a key which started the recording 

 apparatus at headquarters. By studying the record the time 

 intervals could be measured and the position of the gun plotted 

 on the map. This was then telephoned to the artillery. 



There were between thirty and forty sections along the front. 

 They could locate batteries between 10,000 and 15,000 yards 

 away with a mean error of about 50 yards. An idea of the 

 number of locations obtained may be gathered from the fact that 

 each section sent in about one thousand results in the year. 



The greatest difficulty was caused by adverse winds. If the 

 wind were blowing from our side of the line towards the German 

 batteries the sound never reached the microphones, being 

 deflected into the upper air. This meant that in westerly 

 weather Sound Ranging was of little assistance. 



The results obtained by Sound Ranging and other means of 

 location were used in the preparation of maps showing the 

 positions of the enemy batteries. We did not always try to 

 destroy an enemy battery which had been located, it was often 

 advisable to find out as much as possible about it and leave it 

 alone until occasion demanded that its fire should be neutralised. 



