Sound Propagation in Dilute Polyatomic Gases

Sound propagat ion in di lute pure gases, b o t h m o n a t o m i c a n d p o l y a t o m i c , has been considered f r o m the po int o f v iew o f the W a l d m a n n S n i d e r equat ion . It is shown that the c o m m o n l y e m p l o y e d assumption that sound propagat ion in gases is equ iva lent t o the propagat i on o f plane waves is val id o n l y in the region where collisions restore equi l ibr ium faster than it is perturbed b y the sound waves . A systematic t runcat ion procedure f o r an expans ion o f the perturbat ion func t i on in irreducible Cartesian tensors is introduced and then i l lustrated in solutions for three specific kinds o f molecules, hel ium, nitrogen and rough spheres. T h e agreement be tween theory and exper iment is rather g o o d for sound absorpt ion in the region where the rat io o f the collision a n d sound frequencies is greater than 1.5. T h e agreement in the case o f dispersion is g o o d over the whole measured pressure range. One useful result obta ined is t o show the p o l y a t o m i c gas calculat ions in second approx imat ion have as g o o d agreement wi th exper iment as the calculations f o r nob le gases in third approx imat ion . This can be related t o the possession b y the p o l y a t o m i c gas o f a bu lk viscosity which dominates in sound propagat ion .