Octave purity (string instrument)
Octave purity is a quality feature of string instruments with frets . With an octave-pure instrument, each string in the 12th fret sounds exactly one octave higher when fingered than when it is not fingered, with a non-octave-pure instrument it sounds a bit too high when fingered.
Definition of terms
The correct ratio between the scale length of a single string and the frets is called octave . Often the term is also used in relation to the entire instrument or to all strings drawn. Octave purity is a special case of fret purity .
If you place the 12th fret (12 frets correspond to 12 semitone steps, which in turn results in an octave) exactly in the middle of the scale length, the length of a string fingered there is halved and should theoretically sound an octave higher. In fact, however, the string that is being fingered is not only shortened, but also receives additional tension from the pressure on the fingerboard. This increase in tension detunes the string slightly upwards.
For an octave-pure instrument, this effect must be compensated for by making the scale length slightly longer than twice the distance from the nut to the 12th fret. This scale compensation is achieved through structural or mechanical measures on the bridge .
In general, the thicker and / or harder the string, the greater this effect and thus the necessary lengthening of the scale length to achieve octave purity. With gut or nylon strings the effect is relatively small and a straight bridge or a bridge with a slightly inclined bridge insert is sufficient. The effect is greater with metal strings. In the case of the electric guitar and the electric bass , the length of each string can be individually adjusted using special components of the bridge - usually referred to as saddles.
Checking the octave purity
The octave purity can be checked with a trained ear or a commercially available tuner .
With your hearing
First, a harmonica tone that sounds an octave higher is generated on the string . To do this, touch the string with the fingertip of the gripping hand above the 12th fret without pressing and strike it. Then you grab the string in the 12th fret and strike it again. If the string is octave, then the two notes match exactly.
With the tuner
The string is struck empty and the display on the tuner checked. Now the string is gripped in the 12th fret, struck and the display on the tuner checked again. If the string is one octave, the tuner will show a tone with the second measurement that is exactly one octave above the first.
Setting the octave purity
If the pitch of the string that is fingered is too low, the length of the length must be shortened, while the length of the length has to be lengthened if the pitch of the string is too high.
In the case of electric guitars, this is possible thanks to the “tabs” that can be individually adjusted for each string. They are held in place by a screw, so you can pull them up by tightening the screw (this corresponds to lengthening the length of the scale), or by loosening the screw, you can remove them further from the holding point (this corresponds to shortening the length of the scale).
The tabs are used to adapt the length of the vibrating string part to the respective string thickness, for example when the guitarist changes to a thicker or thinner set of strings: A thicker string shortens the vibrating part of the string a little, a thinner string lengthens it (this is the question here only by a fraction of a millimeter), so that the octave purity is minimally missed. These errors can be remedied with the individual saddles.
Older Fender Telecaster models only have three saddles for six strings. The adjustment of the octave purity is only possible with compromises.
With the classic or western guitar, changing the bridge position in order to lengthen or shorten the length accordingly is hardly possible, since the tailpiece is glued to the body and therefore its position is fixed and cannot be changed. However, the bridge can be beveled to lengthen or shorten the strings. This minimal adjustment is usually sufficient for good instruments. In other string instruments, such as. B. mandolins , string instruments , Archtop -Jazzgitarren or Cistern , the web often movable and can be moved or to adjust the intonation slightly oblique.
See also
- Inharmonicity - physically caused partial detuning of vibrating strings