Answer
When a trumpet and a saxophone play the same note, the partial tones have different intensities.
Work Step by Step
The number of partial tones and their relative loudness determine the musical quality of a note.
Many musical sounds are complex, and are composed of many "partial tones" of different frequencies. How many there are, and how intense they are relative to each other, determines the timbre of the note.
This is discussed on page 395.