Our universe is a limitless 'ocean' of continuous flow of infinitely many enerty-waves, of which, only a few - namely, the audible and the ultra and infra sound waves; ultra violet and infrared light rays; X-rays, Gamma rays, Lasers, Microwaves etc - have been discovered and utilized via the modern scientific research. The wave lengths of these waves measure in a wide range - from angstroms, microns and millimeters to centimeters and meters. The frequency (vibrations) and hence the energy and properties - of these waves also differ drastically.
The properties of 'physical sound' have significant relation with the five basic elements (panca tatvas) of Nature's manifestation. for instance, the velocity (in ft/sec) of the sound waves propagated along the flow of air is about 1088; along the flow of water is about 4900; and along the vibrations in the earth is about 16400. The essential practices and rituals of Pranayama, Arghdana, Dipa-Dhupa and Havan - often prescribed along with the mantra-sadhanas - are indeed associated with the conditioning of mind for soothing effects of the sonic powers of the basic elements of air (vayu), water (jala) earth (prathvi) and sun, fire and heat (agni).
Our ears and mouth (and the vocal-cord) play very important role in hearing and pronunciation of a wide variety of sounds and are therefore of significant importance in the experiments of Nada Yoga and Sabda Yoga. No transducer or any advanced technological device could be so sensitive and perfect as these 'natural devices' are. Our ears can simultaneously catch different frequencies of sound in the audible range. so efficient is their connection with the brain that, for a blind person, the ears also work like the eyes - a blind person can recognize different objects by sensing the latter's sound (vibrations).
The relation of mind with the functioning of the ears is equally remarkable. At times, in absent-minded states or because of the disinterest or indifference of our mind, we do not really hear - what the person sitting next to us is telling…… ! Whereas murmuring at some distant place can also be heard by us if our mind keenly concentrates there….. Such familiar experiences clearly explain the need of mental concentration and meditation during mantra japa and other exercises of the Sabda and Nada-Yogas.
The importance of manasika japa also is obvious in the above context. Although, a mantra is enunciated only mentally in this practice of Sabda Yoga, its (the mantra's vibrations are well received in the connecting sensors of the tongue and the ears and are hence registered in the brain as per its sonic pattern and the 'mental signals' of its sadhaka.
In the sadhanas of Nada Yoga, the ears are trained by deep meditation and willpower to grasp the otherwise non-audible sounds existing in the cosmic expansion. By such practices, the sadhaka, while performing the mansika japa of a mantra, can hear (link his mind with) the subliminal sound of this mantra and can thereby attract the corresponding energy-currents of the omnipresent Sabda. This in short is the secret of the supernatural powers attainable by the mantra vidya.