I would like to introduce some definitions and terminology and also establish the difference between sound and music. Music is an arrangement of sounds with different patterns and constructs, including melody, harmony and rhythm. When you listen to a band or a song on the radio, you are listening to music.
For sound healing, sounds are used that are not necessarily musical, although they can be pleasant and calming. When I use the term sound, I can be referring to a single tone created by my voice or the sound generated by striking a bowl. For sound healing, frequency can be used to describe a vibration that is not necessarily heard but felt through bodily application such as placing the tuning fork on the body.
Basically, all music is sound, but all sounds are not necessarily music. For both, the following definitions apply.
Sound is a wave and these waves are measured by how many occur during a specific time frame. Sound is measured in hertz (Hz) which describes how many cycles occur in one second and this is called its frequency. So a tone that has a frequency of 514 Hz is producing 514 cycles per second. (See diagram below)

The range of human hearing is approximately 20 Hz to 20,000 Hz. To give you a relative understanding, the sound of the average male speaking voice is around 110 Hz and the average female speaking voice is around 220 Hz.
The lower the frequency, the lower the sound, and that introduces another term, pitch. Pitch is what most musicians use to describe the fundamental frequency of a tone. An interesting phenomenon of pitch is that it is purely a psychological construct, a mental representation of the frequency of a sound. Sound waves themselves do not have pitch, it takes our human brain to map them to the internal quality we call pitch.
We use notes to represent the pitch of a tone. When you see the note A on a sheet of music, you are seeing a representation of a tone of 440 Hz. Tone and note are used interchangeably, but tone is primarily used in sound healing to describe frequency.
An important component of sound is its loudness or amplitude. Loudness measures the intensity or sonic energy of a sound and is measured in units called decibels (db). The decibel level of a whisper is about 20 db and the level of a loud rock band is around 105 db. Continued exposure to sounds above 100 db can lead to hearing loss, which is something to consider if you listen to music at high volumes regularly.
Earlier, I discussed music as having rhythm. Rhythm is a crucial part of what turns sound into music. Rhythm refers to the length of a note and the relationship between the length of one note and another and usually establishes a rhythm pattern. Rhythm and tempo are related concepts and often confused, as tempo is the pace of a musical piece or the rate you tap your foot to a piece of music. Tempo is also a major factor in conveying emotion. Typically, a song with a fast tempo is thought of as happy or exciting and songs with slower tempos as sad or pensive.
Every tone has what is called a fundamental frequency. But there is also what we call overtones. When I play a note on the piano, for example A, you are actually hearing many pitches at once, not just a single pitch. The one with the slowest vibrational rate or lowest pitch, in this case A, is the fundamental frequency and is what we hear most easily. The other pitches are called the overtones.
Overtones give us the timbre or tone quality of a sound and allow us to identify what we are hearing. Even if they have the same fundamental frequency, the sound of a lion roar and a human scream are different because of their overtones.
In sound healing, when we talk about fundamental frequencies, we’re talking about resonance. A resonant frequency is the natural frequency of the vibration of an object. Every object has a fundamental resonance, each cell in our body, each organ, has its own resonant frequency. Resonance is the probably the most important principle of sound healing.
Some important properties of resonance are: It is easy to get an object to vibrate at its resonant frequencies, hard to get it to vibrate at other frequencies. A vibrating object will pick out its resonant frequencies from a complex excitation and vibrate at those frequencies, essentially "filtering out" other frequencies present in the excitation.
Through entrainment, we can affect the resonance of an object. Entrainment, or forced resonance, is when a vibrating object begins to vibrate at a stronger vibrational rate of an object near it, so that eventually, all are at the same vibrational rate. This term was first used by Christian Huygen in 1666 when he noticed that two pendulum clocks would fall into the same rhythm when placed side by side and that this continued to happen in subsequent experiments. Entrainment is the basis of sound healing.
Sympathetic resonance, or free resonance, is when an object with the same vibrational frequency begins to resonate with a like vibration near it. This has been shown when two tuning forks of the same frequency are placed next to one another and one is struck, the other will begin vibrating even though it was not struck.
Another example of resonance at work and a different application for sound healing can be seen when an opera singer is able to break a glass with their voice. If a singer can match the resonant frequency of the glass with enough amplitude, it will shatter the glass. 
In another blog, I will talk about brainwaves and how sound affects our brainwave states and the results of a small study Atlanta Sound Healing conducted on the bowls and brainwaves.