Each of the multiple frequencies, after the fundamental, is called a harmonic. The fundamental and its harmonics together help create the oboe's rich sound. It is made in three sections, top joint, middle joint and the bell. As the air proceeds down the inside of the instrument, it will be interrupted anyplace there is an open hole. If all keys are closed, the air will escape only at the bell of the instrument.
Sound waves originate from the vibrations of the double reed.
This fundamental sound is rather shrill and unappealing to the human ear. The vibrations disturb the surrounding air by rapidly compressing then decompressing the air. The pitch or frequency of the sound depends on the speed of the reed, the diameter of the bore and the length of the column of air in the instrument. As keys are depressed and released, the length of the column changes, which in turn changes the pitch. The lowest component on the left of the spectrum is the fundamental.