Plot for the upper Q-factor versus its frequency. The data are provided by Barret and Mendez, which have been exploited and discussed in the references (Barret et al. 2005a,b,c, 2006, 2007, 2008; Boutelier et al. 2009, 2010; Mendez 2006). The theoretical curves of upper Q-factor Q₂ of Eq. (10) are plotted with the parameter α = 0.35,0.55,0.8 and the average mass density parameter A = 0.7 (Zhang 2004; Zhang et al. 2007).