A combination of harmonic tones is pleasant to hear. These resonances occur when external vibrations at a system's natural frequency cause constructive interference or vibrations at a larger amplitude than normal. This principle is used in musical instruments to increase the amplitude of the initial vibrational wave and produce an audible musical tone. Resonance is fundamental to the production of sound from acoustic musical instruments.
In musical instruments, standing waves are created by disturbing one end of the vibrational medium. For instance, the double reed in the mouthpiece of an oboe creates a wave which interacts with reflections off of the end boundaries of the vibrating system. The waveform is a function of the length of the instrument. These waves are standing waves and are often amplified through the use of a bell shape at the exit point of the system. On the oboe, a flare on the bell acts as an amplifier, while the timpani uses a bowl-shaped shell which opens upward.
Timbre is the tonal attribute of sound waves that distinguishes sounds of identical pitch, loudness, and duration. A violin, oboe, and timpani drum can play an A of identical frequency, but will sound different.
The three instruments sound different because of two main characteristics: tone and timbre, which are used to describe the quality of the pitch. These two acoustic phenomena arise due to the differing harmonic frequency combinations and the interactions of those differing frequencies.
Musical tone is a dynamic event. A static waveform can not adequately represent the timbre of the event. Timbre is a function of a musical tone's attack, onset, number of overtones, growth, relative intensity of harmonics present, and decay. (Radocy and Boyle 1988) These random fluctuations and overtones are due to the superposition of waves and the harmonics of the vibrating body. (see figure 2)