▁provide ▁music ▁to ▁patients ▁under going ▁an ▁M RI ▁scan , ▁as ▁it ▁would ▁be ▁dangerous ▁to ▁use ▁metal ▁w iring ▁in ▁the ▁sc anner ' s ▁magnetic ▁field . ▁ ▁In ▁Europe , ▁they ▁were ▁installed ▁on ▁the ▁w ards ▁of ▁hosp it als ▁for ▁a ▁time ▁where ▁the ▁patient ' s ▁' radio ' ▁was ▁built ▁into ▁the ▁nur se ▁call ▁and ▁light ing ▁control ▁unit . ▁This ▁unit ▁contains ▁an ▁elect romagnet ic ▁conventional ▁speaker ▁but ▁from ▁there ▁to ▁the ▁patients ▁ears ▁was ▁by ▁t ube ▁- ▁pres umably ▁so ▁that ▁the ▁part ▁contact ing ▁the ▁patient ▁could ▁be ▁easily ▁clean ed . ▁These ▁systems ▁are ▁still ▁in ▁use . ▁ ▁P neum atic ▁inter com s ▁can ▁be ▁applied ▁to ▁motor cycle ▁hel m ets ▁for ▁pilot - pass enger ▁communication . ▁Similar ▁systems ▁are ▁common ▁on ▁ul tr al ight ▁av iation ▁too . ▁They ▁are ▁sometimes ▁preferred ▁over ▁bl uetooth ▁or ▁other ▁radio ▁techn ologies ▁due ▁to ▁their ▁simplicity ▁and ▁absence ▁of ▁batter ies . ▁ ▁Up ▁to ▁the ▁ 2 0 0 0 s ▁ac oust ic ▁tub es ▁were ▁in ▁use ▁in ▁some ▁civil ▁aircraft ▁for ▁mov ies , ▁audio ▁and ▁other ▁audio ▁broadcast ing ▁in - fl ight ▁entertain ment . ▁ ▁The ▁principle ▁of ▁the ▁speaking ▁t ube ▁can ▁be ▁found ▁on ▁certain ▁play ground ▁equipment , ▁which ▁emp lo ys ▁tub ing ▁connecting ▁sound ▁horn s ▁or ▁other ▁speaking ▁boxes ▁to ▁allow ▁voices ▁to ▁travel ▁to ▁separate ▁points , ▁for ▁the ▁am