16. White noise, pink noise...orange noise?


     This page is mostly for fun, so don't take it too seriously!

     You've certainly heard of 'white noise', which is calqued into Chinese as զ⾸ bai2se4 zao4yin1. It is not hard to understand why this term was chosen. White noise contains an equal distribution of energy spread over all frequencies, just as white light is composed of all of the colors of the spectrum of visible light; here is a more detailed explanation. Click here for an audio sample.

      You hear white noise when no signal is being broadcast over a television channel or radio station. It's not just an annoying, unpleasant sound – some people use it to sleep better, e.g. by running a fan during the night. This works because steady sound at all frequencies can mask all kinds of other noises that might disturb a light sleeper. There are even commercial products that emit white noise to block out the snoring noises of one's partner, to help an infant sleep more soundly, or to drown out a conversation you want to keep private! White noise can also be used to determine the frequency response of mechanical equipment.

     The analogy with colors of light does not stop with white noise. Another very commonly used kind of noise is called 'pink noise'. Instead of having equal energy at all frequencies, with pink noise the energy within each successive octave is the same, but it is reduced by 3 dB per octave, i.e. each octave has half the power of the preceding one. This is analogous to light that tends toward the red or lower end of the visible light spectrum. Pink noise gives more weight to the lower frequencies to compensate for the increased number of frequencies of each higher octave. Also, human hearing is most sensitive to sounds in the 1-4kHz range, so pink noise makes the higher frequencies less prominent and less hard on the ear. Some claim that pink noise is the most common kind of noise found in nature, e.g. the churning of ocean waves. Pink noise is also often used for testing the frequency response of audio equipment. You can produce white noise with a program to generate random numbers, and you can turn white noise into pink noise by filtering it – though this is more difficult to do than it sounds!

     A less commonly referred to 'color' of noise is 'brown noise'. This is supposed to simulate Brownian motion, a kind of random motion that shifts in steady increments. 'Brown noise' decreases in power by 6 dB per octave. Listen to a sample here. The reverse of 'brown noise' is 'violet noise' (or 'purple noise'), in which the power of each octave increases by 6 dB.

     There are still other 'colors' of noise, but they are mostly curiosities, good for a little synaesthetic fun, rather than being very practical concepts. Here are some collections of a whole rainbow of noise types:

http://en.wikipedia.org/wiki/Colors_of_noise

http://www.partnersinrhyme.com/blog/audio-tutorials/noise-types-and-colors/

http://www.wired.co.uk/news/archive/2011-04/7/colours-of-noise

     Here is a more recent (Feb. 16, 2016) article from The Atlantic, entitled:
"The Many Colors of Sound: White noise isn't the only sonic hue –
pink, blue, gray, and brown all affect listeners in different ways:

http://theatlantic.com/science/archive/2016/02/white-noise-sound-colors/462972/


     More on the connection between the visual and auditory senses: The "Forry, Wrong Number" unit of "A Moment of Science" mentioned that, in addition to frequency cut-offs, a further reason why it is difficult to hear some words clearly over the phone is the lack of visual cues we get from watching the face of the person talking. How much of a difference do these really make in our understanding of speech? Go on to the next page, and you may be surprised to see just how much difference they can make!

Next: The McGurk Effect (with videos)

 

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