Brain waves are attributed to electrical activities of the brain which are manifest as alternating potential differences at the scalp surface. When acquired through scalp electrodes, such potential differences result in time-continuous signals termed electroencephalogram (EEG). From a physical point of view, it is opportune to model the wave-like qualities of EEG as a finite sum of harmonic oscillations at discrete vibration rates triggered by a central pace-maker. Hence, brain-waves can analoguously be interpreted like sound waves in acoustics which consist of a fundamental oscillation superimposed by higher harmonics. Each of these partial tones is uniquely characterized by 2 quantities, its frequency ("pitch") and its amplitude ("loudness").
In terms of this model, brain-waves are composed of a series of "partial tones" ranging in frequency between 0.25Hz and 64Hz (7 octaves), whereby the "tonal composition" essentially depends on the state of consciousness, such as wakefulness or sleep stages.