Pink noise is acoustical energy distributed uniformly by octave throughout the audio spectrum (the range of human hearing, approximately 20 Hz to 20 kHz). Most people perceive pink noise as having uniform spectral power density -- the same apparent loudness at all frequencies. In pink noise, the total sound power in each octave is the same as the total sound power in the octave immediately above or below it. An octave is a band whose highest frequency is exactly twice its lowest frequency. So-called white noise contains sound power distributed uniformly in absolute terms. True white noise sounds like there is more treble than bass because the human ear/mind interprets sound in terms of octaves, not in terms of absolute frequency. Any given octave represents a frequency band twice as large, in arithmetic terms, as the one below it. For example, the octave from 100-200 Hz is 100 Hz wide, the next octave (200-400 Hz) is 200 Hz wide, the octave above that (400-800 Hz) is 400 Hz wide, and so on. Pink noise can be obtained from white noise by means of a low-pass filter designed so the output spectral power density (that is, the sound power contained within a narrow frequency band of a certain fixed width, such as 1 Hz) drops by 50 percent with each octave as the absolute frequency rises. Pink noise can also be directly generated by a computer-controlled acoustic synthesizer. The terms "pink" and "white" come from optics. The visual color pink has greater spectral power density at the longer optical wavelengths (lower frequencies, near the red end of the visible spectrum) than at the shorter optical wavelengths (higher frequencies, near the violet end of the visible spectrum). Some engineers talk about "brown noise," which is similar to pink noise except that the spectral power density decreases even more rapidly with increasing frequency. White, pink, and brown noise can be generated by an acoustic synthesizer to produce sound effects mimicking surf on a beach, a high wind through trees, a rocket taking off, and other phenomena. White and pink noise are used by audio engineers to test and adjust sound recording and reproduction equipment