Physics of sound
Sound is a mechanical wave, sequence of waves is resulting from an air pressure disturbance produced by vibration, and sound propagates through the medium such as air or water. During the propagation, sound can be reflected or attenuated by the medium. Humans can hear the sound is because the vibration pass the wave in to our ear, this is called Traveling Longitudinal Waves. The propagation of the sound can be affected by the density and pressure. The temperature determines the speed of sound with in the medium, also the medium itself would affect the propagation such as wind (moving medium) if the medium is moving therefore the wave would propagated further; with the medium don’t have viscosity, sound would be easier to propagate, but if the medium have an negative viscosity such as water the it would affects the motion of the sound wave.

On the top is the diagram of the travellingwave the bar above the diagram represent the concentrations of the pressures on the wave, when the curve are compression (the part above the horizontal line) it represent a high pressure; when the curve are rarefaction (the part below the horizontal line) it represent a low pressure and they change from one to another, this pattern repeats indefinitely. Speed of sound = wavelength*Frequency
Wavelength (it can be written as λ) is the distance between two consecutive corresponding points of a waveform. Normally the wavelength is about a meter long. The pitch/frequency of the sound determines the wavelength and the speed of the wave; it can be measured by the formula above. As we known speed is a constant it travels at 343 meters per second at standard temperature and pressure, therefore wavelength and frequency are negative correlation, if the wavelength is high than the frequency would be low (longer the wavelength, the lower the frequency).
In physics, interference shows two different wave patterns (Constructive and destructive), it is a resultant wave of greater or lower amplitude. Amplitude can be defined as the size of the sound wave; it corresponds to the loudness of the sound. Constructive interference is when two waves have the some peak, the result of the new curve would be greater, which means the new the sound would be louder. On the other hand Destructive interference is when two waves has an opposite peaks which means when they join together all the peaks would cancel out , this would create diminished waveform, so the new wave would be silence.

Simple sounds have been known as when a sine wave repeats indefinitely in one pattern and it contain no harmonics, it can only produce electronically. Refer to the interference, when we add two different waves together, it will change the patterns of the waves.
Example:
300Hz wave:
500Hz wave:
They are two different sound, they have a different frequency, when we add them together it will produce the blue curve below:

From the knowledge of interference we knows when both waves have high pressure and reinforce each other they will get a higher pressure in the new wave, when two of them have opposite pressure they will cancel each other out.
Complex wave/ combine wave:

We have discussed the simple sound; in the real would all natural sources vibrate at a combination of frequencies also producing more complex waveforms.

In music, as we known timbre is the distinctive property of a complex sound. It is the character of a sound produced by the relationship of fundamental frequency and harmonics (this is also called overtones). Fundamental frequency is also been known as 1st harmonic, it has the highest amplitude and lowest frequency of a periodic waveform. Harmonic is a component frequency; it is a multiple of the fundamental frequency, in music term used to describe the frequency information contained in a sound is its harmonic series.

The diagram above shows the fundamental frequency followed with harmonic series. As we know harmonic is the multiple of the fundamental frequency, which means each harmonics order are equally spaced by the width of the fundamental frequency and repeatedly adding that frequency. Different sounds contain their harmonics at differing amplitudes and altering the amplitudes of the different harmonics changes the timbre.
Acoustics is another important part of the physics of sound; it is the study of how sound behaves in different place. From the first part of the essay we knows that different materials would have an different effect on the sound waves, in terms to ensure the quality of sounds in a record studio acoustic is one thing we have to consider; Acoustics are the most important component of audio and noise control industries.
When the wavelength of an overtone is a factor of the distance between two parallel surfaces, standing wave will be produced with the frequency which has corresponded from the wavelength, they are referred nodes and antinodes.