Passage: A collection of four or six units is known as a sub-phrase, lasting perhaps ten seconds (see also phrase (music)). A collection of two sub-phrases is a phrase. A whale will typically repeat the same phrase over and over for two to four minutes. This is known as a theme. A collection of themes is known as a song. The whale song will last up to 30 or so minutes, and will be repeated over and over again over the course of hours or even days. This "Russian doll" hierarchy of sounds suggests a syntactic structure that is more human-like in its complexity than other forms of animal communication like bird songs, which have only linear structure.All the whales in an area sing virtually the same song at any point in time and the song is constantly and slowly evolving over time. For example, over the course of a month a particular unit that started as an upsweep (increasing in frequency) might slowly flatten to become a constant note. Another unit may get steadily louder. The pace of evolution of a whale's song also changes—some years the song may change quite rapidly, whereas in other years little variation may be recorded.

Question: What does another unit steadily get?
louder