![]() This frequently occurs in winter and at sundown. Under conditions of a temperature inversion (temperature increasing with increasing height), the sound waves will be refracted downwards, and therefore may be heard over larger distances. The scattering of sound by rain, snow or fog at ordinary frequencies is insignificant. In environmental noise situations, scattering is caused by air turbulence, rough surfaces, and obstacles such as trees. Scattering occurs when sound waves are propagating through the atmosphere and meet a region of inhomogeneity (a local variation in sound speed or air density) and some of their energy is re-directed into many other directions. In reality some sound will enter this zone due to scattering. This will result in the formation of a shadow zone, which is a region in which sound does not penetrate. This results in sound waves being 'bent' upwards (see REFRACTION ). For example, under normal conditions the atmosphere is cooler at higher altitudes. Since the temperature of the atmosphere is not uniform there are local variations in the sound speed. Higher temperatures produce higher speeds of sound. (b) Wind and Temperature Gradients: The speed that sound propagates in a gas depends on the temperature of the gas. Ref.: Cyril Harris, "Absorption of Sound in Air versus Humidity and Temperature," Journal of the Acoustical Society of America, 40, p. Under 'normal' circumstances, atmospheric absorption can be neglected except where long distances or very high frequencies are involved. Precipitation, rain, snow, or fog, has an insignificant effect on sound levels although the presence of precipitation will obviously affect the humidity and may also affect wind and temperature gradients (see next section). ![]() (right) Attenuation as a function of temperature for various percentages of relative humidity. (left) Frequency dependence of attenuation as a function of relative humidity at 20☌. It should be noted, however, it can be as high as 5 dB/100 m at 8 kHz when the temperature is 20☌ and the humidity is 10%. The figures show the variation of the absorption with temperature and relative humidity.įrom the diagrams it can be seen that for the middle of the speech frequency range (2 kHz), the absorption is typically. The amount of ABSORPTION depends on the temperature and humidity of the atmosphere. ![]() High frequencies are absorbed more than low. By far the most important of these is molecular relaxation. These are molecular relaxation and viscosity effects. (a) Air Absorption: There are two mechanisms by which acoustic energy is absorbed by the atmosphere. Radii A and B indicate a doubling of distance. Īttenuation from a point source (left) where the intensity decreases according to the square of the distance from the source (note increasing cone area), and from a line source (right) where the intensity decreases directly according to the distance from the source. A line source will produce cylindrical spreading, resulting in a sound level reduction of 3 dB per doubling of distance. ![]() A busy highway approximates to a line source, that is, equal sound power output per unit length of highway. įor example, in the case of spherical spreading from a point source, which is due to a noise source radiating sound equally in all directions, the sound level is reduced by 6 dB for each doubling of distance from the source (see INVERSE-SQUARE LAW ). Sound propagation losses due to spreading are normally expressed in terms of x dB per doubling of distance from the source. There are two common kinds of geometric spreading: spherical and cylindrical spreading. Geometric spreading is independent of FREQUENCY and has a major effect in almost all sound propagation situations. This refers to the spreading of sound energy as a result of the expansion of the wavefronts. There are several important factors which affect the propagation of sound: geometric spreading, atmospheric effects, and surface effects. This variation is caused by changes in weather conditions and by topographical features such as ground cover, hills and other obstacles between the source and the receiver. The levels of outdoor NOISE, whether they are intrusive or the normal background environment, vary extensively at distances greater than about a hundred meters from the source. The TRANSMISSION of acoustic energy through a medium via a SOUND WAVE.
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