Frequency

Definitions and Units

• Frequency is the number of occurrences of a repeating event per unit of time.
• Temporal frequency is used to distinguish frequency from spatial frequency.
• Ordinary frequency is related to angular frequency by a factor of 2π.
• The period is the interval of time between events, and it is the reciprocal of the frequency.
• The SI unit of frequency is the hertz (Hz), and the SI unit for the period is the second.

Period versus Frequency

• Frequency is the number of cycles or repetitions per unit of time.
• The period is the time taken to complete one cycle of an oscillation or rotation.
• Temporal frequency emphasizes the number of occurrences of a repeating event per unit time.
• Rotational frequency is the instantaneous rate of change of the number of rotations with respect to time.
• Angular frequency is the rate of change of angular displacement or the rate of change of the phase of a sinusoidal waveform.

Wave Propagation

• Frequency has an inverse relationship to the wavelength in nondispersive media.
• The frequency of a sinusoidal wave is equal to the phase velocity divided by the wavelength.
• In vacuum, the frequency of electromagnetic waves is equal to the speed of light divided by the wavelength.
• Monochromatic waves maintain their frequency when traveling between different media.
• Wave period is more commonly used to describe longer and slower waves, such as ocean surface waves.

Measurement

• Frequency can be measured by counting the number of occurrences within a specific time period and dividing by the period.
• Gating error introduces a random error into the count and causes an average error in the calculated frequency.
• A resonant-reed frequency meter was used in the past to measure the frequency of alternating current.
• Stroboscopes can be used to measure the frequency of rotating or vibrating objects by adjusting the flashing light frequency.
• Higher frequencies are usually measured with a frequency counter, which uses digital logic to count the number of cycles during a time interval.
• Frequency can be measured using physical phenomena such as the oscillation of a pendulum.
• Sound waves can be analyzed using Fourier analysis to determine their frequency components.
• Frequency can be estimated by observing the Doppler shift in waves emitted by a moving source.
• Frequency can be measured using interferometry techniques.
• Frequency can be determined by analyzing the spectrum of a signal using a spectrum analyzer.

Examples

• Visible light is an electromagnetic wave with a frequency range of 400 THz to 800 THz.
• Infrared (IR) radiation has frequencies below 400 THz and is invisible to the human eye.
• Ultraviolet (UV) radiation has frequencies above 800 THz and is also invisible to the human eye.
• Microwaves and radio waves have lower frequencies than visible light.
• X-rays and gamma rays have higher frequencies than visible light.
• Sound propagates as mechanical vibration waves of pressure and displacement.
• The audible frequency range for humans is typically between 20 Hz and 20,000 Hz.
• The frequencies of sound determine its pitch and timbre.
• Different species have different hearing ranges, such as dogs perceiving vibrations up to 60,000 Hz.
• In Europe, Africa, Australia, southern South America, most of Asia, and Russia, the frequency of household electrical outlets is 50 Hz.
• In North America and northern South America, the frequency of household electrical outlets is 60 Hz.
• The frequency of the hum in an audio recording can indicate the region where it was made.
• Utility frequency refers to the frequency of alternating current in household electrical outlets.
• The European frequency is close to the tone G, while the North American frequency is between the tones B and B.
• Aperiodic frequency refers to the rate of incidence or occurrence of non-cyclic phenomena.
• It is expressed as a reciprocal second (s) or becquerels in the case of radioactivity.
• Aperiodic frequency is a physical quantity that involves counting the number of entities or events during a given time duration.
• It is the inverse of the (temporal) period.
• Aperiodic frequency is applicable to random processes like radioactive decay.

Frequency Data Sources