The Complete Instrument

The graph below shows the acoustic efficiency of two complete violins. This is the ratio of the sound pressure produced (recorded by a microphone near the f hole) to force applied (electromagnetically at the bridge).

The two curves are for the violins made from the top plates whose properties are shown above. (The measurements were made by Ra Inta, PhD student in Music Acoustics, as part of the study on how violins change with playing and environmental changes over time.)

The air inside and the Helmholtz resonanceThe air inside the body is also important, especially for the low range on the instrument. It can vibrate a little like the air in a bottle when you blow across the top.

In fact if you sing a note near D4 close to the violin, and then hold your ear close to the f-holes, you may hear the air in the body resonating. This is called the Helmholtz resonance. You can see the effect of this resonance at around 300 Hz in these two curves. Resonances increase the sound output over some frequency ranges of both the violin and the guitar, we might say that the lowest resonance (associated with the Helmholtz resonance) falls near the pitch of the second lowest string, and the lowest body mode falls near the pitch of the third lowest. Together these increase the sound radiation at fundamentals of several of the notes in the low range of the instrument. Further body resonances are distributed at higher frequencies. These improve radiation of the fundamentals of higher notes, and to harmonics of lower notes.

Where does the sound energy come from? We have mentioned the importance of resonances in increasing the sound output of the instrument. It is worth making it clear that the body doesn't amplify the sound in the technical sense of the word amplify. An electronic amplifier takes a signal with small power and, using electrical power from the mains, turns it into a more powerful signal. In the violin, all of the sound energy that is produced by the body originally comes from energy put into the string by the bow.

The purpose of the body is to make that conversion process more efficient. In an electric guitar, very little of the energy of the plucked string is converted to sound. The body of an acoustic guitar or violin is more efficient at converting some of that energy into sound.

Timbre vibrato

An interesting, very important and almost characteristic feature of the sound of members of the violin family is timbre vibrato, which is largely due to the acoustic response of the body. We see that the ratio of the sound pressure to the force that the string exerts on the bridge is a very strong function of frequency, because of the many resonances in the body. (This may seem strange to someone with a background in hifi, where the aim is to produce apparatus with negligible dependence on frequency!).

When one plays a note with a particular frequency, some but not all of the hamonics coincide with resonances and so are strongly transmitted in the output sound, while others are weaker. Now consider what happens when the player rocks backwards and forwards the finger that is stopping the string. This produces a pitch vibrato: the pitch of note varies regularly up and down. As it does so, the harmonics of the note also vary up and down in frequency, and so they may move from a strong transmission (due to a resonance) to weak transmission, or vice versa. This means that the spectrum of the output note varies strongly during the vibrato. Perceptually, this makes the sound much more interesting.

It is one of the two most important features that help us identify the sound of a violin. (The other is the way in which a bowed note begins.) The key paper on timbre vibrato is by J. Meyer: "On the Tonal Effect of String Vibrato", Acustica, 76 283-291 (1992). To get an idea of why vibrato is so important to the violin, ask a violinist to play a long soft note on an open string, or two notes simultaneously on two adjacent open strings. On the open string, it will have no vibrato. Now close your eyes? Can you imagine that it is an organ playing? Each time the bow changes direction you can tell that it is a violin, but during sustained, steady bowing it is much less clear. Now ask the violinist to play the same single note, or a double-stopped fifth, on different strings, and to play normally. How important is the difference? We demonstrate this with soundfiles, waveforms and spectra on the page Articulation and vibrato on the violin.

Bowing

The use of the bow is also very important to the violin sound. First, it allows the production of a sustained note, whose loudness can be held nearly constant or, at the performer's choice, varied over time (musicans say 'shaping the note').

There is another important difference between plucking and bowing. A plucked string very quickly loses its high harmonics and, after a few seconds, nearly all of the remaining energy in the string is in its fundamental. Bowing inputs energy continuously to the string and thereby maintains the power in the high harmonics.

See Bows and strings. The bow can be used in a variety of different ways to produce different articulations and sound effects. Some of these are demonstrated with sound files, wave forms and spectra on Articulation and vibrato on the violin.

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