Infrasound in the context of "Sound pressure"

In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid.In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Only acoustic waves that have frequencies lying between about 20 Hz and 20 kHz, the audio frequency range, elicit an auditory percept in humans. In air at atmospheric pressure, these represent sound waves with wavelengths of 17 meters (56 ft) to 1.7 centimeters (0.67 in). Sound waves above 20 kHz are known as ultrasound and are not audible to humans. Sound waves below 20 Hz are known as infrasound. Different animal species have varying hearing ranges, allowing some to even hear ultrasounds.

Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries.

Hearing is one of the most crucial means of survival in the animal world and speech is one of the most distinctive characteristics of human development and culture. Accordingly, the science of acoustics spreads across many facets of human society—music, medicine, architecture, industrial production, warfare and more. Likewise, animal species such as songbirds and frogs use sound and hearing as a key element of mating rituals or for marking territories. Art, craft, science and technology have provoked one another to advance the whole, as in many other fields of knowledge. Robert Bruce Lindsay's "Wheel of Acoustics" is a well-accepted overview of the various fields in acoustics.

In physics, sound energy is a form of energy that can be heard by living things. Only those waves that have a frequency of 20 Hz to 20 kHz are audible to humans. However, this range is an average and will slightly change from individual to individual. Sound waves that have frequencies below 20 Hz are called infrasonic and those above 20 kHz are called ultrasonic. Sound is a longitudinal mechanical wave and as such consists physically in oscillatory elastic compression and in oscillatory displacement of a fluid. Therefore, the medium acts as storage for both potential and kinetic energy.

Consequently, the sound energy in a volume of interest is defined as the sum of the potential and kinetic energy densities integrated over that volume:

The Chelyabinsk meteor (Russian: Челябинский метеорит, romanised: Chelyabinskiy meteorit) was a superbolide that entered Earth's atmosphere over the southern Ural region in Russia on 15 February 2013 at about 09:20 YEKT (03:20 UTC). It was caused by an approximately 18-meter (60 ft), 9,100-tonne (10,000-short-ton) near-Earth asteroid that entered the atmosphere at a shallow 18‐degree angle with a speed relative to Earth of about 19.2 km/s (68,980 km/h; 42,860 mph). The light from the meteor was briefly brighter than the Sun (which is about -26.7 magnitude), visible as far as 100 kilometers (62 miles) away. It was observed in a wide area of the region and in neighbouring republics. Some eyewitnesses also reported feeling intense heat from the fireball.

The object exploded in a meteor air burst over Chelyabinsk Oblast, at a height of about 30 kilometres (18.6 miles). The explosion generated a bright flash, producing a hot cloud of dust and gas that penetrated to 26 kilometres (16 mi), and many surviving small fragmentary meteorites. Most of the object's energy was absorbed by the atmosphere, creating a large shock wave. The asteroid had a total kinetic energy before atmospheric impact equivalent to the blast yield of 400–500 kilotonnes of TNT (1.7–2.1 petajoules), estimated from infrasound and seismic measurements. This was approximately 30 times as much energy as that released by the atomic bomb detonated at Hiroshima.

In broadcasting, local insertion (known in the United Kingdom as an opt-out) is the act or capability of a broadcast television station, radio station or cable system to insert or replace part of a network feed with content unique to the local station or system. Most often this is a station identification (required by the broadcasting authority such as the U.S. Federal Communications Commission), but is also commonly used for television or radio advertisements, or a weather or traffic report. A digital on-screen graphic ("dog" or "bug"), commonly a translucent watermark, may also be keyed (superimposed) with a television station ID over the network feed using a character generator using genlock. In cases where individual broadcast stations carry programs separate from those shown on the main network, this is known as regional variation (in the United Kingdom) or an opt-out (in Canada and the United States).

Automated local insertion used to be triggered with in-band signaling, such as DTMF tones or sub-audible sounds (such as 25 Hz), but is now done with out-of-band signaling, such as analog signal subcarriers via communications satellite, or now more commonly via digital signals. In an emergency, such as severe weather, local insertion may also occur instantly through command from another network or other source (such as the Emergency Alert System or First Warning).