▁discover ▁that ▁effective ▁b aff les ▁must ▁contain ▁a ▁layer ▁of ▁air . ▁ ▁In ▁ 1 9 1 8 , ▁air ships ▁of ▁the ▁Royal ▁Naval ▁Air ▁Service ▁engaged ▁in ▁anti - sub marine ▁war fare ▁experiment ed ▁by ▁trailing ▁di pped ▁hydro ph ones . ▁ ▁B rag g ▁tested ▁a ▁hydro phone ▁from ▁a ▁captured ▁German ▁U - bo at ▁and ▁found ▁it ▁inferior ▁to ▁British ▁models . ▁ ▁By ▁the ▁end ▁of ▁the ▁war , ▁the ▁British ▁had ▁ 3 8 ▁hydro phone ▁officers ▁and ▁ 2 0 0 ▁qualified ▁listen ers , ▁paid ▁an ▁addition ▁ 4 d ▁ ▁per ▁day . ▁ ▁From ▁late ▁in ▁World ▁War ▁I ▁until ▁the ▁introduction ▁of ▁active ▁son ar ▁in ▁the ▁early ▁ 1 9 2 0 s , ▁hydro ph ones ▁were ▁the ▁sole ▁method ▁for ▁sub mar ines ▁to ▁detect ▁targets ▁while ▁sub mer ged ; ▁they ▁remain ▁useful ▁today . ▁ ▁D irection al ▁hydro ph ones ▁A ▁small ▁single ▁cyl ind rical ▁cer am ic ▁trans du cer ▁can ▁achieve ▁near ▁perfect ▁om n id irection al ▁reception . ▁ ▁D irection al ▁hydro ph ones ▁increase ▁sens itivity ▁from ▁one ▁direction ▁using ▁two ▁basic ▁techniques : ▁ ▁F oc used ▁trans du cers ▁This ▁device ▁uses ▁a ▁single ▁trans du cer ▁element ▁with ▁a ▁d ish ▁or ▁con ical - sh aped ▁sound ▁reflect or ▁to ▁focus ▁the ▁signals , ▁in ▁a ▁similar ▁manner ▁to ▁a ▁reflect ing ▁teles cope . ▁ ▁This ▁type ▁of ▁hydro