A very important and overlooked aspect of improvising language. The ears grows tired of too much information in the same register, or over the same scale, and this is when vocabulary becomes less effective and turns into mere displays of technique. Likewise, too much diversity, moving from the bottom of the horn to the top quickly, too often, becomes stale and is a waste of physical and musical resources. Listen to the bebop masters for this balance. It is common for less experienced players to spend too much time in one register of the horn, and they sound like they’re meandering, even if they are playing strong rhythms and strong note choices. The rate of change also affects our perception of consonance, and this has been a major factor in the contemporary interest in large intervals, which define harmonic spaces with less specificity than a series of thirds and seconds. Too much time on one scale spoils it, the details of its subtle consonances and dissonances against the rhythm section are worn away. A solution for keeping scales fresh is to develop a better understanding of these consonances and dissonances within each note, and how placing certain notes on strong beats or weak beats highlights them. Conversely, spending lots of time on one sound or register lets the improvisor make use of other musical elements to develop their solo. Coltrane figured this out during his modal period. 3+ octaves doesn’t go very far when you’re playing over one chords for twenty minutes, so Coltrane stays in one register of the horn for longer through timbral variation, mainly false fingerings and different speeds and depths of vibrato. Vibrato speeds allow him to maintain a vocal, primalist aesthetic, conserving technical speed for later. False fingerings allow Coltrane to stay in one range of the horn while still maintaining a fast and exciting speed of notes. Without false fingerings, he wouldn’t be able to play both fast and stay in one register.