noise processes. They are dominated by high-frequency noise." (density proportional to f 2) over a finite frequency range. It is also known as differentiated white noise, due to its being the result of the differentiation of a white noise signal. Due to the diminished sensitivity of the human ear to high-frequency hiss and the ease with which white noise can be electronically differentiated (high-pass filtered at first order), many early adaptations of dither to digital audio used violet noise as the dither signal. Acoustic thermal noise of water has a violet spectrum, causing it to dominate hydrophone measurements at high frequencies. "Predictions of the thermal noise spectrum, derived from classical statistical mechanics, suggest increasing noise with frequency with a positive slope of 6.02 dB octave−1." "Note that thermal noise increases at the rate of 20 dB decade−1" Grey noise Grey noise is random white noise subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting curve) over a given range of frequencies, giving the listener the perception that it is equally loud at all frequencies. This is in contrast to standard white noise which has equal strength over a linear scale of frequencies but is not perceived as being equally loud due to biases in the human equal-loudness contour. Informal definitions There are also many colors used without precise definitions (or as synonyms for formally defined colors), sometimes with multiple definitions. Red noise A synonym for Brownian noise, as above Similar to pink noise, but with different spectral content and different relationships (i.e. 1/f for pink noise, while 1/f2 for red noise, or an decrease of 6.02dB per octave). In areas where terminology is used loosely, "red noise" may refer to any system where power density decreases with increasing frequency. Green noise The mid-frequency component of white noise, used in halftone dithering Bounded Brownian noise Vocal spectrum noise used for testing audio circuits Joseph S. Wisniewski writes that "green noise" is marketed by producers of ambient sound effects recordings as "the background noise of the world". It simulates the spectra of natural settings, without human-made noises. Pink noise is similar, but has more energy in the area of 500 Hz. Black noise Silence Infrasound Noise with a 1/f spectrum, where . This formula is used to model the frequency of natural disasters. Noise that has a frequency spectrum of predominantly zero power level over all frequencies except for