phase plug
... that the first phase plug in a horn loudspeaker was developed in the 1920s by Bell System engineers?
... that the first <b id="mwlw"><a href="./Phase_plug" id="mwmA" rel="mw:WikiLink" title="Phase plug">phase plug</a></b> in a <a href="./Horn_loudspeaker" id="mwmQ" rel="mw:WikiLink" title="Horn loudspeaker">horn loudspeaker</a> was developed in the 1920s by <a href="./Bell_System" id="mwmg" rel="mw:WikiLink" title="Bell System">Bell System</a> engineers?