What does the knocking in Act 2, Scene 2 of Macbeth represent?

After Macbeth follows through with the assassination of King Duncan, he appears from the king's chamber distraught and overwhelmed with guilt. Macbeth then begins to experience auditory hallucinations and his wife tries her best to calm his nerves. At the end of the scene, the couple hears a mysterious knocking coming from offstage and Macbeth comments that he wishes the knocking would wake King Duncan, which highlights his guilt. In the following scene, the audience learns that Macduff has been knocking at the gate and enters Macbeth's estate at Inverness. The knocking itself is ominous and foreboding, which corresponds to Macduff's purpose in the play. It is Macduff who will oppose Macbeth and eventually kill him in the final battle. At the beginning of act 2, scene 3, the porter answers the door and allows Macduff to enter Macbeth's castle. Shortly after entering the castle, Macduff discovers that the king and his chamberlains have been murdered.

At the end of the scene, Macduff and his companions are arriving at the castle. They apparently had been told by Duncan to come early-- indeed, earlier than Macbeth had expected them. The last line of the scene has Macbeth shouting, "Wake Duncan with thy knocking! I would thou could'st." The exasperated plea is similar to the cliche that something is so loud, it could wake the dead. Macbeth immediately follows up with a statement of guilt. He regrets what he has done, and the guilt will subsequently consume him.

What is the effect of the knocking at the end of Act 2, Scene 2 in Macbeth?

The knocking designates the actual arrival of Macduff and Lennox, but its powerful effect is to startle Macbeth, who has just killed Duncan.  Macbeth's first response to the knocking is: "Whence is that knocking? / How is 'twith me, when every noise appals me?"  That is, "Where is that noise coming from and why does it bother me so much?"  The noise piques his conscience.  The pragmatic Lady Macbeth says they need to get dressed for bed (as if they were awakened by the sound), but Macbeth expresses remorse when he ends the scene with "Wake Duncan with thy knocking! I would  thou couldst!"  The knocking heightens an already suspenseful scene.  At this point, the suspense is so high that Shakespeare introduces comic relief with the porter, who answers the door and jokes about the knocking.  But as he is "the porter of hell gate," we don't forget about the gruesome murder that has just taken place.  

In Macbeth, why is the knocking in Act 2, Scene 2, dramatically effective?

The knocking at the gate of Macbeth's castle is effective because it occurs just after the murder of Duncan when Macbeth and Lady Macbeth are both in a highly anxious state of mind.  The two, having just killed their king, do not have the luxury of relaxing and calming themselves.  Macduff is knocking at their gate because the king had asked him to arrive early that morning and waken him.  Now Macbeth and Lady Macbeth must hurriedly wash their hands and, as Lady Macbeth commanded, get their nightclothes on so that it appears they've been awakened from their sleep.  In reality, they haven't been to sleep yet because of the murder.  Macbeth is so distraught, though, that he declares he cannot return to the king's bedchamber to put the daggers on the guards, which Macbeth forgot to do after he stabbed Duncan to death.  Lady Macbeth chastises him, telling him that it's no big deal to have blood on his hands.  She takes the daggers and puts them on the guards, thus getting blood on her hands.  Both Macbeth and his wife have bloody hands when the knocking begins, so the timing of the knocking emphasizes the anxiety here as they have to clean up and try to appear as though they've just been awakened and have no idea a murder was committed in their house.

What is the significance of the porter's scene in Macbeth, Act 2, Scene 3?

To me, this is a bit of foreshadowing.  The porter keeps talking about Hell and Beelzebub and things like that.  He talks about the kinds of people who might knock for admittance into Hell and compares himself to the gatekeeper of Hell.

This implies that the castle (Inverness) is Hell and that really bad things happen there.  Of course, he is right -- Duncan has already been killed here (although he does not know it yet).

In addition, this implies that Macbeth is the ruler of Hell.  It implies that Macbeth's rule will turn Scotland into a hell as well.

Many critics have assumed that the so-called Drunken Porter Scene (Act 2, Scene 3) was inserted for comic relief. It is funny, but it has a serious purpose. The Porter has to explain why there has been such a prolonged knocking at the gate. What is important is the knocking. When the Porter finally opens the gate, he explains to Macduff that he and all the other members of the household staff were too drunk to respond.

Macbeth had planned to be pretending to be sound asleep when Duncan’s body was discovered. His wife tells him:

Hark, more knocking.
Get on your nightgown, lest occasion call us
And show us to be watchers. 

But Shakespeare wanted Macbeth to be present at the scene of the crime. The knocking forces Macbeth to come down in his nightgown to find out why nobody is opening the gate. This brings him and Macduff together for the only time before their death duel at the end of the play.

The discovery of Duncan’s body is much more dramatic than it would have been without Macbeth’s presence. The audience witnesses Macbeth’s apprehension and understands how he is suffering internally while trying to appear calm and natural.

No doubt the drunken porter elicited a great deal of laughter, especially if a talented clown had the role. Shakespeare knew that comedy would help to beguile his audience and keep them from asking themselves such questions about the plot as:

Why wasn’t Macduff sleeping inside the castle?

Why didn’t Duncan appoint someone else to wake him in the morning, someone such as Banquo who would be accommodated inside the castle?

Why did Macbeth and his wife permit his entire household staff to get so drunk when the King himself was their guest?

Why wasn't Macduff furious when the Porter finally let him in?

Shakespeare had to hope that his audience wouldn’t ask such questions because he wanted Macduff to discover the body and he wanted Macbeth to be present when he did. So the “comic relief” had a very serious purpose.

The idea of having Macbeth appear in his nightgown probably inspired Shakespeare to have everybody appear in nightgowns when Macduff shouts, "Ring the alarum bell" and then calls to all the sleeping guests:

As from your graves rise up and walk like sprites
To countenance this horror.

They will all look like ghosts (or sprites) when they appear onstage in white nightgowns. These sprites will include Banquo, Fleance, Malcolm, Donalbain, Lady Macbeth, and others. It will be a striking visual scene.

What is the significance of the porter in Act 2, Scene 3 of Macbeth?

Shakespeare had a habit of writing a small part into each of his plays that would make audiences laugh, as a bit of comic relief from the tragedy at hand. A good example of this is the gravedigger's role in Hamlet. Others are the Fool in King Lear and Touchstone in As You Like It. Shakespeare decided to make the drunken porter scene in Macbeth comical, but it has a serious purpose.

Macbeth and his wife have planned to pretend they were sound asleep when the King's body is discovered. Lady Macbeth tells her traumatized husband

Get on your nightgown, lest occasion call us
And show us to be watchers.

However, Shakespeare wanted Macbeth to be present when Macduff discovers the body. This would be the only time that Macbeth and Macduff, the protagonist and antagonist, would appear together in the entire play until their death duel at the very end. Shakespeare invented the prolonged knocking to force Macbeth to come down in person to find out why nobody is opening the gate. He is forced to play a different part than the one he was expecting, and he does a terrible job of greeting Macduff and Lennox and of pretending to be  innocent when Macduff goes into the King's chamber and comes out screaming,

O horror, horror, horror!

Before Macbeth appears on the scene, the drunken Porter has finally opened the gate--and we learn that it is Macduff who has been doing all the knocking. The Porter's drunken condition is explanation enough for his dereliction of duty. He tells Macduff that the entire household staff was drunk because they were "carousing till the second cock" (which my edition of the play explains was about 3 A.M.). All the Porter's speech is only intended to explain why there was so much knocking, and the knocking was only included in the play in order to make Macbeth come down in his nightgown.

If the Porter produces a lot of laughter, that is advantageous because it will make the audience overlook some logical questions. Why did such an important thane as Macduff have to spend the night outside the castle? Why did Macbeth and his wife permit the entire household staff to get blind drunk when they were hosting the King himself? Why didn't Duncan ask someone to wake him who would be sleeping inside the castle that night? Banquo was sleeping inside, and so were many others. Why doesn't Macduff, who has a fiery temper, beat the Porter, or at least berate him, for keeping him waiting outside in the cold and rain? But when Macduff finally enters, dripping wet, he asks the Porter

Was it so late, friend, ere you went to bed
That you do lie so late?

Macduff's gentle tone is highlighted by the fact that he calls the Porter "friend."

There are answers to all these questions, but they go beyond the scope of the scene with the drunken Porter. At any rate, the scene was not just thrown in for comic relief, and the knocking at the gate was not invented, as Thomas De Quincey suggested, for dramatic effect.

What is the dramatic significance of the porter/knocking scene in Macbeth?

Many questions have been posted in eNotes about the drunken porter scene in Macbeth(Act 2, Scene 3). Many of the answers deal with the concept of comic relief. It seems likely that Shakespeare inserted comic scenes in some of his tragedies because his audiences liked to laugh. The plays were entertainment, diversion. One might compare the comic interludes to the shows that used to be offered in movie theters. There would be a serious drama plus a cartoon featuring characters like Bugs Bunny or Tom and Jerry, and there might be a short subject with comedians like the Three Stooges. Since Shakespeare was only presenting a single feature (so to speak). he might have been inclined to insert comedy as a sort of bonus to please the members of his diverse audience who wanted that sort of thing.

No doubt Shakespeare had at least one member of his company who was especially good at making people laugh. He couldn't just assign any actor to a role like that of the drunken Porter. Good comedians are rare. If he had a talented comic he would be thinkinig of using him often, and it well might be that the same actor who played the drunken Porter in Macbeth also played the gravedigger in Hamlet. Critics who have questioned the purpose of the drunken Porter scene have also questioned the purpose of the gravedigger scene. Both may have been devised purely to utilize the talents of a particular member of Shakespeare's company.

Furthermore, the same comedian might have played the part of the Fool in King Lear, of Touchstone in As You Like It, and many other parts, including the clownish peasant who brings the two poison adders to Cleopatra in Antony and Cleopatra. In Julius Caesar there is a part for a comic who calls himself a cobbler and is leading a group of Commoners in the very first scene. One might visualize this actor as someone who looked and acted like Bert Lahr in The Wizard of Oz--a natural comedian who could make audiences laugh just with facial expressions and tones of voice.

Shakepeare may not have been so much concerned about "comic relief" as he was about simply providing a little comedy and making use of the special talents of one of the members of his company.

Your question addresses the dramatic (or theatrical) purpose of the scene rather than the thematic purpose. Many debate/discuss the possibilities of the scene involving the Porter who holds the keys to the castle door as a metaphor for the gates of Hell, and the Porter does mention "hell-gate" and "Beelzebub" (another name for the devil).  But these observations don't answer the question of how the scene operates in a theatrical or dramatic sense, merely a thematic one.

The scene is certainly meant to be funny, and the actor playing the Porter would have been one of the "clowns" in Shakespeare's company of actors, but the scene is not simply one of comic relief.  It operates on a much more cohesive level dramatically than this.

In Act II, scene ii, knocking is introduced.  The audience can't be sure what this knocking is, and, at first, neither can Macbeth and Lady Macbeth.  The tension and suspense of this scene are running very high, since the scene, rather than being a presentation of Duncan's murder, is a scene all about the possibility that Macbeth and Lady Macbeth might get caught.

They both begin the scene hearing things: an owl, the bellman, the crickets.  They also might be hearing voices:

Lady Macbeth
Did not you speak?
Macbeth
When?
Lady Macbeth
Now.
Macbeth
As I descended?
Lady Macbeth
Aye.
Macbeth
Hark!

This creating of suspense through real and imagined offstage sounds is a tactic still used today and one you can find in many horror and suspense movies.

So, this scene is all about the tension of whether they will be caught or not.  When Lady Macbeth leaves to return the daggers to the murder scene, Macbeth hears knocking.  And at this moment, he isn't sure what it is, but Lady Macbeth enters and says, "I hear a knocking / At the south entry," identifying it as someone beating to enter the castle.

Again, it sends the message of urgency to the audience.  Is it someone who knows that Duncan has been murdered?  It certainly seems that way, because of the focus that Shakespeare has placed in the scene upon the possibility of Lady Macbeth and Macbeth being discovered.

The knocking builds and continues into scene iii, and, though the Porter is funny, dramatically, the tension of the scene is not relieved because the audience still really wants to know who is out there and whether they might expose Macbeth as the murderer.

Interestingly, once the door is finally opened to admit Macduff (his first entrance into the play), he is already, because of the association made between the knocking and the murder, planted by Shakespeare in the audience's mind as someone who knows what Macbeth has done.  And he, in fact, will be the one thane who refuses to attend Macbeth's banquet in Act III.

So, though the scene is a funny one, it actually serves dramatically to heighten the audience's anticipation concerning the question of whether Macbeth and Lady Macbeth will get caught.  And this is all because the knocking is established as part of the suspense of Act II, scene ii.

The purpose of this scene is known as "comic relief". The audience has just witnessed the killing of Duncan and the drama surrounding it and this comic scene gives the audience a chance to catch their breaths and recover from all the drama that they have just witnessed. In addition, the scene also establishes the evil atmosphere around Macbeth's castle. When the porter curses, he does not use God's name but instead uses the "name of Beelzebub", or the devil. When he hears a knock on the door, he predicts the knock is from one of three people who are all associated with hell, a farmer who committed suicide, someone who committed treason, or a thief. He even refers to people walking "to the “everlasting bonfire.” These references help to symbolize the fact that Macbeth is "raising hell" in Scotland.

Knock, knock! Who's there, in the other devil's
name? Faith, here's an equivocator, that could
swear in both the scales against either scale;
who committed treason enough for God's sake,
yet could not equivocate to heaven: O, come in, equivocator.

Many will tell you that the Porter scene is comic relief, but they are wrong.  His language and actions may seem comical by today's audience, but to Elizabethans (at least the educated ones), his performance is grotesque, and his language is not unlike the devil's.

First of all, the most heinous crime in all of Elizabethan England or in any Medieval honor culture is to kill a king.  Regicide is akin to killing God.  Duncan's murder is not shown on stage for a reason.

Next, the knocking on the door parallels the sound imagery from the scene before, with the bell.  Macduff is the hero of the play.  He was born not of woman (like virgin birth).  He sacrifices his family for the salvation of the country.  He is a Christ-figure, and his knocking is a foreshadowing of the salvation to come.

I think the Porter's speech is grotesque, rather than comic.  Some critics think he's the devil.  All would agree that Macbeth's castle has become a hell, so who else would guard its gates?

The Porter also talks of equivocations (half-truths, paradoxes), like the witches.  The Devil is the great equivocator: he seduces with lies and false promises.  He tempted Christ with "Man shall not live with bread alone" and Eve with "You shall not surely die."

Equivocal morality frightened Elizabethans, almost as much as regicide.  To blur the lines of good and evil was a great fear.  We have all seen what moral relativism has done to the power of the church and the slackening of traditional moral values.  It was indeed a death knell to Christian theology and opened the door of the occult philosophy that was prevalent in the day.