A audio term, basically refering to slightly pitch modulating a tone to give it a smoother sound. So, if I have a tone at 440 Hz, I might modulate that using a 5 Hz sine wave, so it swings between 435 and 445 Hz.

More advanced techniques increase the amount of modulation over time, so intiially it may have no vibrato, but after a second it might be 1% of the original frequency, 2% after 2 seconds, and so on.

Until the last half of the twentieth century, vibrato as a form of expression in music was largely taken for granted. All forms of legato and sustained playing on any string instrument in the orchestra is subject to the enriching qualities of vibrato.

A proper vibrato is acheived by the shaking back and forth of the wrist and elbow without picking up the finger that is depressing the string in a proper position. This is done by a performer to increase the warmth and tone color of a given passage; as such, when a composer marks the score senza espress it is generally taken to mean 'without vibrato.' It lessens the chance for a performer to add expression or any interpretation. Igor Stravinsky was proud and fond of using this.

Vibrato is such a widely used practice that actually seeing the word in the score is indicative of a place where the string player should grossly over-exaggerate the vibrato used, either to increase drama and tension or to parody. The generally accepted practice of using vibrato in most or all expressive, sustained, or non-stacatto passages leads to the combined vibrati of an entire string section, and this is what gives the section its tonal color, its character, and its ability to inspire emotion in the modern listener.

At different times in the history of music, the term vibrato has referred to different techniques among different instruments. In this era of sound recordings, so-called period instruments, and historially informed performance practice, the sound of these different techniques (as we understand them today) can be easily experienced.

As far as research can suggest, vibrato was not used in plainsong, Gregorian chant, or organum. It is not mentioned in the records of the Council of Trent, nor in other treatises discussing polyphony at the time of Palestrina.

The earliest mentions of vibrato occur in the Baroque period, where it is discussed as an ornament to be employed by singers, a slight variation of the pitch on a tone. I am no expert in Baroque ornamentation, but as an ornament, the vibrato was not present at every point of the vocal line, it was a rarity, not the norm. There is some confusion in the literature of the period with the ornamental tremolo, a pulsing variation of volume on a held tone.

Now, the viol family of instruments (of which a barely surviving example is the viola da gamba) were the primary string instruments in the early Baroque, gradually developed into the violin and cello. None of the period treatises on viol playing mention the use of vibrato, and it is not currently accepted as a part of the technique. However, the string instruments are particularly well suited to produce vibrato, using the technique Beltane describes above (btw, make sure you have a relaxed wrist and arm when you do this, it’s a long road back from tendonitis).

Precisely when vibrato became a string instrument technique is likely a question that cannot be answered, however, it was during the Classical period, the age of Haydn, Mozart, and Beethoven, among others, that vibrato began to be used expressively by instruments. This use of vibrato may have developed from two things: the imitation of the voice and/or the need for larger numbers of players to play in tune.1

When Joseph Tribensee arrived in Vienna, he was praised for the vocal-like sound he produced from his clarinet. This attention resulted in the Mozart Clarinet Concerto and several other works written for his talents. But what we can conclude from this praise is that he had developed the technique of vibrato on the clarinet, and probably tremolo as well. Modern clarinet (and saxophone) technique uses the lower jaw to tense and relax the reed and thereby produce vibrato. The diaphragm may be used to produce vibrato or tremolo by varying air pressure.

Throughout the Classical and the Romantic periods, the use of vibrato was becoming less an ornament and more an expected component of the string sound. In the work of Richard Strauss, Anton Bruckner, and Gustav Mahler, among others, vibrato is required almost constantly, and expressively lies in the subtle variation of vibrato at different points in the melody. The use of the instruction espressivo asks for even still more vibrato while indicating places in the melody to play out.

At the beginning of the Modern period, partly to combat such romantic excess, partly to evoke a more brutal sound, Igor Stravinsky uses the instruction senza espressivo. Arnold Schoenberg, looking for more control over the sound, also uses molto espressivo, con espressivo, and senza espressivo. In the high Modernism of the Twentieth Century, composers forsake the identification of vibrato with expressive strength, and simply ask molto vibrato, senza vibrato, and even wide vibrato of a semitone.

SUGGESTED LISTENING ASSIGNMENT
Start with the Korngold Violin Concerto in the recording by Gidon Kramer as an example of an expressive use of vibrato. Compare with the two string quartets of Penderecki, if possible follow along with the score. The quartets use variable amounts of vibrato (along with other noises suggesting the destruction of the instruments themselves) as expression. Notice the lean, incisive quality of the senza vibrato pitches and the wide, sinuous sound of the molto vibrato. Then try Tabula Rosa by Arvo Part (again, choose the Kramer recording), or the piece Spiegel im Spiegel performed by Theatre of Voices, and compare with one of the Brandenburg Concertos performed under the direction of Trevor Pinnock.

1 As good as players are, it is tricky to get many of them to play the very same pitch simultaneously. Vibrato, by varying the pitch each of them produce, minimizes beat frequency interference. This same trick is used in piano tuning to give the sound a bit of a shimmer, an approximation of vibrato.

This write-up was contributed at the request of m_turner. Here is a gentle musing for that gentle muse.

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