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Binaural beats, or binaural tones, are auditory processing artifacts, or apparent sounds, caused by specific physical stimuli. This effect was discovered in 1839 by Heinrich Wilhelm Dove and earned greater public awareness in the late 20th century based on claims coming from the alternative medicine community that binaural beats could help induce relaxation, meditation, creativity and other desirable mental states. The effect on the brainwaves depends on the difference in frequencies of each tone: for example, if 300 Hz was played in one ear and 310 in the other, then the binaural beat would have a frequency of 10 Hz. The brain produces a phenomenon resulting in low-frequency pulsations in the amplitude and sound localization of a perceived sound when two tones at slightly different frequencies are presented separately, one to each of a subject's ears, using stereo headphones. A beating tone will be perceived, as if the two tones mixed naturally, out of the brain. The frequencies of the tones must be below 1,000 hertz for the beating to be noticeable.〔 The difference between the two frequencies must be small (less than or equal to 30 Hz) for the effect to occur; otherwise, the two tones will be heard separately, and no beat will be perceived. Binaural beats are of interest to neurophysiologists investigating the sense of hearing.〔 〕 Binaural beats reportedly influence the brain in more subtle ways through the entrainment of brainwaves and provide other health benefits such as control over pain.〔(Hemispheric-synchronisation during anaesthesia: a double-blind randomised trial using audiotapes for intra-operative nociception control ), Jan 2000, Kliempt, Ruta, Ogston, Landeck & Martay〕 == Acoustical background == For sound localization, the human auditory system analyses interaural time differences between both ears inside small frequency ranges, called critical bands. For frequencies below 1000 to 1500 Hz interaural time differences are evaluated from interaural phase differences between both ear signals.〔Blauert, J.: Spatial hearing - the psychophysics of human sound localization; MIT Press; Cambridge, Massachusetts (1983), ch. 2.4〕 The perceived sound is also evaluated from the analysis of both ear signals. If different pure tones (sinusoidal signals with different frequencies) are presented to each ear, there will be time-dependent phase and time differences between both ears (see figure). The perceived sound depends on the frequency difference between both ear signals: * If the frequency difference between the ear signals is lower than a few hertz, the auditory system can follow the changes in the interaural time differences. As a result, an auditory event is perceived, which is moving through the head. The perceived direction corresponds to the instantaneous interaural time difference. * For slightly bigger frequency differences between the ear signals (more than 10 Hz), the auditory system can no longer follow the changes in the interaural parameters. A diffuse auditory event appears. The sound corresponds to an overlay of both ear signals, which means amplitude and loudness are changing rapidly (see figure in the chapter above). * For frequency differences between the ear signals of above 30 Hz, the cocktail party effect begins to work, and the auditory system is able to analyze the presented ear signals in terms of two different sound sources at two different locations, and two distinct signals are perceived. Binaural beats can also be experienced without headphones; they appear when playing two different pure tones through loudspeakers. The sound perceived is quite similar: with auditory events that move through the room, at low-frequency differences, and diffuse sound at slightly bigger frequency differences. At bigger frequency differences, apparent localized sound sources appear.〔Slatky, Harald (1992): ''(Algorithms for direction specific Processing of Sound Signals - the Realization of a binaural Cocktail-Party-Processor-System )'', Dissertation, Ruhr-University Bochum, ch. 3〕 However, it is more effective to use headphones than loudspeakers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「binaural beats」の詳細全文を読む スポンサード リンク
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