White light is the presence of all of the visible light waves. White noise is the presence of all of the sound waves within the range of human hearing.
Because our sensory systems are optimized to detect change, noises at night are likely to jar us awake. White noise machines or smartphone apps (or fans) mask other noises. The frequencies from those other noises blend into the white noise as long as the loudness of the other noises is the same or lower than the white noise. If they blend in well enough, our brains won’t detect them, and we sleep right through the sound. (Mileage varies. Some people are more sensitive to other noises when presented inside of white noise.)
Side note: Pink noise is like white noise in that all of the frequencies are there, but with pink noise, the higher frequencies have decreased loudness. LiveScience has a nice explanation of the difference. Why is it called pink noise? In light, the higher frequencies are on the blue end of the spectrum. If those higher frequencies in white light are reduced, the light would appear more pink. Some people prefer pink noise over white because white noise sounds too high-pitched.
World War II (source: 99 Percent Invisible, Episode 208: Vox Ex Machina)
The 1939 World’s Fair in New York debuted the first voice synthesizer, created by sound engineer Homer Dudley of Bell Labs. After the 1941 attack on Pearl Harbor, the US called on Dudley to solve a serious communication problem. For allied military leaders to talk with each other, they had to use shortwave radio – that anyone could listen in on. They had been using a scrambling technique that would switch the frequencies of the voices, so that high frequency sounds would transmit as low frequency and vice versa. Decoding those transmissions was as easy as it appears – for anyone listening in.
Homer Dudley created a 2,000 square foot, 50-ton computer that compressed and digitized voices then masked them in white noise, on the fly. The trick? Two identical vinyl records of recorded white noise – for each conversation. At least 3,000 pairs of these records were made – each with a different white noise pattern. One set stayed in Washington, DC; the other set was sent to London. Each pair had a codename, such as wild dog.
Before a call, the communication officers would decide which record to use. At the Pentagon in Washington, DC, the communication officers would open a short wave radio connection to London. At the designated time, each side would start their records. The voice from, say Truman, would be sent from his microphone, through the machine that digitized his voice, then mixed his digitized voice with the white noise from the record, and finally sent it out over the shortwave radio frequency. To anyone listening in on that radio frequency, they would hear only white noise since Truman’s voice would blend into the white noise. Across the pond in London, the signal would be intercepted, run through the machine where the white noise playing on the vinyl record would be subtracted, and Churchill would hear Truman’s digitized voice. After the call, the records were destroyed. For the next call, a new pair was used.
This device “was involved in virtually every major military operation after 1942. It was even critical in the planning of the Manhattan Project and the dropping of the atomic bombs over Japan.”
Another side note: When this technology was declassified in the 1970s, researchers put it to good use. It’s digital compression that smooshes our voices enough to be sent through cell phone towers. Our MP3 audio files and streaming video files use this same compression technology.
This Memorial Day weekend, when you turn on your white noise generator, give a special nod to those fought in World War II.