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In vibration analysis, there is a detailed explanation of frequency. Many people do not have a good understanding of vibration frequencies, so the editor will provide a detailed introduction based on the knowledge of our company’s technical staff. Frequency refers to the number of periodic changes that occur per unit of time; it is a measure that indicates how frequently a periodic motion takes place. It is commonly denoted by the symbols f or u, with its unit being per second. To honor the contributions of the German physicist Hertz, the unit of frequency was named hertz, abbreviated as “Hz”. Every object has a frequency that is determined by its own properties and is independent of the amplitude; this is called the natural frequency. The concept of frequency is applied not only in mechanics and acoustics, but also commonly in electromagnetism and radio technology. Audio sound is mechanical vibration that can pass through materials in various states of matter. These substances that can transmit sound are called media. Sound cannot travel in a vacuum. The sounds we hear are also sound waves with a certain frequency. The frequency range of human hearing is approximately 20–20,000 Hz; frequencies outside this range cannot be detected by our ears. Frequencies below 20Hz are infrasound, while those above 20kHz are ultrasound. The higher the frequency of a sound, the higher its pitch; the lower the frequency, the lower its pitch. In astronomical tide theory, due to the long periods of various celestial activities, it is inconvenient to express tidal frequencies in hertz; the common unit for frequency is cph, which stands for cycle per hour. For example, the period of the common M2 semidiurnal tide is about 12.42 hours, so its frequency is typically expressed as 0.08051 cph (cycles per hour). The reciprocal of the angular frequency period is called frequency (denoted by the symbol f), that is, f = 1/T. It represents the number of periodic cycles per unit of time, that is, it indicates the rate of change (how fast or slow). The international unit of frequency is the hertz. Through the above introduction, everyone should now have a general understanding of various frequencies. There are many issues in the vibration analysis of rotating equipment, and frequency is one of the factors that we need to take into account. For more details, please contact us.