Final answer:
The old man's 'heartbeat' affects the narrator by exacerbating his guilt and leading to his psychological breakdown, symbolizing the manifestation of his internal struggle and culminating in his confession of murder.
Step-by-step explanation:
The old man's heartbeat significantly affects the nameless narrator of Edgar Allan Poe's "The Tell-Tale Heart," where the beating sound exerts an escalating psychological pressure on him. As for the sound's nature, it can be interpreted as the narrator's guilt or paranoia manifesting as auditory hallucinations. The repetitious, pounding noise that the narrator perceives exacerbates his mental instability, driving him to the point of confessing his crime to alleviate the intolerable burden of his conscience. This heartbeat symbolizes his internal battle with his sense of guilt over the murder of the old man. As the true source of the sound is ambiguous, it acts as catalyst for the narrator's ultimate unraveling.