Sound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations
For humans, hearing is normally limited to frequencies between about 12 Hz and 20,000 Hz (20 kHz), although these limits are not definite. The upper limit generally decreases with age. Other species have a different range of hearing. For example, dogs can perceive vibrations higher than 20 kHz. As a signal perceived by one of the major senses, Sound is used by many species for detecting danger, navigation, predation, and communication. Earth's atmosphere, water, and virtually any physical phenomenon, such as fire, rain, wind, surf, or earthquake, produces (and is characterized by) its unique sounds. Many species, such as frogs, birds, marine and terrestrial mammals, have also developed special organs to produce sound. In some species, these have evolved to produce song and speech. Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound.
Speed of sound
: Speed of sound
The speed of Sound depends on the medium through which the waves are passing, and is often quoted as a fundamental property of the material. In general, the speed of Sound is proportional to the square root of the ratio of the elastic modulus (stiffness) of the medium to its density. Those physical properties and the speed of Sound change with ambient conditions. For example, the speed of Sound in gases depends on temperature. In 20 °C (68 °F) air at the sea level, the speed of Sound is approximately 343 m/s (1,230 km/h; 767 mph) using the formula "v = (331 + 0.6T) m/s". In fresh water, also at 20 °C, the speed of Sound is approximately 1,482 m/s (5,335 km/h; 3,315 mph). In steel, the speed of Sound is about 5,960 m/s (21,460 km/h; 13,330 mph). The speed of Sound is also slightly sensitive (a second-order anharmonic effect) to the Sound amplitude, which means that there are nonlinear propagation effects, such as the production of harmonics and mixed tones not present in the original Sound