The acoustic intensity is given by the product of acoustic pressure p and particle velocity v, but in the literature, expressions like p2/ρc or ρcv2 are often quoted. These are plane-wave approximations and are not in general valid. Assuming continuous-wave operation, the relations between the approximate formulas and the exact one are discussed in terms of the specific acoustic impedance. Further remarks deal with the vector character of the acoustic intensity and its connection with heat generation in the medium, as considered in a recent paper by Nyborg [J. Acoust. Soc. Am. 70, 310–312 (1981)].