God is a vibration, a pulse, a frequency, a sound. This sound envelops the brain and dwells deeply in the heart. The cosmic bodies making up the universe emit this sound. It is the divine sound of the OM.Sukha Yoga
There is a subtle body. In this dimension, one can see objects, shapes, and colours. Different sound frequencies fill this energetic realm. As the energy increases and kept sealed inside, the mind becomes fixed to the frequency, then one could hear through the mind only. In Samadhi (absorption), the mind breathes on its own as it watches the soul travelling.
By practising stillness and silencing the breath, the senses dissolve and the union of the kundalini and bindu takes place. The union actually happens first in the hips. However, it is in the heart where it becomes spiritual. It is also the most joyous and peaceful. Although later on, they shall eventually rise in the brain.
At first, the unified energy manifests as a sensation, an electrical current which rises through the energy and astral anatomy. It transcends and becomes a sound wave, known as nada (sound). The emergence of the nada varies depending on the level at which the chakras are made open. It could manifest as a white light the mind sees inside. By listening to the nada and following the white light, one can easily attain sense-withdrawal and the state of dissolution (laya and pratyahara), resulting in various energetic, psychic, cosmic and spiritual experiences.
The nada emerges through the union of the absolute prana (bindu/soma/consciousness) and the absolute apana vayu (kundalini shakti). Consciousness is passive, neutral, inactive and formless. From the brain, the soma descends. On the other hand, the kundalini is dynamic, energetic and restless. Normally, the apana vayu exits the body predominantly during sexual arousal. This is easily reversed by the application of the bandhas. From the hips, the kundalini rises, piercing through the chakras of the astral system, producing different vibrations and frequencies.
At times, white light could be seen too as the mind gets absorbed by the nada. This could be attained by fixing the internal eyes between the eyebrows while the eyelids are closed. It would be helpful to cover the eyes to steady them behind the closed eyelids. As the mind hears the anahata nada, the mind sees the light too. This could powerfully lead one to samadhi.
The subtle sound vibrations are so low to be heard by the external ear and only by adjusting the level of concentration one can experience it. It helps to breathe lightly while the mind scans the frontal brain for the unmade sound. The centre of the eyebrows is always a good starting point. Then, listen closely across the temples, the left hemisphere generally hears a slightly higher sound. The right is usually humming and fleeting. Another technique which I do is to lightly crimp the optical nerves for successive times then stop the neural activity in the brain. This certainly emerges the nada.
The bhramari pranayama helps amplify the nada during meditation. When practising it, pay utmost attention to the vibration of the sound in the brain, increase or lessen the vibration of the vocal cords so that you are hearing more from the inside of the brain rather than from the sound of the throat.
Adjust the level of pressure as well the placement of your fingers on the cartilage of the ears until both sides yield the same vibration inside. You may find that one side needs more pressure than the other. Try to produce a humming sound in the same melodious way as the honey bee. It should not be a strong and forced sound. After each repetition as you recover the breath, concentrate on the source of the sound in the brain. This residual sound is always present, constantly vibrating around.