The field of physics contains words that also are used in common, everyday language. Examples are intensity, power, work, and energy. In physics these words have very specific and well-established definitions. However, they are often misused in the underwater acoustics literature. The most prevalent error probably is the use of intensity to describe the mean square pressure. Another common mistake is to use power when referring to instantaneous squared pressure and to refer to the sum of squared pressure over time as energy. The descriptions below conform to the physics definition of the terms as they apply to the study of acoustics. They are presented in this report to help remove the confusion that surrounds this topic.

Acoustic Energy Density—the energy per unit volume in the sound field. Two types of mechanical energy density exist in an acoustic field, potential energy density and kinetic energy density. The potential energy density measures the ability of the deformed fluid (deformed by the presence of sound) to do work. The acoustic kinetic energy density measures the ability of the fluid to do work because of the fluid motion associated with the acoustic field. The mean square pressure is proportional to the average potential energy density. Therefore, the integral of the pressure squared over a time interval is simply the mean squared pressure multiplied by the duration of the time interval, or proportional to the average potential energy density multiplied by the duration of the time interval. This time integral is not equal to energy, as it is sometimes mistakenly called. Similarly, the acoustic kinetic energy density is proportional to the mean squared particle velocity amplitude. The standard units of energy are joules, so that acoustic energy density (either potential or kinetic, or the sum of the two) has decibel units of dB re 1 J/m³. Both types of acoustic energy density and acoustic energy (obtained by summing the energy densities over a specified volume of fluid) are second order in acoustic field variables.

Acoustic Impedance—a measure of the resistance to acoustic motion. There are two types of impedance that measure significantly different properties. Characteristic impedance is a property of the fluid medium itself and is equal to the product of the fluid ambient density (mass per unit volume in the absence of sound) and the speed of sound. The second type is specific acoustic impedance. It is a property of the sound field at a given point in space and is equal to the ratio of the acoustic pressure amplitude to acoustic velocity amplitude. As in the discussion of the decibel, the particle motion in acoustic fields can be quite complicated so that care is required in dealing with specific acoustic impedance. For example, acoustic velocity at a given frequency can have a component that is in quadrature with the acoustic pressure as well as one in phase, so that the specific acoustic impedance can