▁to ▁a ▁pressure ▁change , ▁such ▁as ▁a ▁sound ▁wave . ▁Some ▁pie zo elect ric ▁trans du cers ▁can ▁also ▁serve ▁as ▁a ▁sound ▁project or , ▁but ▁not ▁all ▁have ▁this ▁cap ability , ▁and ▁some ▁may ▁be ▁destroyed ▁if ▁used ▁in ▁such ▁a ▁manner . ▁ ▁A ▁hydro phone ▁can ▁detect ▁air bor ne ▁sounds , ▁but ▁will ▁be ▁ins ens itive ▁because ▁it ▁is ▁designed ▁to ▁match ▁the ▁ac oust ic ▁imped ance ▁of ▁water , ▁a ▁dens er ▁fluid ▁than ▁air . ▁Sound ▁travel s ▁ 4 . 3 ▁times ▁faster ▁in ▁water ▁than ▁in ▁air , ▁and ▁a ▁sound ▁wave ▁in ▁water ▁ex ert s ▁a ▁pressure ▁ 6 0 ▁times ▁that ▁ex ert ed ▁by ▁a ▁wave ▁of ▁the ▁same ▁amplitude ▁in ▁air . ▁Similarly , ▁a ▁standard ▁micro phone ▁can ▁be ▁buried ▁in ▁the ▁ground , ▁or ▁imm ers ed ▁in ▁water ▁if ▁it ▁is ▁put ▁in ▁a ▁water proof ▁container , ▁but ▁will ▁give ▁poor ▁performance ▁due ▁to ▁the ▁similarly ▁bad ▁ac oust ic ▁imped ance ▁match . ▁It ▁is ▁also ▁considered ▁a ▁sensor . ▁ ▁History ▁ ▁The ▁first ▁hydro ph ones ▁consisted ▁of ▁a ▁t ube ▁with ▁a ▁thin ▁memb rane ▁covering ▁the ▁sub mer ged ▁end ▁and ▁the ▁observer ' s ▁ear ▁on ▁the ▁other ▁end . ▁The ▁design ▁of ▁effective ▁hydro ph ones ▁must ▁take ▁into ▁account ▁the ▁ac oust ic ▁resistance ▁of ▁water , ▁which ▁is ▁ 3 7 5 0 ▁times ▁that ▁of ▁air ; ▁hence ▁the ▁pressure