From Longman Dictionary of Contemporary Englishyou could hear a pin dropyou could hear a pin dropQUIETused to emphasize how quiet a place is You could have heard a pin drop in there. → unheard of → hear
Examples from the Corpus
you could hear a pin drop• After he finished telling the story you could have heard a pin drop.• It was so quiet in the hall you could hear a pin drop.• You could hear a pin drop in the auditorium during Norvell's speech.you could hear a pin dropyou could hear a pin dropspokenQUIET used to say that it is very quiet and no one is speaking → pin 