In some cases the best strategy for dealing with unwanted noise is not to block it or cancel it, but to obscure it. In well-designed restaurants, it is possible to sit a short distance away from another table and not be able to make out what the neighboring diners are saying. That is because the whole room is filled with the soft babble of many people talking, drowning out particular conversations. Engineers have learned that this is an excellent way to obscure the sound of a human voice.
This technique, called “sound masking,” is used widely in offices, where most employees now labor in cubicles that are not fully enclosed by walls. Some offices are even designed without interior dividing walls. These kinds of environments can be filled with noisy distractions that disrupt employees' concentration and make them less productive.
To address this problem, some noise-control companies sell sound-masking systems that make work areas seem quieter. The simplest approach, which was first developed in the 1960's, entails generating a type of unobtrusive sound called “white noise,” which has roughly equal amounts of energy in each frequency across the entire range of human hearing (20 to 20,000 hertz). White noise is essentially a background hum that people are barely aware of, similar to radio static. This type of noise is called “white” because it contains all audio frequencies, just as white light contains all optical frequencies.
Although white noise is effective at masking sounds, acoustic experts are finding that “pink noise” can be an even less intrusive sound-masking tool. Pink noise contains roughly the same amount of sound energy in every octave, the range between a sound of a particular frequency and a sound with twice that frequency. Compared with white noise, pink noise has more energy at lower frequencies, making it sound more like a rumble than the “hiss” of white noise.
(Source: How stuff works?)