Violin, oboe, and timpani drum all produce sound waves, albeit in different media and manner. The violin produces its sound waves with the motion of vibrating strings. A rosin-coated bow is drawn across a string displacing the string from its original position, where it remains until the force of the bow's motion becomes greater than the force of friction between the bow and the string. At this point, the bow allows the string to return to the original position.
This repeated periodic motion causes vibrational waves to propagate along the sting, exciting the air surrounding the string, and producing a musical note. The bridge transmits the strings' vibrations to the soundboard to amplify the sound.
The oboe generates its vibration in a double reed seated within the instrument and partially within the mouth of the musician. The musician blows air through the reed, which in turn, sets up vibrations and the necessary pressure within the oboe's internal column of air. (Benade p.35) The frequency of waves produced is dependent on the vibrations of the reed, diameter of the internal chamber, and length, as dictated by the opening and closing of holes in the chamber. The location of the holes in the instrument set the places in the sound wave where anti-nodes are located in the waveform.
A timpani drum creates sound waves by means of the vibrating membrane which covers the bowl-shaped shell. The percussionist strikes the timpani with a mallet (hard or soft) or his fingers, displacing the drum head membrane. As the membrane returns to the initial position, it begins a vibration cycle. This motion displacement causes a disturbance in the air within the drum, which is the sound wave. The waves produced are often very intense with large amplitudes. To circumvent air concussion splitting the membrane in loud passages, a small hole is drilled in the bottom of the shell.
When two identical traveling waves move in opposite directions along a line, they form a standing wave. (see figure 1) This waveform does not travel through air or along a string, it stands in place. Like its two component waves, it is a sinusoidal wave and oscillates at the same frequency as the components. Constructive interference increases the amplitude of the sound wave and the volume of the pitch. When the wavelength is a multiple of the instrument length, constructive interference occurs and is called resonance.