531. Why does white noise cancel out the wide range of frequencies in the real world? What range of frequencies does this technology affect? Can you block out the low thud of a neighbor walking in the unit above yours? — EH, Chelmsford, MA
In the context of sound, a source of white noise emits random, non-repetitive sound waves that have equal acoustic powers at all frequencies. That means that the source emits the same amount of energy each second at each frequency, over the entire audible spectrum. What white noise does is to numb your hearing by creating a featureless, uniform background noise at every frequency you can hear. Since your sensitivity to sound volume is logarithmic, meaning that the acoustic power in a sound has to double before you notice that it's substantially louder, this uniform background makes it extremely difficult for you to hear small sounds. Regardless of a small sound's frequencies, the white noise is already exposing your ears to those frequencies and the small sound only makes a small change in the volumes of these frequencies. For an analogy, think about how much more you would notice a small blinking red light in the dark than in bright white sunlight. Similarly, white noise creates the acoustic equivalent of white illumination, making it hard for you to notice small noises that would be very easy to hear against complete silence. If the sounds your neighbor makes are small enough, this numbing effect should make them much less noticeable.
There are also much more sophisticated devices that really cancel noise out. However, these look like earphones and must be worn directly on your ears. These devices use microphones to measure the pressure fluctuations in the sounds and then cause the earphones to create exactly the opposite pressure fluctuations. With these noise cancellation devices properly adjusted, the air pressure fluctuations that are sound never reach your ears at all—they are simply cancelled away to nothing before they arrive.