Intro to Sound and Waves — Understanding Frequency, Amplitude, and Wavelength Through Ripples and Guitar Strings

Sound Is a Wave Carried by Vibrating Air

Sound is created when an object vibrates. Pluck a guitar string, and the string trembles, pushing and pulling on the surrounding air. As air particles repeatedly compress and expand near their resting positions, that change travels all the way to your ear, and you perceive it as sound.

Sound isn’t an object flying to your ear — it’s a pattern of vibration traveling through a medium.

Think of a ripple on water. A leaf floating on the surface doesn’t get carried far away — it just bobs up and down. But the shape of the ripple itself moves outward. Sound works similarly: individual air particles don’t travel far, but the change in pressure does.

파동의 기본 요소

진폭 얼마나 크게 흔들리는가

소리에서는 대체로 큰 소리와 관련됩니다.

주파수 1초에 몇 번 흔들리는가

소리에서는 높은음·낮은음과 관련됩니다.

파장 마루와 마루 사이 거리

파동이 공간에서 반복되는 간격입니다.

1. Four Words for Understanding Waves

Concept	Simple meaning	What it feels like in sound
Amplitude	How large the vibration is	Roughly, loudness
Frequency	How many vibrations occur per second	High pitch vs. low pitch
Wavelength	The spatial distance of one repeating cycle	Depends on frequency and speed
Medium	The material the wave travels through	Air, water, metal, and so on

Sound feels hard to grasp mainly because we can’t see it — but the core idea is simply “vibration gets transmitted.” Strings, drumheads, vocal cords, and speakers all make sound by vibrating the air around them.

2. High Pitch and Loud Volume Are Different Things

Many people mix up “high” sounds and “loud” sounds. But they’re actually separate concepts.

High pitch means a high frequency — the vibration repeats more often per second.
Low pitch means a low frequency — the vibration repeats less often per second.
Loud sound means large amplitude — a bigger change in air pressure.
Quiet sound means small amplitude.

For example, a violin can play a high note softly, and a large drum can produce a loud, low note. Pitch is about frequency; loudness is about amplitude.

How sound is made and heard

An object vibrates

Something — a guitar string, vocal cords, a speaker diaphragm — begins to vibrate.

The surrounding air compresses and expands

Air particles don't fly off; they push and pull the particles near them.

The pressure change travels like a wave

The pattern of vibration is carried through the air all the way to your ear.

The eardrum and cochlea interpret the vibration

The ear converts pressure changes into nerve signals, and the brain interprets them as sound.

3. Seeing Frequency Through Stringed Instruments

Guitars and violins are great for understanding frequency.

Condition	String behavior	Sound
String shortens	Vibrates faster	Higher pitch
String lengthens	Vibrates slower	Lower pitch
String plucked hard	Amplitude increases	Louder sound
String plucked softly	Amplitude decreases	Quieter sound

On a stringed instrument, pressing a finger down on the string shortens the vibrating length. A shorter string vibrates faster, raising the frequency and producing a higher note. Conversely, thicker, longer strings tend to produce lower notes.

4. Resonance Is What Happens When Vibrations Sync Up and Amplify

If you time your pushes on a swing correctly, even a small push can build up a big swing. That’s a good analogy for resonance. Many objects have a natural vibration pattern they “prefer,” and when an outside force pushes at a similar rhythm, the vibration grows larger.

Examples of resonance

Pushing a swing at just the right timing
A guitar's body amplifying the string's sound
Blowing across an empty bottle produces a specific note

Three elements of sound

Loudness: amplitude
Pitch: frequency
Timbre: the combination of waveform and overtones

How sound travels

Sound through air
Sound through water
Vibrations traveling through walls
Ordinary sound cannot travel through a vacuum

Everyday observations

Sound gets quieter with distance
The muffled low tone of a room with the door shut
The changing pitch as an ambulance passes
How room shape affects echo

5. The Doppler Effect: When Motion Changes How Frequency Sounds

You’ve probably noticed that an ambulance siren sounds higher-pitched as it approaches, then lower as it passes and moves away. That’s the Doppler effect.

When a sound source moves toward you, the waves get compressed ahead of it, so the frequency sounds higher. When it moves away, the waves stretch out, so the frequency sounds lower. The siren itself isn’t actually changing pitch — what you hear is different because of the relative motion between the observer and the sound source.

This same principle extends beyond sound to light. In astronomy, the redshift seen in light from receding galaxies can be understood through the same broad idea.

6. One-Page Summary

Sound is a pressure wave carried through a medium like air.
Amplitude roughly corresponds to loudness, and frequency corresponds to pitch.
High pitch and loud volume are different things.
On a stringed instrument, a shorter, faster-vibrating string produces a higher note.
Resonance is when the timing of vibrations lines up, making the vibration grow larger.
The Doppler effect is when frequency sounds different because of relative motion between a sound source and an observer.

Continue to the Next Lecture

Once you understand sound, a single thread connects music, speech, soundproofing, ultrasound, seismic waves, and even the wave nature of light. That’s how science works — one concept you truly grasp keeps opening doors to other worlds.