- The amount (or the process) by which the threshold of audibility for one sound is raised by the presence of another (masking) sound.
- Increases in noise levels can decrease the ability of an animal to detect biologically important sound when the increased noise level rises above the level of sound for which the animal is listening. This effect is commonly known as masking. Masking of significant sounds (e.g., calls of other animals, predators, sounds of hazards, such as approaching boats, etc.) can occur when ambient noise levels increase. Marine mammals have evolved in the highly variable noise environment of the ocean, and presumably are well adapted for tolerating the natural variations in ocean noise that could at times cause masking. However, the determination of an animal's ability to tolerate changes in noise levels requires a better understanding of: 1) the functional importance of faint sound signals from the same species, predators, prey, and other natural sources; 2) signal detection abilities of marine mammals in the presence of background noise, including directional hearing abilities at frequencies where masking is an issue; and 3) abilities of marine mammals to adjust the intensities and perhaps frequencies and timing of emitted sounds to minimize masking effects. Source: Outline for Acoustics Environmental Assessment