Frequency

Frequency is defined in the New Oxford American Dictionary as:

"1. The rate at which something occurs or is repeated over a particular period of time...

2. The rate at which a vibration occurs that constitutes a wave, ... usually measured per second."

Note that in both definitions, the word "rate" is used as a synonym. In some cases, in the discussion of sonic or musical phenomena, those two words may be used interchangeably. However, most of the time 'frequency' is reserved for discussions of periodically oscillating sound waves or signals (especially at an audible rate), whereas 'rate' is used to discuss the speed with which discrete events occur.

With regard to sound, frequency most commonly is used to refer to the number of cyclic vibrations back and forth by a sound-producing object, and the resulting cyclically-varying signal that that sound produces. When an object vibrates repeatedly back and forth, or an electrical signal's voltage fluctuates repeatedly up and down, or any signal (analog or digital) exhibits a repetitive pattern, one full repetition of that pattern is called a cycle. Audio signals (especially pitched sounds) are thus described the the number of cycles of repetition they exhibit per second. So, frequency (often abbreviated f or ƒ in formulas and charts) is a measurement of "cycles per second" that occur in an audio wave. The name Hertz is synonymous with "cycles per second", and is more commonly used because it's shorter to say; in writing, Hertz is usually abbreviated as Hz (with no period afterward).

The range of human hearing is usually stated as being 20 Hz to 20,000 Hz. However, each individual has a somewhat different frequency range of hearing capability. Most people cannot hear frequencies all the way up to 20,000 Hz, so the actually upper end of the range is usually somewhere between 16,000 Hz and 20,000 Hz. And on the low end of the range, the precise frequency at which a person begins to hear repetitions as a continuous low tone (rather than as a series of discrete events) is variable, usually somewhere between 20 Hz and 40 Hz.

[This article was started by cdobrian.]

Frequency is the measurement of repetition in time. In relationship to music frequency is  the repeating pattern of amplitudes that occur during "one second”. Frequency is also the speed of the vibration that will the determine “the pitch of the sound.” The measurement unit of frequency is Hertz or Hz which is just the short version. [This additional information was provided by Usbaldo Castellanos on 1/8/15.]