Naad Focus
Srila Yogeshwarananda wanted me to mention that listening to naad and focusing on or in it are two separate achievements. Beginning students should listen to naad. For them that is the achievement. For the advanced students, there should be distinction made between listening and focusing with listening being the elementary practice, and focusing being the advanced one.
Listening to naad is a passive practice for the core-self. Focusing is a gentle active practice which has listening as its basis.
It is natural to think that focusing has to do with only a target, but that is not the truth. Focusing has more to do with holding a focus on whatever is the target, rather than focusing on a target.
While listening to naad, after one became proficient doing that, one should focus gently on a node or zone of naad and hold the focus, noting how the focus may be broken or shifted from time to time and noting what it takes to resume the focus each time there is a breach of focus.