Throw a stone into a calm lake and ripples spread outward, but a leaf floating on the water only bobs up and down in place instead of drifting off with them.
When the material only shakes in place and just the shaking is passed along like this, we call it a wave, and the material that carries it is the medium.
Sound is also a wave, with air as its medium: how often the air vibrates (frequency) sets the pitch, and how strongly it vibrates (amplitude) sets the loudness.
The first picture is a snapshot of a wave traveling along a rope at 1 m/s, so as you raise the frequency the wavelength shortens to match, and you can check that frequency times wavelength always stays 1 m/s.
What a Wave Looks Like
seegongsik.com
A snapshot of a wave moving along a rope at 1 m/s (the picture is 100 cm wide)
8 cm
3 Hz
💡 Words That Describe Waves
①Wave: a vibration started in one place spreading through a medium; the medium only vibrates in place and does not travel along with it.
②Amplitude (A): the height from the center of the vibration to a crest (or trough); the larger it is, the more energy the wave carries.
③Wavelength (λ): the distance from one crest to the next (or trough to trough).
④Frequency (f): the number of vibrations per second, measured in hertz (Hz).
⑤In the picture, 2 Hz gives a wavelength of 50 cm and 4 Hz gives 25 cm.
Transverse and Longitudinal Waves
seegongsik.com
Transverse and longitudinal waves: the direction the medium shakes and the direction the wave travels
🔍 Kinds of Waves
①Transverse wave: the medium vibrates at right angles to the direction the wave travels, as in a wave on a rope, earthquake S waves and light.
②Longitudinal wave: the medium vibrates along the direction of travel, as in sound and earthquake P waves.
③In a longitudinal wave, crowded regions (compressions, C) and spread-out regions (rarefactions, R) alternate and move forward.
④Sound is passed along by air particles vibrating back and forth, so it cannot travel through a vacuum.
Three Elements of Sound
seegongsik.com
Comparing pitch, loudness and timbre by waveform
High and low sounds
Wave speed
v = fλ (speed = frequency × wavelength)
The medium sets the speed; in the same medium, raising the frequency shortens the wavelength to match, so the product stays the same.
🎵 Three Elements of Sound
①Pitch is set by frequency: the higher the frequency, the higher the sound. People hear roughly 20 to 20,000 Hz.
②Loudness is set by amplitude: the larger the amplitude, the louder the sound.
③Timbre is set by the waveform: a tuning fork gives a smooth waveform and a guitar a complex one made of several vibrations, so they sound different at the same pitch.
Exam Key Points
Summary
v = fλ · pitch = frequency · loudness = amplitude · timbre = waveform
Remember the wave speed formula together with the three elements of sound.
🎯 Exam Points
①In transverse waves the vibration is perpendicular to the travel (rope waves, light); in longitudinal waves it is parallel (sound).
②v = fλ: wave speed = frequency × wavelength.
③A high sound has a large frequency; a loud sound has a large amplitude.
④Sound travels at about 340 m/s in air, a little faster when it is warmer.
⑤Sound needs a medium, so it cannot be heard in a vacuum.
Examples and Unit Test
Example 1
What is the speed of a wave with a frequency of 50 Hz and a wavelength of 4 m? (v = fλ)
1
Wave speed is v = fλ, frequency times wavelength.
2
v = 50 Hz × 4 m = 200 m/s.
▸ 200 m/s
It vibrates 50 times a second and moves 4 m with each vibration, so it covers 200 m every second.
Example 2
What determines the pitch and the loudness of a sound?
1
Pitch is set by frequency: the larger the frequency, the higher the sound.
2
Loudness is set by amplitude: the larger the amplitude, the louder the sound. Timbre is set by the waveform.
▸ Pitch = frequency / loudness = amplitude
In the lower picture, a high sound has a tighter waveform and a loud sound a taller one.
Unit-test style
Which statement about waves and sound is correct?
①The speed of a wave is found with v = f ÷ λ
②The pitch of a sound is determined by frequency
③The loudness of a sound is determined by frequency
④Sound is a transverse wave
⑤The unit of frequency is m
▸ ② The pitch of a sound is determined by frequency
1
Pitch rises as frequency increases, so it is set by frequency; ② is correct.
2
Wave speed is v = fλ (①), loudness is set by amplitude (③), sound is a longitudinal wave (④), and the unit of frequency is the hertz (⑤).