<unk>  or<unk>  tones are auditory processing artifacts, that is, apparent sounds, the perception of which arises in the brain independent of physical stimuli. This effect was discovered in 1839 by Heinrich Wilhelm Dove.
The brain produces a phenomenon resulting in low-frequency pulsations in the loudness 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<unk> ng tone will be perceived, as if the two tones mixed naturally, out of the brain. The frequency of the tones must be below about 1,000 to 1,500 hertz for the<unk> ng to be heard. The difference between the two frequencies must be small (below about 30 Hz) for the effect to occur; otherwise the two tones will be heard separately and no<unk> will be perceived.
Interest in<unk>  can be classified into two categories.  First, they are of interest to neurophysiologists investigating the sense of hearing.  Second, that<unk>  may influence the brain in more subtle ways through the entrainment of brainwaves   and can be used to produce relaxation and other health benefits.[citation needed]
Binaural beats
Ventricular escape beat
Pulsus alternans
Sine wave pattern
Precordial thump
Inverted P wave
Fusion beats
Pulsus bisferiens