# Frequency      Number of occurrences of a repeating event per unit of time.

- Frequency

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## Sound

Vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.          Only acoustic waves that have frequencies lying between about 20 Hz and 20 kHz, the audio frequency range, elicit an auditory percept in humans.

## Light

Electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye.          Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750-420 terahertz, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths).

## Wave

Propagating dynamic disturbance of one or more quantities.                       Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (resting) value at some frequency.

## Revolutions per minute

Number of turns in one minute. It is a unit of rotational speed or the frequency of rotation around a fixed axis.

## Cycle per second  The cycle per second was a once-common English name for the unit of frequency now known as the hertz (Hz).

## Second

Base unit of time in the International System of Units (SI) (Système International d’unités), commonly understood and historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each (24 × 60 × 60 = 86400). The second is also part of several other units of measurement like meters per second for speed, meters per second per second for acceleration, and cycles per second for frequency.

## Oscillation

Repetitive or periodic variation, typically in time, of some measure about a central value or between two or more different states.    The systems where the restoring force on a body is directly proportional to its displacement, such as the dynamics of the spring-mass system, are described mathematically by the simple harmonic oscillator and the regular periodic motion is known as simple harmonic motion.

## Phase (waves)

In physics and mathematics, the phase of a periodic function.        In the adjacent image, the top sine signal is the test frequency, and the bottom sine signal represents a signal from the reference.

## Vibration

Mechanical phenomenon whereby oscillations occur about an equilibrium point.       Note: angular frequency ω (ω=2 π f) with the units of radians per second is often used in equations because it simplifies the equations, but is normally converted to ordinary frequency (units of Hz or equivalently cycles per second) when stating the frequency of a system.

## Sine wave

Mathematical curve defined in terms of the sine trigonometric function, of which it is the graph.    where λ (lambda) is the wavelength, f is the frequency, and v is the linear speed.

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