1. Wave: A periodic vibration that occurs in any part of a material and spreads around it.

2. Transverse Wave: Waves in which the direction of vibration of the medium and the direction of wave travel are perpendicular to each other
Example) Water wave, electromagnetic wave, seismic wave S wave

3. Seismic wave: A wave in which the direction of vibration of the medium and the direction of wave travel are parallel to each other Example) Sound (sound wave), seismic wave P wave

4. Wave expression (transverse wave)
Goal: where the medium is at the lowest
Wavelength: distance from floor to floor, valley to valley
Amplitude: Distance from the center of vibration to the floor or valley
Period: The time it takes for the point that used to be a floor to become a floor again in the process of propagating the wave.
Frequency: The number of times the wave vibrates in 1 second.

5. The three elements of sound
Loudness: The larger the amplitude of the wave, the louder the sound.
Sound level: The higher the frequency of the wave, the higher the sound.
Voice: Different instruments have different waveforms, so the tone is different.