- Steel String Acoustic Guitar Fretboard
A standing wave is a wave which does not travel. Though standing waves typically are not those which reach our ears, most acoustic musical instruments utilize them. A standing wave in a musical instrument allows for consistency and predictability of the wave in order to produce a tone.
In wind instruments, a standing wave is “trapped” by resonance in a resonating chamber, and in string instruments the standing wave is continuously reflected between the string's two anchor points.
Standing waves are rarely pure, meaning that they vibrate at many frequencies simultaneously, and the series of harmonic partial frequencies are musically utilized in various ways.
In a standing wave system like an instrument, the fundamental frequency is the frequency that has the longest wavelength possible. The fundamental frequency in such a system is therefore the lowest frequency possible. In a vibrating string system, such as that of a guitar or piano, the wavelength of the wave which produces the fundamental frequency equals the length of the entire string from anchor to anchor. The fundamental frequency is what defines the tone produced by a standing wave system, because the pitch of this frequency is the loudest and most identifiable.
In such a system, there are many other frequencies besides the fundamental. These are known as partials, which can be defined as either harmonic or inharmonic. Harmonic partial frequencies, as opposed to inharmonic frequencies, are defined to be integer multiples higher than the fundamental frequency (in Hz). This also means that the wavelengths of harmonics are even fractions of the fundamental frequency's wavelength, of the form
Harmonic frequencies are usually much louder than inharmonic frequencies. Harmonic frequencies also have more 'sustain' than inharmonic frequencies. 'Sustain' is a term used in music to describe the length of time before the string completely stops vibrating, and it becomes silent. More sustain refers to vibrations which last for longer periods of time.
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To discover different string harmonics, press the finger (click and hold down the mouse) on different parts of the string.
A More Mathematical Explanation
The Harmonic Series In Practice
String InstrumentsThe list of harmonic partials for a par [...]
The Harmonic Series In Practice
The list of harmonic partials for a particular wave is sometimes referred to as a scale of harmonics, or harmonic series. In the case of some string instruments, including guitar and violin, a player is able to dampen, or mute the fundamental frequency, along with other lower frequencies of the harmonic series, giving the listener the ability to better hear the more quiet, high partials.
If the player lightly touches the string halfway between the two anchor points while it is vibrating, the fundamental frequency, along with some other frequencies, will be dampened, and the listener will be better able to hear the other harmonic partials. They will be best able to hear the 'octave' harmonic, with half the wavelength, and twice the frequency in Hz as the fundamental frequency. The listener can still hear these harmonic frequencies because the pressed finger lies on a node between two wavelengths, and therefore does not interfere with the waves.
Standing waves in wind instruments work much like they do in string instruments, but the vibrating component is not a string. Vibration in string instruments comes from reeds in most woodwind instruments, and from the players lips in brass instruments. Both types of wind instruments utilize these vibrations to produce a standing wave with the column of air trapped inside the instrument (known as the resonating chamber). The wave's shape can then be disrupted to change the tone produced in numbers of different ways. Strategically covering and uncovering holes in the resonating chamber (i.e. a Clarinet), changing the resonating chambers shape (i.e. a Trombone), changing the vibration of the lips by altering embrasure, or "mouth shape" (i.e. a Bugle), etc. are all ways to alter the waveform and thereby alter the tone and pitch.
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