Stretched tuning

Last updated
If the widths of the keys of a piano keyboard were stretched as the intervals between the corresponding notes are in stretched tuning, it would look something like the above. Stretched tuning as keyboard.png
If the widths of the keys of a piano keyboard were stretched as the intervals between the corresponding notes are in stretched tuning, it would look something like the above.

Stretched tuning is a detail of musical tuning, applied to wire-stringed musical instruments, older, non-digital electric pianos (such as the Fender Rhodes piano and Wurlitzer electric piano), and some sample-based synthesizers based on these instruments, to accommodate the natural inharmonicity of their vibrating elements. In stretched tuning, two notes an octave apart, whose fundamental frequencies theoretically have an exact 2:1 ratio, are tuned slightly farther apart (a stretched octave). "For a stretched tuning the octave is greater than a factor of 2; for a compressed tuning the octave is smaller than a factor of 2." [3]

Contents

Melodic stretch refers to tunings with fundamentals stretched relative to each other, while harmonic stretch refers to tunings with harmonics stretched relative to fundamentals which are not stretched. [4] For example, the piano features both stretched harmonics and, to accommodate those, stretched fundamentals.

Fundamentals and harmonics

In most musical instruments, the tone-generating component (a string or resonant column of air) vibrates at many frequencies simultaneously: a fundamental frequency that is usually perceived as the pitch of the note, and harmonics or overtones that are multiples of the fundamental frequency and whose wavelengths therefore divide the tone-generating region into simple fractional segments (1/2, 1/3, 1/4, etc.). (See harmonic series.) The fundamental note and its harmonics sound together, and the amplitude relationships among them strongly affect the perceived tone or timbre of the instrument.

In the acoustic piano, harpsichord, and clavichord, the vibrating element is a metal wire or string; in many non-digital electric pianos, it is a tapered metal tine (Rhodes piano) or reed (Wurlitzer electric piano) with one end clamped and the other free to vibrate. Each note on the keyboard has its own separate vibrating element whose tension and/or length and weight determines its fundamental frequency or pitch. In electric pianos, the motion of the vibrating element is sensed by an electromagnetic pickup and amplified electronically.

Intervals and inharmonicity

In tuning, the relationship between two notes (known musically as an interval) is determined by evaluating their common harmonics. For example, we say two notes are an octave apart when the fundamental frequency of the upper note exactly matches the second harmonic of the lower note. Theoretically, this means the fundamental frequency of the upper note is exactly twice that of the lower note, and we would assume that the second harmonic of the upper note will exactly match the fourth harmonic of the lower note.

On instruments strung with metal wire, however, neither of these assumptions is valid, and inharmonicity is the reason.

Inharmonicity refers to the difference between the theoretical and actual frequencies of the harmonics or overtones of a vibrating tine or string. The theoretical frequency of the second harmonic is twice the fundamental frequency, and of the third harmonic is three times the fundamental frequency, and so on. But on metal strings, tines, and reeds, the measured frequencies of those harmonics are slightly higher, and proportionately more so in the higher than in the lower harmonics. A digital emulation of these instruments must recreate this inharmonicity if it is to sound convincing.

The theory of temperaments in musical tuning do not normally take into account inharmonicity, which varies from instrument to instrument (and from string to string), but in practice the amount of inharmonicity present in a particular instrument will effect a modification to the theoretical temperament which is being applied to it.

Vibration of wire strings

When a stretched wire string is excited into motion by plucking or striking, a complex wave travels outward to the ends of the string. As it travels outward, this initial impulse forces the wire out of its resting position all along its length. After the impulse has passed, each part of the wire immediately begins to return toward (and overshoot) its resting position, which means vibration has been induced. Meanwhile, the initial impulse is reflected at both ends of the string and travels back toward the center. On the way, it interacts with the various vibrations it induced on the initial pass, and these interactions reduce or cancel some components of the impulse wave and reinforce others. When the reflected impulses encounter each other, their interaction again cancels some components and reinforces others.

Within a few transits of the string, all these cancellations and reinforcements sort the vibration into an orderly set of waves that vibrate over 1/1, 1/2, 1/3, 1/4, 1/5, 1/6, etc. of the length of the string. These are the harmonics. As a rule, the amplitude of its vibration is less for higher harmonics than for lower, meaning that higher harmonics are softer—though the details of this differ from instrument to instrument. The exact combination of different harmonics and their amplitudes is a primary factor affecting the timbre or tone quality of a particular musical tone.

In an ideal plain string, vibration over half the string's length will be twice as fast as its fundamental vibration, vibration over a third will be three times as fast, and so on. In this kind of string, the only force acting to return any part of it to its resting position is the tension between the string's ends. Strings for low and mid-range tones, however, typically consist of a core that is wound with another, thinner piece of wire. This makes them naturally resistant to being bent, adding to the effect of string tension in returning a given part of the string toward its resting position; the result is a comparatively higher frequency of vibration of wound strings. Since rigidity is constant, its effect is greater for shorter wavelengths, i.e. in higher harmonics.

Tines and reeds

Tines and reeds differ from strings in that they are held at one end and free to vibrate at the other. The frequencies of their fundamental and harmonic vibrations are subject to the same inharmonicity as strings. However, because of the comparative thickness of the bars that terminate the tines in an electric piano, the larger (and stronger) vibrations tend to "see" termination points slightly deeper in the bar than do smaller, weaker vibrations. This enhances inharmonicity in tines.

Effects on tuning

Inharmonicity alters harmonics beyond their theoretical frequencies. As the overtone series progresses, each partial becomes proportionally sharper. Thus, in our example of an octave, exactly matching the lowest common harmonic causes a slight amount of stretch; matching the next higher common harmonic causes a greater amount of stretch; and so on. If the interval is two octaves plus a fifth (the favored means of cross-checking the stretch of the upper treble of the piano), exactly matching the upper note to the sixth harmonic of the lowest requires great sophistication of octave stretch to make the lower individual octaves, its double and triple octaves, and their other intervallic relationships to sound pure and balanced.

Solving such dilemmas is at the heart of precise tuning by ear, and all solutions involve some stretching of the higher notes upward and the lower notes downward from their theoretical frequencies. In shorter pianos, the wire stiffness in the tenor and bass registers is proportionately high, and causes greater inharmonicity and hence greater stretch, negatively affecting timbre and creating serious compromises to what is considered acceptable tuning. On large grand pianos, and in particular concert grand pianos, this effect is greatly reduced. Online sources suggest that the total amount of "stretch" over the full range of a piano may be on the order of ±35 cents: this also appears in the empirical Railsback curve.

See also

Related Research Articles

<span class="mw-page-title-main">Harmonic series (music)</span> Sequence of frequencies

A harmonic series is the sequence of harmonics, musical tones, or pure tones whose frequency is an integer multiple of a fundamental frequency.

<span class="mw-page-title-main">Musical tuning</span> Terms for tuning an instrument and a systems of pitches

In music, there are two common meanings for tuning:

<span class="mw-page-title-main">Piano</span> Keyboard instrument

The piano is a keyboard instrument that produces sound when the keys are pressed. Most pianos have a row of 88 black and white keys: 52 white keys for the notes of the C major scale and 36 shorter and thinner black keys raised above the white keys and set further back, for sharps and flats. This means that the piano can play 88 different pitches, spanning a range of a bit over seven octaves. The black keys are for the "accidentals", which are needed to play in all twelve keys.

<span class="mw-page-title-main">Violin</span> Bowed string instrument

The violin, colloquially known as a fiddle, is a wooden chordophone, and is the smallest, and thus highest-pitched instrument (soprano) in regular use in the violin family. Smaller violin-type instruments exist, including the violino piccolo and the pochette, but these are virtually unused. Most violins have a hollow wooden body, and commonly have four strings, usually tuned in perfect fifths with notes G3, D4, A4, E5, and are most commonly played by drawing a bow across the strings. The violin can also be played by plucking the strings with the fingers (pizzicato) and, in specialized cases, by striking the strings with the wooden side of the bow.

<span class="mw-page-title-main">Harmonic</span> Wave with frequency an integer multiple of the fundamental frequency

In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the fundamental frequency of a periodic signal. The fundamental frequency is also called the 1st harmonic; the other harmonics are known as higher harmonics. As all harmonics are periodic at the fundamental frequency, the sum of harmonics is also periodic at that frequency. The set of harmonics forms a harmonic series.

<span class="mw-page-title-main">Overtone</span> Tone with a frequency higher than the frequency of the reference tone

An overtone is any resonant frequency above the fundamental frequency of a sound. In other words, overtones are all pitches higher than the lowest pitch within an individual sound; the fundamental is the lowest pitch. While the fundamental is usually heard most prominently, overtones are actually present in any pitch except a true sine wave. The relative volume or amplitude of various overtone partials is one of the key identifying features of timbre, or the individual characteristic of a sound.

<span class="mw-page-title-main">String instrument</span> Class of musical instruments with vibrating strings

In musical instrument classification, string instruments or chordophones, are musical instruments that produce sound from vibrating strings when a performer plays or sounds the strings in some manner.

<span class="mw-page-title-main">Timbre</span> Quality of a musical note or sound or tone

In music, timbre, also known as tone color or tone quality, is the perceived sound quality of a musical note, sound or tone. Timbre distinguishes different types of sound production, such as choir voices and musical instruments. It also enables listeners to distinguish different instruments in the same category.

<span class="mw-page-title-main">Pitch (music)</span> Perceptual property in music ordering sounds from low to high

Pitch is a perceptual property that allows sounds to be ordered on a frequency-related scale, or more commonly, pitch is the quality that makes it possible to judge sounds as "higher" and "lower" in the sense associated with musical melodies. Pitch is a major auditory attribute of musical tones, along with duration, loudness, and timbre.

<span class="mw-page-title-main">Inharmonicity</span>

In music, inharmonicity is the degree to which the frequencies of overtones depart from whole multiples of the fundamental frequency.

<span class="mw-page-title-main">Pseudo-octave</span>

A pseudo-octave, pseudooctave, or paradoxical octave in music is an interval whose frequency ratio is not 2:1, that of the octave, but is perceived or treated as equivalent to this ratio, and whose pitches are considered equivalent to each other as with octave equivalency.

Piano acoustics is the set of physical properties of the piano that affect its sound. It is an area of study within musical acoustics.

<span class="mw-page-title-main">Aliquot stringing</span> Use of additional unstruck strings in a piano to enrich the tone

Aliquot stringing is the use of extra, un-struck strings in a piano for the purpose of enriching the tone. Aliquot systems use an additional string in each note of the top three piano octaves. This string is positioned slightly above the other three strings so that it is not struck by the hammer. Whenever the hammer strikes the three conventional strings, the aliquot string vibrates sympathetically. Aliquot stringing broadens the vibrational energy throughout the instrument, and creates an unusually complex and colorful tone.

<span class="mw-page-title-main">Sympathetic string</span>

Sympathetic strings or resonance strings are auxiliary strings found on many Indian musical instruments, as well as some Western Baroque instruments and a variety of folk instruments. They are typically not played directly by the performer, only indirectly through the tones that are played on the main strings, based on the principle of sympathetic resonance. The resonance is most often heard when the fundamental frequency of the string is in unison or an octave lower or higher than the catalyst note, although it can occur for other intervals, such as a fifth, with less effect.

<span class="mw-page-title-main">Piano tuning</span> Profession

Piano tuning is the act of adjusting the tension of the strings of an acoustic piano so that the musical intervals between strings are in tune. The meaning of the term 'in tune', in the context of piano tuning, is not simply a particular fixed set of pitches. Fine piano tuning requires an assessment of the vibration interaction among notes, which is different for every piano, thus in practice requiring slightly different pitches from any theoretical standard. Pianos are usually tuned to a modified version of the system called equal temperament.

Musical acoustics or music acoustics is a multidisciplinary field that combines knowledge from physics, psychophysics, organology, physiology, music theory, ethnomusicology, signal processing and instrument building, among other disciplines. As a branch of acoustics, it is concerned with researching and describing the physics of music – how sounds are employed to make music. Examples of areas of study are the function of musical instruments, the human voice, computer analysis of melody, and in the clinical use of music in music therapy.

<span class="mw-page-title-main">String (music)</span> Sound producing musical instrument component

A string is the vibrating element of chordophones such as the guitar, harp, piano, and the violin family, that produces sound. Strings are lengths of a flexible material that a musical instrument holds under tension so that they can vibrate freely, but with control. This is to make the string vibrate at the desired pitch, while maintaining a low profile and sufficient flexibility to play in action.

<span class="mw-page-title-main">Acoustic resonance</span> Resonance phenomena in sound and musical devices

Acoustic resonance is a phenomenon in which an acoustic system amplifies sound waves whose frequency matches one of its own natural frequencies of vibration.

<span class="mw-page-title-main">Violin acoustics</span> Area of study within musical acoustics

Violin acoustics is an area of study within musical acoustics concerned with how the sound of a violin is created as the result of interactions between its many parts. These acoustic qualities are similar to those of other members of the violin family, such as the viola.

Sympathetic resonance or sympathetic vibration is a harmonic phenomenon wherein a passive string or vibratory body responds to external vibrations to which it has a harmonic likeness. The classic example is demonstrated with two similarly-tuned tuning forks. When one fork is struck and held near the other, vibrations are induced in the unstruck fork, even though there is no physical contact between them. In similar fashion, strings will respond to the vibrations of a tuning fork when sufficient harmonic relations exist between them. The effect is most noticeable when the two bodies are tuned in unison or an octave apart, as there is the greatest similarity in vibrational frequency. Sympathetic resonance is an example of injection locking occurring between coupled oscillators, in this case coupled through vibrating air. In musical instruments, sympathetic resonance can produce both desirable and undesirable effects.

References

  1. Campbell, Murray and Greated, Clive (1994). The Musician's Guide to Acoustics, p.257-58. ISBN   9780191591679. "In a properly tuned instrument the notes will be progressively sharper in the treble compared with the frequencies calculated for a particular tempered scale (Schuck and Young 1943). Likewise in the bass the notes will becomes progressively flatter. This effect is known as octave stretching."
  2. Carterette, Edward (1978). Hearing, p.525. ISBN   9780323142755. "The tuning of pianos is usually stretched, that is, the high tones are higher and the lower notes are lower than would correspond to the tempered scale. This can be ascribed partly to the inharmonicity of piano strings (Schuck and Young, 1943)..."
  3. Hartmann, William M. (1997). Signals, Sound, and Sensation, p.275. ISBN   9781563962837.
  4. Hartmann (1997), p.276.

Further information