The sound of the beating heart is used to create an atmosphere of terror, guilt, and questionable sanity. The heart makes ‘‘a low, dull, quick sound, such as a watch makes when enveloped in cotton,’' and continues even after the old man is dead: ‘‘many minutes, the heart beat on.’’ The beating heart is compared to the ticking of a watch, repetitively beating, driving the narrator to the brink of insanity. There is a strong theme of time, as seen in the watch references, and the sense of running out of time. The old man's life ran out of time, and the beating heart that the narrator hears counts down his time as a free man.
The narrator opens the old man's door "cautiously--oh so cautiously..." to keep it from creaking, an unnerving sound of itself. But, the eerieest of sounds is the old man's "groan of mortal terror"; he knows in advance that the narrator plans to kill him. Describing this groan as not of grief or pain, the narrator says, I
It was the low stifled sound that arises from the bottom of the soul when overcharged with awe. I knew the sound well...It has welled up from my own bosom, deepening with its dreadful echo, the terrors that distracted me.
His echoings of this terror create suspense; How can the narrator feel a similar terror? Is he horrorified by what is in his own mind?
Later, in the narrator's increased sensitivity,much like that of Roderick Usher in "The House of Usher,"the narrator hears "a low, dull, quick sound, such as a watch makes when enveloped in cotton." This is the sound of the old man's heart, the narrator explains. As this sound "grows louder" to the narrator, he expresses that he is excited to "uncontrollable terror." In a bizarre state of mind, the narrator fears that a neighbor will hear this sound,so, shrieking in madness, he rushes into the room and kills the old man.
After the police officers arrive, he fancies "a ringing in my ears," the sound of a watch again. This must be his conscience. Now he cannot silence it with murder,so he confesses.
In the very first paragraph, sound sets a rather ominous tone: "I heard all things in the heaven and in the earth. I heard many things in hell." Mentioning hell right off the bat makes it suspenseful, and hooks the reader. Then, as he stalks the old man, he opened his lantern" cautiously—oh, so cautiously—cautiously (for the hinges creaked)". The reader can get a sense of the utter and complete silence, and how loud that creaking would be; this mention of sound here just intensifies the tension. Then, during the murder, it is pitch black, so sound is the only clue we have to what is going on. In paragraph 10, the narrator describes the old man's heart beating faster and faster, which, like a soundtrack for a movie, sets the pace and mood for the horrific murder.
And then, of course, it is the narrator hearing the old man's heart beating beneath the floorboards that leads to his confession. The sound drives the man insane-he rants, raves, makes noise to cover it up, but to no avail: "I felt that I must scream or die! and now—again!—hark! louder! louder! louder! louder!" In that super intense last paragraph, the heartbeat IS what causes the suspense, the tension, and the narrator's confession. Poe's story is a great example of using sound-almost as a separate character-to create the suspense.