Frequency is the number of repetitions (cycles) completed in a specific time interval. In telecommunications, it refers to the rate of change of amplitude of an electrical, electromagnetic, or optical signal. When measured in cycles per second, we use the unit of Hertz (Hz) to describe frequency.
You get a good sense of what frequency is by considering acoustical energy, which is essentially waves of air pressure generated by something vibrating. To the human ear, the frequency of these waves of air pressure is interpreted as pitch. Two different guitar strings create different tones because the strings each vibrate with different frequencies. Imagine a guitar with six identical strings; the sound created from each would be identical . . . and hardly musical! So the strings are given different diameters and, more importantly, tightened to different tension levels so that, when they are plucked, they vibrate at different speeds. The speed of the vibration sets up corresponding waves of air pressure that impact your eardrum and are interpreted as different pitches or tones. The A directly above Middle C, for example, vibrates at exactly 440 Hz. Middle C itself, vibrates at approximately 262 Hz.
The middle graph on the visual shows a 1 Hz wave and a 2 Hz wave.
Many transmission systems use the frequency of a signal to convey information. For example, your FM (for frequency modulation) radio station is given a specific electromagnetic (wireless) passband and alters the frequency of the electromagnetic pulses in direct proportion to the amplitude of the sound to be conveyed. This is an analog signaling technique. In frequency shift keying (FSK), a carrier is established and the frequency of the carrier altered to convey digital information.
|| Wavelength vs. Frequency|
|| Amplitude, Frequency, and Phase|