Equations 4 and 5 show that amplitude is a particularly sensitive measurement for gas formation in a battery. The acoustic impedance of most gases is on the order of hundreds of Rayl (1 Rayl = 1 kg/m 2 s), while solids and liquids are typically greater than 10 6 Rayl. This means a solid/gas interface will have a reflection coefficient >0.99. An ultrasonic wave encountering a solid-gas or liquid-gas interface will have most of its amplitude reflected, shown schematically in Figure .