Different animal species have different auditory ranges. In air at atmospheric pressure, they represent sound waves with wavelengths from 17 m (56 feet) to 1.7 cm (0.67 inches). Only waves with frequencies between about 20 Hz (infrasound) and 20 kHz (ultrasound), the range of audio frequencies, arouse an auditory perception in humans. In human physiology and psychology, the sound is the reception of sound waves and their perception by the brain. Sound perception in humans, animals and plants Rain noise is fairly constant on all frequencies, while wind noise is one of the predominant natural factors influencing low-frequency ambient noise levels. Furthermore, account should be taken of anthropogenic noise, such as ship noise, particularly commercial ships, which in recent centuries has increased ambient noise levels at frequencies below 1 kHz. In shallow waters, the main natural sources of noise arise from waves hitting the shore, local wind, rain, and biological sounds such as shrimp and marine mammals. In deep water, the main natural sources of noise arise from waves generated by tidal and wind cycles, seismic disturbances such as earthquakes and volcanism, lightning, rain, ocean turbulence, and marine mammals. When sound is propagated in water, a distinction must be made between deep water and shallow water. In environmental noise, air turbulence, rough surfaces, and obstacles such as trees may cause scattering. 4įurthermore, scattering occurs when sound waves propagates through atmosphere and meet a region of inhomogeneity therefore, some of their energy is redirected into many other directions. Precipitation, rain, snow, or fog has an insignificant effect on sound levels although the presence of precipitation will obviously affect the humidity and may affect wind and temperature gradients. When sound propagates in air, high frequencies are absorbed more than low because of the molecular relaxation phenomenon, and the amount of absorption depends on the temperature and humidity of the atmosphere. In addition, during propagation, waves may be reflected, refracted, or attenuated by the medium. The third factor is the viscosity: the average viscosity determines the speed at which the sound is attenuated, but for water or air the attenuation due to viscosity is negligible. The second is the motion of the medium: if the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement. The first is the relationship between the density and the pressure of the medium, which is influenced by temperature and determines the speed of sound within the medium. The behavior of sound propagation is generally influenced by three factors. The frequency bands of naturally audible sound are divided into: 3 The speed of propagation of sound depends on the characteristics of the medium in particular, it is directly proportional to elasticity and inversely proportional to density. Sound waves are characterized by frequency (Hz), intensity (dB) and timbre, which at the same frequency distinguishes one sound from another. In longitudinal waves particles oscillate along the direction of propagation, while in transversal ones the direction of particles oscillation is at right angle to the direction of propagation. Sound is transmitted through water and air with longitudinal waves, through solids with longitudinal and transverse waves. While the sound source continues to vibrate the medium, vibrations propagate far from the source at the speed of sound, forming the sound wave. 1, 2 Sound waves are generated by a sound source that creates vibrations in the surrounding medium. Sound is a vibration that propagates in the form of a sound wave through a medium that can be a liquid as water, a gas as air or a solid material.
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