**2. Specificity of industrial and transport noise monitoring**

## **2.1 Physical characteristics**

Depending on the frequency, acoustic vibrations are divided into infrasonic, sonic and ultrasonic. According to their physical nature, the acoustic vibrations of these ranges are the same, and their separation is somewhat arbitrary and is associated with the physiological feature of the human auditory analyzer. It is believed that a person hears sounds with frequencies from 16 Hz to 20 kHz. The area of sound frequencies or acoustic vibrations of the air in the infra-, ultra-, and hypersonic ranges is not perceived by the human ear. It should be noted that modern regulatory documents give a slightly different frequency gradation for infrasound. Infrasound is commonly understood as acoustic vibrations with a frequency below 22 Hz. At high levels of sound pressure (SPL) infrasound (over 120 dB), a person has a feeling of pulsation, pressure, and even pain in the eardrum. The physical features of infrasound include a long wavelength and low absorption in the atmosphere and the resulting ability of infrasound to propagate over long distances from the source without significant loss of energy. It should be kept in mind that sound propagates spherically and the decrease in sound pressure is inversely proportional to the square of the distance from the source [13, 14].

The audio frequency range includes acoustic vibrations from 20 Hz to 20 kHz, which are perceived by the human ear. Noise is a disorderly combination of sounds of different strength and frequency. According to the predominance of acoustic energy in one or another part of the spectrum, noise is divided into low-frequency (up to 500 Hz), medium-frequency (from 500 to 1000 Hz) and high-frequency (from 1000 to 8000 Hz).
