Resonance, a principle of sound waves, plays a crucial role in distinguishing sounds and even in healing therapies. In music, resonance amplifies the sound, like the soundboard in a piano resonating with the vibrations of the strings. Similarly, a singer's body resonates with the vibrations of the vocal folds. Resonance is not limited to music; it can cause any vibrating body to start vibrating. This principle is used in sound therapies, where weak or unhealthy vibrations are modified through resonance to be more intense and harmonious. Research is verifying ways in which resonance and sound vibrations can be used to realign damaged tissue, a field known as acoustic bioengineering.

“Resonance” is used in spoken English to suggest agreement. “Her ideas really resonate with how I feel.” 

But it is also a principle of sound waves. Resonance occurs when a vibrating body brings another one into vibration. 

This can be demonstrated by playing a low note on the piano. This vibration includes the full length of the string, set into motion when the hammer strikes it. But the string will also vibrate in halves and thirds and fourths, which will produce pitches sounding an octave above, a fifth above that, and another octave above, etc.. If I release the dampening on those higher strings, they will vibrate sympathetically. This is resonance. The vibrating low string causes the other strings to resonate with it because they vibrate at the same frequencies.

All musical sound requires something that vibrates, like a string or column of air; something to initiate this vibration, like a bow or a flow of wind; and finally, something to resonate with the sound and increase its volume. On the piano, the string is put into motion by the strike of a hammer, which initiates the vibration; the soundboard resonates with the vibrations of the strings, creating a full sound that can be heard in a large concert hall. 

For a singer, the vocal folds [more accurate term for vocal cords] are put into motion by exhaled air, the initiator; they are amplified by the head and chest cavities, which resonate with the vibrations of the vocal folds. A singer’s body is her instrument, and for this reason singers have to be extremely careful about their health, especially their respiratory systems. A simple cold can ruin opening night for an opera star.

Resonance is not only a principle of musical sound.  Any vibration can cause another body that shares its frequency to begin vibrating.  When a truck passes by your house and a loose air vent starts to rattle, this is an example of resonance: the vibrations of the truck cause the air vent to resonate with it.  If the air vent is well secured, the resonance will be arrested and the vent won’t rattle.

A dramatic example of resonance occurred on a windy day in 1940 when gusts of wind caused the Tacoma-Narrows bridge to resonate. The bridge began to move in synchronous wave frequencies to those of the wind, in a graphic display of resonance.

The concept of resonance is used for healing in sound therapies. Our bodies, like all matter, are energetic vibrations; our tissues are made of cells that vibrate as part of their metabolic function. In 2002, Professor James Gimzewski of UCLA, with his colleague, Dr. Andrew Pelling, found that cells made sounds which lie in the range of human hearing. These vibrations can be assessed to distinguish healthy cells from cancerous cells. The irregular vibrations of cancerous brain cells could be readily distinguished from the regular vibrations of the healthy cells.

The idea of traditional sound therapy is that weak or unhealthy vibration can be modified through resonance to be more intense and harmonious. In a sound meditation or sound bath, the body is exposed to a wide range of frequencies produced by gongs, chimes and singing bowls. While there are different theories about which vibrations are best for which parts of the body, scientific research is beginning to verify ways in which resonance and sound vibrations can be used to realign damaged tissue - known as acoustic bioengineering.

Utkan Demirci and Sean Wu of Stanford University have used acoustic waves to manipulate heart cells. By changing the frequency and amplitude of a wave form, they are able to move heart cells, guide them to new positions and secure them in place.