Waves and Sound

Grade 9 · Science · Study Guide

Explore how energy travels through waves and how sound is produced, transmitted, and perceived.

A wave is a disturbance that transfers energy from one place to another without transferring matter. Waves are broadly classified into two types. Transverse waves oscillate perpendicular to the direction of wave travel — like shaking a rope up and down while it moves forward. Light is a transverse wave. Longitudinal waves oscillate parallel to the direction of travel, creating alternating compressions and rarefactions. Sound is a longitudinal wave.

Key wave properties include wavelength (the distance between two consecutive identical points, such as crest to crest), frequency (the number of complete waves passing a point per second, measured in Hertz), amplitude (the maximum displacement from the rest position — related to energy and intensity), and wave speed (v = fλ, where v is speed, f is frequency, and λ is wavelength). The speed of sound in air at room temperature is approximately 343 m/s — much slower than light's 300,000,000 m/s, which is why you see lightning before you hear thunder.

Sound is produced by vibrating objects. When a guitar string vibrates, it creates compressions and rarefactions in the surrounding air that travel outward as sound waves. The pitch of a sound corresponds to frequency — higher frequency means higher pitch. The loudness of sound is measured in decibels (dB) and relates to amplitude. Sound requires a medium to travel through (it cannot travel in a vacuum). The Doppler effect describes how the perceived frequency of a wave changes when the source and observer are moving relative to each other — the classic example is an ambulance siren sounding higher as it approaches and lower as it recedes.

Comparing Transverse and Longitudinal Waves
Feature	Transverse Wave	Longitudinal Wave
Particle motion	Perpendicular to wave travel	Parallel to wave travel
Examples	Light, water surface waves	Sound, seismic P-waves
Propagation medium	Can travel through vacuum (light)	Requires a medium
Features	Crests and troughs	Compressions and rarefactions

Key terms

Wave: A disturbance that transfers energy through matter or space without permanently displacing the medium.
Transverse Wave: A wave where particles oscillate perpendicular to the direction of wave propagation (e.g., light).
Longitudinal Wave: A wave where particles oscillate parallel to the direction of wave propagation (e.g., sound).
Wavelength (λ): Distance between two consecutive points in phase on a wave (e.g., crest to crest).
Frequency (f): Number of complete wave cycles passing a point per second; measured in Hertz (Hz).
Amplitude: Maximum displacement of a wave from its rest position; related to the wave's energy and intensity.
Doppler Effect: The perceived change in frequency of a wave when the source and observer are in relative motion.

Flashcards

Transverse Wave: Particles move perpendicular to wave direction; e.g., light.
Longitudinal Wave: Particles move parallel to wave direction; e.g., sound.
Frequency: Wave cycles per second; measured in Hertz (Hz); determines pitch in sound.
Amplitude: Maximum displacement from rest; determines loudness in sound.
Wave Speed Formula: v = fλ (speed = frequency × wavelength).
Doppler Effect: Change in perceived frequency when source and observer move relative to each other.

Practice questions

What type of wave is sound?
1. Transverse wave
2. Electromagnetic wave
3. Longitudinal wave
4. Surface wave
Show answer
Answer: Longitudinal wave
Sound travels as compressions and rarefactions — regions of high and low pressure — where particle movement is parallel to the wave's direction of travel.
Sound can travel through a vacuum.
Show answer
Answer: False
Sound requires a medium (gas, liquid, or solid) to travel. In the vacuum of space, there is no medium, so sound cannot propagate — movie explosions in space are scientifically inaccurate!
The number of wave cycles passing a point per second is called ___.
Show answer
Answer: frequency
Frequency (f) is measured in Hertz (Hz). Human hearing ranges from about 20 Hz to 20,000 Hz. Higher frequency = higher pitch.
A wave has a frequency of 200 Hz and a wavelength of 1.5 m. What is its speed?
1. 133 m/s
2. 200 m/s
3. 300 m/s
4. 133.3 m/s
Show answer
Answer: 300 m/s
v = fλ = 200 Hz × 1.5 m = 300 m/s. This is close to the speed of sound in air at room temperature (343 m/s).
The Doppler effect explains why an approaching ambulance siren sounds:
1. Lower in pitch
2. Higher in pitch
3. The same pitch
4. Silent
Show answer
Answer: Higher in pitch
As the source approaches, sound waves are compressed (shorter wavelength, higher frequency). As it recedes, waves stretch out (lower frequency). This is the Doppler effect.

Fun facts

Sound travels about 4 times faster in water than in air — whales can communicate across entire ocean basins.
The loudest sound ever recorded was the 1883 eruption of Krakatoa, heard 5,000 km away.
Bats use echolocation — they emit ultrasonic waves (above 20,000 Hz) and listen for echoes to navigate and hunt in total darkness.

Get a personalized course on Waves and Sound built around your child.