hear only their own voices as they speak into the microphone.
The producer is in a sound-proof glass compartment, where he hears
the voices as they will be heard on the air. From that room he
gives signals, not only to the actors, but also to the person who
produces sound effects. The producer may come on to the stage
himself, but even then he does not hear the voices of the artists
directly, because, in order to be in the position of a listener-in, he
wears telephone ear-pieces, attached to a long flex, which enable
him to hear what is going on, but only through the microphones,
and still to give his signals as required. It is apparent, therefore,
that the rehearsals and the control of the artists during the rehearsals
in relation to many details are essential to the success of the play.