Atmospheric Acoustics

Atmospheric sound attenuation

As sound propagates in the atmosphere, several interacting mechanism attenuate and change the spectral or temporal characteristics of the sound received at a distance from the source. The attenuation means that sound propagating through the atmosphere decreases in level with increasing distance between source and receiver. The total attenuation, in decibels, can be approximated as the sum of three nominally independent terms, as given

Sound energy spreads out as it propagates away from its source due to geometrical divergence. At distances that are large compared with the effective size of the sound source, the sound level decreases at the rate of 6 dB for every doubling of distance. The phenomenon of geometrical divergence, and the corresponding decrease in sound level with increasing distance from the source, is the same for all acoustic frequencies. In contrast, the attenuation due to the other two terms in the equation depends on frequency and therefore changes the spectral characteristics of the sound.

Air absorption

Dissipation of acoustic energy in the atmosphere is caused by viscosity, thermal conduction, and molecular relaxation. The last arises because fluctuations in apparent molecular vibrational temperatures lag in phase the fluctuations in translational temperatures. The vibrational temperatures of significance are those characterizing the relative populations of oxygen (O₂) and nitrogen (N₂) molecules. Since collisions with water molecules are much more likely to induce vibrational state changes than are collisions with other oxygen and nitrogen molecules, the sound attenuation varies markedly with absolute humidity. See Molecular structure and spectra, Viscosity

The total attenuation due to air absorption increases rapidly with frequency. For this reason, applications in atmospheric acoustics are restricted to sound frequencies below a few thousand hertz it the propagation distance exceeds a few hundred meters. See Sound absorption

Effects of the ground

When the sound source and receiver are above a large flat ground surface in a homogeneous atmosphere, sound reaches the receiver via two paths. There is the direct path from source to receiver and the path reflected from the ground surface. Most naturally occurring ground surfaces are porous to some degree, and their acoustical property can be represented by an acoustic impedance. The acoustic impedance of the ground is in turn associated with a reflection coefficient that is typically less than unity. In simple terms, the sound field reflected from the ground surface suffers a reduction in amplitude and a phase change.

When the source and receiver are both relatively near the ground and are a large distance apart, the direct and reflected fields become nearly equal and cancel each other.

Refraction of sound

Straight ray paths are rarely achieved outdoors. In the atmosphere, both the wind and temperature vary with height above the ground. The velocity of sound relative to the ground is a function of wind velocity and temperature; hence it also varies with height, causing sound waves to propagate along curved paths.

The speed of the wind decreases with decreasing height above the ground because of drag on the moving air at the surface. Therefore, the speed of sound relative to the ground increases with height during downwind propagation, and ray paths curve downward. For propagation upwind, the sound speed decreases with height, and ray paths curve upward (see illustration). In the case of upward refraction, a shadow boundary forms near the ground beyond which no direct sound can penetrate. Some acoustic energy penetrates into a shadow zone via creeping waves that propagate along the ground and that continually shed diffracted rays into the shadow zones. The dominant feature of shadow-zone reception is the marked decrease in a sound's higher-frequency content. The presence of shadow zones explains why sound is generally less audible upwind of a source.

Curved ray paths

Refraction by temperature profiles is analogous. During the day, solar radiation heats the Earth's surface, resulting in warmer air near the ground. This condition is called a temperature lapse and is most pronounced on sunny days. A temperature lapse is the common daytime condition during most of the year, and also causes ray paths to curve upward. After sunset there is often radiation cooling of the ground, which produces cooler air near the surface. In summer under clear skies, such temperature inversions begin to form about 2 hours after sunset. Within the temperature inversion, the temperature increases with height, and ray paths curve downward.

The effects of refraction by temperature and wind are additive and produce rather complex sound speed profiles in the atmosphere.

Effects of turbulence

Turbulence in the atmosphere causes the effective sound speed to fluctuate from point to point, so a nominally smooth wave front develops ripples. One result is that the direction of a received ray may fluctuate with time in random manner. Consequently, the amplitude and phase of the sound at a distant point will fluctuate with time. The acoustical fluctuations are clearly audible in the noise from a large aircraft flying overhead. Turbulence in the atmosphere also scatters sound from its original direction. See Turbulent flow

a branch of acoustics in which the propagation and generation of sound in the meteorological atmosphere are studied and the atmosphere is investigated by acoustical methods. As a research method it is also a branch of atmospheric physics. The study of sound propagation in the atmosphere began with the origin of acoustics. In the late 17th and the 18th centuries, W. Durhem (England) investigated the dependence of sound velocity on wind velocity, and Bianconi (Italy) and C.-M. la Condamine (France) studied the effect of temperature on sound velocity. The Soviet scientists N. N. Andreev and I. G. Rusakov (1934) and D. I. Blokhintsev (1947) made a large contribution to the research on sound propagation in an inhomogeneously moving medium.

Sound propagation in free air has a number of peculiarities. Owing to the thermal conductivity and viscosity of the atmosphere, sound wave absorption is greater the higher the frequency of the sound and the lower the density of the air. Consequently, the sharp sounds of nearby shots or explosions become deadened at great distances. The inaudible sounds at very low frequencies (known as infrasonic), having periods of several seconds to several minutes, are not greatly attenuated and can be propagated thousands of kilometers and even circle the globe several times. This makes it possible to detect nuclear explosions, which are a powerful source of such waves.

There are important problems in atmospheric acoustics connected with phenomena that occur during the propagation of sound in an atmosphere which from an acoustical viewpoint is a moving inhomogeneous medium. The temperature and density of the atmosphere decrease with increasing height; at great heights the temperature again rises. Upon these regular heterogeneities are superimposed variations in the temperature and the wind which depend on meteorological conditions, as well as random turbulent pulsations of different degrees. Inasmuch as the wind velocity is controlled by the air temperature and sound is “carried” by the wind, then all the heterogeneities mentioned have a strong effect on sound propagation. Bending of the sound rays—refraction of sound—occurs, with the result that a deflected sound ray can be returned to the earth’s surface, thus forming acoustic audibility zones and zones of silence; sound scattering and attenuation occur in turbulent anomalies, strong absorption at great heights, and so on.

It is necessary to solve the complex inverse problem in acoustic sounding of the atmosphere. The distribution of temperature and wind at great heights is obtained from measurements of the time and direction of arrival of sound waves created by ground-level explosions or the explosions of bombs released from a rocket. For the investigation of turbulence, the temperature and wind velocity are determined by measuring the propagation time for sound over small distances; to achieve the required accuracy ultrasonic frequencies are utilized.

The problem of industrial noise propagation, particularly of shock waves produced by the motion of supersonic jet aircraft, has become highly important. If atmospheric conditions are favorable for focusing these waves, the pressures at ground level can attain values that are dangerous to structures and human health.

Various sounds of natural origin are observed in the atmosphere. Long peals of thunder occur because of the great length of a lightning discharge and because when sound waves are refracted they propagate along different paths and arrive with various delays. Certain geophysical phenomena such as the auroras, magnetic storms, strong earthquakes, hurricanes, and sea waves are sources of sound, particularly of infrasonic waves. Their study is important not only for geophysics but, for example, for timely storm warnings. Various audible noises are produced either by collision vortices with various objects (the whistling of the wind) or by the vibrations of some object in the air flow (the humming of wires, the rustling of leaves, and so on).