Acoustic impedance, denoted Z and measured in Pa·m −3 ·s in SI units, is defined by [2] = ^ ^ (), where ^ is the Laplace transform of sound pressure, [citation needed] ^ is the Laplace transform of sound volume flow rate. Specific acoustic impedance, denoted z and measured in Pa·m −1 ·s in SI units, is defined by [2] = ^ ^ (), where ^ is the Laplace transform of sound pressure,