![]() Such work is related to the use of sonar and radar techniques to determine the structure of media (water and air) and the position of objects in them, to seismic techniques used in petroleum exploration and to the use of ultrasound in medicine. More esoteric work, such as measuring the velocity and absorption of high ultrasonic sound in various liquids to deduce the detailed architecture of their constituent molecules and the energy needed to change their structure, has application in various fields of engineering. This research has applications in urban and regional planning, for locating buildings and for sound barriers near expressways, airports, railways, etc. ![]() Canadian research into the transmission of sound in the atmosphere has been extensive areas studied include the effect of temperature gradient, turbulence, obstruction, ground reflections and ground impedance. In Canada, acoustics research takes place in government laboratories (eg, National Research Council), the universities (eg, Sherbrooke, Toronto) and industry. Sound travelling in the solid materials of a building is called structureborne sound. Although in everyday language the word "sound" is mostly used to refer to sound in air, propagating vibration in solids is an acoustical phenomenon. Seismic waves (whether on Earth or the moon) have surface and shear components and are a legitimate part of acoustics. In plates, rods and beams, bending waves are important. Longitudinal waves can also propagate in solids. In solids, several different types of acoustic waves, such as shear waves (which cause changes in shape but not in volume) and surface waves, occur. In materials such as concrete and steel, it is higher still. The sound speed in water, for example, is about 1500 metres per second at ordinary temperatures. The velocity of sound is generally greater in liquids and solids than it is in gases. The velocity of sound in air increases with increasing temperature at normal room temperatures around 22☌, the velocity of sound is about 345 metres per second. In air, these pressure variations stimulate the ear, which sends signals that are processed by the brain as sound. This kind of propagation is called a longitudinal wave. Sound waves in gases generate alternating small increases and decreases in pressure about the equilibrium value that propagate through the gas the molecules oscillate back and forth along the direction of propagation. Physical acoustics includes both linear processes such as the propagation of sound from traffic, and non-linear processes such as the shock waves that are generated by planes flying faster than the speed of sound. It deals with airborne, audible sound, infrasound and ultrasound. Physical acoustics encompasses propagation and absorption of sound at all frequencies in air and other gases, liquids, semi-solids and solids. Knowledge of the physical principles of acoustics has practical application in planning recreational and other developments to reduce the adverse effects of noise, but, because of its importance as a modern environmental health problem, noise and its control are treated as a separate branch of acoustics. It can be divided into various branches, but the boundaries between the various branches are often ill-defined.
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