Unit 8: Wave Motion and Sound - Explained
This unit introduces the concepts of waves, their properties, and how they propagate. It focuses
on mechanical waves, particularly sound waves, and their characteristics.
8.1 Wave Propagation
A wave is a disturbance that travels through a medium, transferring energy without
transferring matter. Examples include water waves, sound waves, and light waves.
A wave pulse is a single disturbance that travels through a medium (like a ripple created
by dropping a stone in water). A continuous wave consists of a series of repeating
disturbances.
8.2 Types of Waves
Transverse Waves: The vibrations of the particles in the medium are perpendicular to
the direction of wave propagation.
o Examples: Light waves, waves on a string, S-waves (secondary waves) in
earthquakes.
Longitudinal Waves: The vibrations of the particles in the medium are parallel to the
direction of wave propagation.
o Examples: Sound waves, P-waves (primary waves) in earthquakes.
Mechanical Waves vs. Electromagnetic Waves:
o Mechanical waves require a medium to propagate. They travel through a medium
by causing the particles of that medium to vibrate. Sound waves, water waves,
and seismic waves are all mechanical waves.
o Electromagnetic waves do not require a medium and can travel through a
vacuum (like space). They consist of oscillating electric and magnetic fields that
are perpendicular to each other and to the direction of wave propagation. Light,
microwaves, radio waves, and X-rays are all examples of electromagnetic waves.
8.3 Key Features of a Wave
Crest: The highest point on a transverse wave.
Trough: The lowest point on a transverse wave.
Compression: A region in a longitudinal wave where the particles are closest together.
Rarefaction: A region in a longitudinal wave where the particles are furthest apart.
Amplitude (A): The maximum displacement of a particle from its equilibrium position.
A larger amplitude generally corresponds to a more intense wave (e.g., a louder sound
wave or a brighter light wave).
Wavelength (λ): The distance between two corresponding points on adjacent waves (e.g.,
crest to crest, trough to trough, compression to compression).
Frequency (f): The number of complete waves passing a given point per second.
Measured in Hertz (Hz).
Time Period (T): The time taken for one complete wave to pass a given point. The
reciprocal of frequency (T = 1/f).
Wave Speed (v): The speed at which a wave propagates through a medium. It depends
on the properties of the medium.
Wave Equation: The relationship between wave speed, frequency, and wavelength:
v = fλ.
8.4 Properties of Waves
Reflection: The bouncing back of a wave when it encounters a boundary between two
different media.
Refraction: The bending of a wave as it passes from one medium to another, due to the
change in wave speed.
Diffraction: The spreading out of waves as they pass through an opening or around an
obstacle.
Interference: The superposition of two or more waves that meet, resulting in a new wave
pattern.
o Constructive Interference: When two waves meet in phase (crests align with
crests, troughs with troughs), their amplitudes add up, resulting in a larger
amplitude.
o Destructive Interference: When waves meet out of phase (crests align with
troughs), their amplitudes subtract, resulting in a smaller amplitude or even
cancellation.
8.5 Sound Waves
Sound waves are longitudinal mechanical waves that are produced by vibrating objects.
They travel through a medium by causing the particles of the medium to vibrate in the
same direction as the wave propagation.
Speed of Sound: The speed of sound depends on the medium through which it travels.
Sound travels faster in denser media. For example, sound travels faster in solids than in
liquids and faster in liquids than in gases.
Intensity of Sound: The amount of energy carried by a sound wave per unit area per
second. Measured in watts per square meter (W/m2). Intensity decreases with the square
of the distance from the source (inverse square law).
Characteristics of Sound:
o Pitch: The perceived highness or lowness of a sound, determined by the
frequency of the sound wave. Higher frequency corresponds to higher pitch.
o Loudness: The perceived intensity of a sound, determined by the amplitude of the
sound wave. Larger amplitude corresponds to louder sound.
o Timbre (Quality): The characteristic quality of a sound that distinguishes it from
other sounds of the same pitch and loudness. It is determined by the complex
overtones present in the sound wave.
