pressed ▁air ▁gram oph ones ▁were ▁gram oph ones ▁which ▁employed ▁com pressed ▁air ▁and ▁a ▁p neum atic ▁ampl ifier ▁to ▁ampl ify ▁the ▁recorded ▁sound . ▁ ▁One ▁of ▁the ▁earliest ▁versions ▁was ▁the ▁, ▁designed ▁by ▁the ▁Irish ▁engineer ▁Sir ▁Charles ▁Par s ons . ▁It ▁was ▁capable ▁of ▁producing ▁sufficient ▁volume ▁to ▁broadcast ▁public ▁music ▁performances ▁from ▁the ▁top ▁of ▁the ▁Black pool ▁Tower , ▁and ▁was ▁said ▁to ▁be ▁loud ▁enough ▁to ▁cause ▁people ▁to ▁vac ate ▁the ▁front ▁rows ▁of ▁seats ▁in ▁an ▁aud itor ium . ▁The ▁A ux et oph one ▁was ▁sold ▁in ▁the ▁United ▁States ▁as ▁the ▁Victor ▁A ux et oph one . ▁ ▁A ▁ ▁was ▁real ised ▁by ▁using ▁a ▁sensitive ▁val ve , ▁which ▁required ▁little ▁force ▁to ▁operate , ▁to ▁mod ulate ▁the ▁flow ▁of ▁a ▁stream ▁of ▁com pressed ▁air . ▁The ▁basic ▁principle ▁of ▁the ▁val ves ▁used ▁in ▁these ▁devices ▁was ▁to ▁pass ▁the ▁stream ▁of ▁com pressed ▁air ▁through ▁two ▁partially ▁over la pping ▁comb s . ▁The ▁sound ▁v ibr ations ▁to ▁be ▁ampl ified ▁were ▁applied ▁to ▁one ▁of ▁the ▁comb s , ▁causing ▁it ▁to ▁move ▁later ally ▁in ▁relation ▁to ▁the ▁other ▁comb , ▁varying ▁the ▁degree ▁of ▁overlap ▁and ▁so ▁alter ing ▁the ▁flow ▁of ▁com pressed ▁air ▁in ▁sympathy ▁with ▁the ▁sound ▁v ibr ations . ▁ ▁The ▁, ▁designed ▁for ▁large ▁h alls , ▁was ▁a ▁com pressed ▁air ▁gram oph one ▁which ▁employed ▁com pressed ▁air