String Instrument Acoustics - Sound Wave Panorama
The end goal of all our efforts to craft or play a musical instrument is the sweet, pleasing musical sounds we eventually hear and enjoy. Musical sounds may be generally described as noises that are smooth, soothing, pleasant, melodic, recognizable, identifiable, and inspiring. We each possess an individual instrument called the ear, which allows us to hear an array of the multifarious sounds existing in the universe. Our individual personal makeup allows us to define the way in which we are affected by these sounds. Whether a sound is pleasing or not is a matter of personal makeup, subconscious impressions based on our experiences, likes and dislikes, values, and psychological and emotional characteristics among other factors. We all hear differently. Numerous listeners can hear the same sound at the same moment in time, yet each will hear and describe the sound differently.
Sounds also have very specific and exacting physical properties, which must and do abide in exact accordance with laws of nature, sound wave physics. Sound is governed by the laws of wave motion, but the way we each individually hear sound and interpret it for our own needs is more philosophical, psychological, emotional and personal. The study of psycho-acoustics has reached the conclusion that the human ear and the spectacular way it is linked to the perceptive regions of the brain actually adjusts the sounds we hear and modifies them within certain limits, according to patterns etched in our personal subconscious reservoir experience field, adding or removing frequencies and modifying other aspects of the sound wave to make the sound meet our personal needs.
What then creates the sound of a string instrument and how is that brought into a sphere of pleasantness for the listener? As the pick, bow, or finger is drawn across the strings, sound waves emanate from the vibration caused on the string. The string vibrates at a certain resonant harmonic frequency depending on its length, tension and mass. Along with the main resonant frequency of the string, several other waves called partials or partial waves (0vertones and harmonics) will be produced. The motion of the string and the vibrations of the instrument’s body, resonating together, produce all these overtones or harmonics. The string itself is relatively small and does not send much sound energy into the air. The string sound wave is transmitted through the bridge and through the surrounding air into the body of the instrument. The internal sound box (body) of the instrument and the resonating top act like an amplifier to increase and strengthen the sound waves and project them back into the air, reaching the ear of the listeners. The outside surfaces of the plates, top and back, generate most of the sound wave support. The horizontal vibration of the string also sends sound wave energy into the bridge, which actually rocks or moves during this wave motion. Some of this sound energy is carried through the bracing, sound post, and or bass bar, and through the internal air space to the back and sides of the instrument. Thus, the entire instrument resonates and magnifies the small sound wave initiated by the attack stroke on the string. This sound wave, magnified by support resonance from the instrument’s body and enriched by partial waves or harmonics and overtones, travels through the air and upon reaching the ear of the listener, inscribes upon it a recognizable musical sound. The ear then carries this sound into the brain, where it is processed, interpreted, and presented to the listener.