Sound Waves Study Pack
Kibin's free study pack on Sound Waves includes a 6-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Sound Waves Study Guide
Unpack the physics of longitudinal waves — from compression and rarefaction to the speed of sound in different media, pitch, decibels, the Doppler effect, and wave behaviors like interference and diffraction.
Key Takeaways
- •Sound waves are longitudinal mechanical waves that travel through a medium by compressing and rarefying the particles of that medium in the same direction as wave propagation.
- •The speed of sound depends on the medium's elasticity and density — sound travels faster in solids than liquids, and faster in liquids than gases, and increases with temperature in air.
- •Frequency determines pitch: humans perceive sound in the range of approximately 20 Hz to 20,000 Hz, with infrasound below and ultrasound above that range.
- •Amplitude corresponds to the loudness of a sound and is measured in decibels (dB) using a logarithmic scale, where every 10 dB increase represents a tenfold increase in sound intensity.
- •The Doppler effect describes the apparent change in frequency heard when a sound source and an observer are moving relative to each other — approaching sources produce higher perceived frequencies, receding sources produce lower ones.
- •Wave behaviors including reflection, refraction, diffraction, and interference apply to sound waves and explain phenomena like echoes, the bending of sound around obstacles, and standing waves in enclosed spaces.
What Sound Waves Are and How They Form
Sound is a mechanical disturbance that propagates through matter by transferring energy from one particle to the next — it cannot travel through a vacuum because it requires a physical medium to exist.
Longitudinal Wave Structure
- •Sound waves are longitudinal, meaning particle displacement occurs parallel to the direction the wave travels, unlike transverse waves where displacement is perpendicular.
- •As a sound wave moves through air, it alternately pushes molecules together into regions of higher pressure called compressions and pulls them apart into regions of lower pressure called rarefactions.
- •One complete cycle consists of one compression and one adjacent rarefaction, and the distance spanning that cycle is the wavelength.
Mechanical Wave Requirement
- •Because sound relies on particle-to-particle interaction, it needs a physical medium — solid, liquid, or gas — to propagate.
- •In the absence of matter, such as in outer space, sound cannot travel at all, distinguishing it fundamentally from electromagnetic waves like light.
Speed of Sound in Different Media
The rate at which a sound wave travels depends on two properties of the medium: how elastic it is (how readily it restores its shape after compression) and how dense it is.
Elasticity and Density as Governing Factors
- •Higher elasticity allows disturbances to be transmitted more rapidly between particles, increasing wave speed.
- •Higher density means more mass per unit volume must be moved, which resists wave propagation and reduces speed.
- •Speed of sound is approximated by v = √(B/ρ), where B is the bulk modulus (a measure of elasticity) and ρ is the medium's density.
Speed Comparisons Across States of Matter
- •Sound travels fastest in solids (e.g., approximately 5,120 m/s in steel), slower in liquids (approximately 1,480 m/s in water at 20°C), and slowest in gases (approximately 343 m/s in air at 20°C).
- •Solids have much higher bulk moduli than liquids or gases, which more than compensates for their greater density.
Temperature Dependence in Air
- •In air specifically, speed increases with temperature because warmer molecules move faster and transmit collisions more quickly.
- •A useful approximation is v ≈ 331 + 0.6T m/s, where T is temperature in degrees Celsius.
Frequency, Wavelength, and the Perception of Pitch
The physical properties of frequency and wavelength determine how humans and other animals perceive the character of a sound, most notably its pitch.
Frequency and Pitch
- •Frequency is the number of complete wave cycles passing a fixed point per second, measured in hertz (Hz).
- •The human auditory system detects frequencies between approximately 20 Hz and 20,000 Hz; this range is called the audible spectrum.
- •Higher frequency corresponds to higher perceived pitch — a 4,000 Hz tone sounds much higher than a 200 Hz tone.
Infrasound and Ultrasound
- •Infrasound refers to frequencies below 20 Hz; some large animals such as elephants use infrasound for long-distance communication.
- •Ultrasound refers to frequencies above 20,000 Hz and is used in medical imaging (sonography) and industrial applications because its short wavelengths can resolve fine structural details.
Frequency–Wavelength Relationship
- •Frequency and wavelength are inversely related through the wave equation: v = fλ, where v is wave speed, f is frequency, and λ is wavelength.
- •At a fixed speed, doubling the frequency halves the wavelength — high-pitched sounds in air have shorter wavelengths than low-pitched sounds.
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About this Study Pack
Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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Question 1 of 25
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What type of wave is a sound wave, and how does particle displacement relate to the direction of wave travel?
Card 1 of 30
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Concept 1 of 5
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Longitudinal Wave Structure of Sound
Explain what it means for sound to be a longitudinal mechanical wave. How do compressions and rarefactions form, and why does sound require a medium to travel?
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