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Know What You Are Working With
If you are going to spend many nights and weekends getting the sound "juuuust right" then you better know exactly what those sounds you are listening to really are in the physical world. DO NOT SKIP THE NEXT FEW SECTIONS on sound, they essential building blocks to recording and manipulating audio (and a lot shorter than most articles on sound).
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From End To End
When listening to Eric Clapton Unplugged, the sounds that you hear when you listen to music are coming from speakers that are vibrating back and forth. The speakers' motion creates compression waves that move through the air until they reach your eardrum.
When the waves reach the eardrum, the eardrum also vibrates. The eardrum is connected to tiny bones in your middle ear which vibrate along with the eardrum. The tiny bones in the middle ear transmit those same vibrations to the fluid in your inner ear, which causes the hair cells in the inner ear to bend.
When the hair cells in your inner ear bend an electrical analog signal is produced and received by your auditory nerve which relays the signal to the brain. Once the brain receives the signal it is processed by the brain.
Though there are studies still being conducted on how the brain processes sound, there is a general consensus that first there is a reflex reaction processed by the brain (which may cause a physical reaction like closing of the eyes or turning of the head). After that (chronologically) there are several parts of the brain that will do the conscious (active listening) and unconscious (background environment) processing of sound.
The brain will also reference sound in relation to the sounds that came before and after it. For example when listening to your friend talk, you are able to understand the meaning of each word because of the words that were said before and after. The same is true for sound, the brain makes connections between sounds to create context.
How the brain processes sounds is paramount to music listening. Keep this in mind when composing, capturing, mixing, and distributing music.
An Intro Waves
A sound wave is a transmission of kinetic energy within a medium (gas, liquid, solid). When sound waves are in a gas, they are compression waves (aka longitudinal waves). In the previous example of listening to Eric Clapton, when the speaker moves forward it is pushing the atoms away from the speaker and forcing those atoms to push on the adjacent atoms. This process repeats itself which is how sound waves travels from one point in space to another.
Once an atom pushes on the adjacent atoms, it slows down and the adjacent atoms (that were pushed upon) move away from the original atom. When atoms are pushing on each other this is the high pressure portion of the wave, when the atoms are moving away from each other this is the low pressure portion of the wave. The highest point of the wave is known as the peak (aka crest or apex) and the lowest point is known as the trough (aka rarefaction). In audio production the graphical representation of a sound wave is converted from a compression wave to a transverse wave and called a "waveform". The distance from crest to crest (or trough to trough) is called the wavelength, regarding sound this correlates to the frequency.
If sound is the result from high or low pressure waves, then what is silence?
Silence is when the air molecules are just "resting" at the current environment's atmospheric pressure. Any pressure above this environmental resting pressure is "high" pressure and anything lower is "low" pressure. The difference between the resting pressure and the wave's pressure at any point is known as the amplitude of the wave. It is a measure of energy and when referring to sound it directly correlates to the volume of the sound.
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When Waves Collide!
Wave are colliding all the time, so get used to dealing with this in all aspects of audio production.
For now we will skip how our amazing brains can dissect sounds from separate sources even with our eyes closed in a crowded subway while carrying on a conversation...