A noise barrier (also called a soundwall, sound berm, sound barrier, or acoustical barrier) is an exterior structure designed to protect sensitive land uses from noise pollution. Noise barriers are the most effective method of mitigating roadway, railway, and industrial noise sources – other than cessation of the source activity or use of source controls.

In the case of surface transportation noise, other methods of reducing the source noise intensity include encouraging the use of hybrid and electric vehicles, improving automobile aerodynamics and tire design, and choosing low-noise paving material. Extensive use of noise barriers began in the United States after noise regulations were introduced in the early 1970s.

Noise barriers have been built in the United States since the mid-20th century, when vehicular traffic burgeoned. In the late 1960s, acoustical science technology emerged to mathematically evaluate the efficacy of a noise barrier design adjacent to a specific roadway. By the 1991s, noise barriers that included use of transparent materials were being designed in Denmark and other western European countries. Below, a researcher collects data to calibrate a roadway noise model for Foothill Expressway.

The sound barrier, in aerodynamics, is the point at which an aircraft moves from transonic to supersonic speed. The term, which occasionally has other meanings, came into use during World War II, when a number of aircraft started to encounter the effects of compressibility, a collection of several unrelated aerodynamic effects that "struck" their planes like an impediment to further acceleration. By the 1950s, new aircraft designs routinely "broke" the sound barrier.

Some common whips such as the bullwhip or sparewhip are able to move faster than sound: the tip of the whip breaks the sound barrier and causes a sharp crack—literally a sonic boom.Firearms since the 19th century have generally had a supersonic muzzle velocity.

The sound barrier may have been first breached in nature some 150 million years ago. Some paleobiologists report that, based on computer models of their biomechanical capabilities, certain long-tailed dinosaurs such as Apatosaurus and Diplodocus may have possessed the ability to flick their tails at supersonic velocities, possibly used to generate an intimidating booming sound. This finding is theoretical and disputed by others in the field. Meteorites entering the Earth's atmosphere usually, if not always, descend faster than sound.

Charles Elwood "Chuck" Yeager ( /ˈjeɪɡər/; born February 13, 1923) is a retired major general in the United States Air Force and noted test pilot. He was the first pilot to travel faster than sound (1947). Originally retiring in 1975 as a brigadier general, Yeager was promoted to major general on the Air Force's retired list in 2005 for his military achievements.

His career began in World War II as a private in the United States Army Air Forces. After serving as an aircraft mechanic, in September 1942 he entered enlisted pilot training and upon graduation was promoted to the rank of flight officer (the World War II USAAF equivalent to warrant officer) and became a North American P-51 Mustang fighter pilot.

After the war he became a test pilot of many kinds of aircraft and rocket planes. Yeager was the first man to break the sound barrier on October 14, 1947, flying the experimental Bell X-1 at Mach 1 at an altitude of 45,000 ft (13,700 m). Although Scott Crossfield was the first man to fly faster than Mach 2 in 1953, Yeager shortly thereafter set a new record of Mach 2.44.