Harmonics is singing several notes at once (up to 3 notes, 2 being more common). This very special music comes from the shamanic traditions of the world. Many shamanic cultures throughout the world have used this droning voice that creates clearly audible overtones above the main tone. Some Native American sources call it Spirit Voices. The Tibetan Buddhists have also perfected this type of singing, sounding notes an octave below the bass tone. In North America, the Inuit culture has a similar technique called ‘throat chanting’. Up close, face to face, a couple of Inuit Throat Singers will make throat like sounds often imitating animals. These sounds bouncing off one another create complex overtones.
These special tones have a profound effect on our consciousness. They bring us to other levels of awareness initiating trance like states of perception. As the harmonics resound into different octaves of manifestation, they touch off subtle reactions in our own mind stream. These sounds will bring us into contact with other dimensions.
There are many different dimensions sharing this time-space continuum with us. Crystals also have the potential and ability of vibrating easily into other realms of existence. Sound is a privileged door into levels of perception and reality that can bring many benefits. Harmonics give very special healing and can work on all levels of being, physical, emotional, mental and spiritual. They give deep relaxation, reorganization of mis-aligned energies, realignment of etheric and astral energies, rejuvenation, understanding problems and finding their solution, clearing and purifying the energy channels and opening new pathways in the brain. Modern research on harmonics has found that listening to harmonic sounds will charge the cortex of the brain, reduce respiration and heart rate and lower brain wave activity. Mark Ryder, PhD of Southern Methodist University, found that people who listened to the music of the Harmonic Choir as opposed to other types of music, showed decreased heart rate, slower respiration and lower galvanic skin response.1
Harmonics are actually present in all sounds. The clarity and number of harmonics present in any given sound will determine its texture and sound quality. As you listen to harmonics, a whole new way of hearing occurs. You will hear things you had not noticed before and your perceptions will change.
Please be careful if you want to listen to my recording of harmonic sounds, Mysteries. Do not listen to it when driving a motor vehicle. I tried it and nearly took the ditch as I closed my eyes, completely taken up by the music. 
The central piece can have dramatic effects on the consciousness of people. It is good to be aware of that and to be receptive to the reactions of the people around us. Too much transformation too quickly can be difficult for certain people.