Background
If you want to find the secrets of the universe, think in terms of energy, frequency and vibration
 -Nikola Tesla
Sound is according to ANSI;
(a) Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation.
(b) Auditory sensation evoked by the oscillation described in (a).
In physics, sound is a vibration that propagates as a typically audible mechanical wave of pressure and displacement, through a medium such as air or water. In physiology and psychology, sound is the reception of such waves and their perception by the brain. – wikipedia
Basically, sound is a disturbance of molecules around us such as walls, air, water… When any kind of event happens that takes these mediums away from their restful state (the molecules are compressed together) and forces them to vibrate (these compressed molecules shoot off in every direction from the point of disturbance), our ears get a disturbance which is perceived as sound. Then, these moving molecules fades out according to their physical properties and turn back their restful state. (Figure 1.)
Figure 1.
Everything around us in the world have molecules and some of them are tend to move faster some of them are not. For instance; if you strike a wall, the molecules in the wall moves much faster than in air. Molecules in the concrete wall are much close together than air. But, the sound you hear fades out much faster because the material is more rigid that air. (Figure 2.)
Figure 2 – Shows molecule structure of different mediums and sound wave transmission speed
Sound waves travels at different speed on different mediums because of  molecules are spaced differently on each mediums. The distance between molecules in solid objects are the closest ones according to liquids and air.
Speed of sound through air = 340 meters per second
Also, speed of sound is altered by altitude and temperature.
Speed of sound through steel = 5100 meters per second
Sound energy is absorbed faster than in a gas or liquid. It will therefore be either partially or completely absorbed. The amount of absorption will depend on the nature and size of the object. And the frequency of the sound wave.
The sound wave once converted into analog electrical, digital electrical or digital optical signal sound travels much faster.
Speed of electrical sound wave = 200.000 apprx. kilometers per second
Speed of Sound = Wavelength x Frequency
Quantifying Sound
When scientists work with sound they use terms that described the physical attributes of sound. Frequency, pitch, amplitude, wavelength and speed of sound are just a few. The following paragraphs outline these basics of audio terms.
Sound Wave: Sound waves are just like water waves. Think of a wave in water, when you drop a stone from above on it. The waves appears at center and propagate through outside… Propagation of sound happens in all directions not just on a level surface like water waves do.
A sound wave is the completion of a compression and a rarefaction cycle (Figure 1.). The compression part of the wave is a grouping of medium’s(solid, liquid, gas) molecules. Rarefaction part is the separation of these molecules. This cycle continues until the sound wave loses all energy (Figure 2.).
Wavelength: Wavelength and the speed of sound are basics of audio terms that are dependent on each other. The physical distance between two adjacent points on two cycles of a the sound wave is called the wave length and is denoted by the Greek letter Lambda (λ).Â
Sound travels at a rate of 1130 feet per second at 70 degrees Fahrenheit. If you want to know the length of a 100 hertz sound wave, divide 100 into 1130 and you will get 11.3 feet. That is the distance it takes for a 100 hertz waveform to complete one compression and rarefaction cycle.
You can also use the same math to find what frequency is 10 feet long by dividing 10 feet into the speed of sound. 1130 divided by 10 equals 113 Hertz. This basic math is very important to the physical design of recording studios and the acoustic materials that are used to control those frequencies in a recording space.
Frequency: Frequency is the cycle count of a sound wave cycles in 1 unit time. E.g: 440 hz (which is A note) means molecules are cycling 440 times in 1 second.
1000 cycles per second = 1000 hertz = 1 kilohertz
1000 cps = 1000 Hz = 1 Kh
Humans can hear frequencies between 20 Hz to 20.000 Hz (20 Kh) which is called audible spectrum.
Pitch: Pitch is the relative highness or lowness of a frequency. Relative to any pitch is a series of other pitches that would define musical scales, chords, etc…
Resonance: Resonance is vibrating of objects in a particular frequency range.
Timbre: This identity is based on information gained from frequency transients, noisiness, unsteadiness, perceived pitch and the spread and intensity of overtones in the sound over an extended time frame. E.g the difference between piano and obua.
The Perception of Sound: There are psychological aspects to sound as well as physical. While the psychological aspects are harder to measure, the study of these effects are critically important to the music producer and engineer.
Amplitude:Â Amplitude is the size of the vibration, and this determines how loud the sound is.