Final answer:
Asaf has become habituated to the train sounds, meaning the repeated exposure to the noise has resulted in it becoming less noticeable.
Step-by-step explanation:
Asaf has likely become habituated to the sound of the trains outside their apartment after living there for 3 months. Habituation is a process in which there is a decrease in response to a stimulus after repeated presentations. For instance, when Asaf first moved into their apartment, the noise from the trains was new and bothersome. However, over time, their brain has learned to ignore this repetitive stimulus, making it less noticeable or bothersome. This is a common experience where an individual's reaction to a certain stimulation diminishes when it is encountered frequently.