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a0
Usual abbreviation for the "air mode" of a violin or other instrument. This mode is a Helmholtz resonance, involving air flow in and out of the f-holes, but modified by the fact that the walls of the box are not rigid so that some breathing motion of the box also occurs. A0 is usually found around 280 Hz in a violin.
a1
Label for the second "air mode" of a violin. The mode is an "organ pipe" standing wave, with approximately a half-wavelength fitting into the length of the body cavity. It is usually around 480 Hz in a violin. It has a nodal line near the bridge, and is therefore not driven very effectively by the strings.
accelerometer
Device for measuring vibration of a structure, producing an output signal proportional to acceleration. They work by having some kind of force-measuring sensor, with a mass attached to it so that when the device is forced to vibrate a force is produced by Newton's law, proportional to acceleration. There are two types: the force measuring element can be either a piezoelectric crystal or a cantilever beam fitted with a strain
gauge.
b0
Usual name for a low-frequency resonance mainly determined by a cantilever motion of the projecting piece of the fingerboard. It typically falls at about the same frequency as the air resonance A0. There is a school of thought that adjusting the frequency of B0 to be close to matching that of A0 is a good thing.
b1- b1+
Usual names for two strong resonances of a violin body, responsible for a lot of the sound radiation at low frequencies. Between them they are responsible for what older literature called the "main body resonance". Other labels for these modes are P1 and P2, or T1 and C3. For a violin, B1- is typically found around 450-480 Hz, B1+ around 530-570 Hz.
b1-,b1+
Usual names for two strong resonances of a violin body, responsible for a lot of the sound radiation at low frequencies. Between them they are responsible for what older literature called the "main body resonance". Other labels for these modes are P1 and P2, or T1 and C3. For a violin, B1- is typically found around 450-480 Hz, B1+ around 530-570 Hz.
cbr
Acronym for a vibration mode of the violin body, coined by Bissinger: "C bouts rhomboidal". This is a mode usually in the general vicinity of 400 Hz for a violin, which shows up in input admittance measurements but is not a strong radiator of sound. This mode was labelled by Jansson "C2", and by Marshall "Vertical translation of C bouts".
chladni pattern
Traditional way to visualise a vibration mode shape (or more properly, an ODS) by forcing the system to vibrate and then sprinkling powder such as sand or tea leaves on the surface. The powder collects at nodal lines.
damping
“Damping” describes how quickly the energy in a vibration is dissipated. It is easiest to think about the response to tapping an object: there is an initial sound which dies away with time, fast if the damping is high and slow if the damping is low. If the vibrating structure is being driven in more-or-less steady vibration, as in a violin, then damping has a different consequence. It determines how strong the resonances of the structure are. If damping is low, a resonance will respond with a much bigger amplitude to a given level of excitation. As damping increases this resonant enhancement gets less, until if damping is too high there is no “resonance” at all.
Damping comes from many sources, which can be broadly grouped into three types. (1) “Material damping” describes energy loss due to “internal friction” within the body of the material the structure is made of. (2) “Boundary damping” describes energy loss associated with joins between different pieces of a structure: for example at a bolted join in a metal structure, there may be some frictional rubbing between the parts, or pumping of air in and out of small gaps. (3) Viscous and radiation damping. The structure is surrounded by air. Some energy is lost because it is carried away as sound waves. Some is also lost to viscosity of the air because of the local air flow around the edges of the structure.
Variation in material damping is one of the important factors in the difference between strings of different materials, or between different wood specimens.
decibel
The decibel is a unit of level, used to describe for example loudness of sounds. It is a measure based on logarithms, and it describes the RATIO between two signals. 20dB is defined to be a factor of 10 in amplitude. 6dB is approximately a factor of 2. When decibel numbers are quoted, they are relative to some agreed reference level.

fft
Acronym for "Fast Fourier transform". This the method used by a computer program to convert a recorded time signal into a frequency spectrum.
frf
Acronym for "frequency response function". Related to "spectrum", also known as "transfer function". If you apply a force to a structure and measure the response somewhere (on the structure or with a microphone in the air) the FRF is a response per unit input force, as a function of frequency. It will have peaks at the resonances.
hz
Abbreviation for Hertz, unit of frequency - "cycles per second". Named after 19th century German scientist.
input admittance
A commonly-measured FRF for a violin: the velocity response at a string notch on the bridge to a force applied at the same point. Usually, the force and response would be aligned in the bowing direction. For practical reasons, input admittance is usually measured approximately, by applying the force to one corner of the bridge and measuring response on the other corner. An alternative name is "driving-point mobility".
nodal line
A nodal line marks out the positions on a vibrating structure where a particular vibration mode is NOT vibrating. These lines separate regions where (at a given moment) the structure is moving in positive and negative directions.
normal mode
A normal mode (or "eigenmode", or just "mode") is a pattern of vibration of a structure which corresponds to an individual resonant frequency (or "natural frequency", or "eigenfrequency"). Once the system is started off vibrating with one particular mode shape it will continue in that shape, oscillating at the corresponding natural frequency and gradually dying away because of the effect of damping. Any possible vibration of the system can be described as a mixture of the normal modes, each responding independently to the applied force. Mode shapes can be deduced from Chladni patterns, or more high-tech measurements such as laser holographic methods or "experimental modal analysis".
ods
Acronym for "operating deflection shape". This is the pattern of response which you see if the structure (violin body or whatever) is driven with a sinusoidal force at a particular frequency. The ODS pattern will change as the frequency changes. Near a resonance, the ODS will be roughly the same as the mode shape, but not exactly the same because it will contain contributions from all the other modes as well.
q value
A way to measure and describe the level of damping for a given vibration mode: high Q means low damping and conversely. The Q stands for "quality", because in the world of quartz crystal oscillators, a "high quality" oscillator has very low damping. The Q factor relates to the number of cycles it takes for the vibration to decay by a given factor.
spectrum
Description, for a function of time, of the resolution of a signal into components, each of different frequency and (usually) different amplitude and phase.
transient
A rather broad term, covering any aspect of sound or vibration which is not a steady periodic response. There are two main types of "transient" relevant to violins. The motion of the string in response to a bow gesture will go through some kind of transient before (usually) settling into the preferred Helmholtz motion. The nature and length of that transient are a big part part of judging the skill of the player, as well as of the sound. The second type of transient is the response of the violin body to some change in what the string does. The body resonances take time to build up or to die down following a change in excitation. This type of transient is most familiar through the "body knock" sound when you tap on the corner of the bridge. Recognition of a particular instrument (as opposed to a particular player) probably depends a lot on these transient sounds, which are unique to each instrument body.


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