Answer: Sensory Adaptation.
Step-by-step explanation:
Sensory adaptation is a reduction in the responsiveness of the sensory system to a stimulus after being constantly exposed to it. It is useful because it helps our brain to focus its attention on other, more important things.
In this case, Thad has been experiencing the same stimulus (the air conditioner sound) for a while, so his brain has accustomed to it and turned its attention to the television.