Quantities that measure properties of the sound field and those that measure properties of the fluid medium itself must be clearly distinguished. For example, specific acoustic impedance is a property of a received sound field, whereas characteristic acoustic impedance is a property of the medium (see the “Physics of Sound” section of the Glossary). Another example is sound speed; group speed and phase speed are properties of a sound field, whereas medium sound speed obviously is a property of the medium. Acoustic density is the perturbation of the fluid density from its ambient value by the presence of sound and so is a property of the sound field. In contrast, the fluid ambient density is the density of the medium in the absence of sound. The same relationship holds for acoustic pressure and hydrostatic pressure.

Likewise, clear distinctions must be made between the properties of an acoustic source and those of a received field. For example, the sound level at a receiver is reduced from the source level by the transmission loss between source and receiver. (This transmission loss is quite large over short distances at close range from point-like sources as a result of spherical spreading.) The character of a received signal is due not only to the source of the signal but also the medium through which the signal has traveled. The received level is directly measured, whereas source level must be derived for many types of sources. For controlled, man-made sources that intentionally transmit sound such as sonars and air-gun arrays, the source level in most cases is well known. However, to derive the source level for uncontrolled and naturally occurring sources using underwater acoustic measurements of the received field, the location of the source must be known or determined, and the propagation conditions from source to receiver location must be accurately modeled. This effort has been accomplished successfully in situations for naturally occurring discrete sources that can be modeled as simple points in space such as individual vocalizing animals. However, estimating the source levels of spatially diffuse naturally occurring sources—chorusing fish schools, snapping shrimp colonies, ocean surface breaking waves, oscillating bubble clouds—is a topic of research. In other cases, the propagation conditions from source to receiver are too complicated to model with reliable accuracy at present and are areas of modern-day research. Examples of such naturally occurring sources in this category are earthquakes, surf, and lightning.

Ambient Noise—the noise associated with the background din emanating from a myriad of unidentified sources. Its distinguishing features are that it