Here is an animation showing the standing wave patterns that are produced on a medium such as the air inside of a flute. This type of medium would be said to be open at both ends, that is, able to move at both ends.
When a note is played on a flute, vibrations, or waves, travel back and forth through the medium and are reflected at each open end. Certain sized waves can survive on the medium. These certain sized waves will not cancel each other out as they reflect back upon themselves. These certain sized waves are called the harmonics of the vibration. They are standing waves. That is, they produce patterns which do not move.
On a medium such as the air in the tube of a flute several harmonically related standing wave patterns are possible. The first five of them are illustrated above. It is important to understand that for any one given medium open at each end only certain sized waves can stand. We say, therefore, that the medium is tuned.
The first pattern has the longest wavelength and is called the first harmonic. It is also called the fundamental.
The second pattern, or second harmonic, has half the wavelength and twice the frequency of the first harmonic. This second harmonic is also called the first overtone. This can get confusing with the second member of the harmonic group being called the first member of the overtone group.
The third harmonic, or pattern, has one third the wavelength and three times the frequency when compared to the first harmonic. This third harmonic is called the second overtone.
The other harmonics follow this pattern regarding wavelengths, frequencies, and overtone naming conventions described above. This is summarized below.
Open at both ends:
|Harmonic||Overtone group||Frequency relationship|
|2nd||1st overtone||2 times the fundamental frequency|
|3rd||2nd overtone||3 times the fundamental frequency|
|4th||3rd overtone||4 times the fundamental frequency|
|5th||4th overtone||5 times the fundamental frequency|
Depending upon how the flute is blown, different harmonics can be emphasized. In the above animation all harmonics have the same maximum amplitude. This is for purposes of illustration. Actually, the higher harmonics almost always have maximum amplitudes much less than the fundamental, or first harmonic.
It is the fundamental frequency that determines the note that we hear. How many of t is the upper harmonic structure that determines the timbre of the instrument.