Mantra’s Four stages and Brain waves Electrical pulses

The Vedas recognize four stages of the manifestation of sound.

(i) Vaikhari: The grossest form of sound, Vaikhari is normal audible sound, including speech.

(ii) Madhyama: More subtle is Madhyama, a whispered sound with no audible effect.

(iii) Pashyanti: As you move within, you experience Pashyanti. These are mental sounds such as songs in the head or a dream. It is also the visualization of sounds such as silently listening to your thoughts or the silent repetition of a mantra.

(iv) Para: Finally you reach Para or transcendental sound.

The basic aim of any form of meditation, including the mantra-based meditation, is to induce certain sympathetic frequencies in the oscillation of neurotransmitters, coherently and synergistically, in order to achieve the desirable effects: like achieving calmness, controlled blood pressure, clarity of thoughts, immunity from outside disturbance, achieving restful alertness, happiness, peace, bliss, and enlightenment.
So, the Brain waves are produced by the synchronized Electrical pulses from a large number of neurotransmitters Communicating with each other simultaneously. Broadly speaking, there are the following five types of brain waves to be mentioned in the present context:
(i) Gamma waves: for inspiration, higher learning, and focus. Their frequencies are > 40 Hz.

(ii) Beta waves: for alertness, concentration, and cognition. Their frequencies are in the range of 13 – 40 Hz.

(iii) Alpha waves: for visualization, relaxation, and creativity. Their frequencies are in the range of 7 – 13 Hz.

(iv) Theta waves: for meditation, intuition, and memory. Their frequencies are in the range of 4–7 Hz

(v)This is pre-manifest sound in its subtlest state as light or pure silence. This is the root of the potential of all sounds, sometimes referred to as the ―unstuck sound.
The important thing is that as we go from various levels of sound from Vaikhari to Para, not only the intensity of sound changes, simultaneously its frequency can also change thereby synchronizing with the sympathetic nervous system of the meditator.