▁ex ert ed ▁by ▁a ▁wave ▁of ▁the ▁same ▁intensity ▁in ▁air ▁is ▁increased ▁by ▁a ▁factor ▁of ▁ 3 7 5 0 ▁in ▁water . ▁▁▁▁ ▁The ▁American ▁Sub marine ▁Sign aling ▁Company ▁developed ▁a ▁hydro phone ▁to ▁detect ▁under water ▁b ells ▁r ung ▁from ▁l ighth ouses ▁and ▁light ships . ▁The ▁case ▁was ▁a ▁thick , ▁h ollow ▁br ass ▁disc ▁ ▁in ▁diameter . ▁ ▁On ▁one ▁face ▁was ▁a ▁▁ ▁thick ▁br ass ▁dia ph rag m ▁which ▁was ▁couple d ▁by ▁a ▁short ▁br ass ▁rod ▁to ▁a ▁carbon ▁micro phone . ▁ ▁World ▁War ▁I ▁ ▁Early ▁in ▁the ▁war , ▁French ▁President ▁Raymond ▁Po inc ar é , ▁himself ▁a ▁phys ic ist , ▁provided ▁Paul ▁L ange vin ▁with ▁the ▁facilities ▁needed ▁to ▁work ▁on ▁a ▁method ▁to ▁locate ▁sub mar ines ▁by ▁the ▁e chos ▁from ▁sound ▁p uls es . ▁They ▁developed ▁a ▁pie zo elect ric ▁hydro phone ▁by ▁increasing ▁the ▁power ▁of ▁the ▁signal ▁with ▁a ▁vac u um ▁t ube ▁ampl ifier ; ▁the ▁high ▁ac oust ic ▁imped ance ▁of ▁pie zo elect ric ▁materials ▁facil itated ▁their ▁use ▁as ▁under water ▁trans du cers . ▁The ▁same ▁pie zo elect ric ▁plate ▁could ▁be ▁v ibr ated ▁by ▁an ▁elect rical ▁oscill ator ▁to ▁produce ▁the ▁sound ▁p uls es . ▁▁▁▁▁ ▁Later ▁in ▁the ▁war , ▁the ▁British ▁Admir alty ▁bel ated ly ▁conven ed ▁a ▁scientific ▁panel ▁to ▁advise ▁on ▁how ▁to ▁combat ▁U