The Attenuation of Audible Sound in Fog and Smoke

Abstract

Measurements of the rate of decay of sound in a reverberation room, first with no fog or smoke in the room and then with fog or smoke of known concentration and particle size added to the room, show that the attenuation of sound in a number of aerosols is in approximate agreement with values predicted by the theories of Sewell, Epstein, and Oswatitsch. In a water fog having a concentration of 2.0×10−6 grams/cm3 and average droplet radius of 6.25×10−4 cm, the attenuation owing to the fog increased from about 5 db/sec. at 500 c.p.s. to 13 db/sec. at 8000 c.p.s. In a quite similar fog of mineral oil, the corresponding attenuation increased from 1.6 db/sec. at 500 c.p.s. to 21 db/sec. at 8000 c.p.s. At very low frequencies, a fog of water is much more absorptive than is a fog of oil; the difference is ascribed to a “relaxation” effect of evaporation from and recondensation the droplets, which is much greater for water than for oil. The attenuation of sound in smoke may become rather high, amounting to 58 db/sec. at a frequency of 6000 c.p.s. for a moderately dense smoke of NH4Cl (180 g in a volume of 6080 cu. ft.).

Sound Scattering Laws for Moving Microinhomogeneous Media

“…It equals to 27 (15) Unlike (14) it is also proportional to Mach number squared and has no components independent of or linear in . However, (15), just like (12), could be used as correction estimate for (14), being fair up to order only -if it is necessary to write it out with accuracy up to . It is necessary for instance in scattered sound field calculation for wave propagating in normal direction to particles velocity, say, for horizontal sound waves in rain.…”

“…In ideal fluid dependence of on Reynolds number is missing. Due to linearity of basic equation (1), in the case of particles weak chaotic motion, superposition of more rapid ordered motion (say, for particles buffeted by local wind or flow) over their, modified Rayleigh c c (14) with Mach number of that rapid motion, will be valid as well. Evaluation of raindrops motion effect on sound scattering in ideal fluid is provided in [32].…”

“…Although Tyndall and Rayleigh as far back as formulated its general concepts in the 19th century, various aspects of it still attract scientists' efforts . They are pretending to explain observable experimental data (wave attenuation laws) by simple theoretical model or concept, say, by original Rayleigh law [1,14], by statistical properties of media frozen turbulence [4,7,[8][9][10][11][12][13] leading to Rayleigh law as well or even by supposed underwater sound field fractal nature [15]. Weakness of first model observed for moving media will be discussed below.…”

“…Papers [23][24][25][26][27][28][29][30][31] were devoted to isolate moving body or localized flow scattering problem solutions giving proper weight to ambient fluid motion. It is obvious that, in general, most of natural microinhomogeneous media elements including "movable" elements examined in [14][15][16][17][18][19][20][21][22] are in motion. That is why our papers [32,33] were related directly to low frequency scattering of sound waves in microinhomogeneous moving medium with detailed analysis of rain drops scattering and attenuation.…”

Prediction of Turbulent Jet Mixing Noise Reduction by Water Injection

“…Epstein and Carhart [12] considered sound attenuation by particles, allowing the particles to oscillate and considering both momentum and heat exchange. They compared the theory with the measurements with water droplets by Knudsen et al [13]. Marble et al [14,15] conducted theoretical investigations to determine the sound attenuation in fans and ducts by vaporization of liquid droplets.…”