What is SOUND?
To imagine invisible sound is fundamental for further understanding of sound science.
"In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid."
"In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Only acoustic waves that have frequencies lying between about 20 Hz and 20 kHz elicit an auditory percept in humans."
"Only acoustic waves that have frequencies lying between about 20 Hz and 20 kHz elicit an auditory percept in humans."
by Wikipedia https://en.m.wikipedia.org/wiki/Sound
The expressions above tell us that sound has 2 aspects: Physical and Psychological.
If sound is received by mechanical sensors, it is a kind of Machine recognition.
On the other hand, if it is received by human ears, it can be called Human recognition. And it deeply relates to, and depends on sensitivities and feelings of individuals.
And for us, humans, only sound (acoustic waves) with frequency between 20 Hz to 20 kHz is audible, so we can focus on those range, when we talk about the sound of music.
Therefore, musicians and sound engineers put their efforts to make physical sound and deliver its vibration to ears and bodies of humans to move their hearts.
What is FREQUENCY?
How it relates to High sound and Low sound
Frequency is the number of repeating waves per unit of time. One cycle of frequency is a wave having 1 top (mountain) and 1 bottom (valley). And it is measured as Herz (Hz), which is defined as cycles per one second.
Red line indicates 1Hz. It has 1 top and 1 bottom in 1 sec. Frequency can be expressed with sin curve.
Audible frequency of humans is 20Hz to 20kHz, lower frequency is lower sound, higher frequency is higher sound.
Pure tone and Complex tone
Sounds of the sin wave are called "pure tones". A sound used in audiometry (medical check of ears) is a pure tone. The pure tone is a sound made artificially.
There are no pure tones in the natural world. Plural frequencies are mixed in a naturally occurring sound, such as a human-voice, bird-song, and wind-blow. Those sounds are called "complex tones". Sound in nature contains an infinite number of frequencies.
Green and blue are waves of pure tones. Red is complex tone of green and blue.
Loud sound and Weak sound
Physical loudness and human-being recognition of loudness resemble each other but there are some differences on the other hand.
Physically loudness is measured by unit of Pa (Pascal). It is the same measurement unit of atmospheric pressure of weather. Therefore it can be called sound pressure.
Sound becomes louder physically when the amplitude is bigger. But humans do not recognize it as 2 times louder sound, even if the amplitude becomes 2 times bigger.
As sound pressure becomes strong, the degree of humans recognition becomes relatively smaller. There is a formula to exchange physical loudness (Pascal :Pa) to psychological loudness (decibel :dB).
x-axis :µ Pa (Micro Pascal)
|0dB||= 20 µ Pa|
|40dB||= 2,000 µ Pa (Point A)|
|100dB||= 2,000,000 µ Pa|