What Is a Pink Noise?
Pink noises are noises whose power density is inversely proportional to their frequency.
It is called pink noise because light with the same frequency component appears pink. It is characterized by a “zing” sound, like the sound of strong rain or a waterfall. Pink noises have a constant energy per octave band and are used as a signal source for various acoustic measurements.
A similar type of pink noise is white noise, which has a constant power regardless of its frequency. It is considered to have a calming effect because it sounds pleasant to the ear. Pink noises are also called “1/f Fluctuations” because their power is inversely proportional to their frequency, and in nature, they are sometimes compared to the tides on a sandy beach.
Uses of Pink Noises
Pink noises are used as a signal source over a wide frequency range for testing audio products such as amplifiers, speakers, and headphones, as a signal source for testing electronic equipment, and as a sound source for human hearing tests and healing.
Principle of Pink Noises
Pink noises are also called 1/f Noises because their power is inversely proportional to their frequency. Here, f is the frequency. The general expression for noise broadly speaking is S(f)∝1/f^α.
Note that S(f) is the spectral density, f is the frequency, and α is 0=<α<=2. For pink noises, α is around 1. In this case, the spectral density is attenuated by a factor of 10 in frequency, α x 10dB.
Noise with α around 1 is found in nature, including tidal noise, and has been the subject of much research. Also, α = 0 is white noise, and α = 2 is called red noise.
Other Information on Pink Noises
1. Effects of Pink Noises
Pink noises are often used for testing audio products and electronic equipment. Another important use is as a sound source. Because of its high low-frequency content, pink noises, when heard by the ear, have a soothing effect by blocking out external sounds. It is also a pleasant sound because it is close to natural noises that humans can easily hear.
Therefore, it is used in some cases as an environmental sound to improve concentration and productivity. Another benefit of pink noises is its positive effect on sleep. It is said that the brain converts short-term memories into long-term memories during sleep.
Among non-REM sleep, slow-wave sleep, a particularly deep state of sleep, is necessary for long-term memory. Research has shown that acoustic stimulation with pink noises during slow-wave sleep improves memory.
2. How to Create Pink Noises
Pink noises can be easily created by using a noise generator, by using an electronic circuit such as a diode, or by using a programming language.
Noise generators include those used in electromagnetic interference EMI tests and acoustic characteristic tests, as well as those built into function generators. The noise generator can generate white noise, pink noises, etc., by selecting the desired type of noise. Noise generators attached to synthesizers can also be used.
To use a noise generator circuit with a zener diode, transistor, or operational amplifier, first create white noise. Then, a power attenuation inversely proportional to the frequency is applied to make pink noises. Power attenuation requires -3 dB/oct, so a circuit called a lag-lead filter is used.
Another method is to use a programming language such as C, Java, or Python. Noise is generated in software using random numbers.
3. Color Representation of Noise
It is common to represent noise in terms of color. These are called “colored noise,” and include white, pink, red, brown, blue, violet, purple, and gray. Noise is determined by its frequency components, and noise is represented by the color of light having the same frequency components.
White noise contains all frequencies uniformly and is perceived as a “sough, sough” like the sound of a ⾵ sound or the sound of a low river. Red noise is a noise with a power density that decreases by 6 dB per octave of frequency. Pink noises are between white and red, with a power density of 3dB lower per octave of frequency. It sounds like a “zing,” like the sound of rain or a waterfall.
Blue noise is also a noise whose power density increases by 3 dB per octave with increasing frequency. It has a limited frequency range. Purple noise is noise whose power density increases by 6 dB per octave. It has a finite frequency range and is equal to the derivative of white noise.
Gray noise is noise with a power density close to the auditory equal loudness curve. The equal loudness curve is an equal-hearing curve with a large attenuation of power density around 2,000 Hz and a smaller attenuation at lower and higher frequencies.