How does the porters scebe increase tension and suspende ?

In the scene just prior to the porter's, the audience witnesses the reaction of the Macbeth to Duncan's murder , and it is a very tense scene

In the scene just prior to the porter's, the audience witnesses the reaction of the Macbeths to Duncan's murder , and it is a verytense scene.  Macbeth is in a panic, thinking he'll never have clean hands again, and Lady Macbeth is chastising him for bringing the daggers from the room and calling him a coward for being unable to go back or pull himself together.  They hear the knocking at the door even...