Ultrasonic Absorption [antikvár]

CHAPTER 1 INTRODUCTION 1.1. General remarks The quantities of prime interest in this work are the velocity and the absorption coefficient of the ultrasonic waves, and their dependence on temperature and pressure of the medium on the one hand, and on the frequency of the wave on the other hand. A few, order of magnitude, values of these quantities in gases, liquids, and solids, -will serve as a useful orientation to the subject. As shown in any first year textbook, the velocity of sound in an ideal gas is given by the expression = (yRT/M)*; here R is the gas constant, T the absolute temperature, M the molecular weight, and y the ratio of the constant pressure to constant volume specific heats. In air at room temperature 340 m/sec. In a liquid is commonly of the order 1200 m/see and in a solid of the order 5000 m/sec. In contrast to gases the velocity of sound in a typical solid depends but little on temperature, the variation being less than 10 or 15 per cent from absolute zero to temperatures close to its melting-point. The variation of with frequency (dispersion) is small. In solids it is usually less than 1 per cent. In fluids it is seldom larger than 25 per cent, except in the case of a gas at such high frequencies that the wavelength of the sound wave is smaller than the mean free path of the molecules. In contrast to the velocity, the absorption, as a rule, is both very temperature and frequency dependent. A plane sound wave of frequency 10® c/s (cycles/second) can travel hundreds of metres without any appreciable attenuation in its intensity. On the other hand its intensity is, in general, barely measurable after 10 cm, in a gas above 1 Me/s (10® c/s), and in a liquid or a solid above 50 Me/s. For the values of given above, the wavelength at these frequencies is 0-03 cm in a gas, 0'002 cm in a liquid,