The "cocktail effect" in acoustics refers to the masking effect of the human ear. In a noisy crowd at a cocktail party, two people can have a smooth conversation. Although the surrounding noise is very loud, what they hear in their ears is the other person's voice, and you seem to be unable to hear various noises other than the content of the conversation. Because you have put your respective focus (this is the selectivity of attention) on the topic of conversation.