Excess attenuation of an acoustic beam by turbulence

Abstract

A theory based on the concept of a spatial sinusoidal diffraction grating is presented for the estimation of the excess attenuation in an acoustic beam. The equation of the excess attenuation coefficient shows that the excess attenuation of acoustic beam not only depends on the turbulence but also depends on the application parameters such as the beam width, the beam orientation and whether for forward propagation or back scatter propagation. Analysis shows that the excess attenuation appears to have a frequency dependence of cube-root. The expression for the excess attenuation coefficient has been used in the estimations of the temperature structure coefficient, C T 2 , in sodar sounding. The correction of C T 2 values for excess attenuation reduces their errors greatly. Published profiles of temperature structure coefficient and the velocity structure coefficient in convective conditions are used to test our theory, which is compared with the theory by Brown and Clifford. The excess attenuation due to scattering from turbulence and atmospheric absorption are both taken into account in sodar data processing for deducing the contribution of the lower atmosphere to seeing, which is the sharpness of a telescope image determined by the degree of turbulence in the Earth's atmosphere. The comparison between the contributions of the lowest 300-m layer to seeing with that of the whole atmosphere supports the reasonableness of our estimation of excess attenuation.

Measurements and predictions of hooded crow (Corvus corone cornix) call propagation over open field habitats

“…This assumption has been based on some early work ͑Lighthill, 1952; Ingard, 1953;Lighthill, 1953͒. However, more recent work does not support this notion ͑Brown and Clifford, 1976; Piercy et al, 1977;Pan, 2003͒ and points to errors in Lighthill's work ͑Brown and Clifford, 1976͒. The more recent work points to a very small effect of turbulence and a weak frequency dependence of approximately 1.63͑f / 1000͒ 1/3 in dB/km, where f is the frequency ͑Brown and Clifford, 1976; Pan, 2003͒.…”

Wind tunnel investigation of sound attenuation in turbulent flow

“…(1) the streamwise integral scale corresponding to the location of the transducers is l ffi 7 Á 10 À3 m. For the maximum sound path propagation s = 0.25 m the first Frensel's zone is ffiffiffiffi ffi ks p ¼ 2:7 Á 10 À2 m , where k is the wavelength. All parameters of the turbulence, which have been stated in Section 1 to be important for the attenuation of sound waves, could be varied independently over a broad range by varying the size of the mesh M, the length of the measuring sound path s (s = 0.05 mÄ0.26 m), the degree of turbulence (intensity of the turbulence) by varying mean flow velocity U (U = 5-20 m/s) only, since it has been shown [18] that the velocity inhomogeneities are the main factor in sound wave attenuation caused by turbulence.…”

“…Detailed review of experimental works related to so-called excess attenuation (beyond that due to classical and molecular absorption) presented in [21]. It has been established that sound was more influenced by wind fluctuations than by temperature fluctuations [18], Cheinet et al [5].…”

“…Experimental problems arise from variety of factors, for example, the wave characteristics and atmospheric fluctuations cannot be measured simultaneously along the propagation path. The other environmental effects, such as a ground reflection and atmospheric stratification also must be considered that make the results hard to interpret [18] and references therein, [19]. It must be emphasized that characterization of the mentioned environmental effects including characterization of outdoor wind speed regimes requires very comprehensive instrumentation to obtain meaningful data ( [15] and references therein).…”