Introduction to Sound Waves
Sound is the earliest physics laboratory available to any student, yet it is also the most frequently misread. Unlike light, sound cannot cross a vacuum; it demands a material medium, and that single constraint governs everything from the pitch of a tuning fork to the thunder of a supersonic jet.
This quiz tests the mechanical foundations of sound: how it is produced, how it propagates, and how it differs from electromagnetic waves. You will meet the vocabulary that later lessons on superposition, standing waves, resonance and the Doppler effect will reuse.
What to expect
- Sound as a mechanical wave requiring inertia and elasticity
- Longitudinal versus transverse particle motion
- Compressions, rarefactions, crests and troughs
- Speed of sound in gases, liquids and solids
- Frequency, wavelength, amplitude and the human hearing range
- Infrasonic and ultrasonic waves and everyday applications
Answer all fifteen questions carefully. Each explanation is designed to reinforce the physical intuition, not just the final answer.
Q1. Sound is best described as which of the following?
Sound is a self-propagating disturbance in an elastic medium. It transports energy and momentum, but the medium particles merely oscillate about equilibrium and are not carried along.
Q2. Which pair of medium properties must act together for sound to propagate?
Propagation requires inertia (the mass that oscillates) and elasticity (the restoring force). Remove either and propagation fails, which is why a vacuum transmits no sound.
Q3. Why does a vacuum transmit no sound?
Sound needs a material medium. In a vacuum there are no particles to displace, so no disturbance can be passed on and no energy can be transported.
Q4. In a sound wave travelling through air, the individual air particles:
A particle oscillating at 440 Hz completes 440 back-and-forth cycles each second, yet its net displacement over any full cycle is zero. Energy travels; matter merely vibrates in place.
Q5. Sound waves in air are:
In a gas the particles are displaced parallel to the direction in which the disturbance travels, so sound in air is a longitudinal wave.
Q6. In a longitudinal sound wave, a region where particles are crowded together is called a:
A compression is a region of increased density and pressure. A rarefaction is the opposite, a region where particles are spread apart. Crests and troughs belong to transverse waves.
Q7. The terms crest and trough are most naturally associated with:
Crests and troughs describe the maximum and minimum displacements of a transverse wave. Longitudinal waves are described instead by compressions and rarefactions.
Q8. Which statement about the speed of sound is correct?
Solids have far stiffer interatomic bonds, so sound travels much faster in steel (about 5,960 m/s) than in air (about 343 m/s at 20 °C).
Q9. The speed of sound in a medium is governed by:
Density alone does not decide speed. It is the ratio of elastic modulus to density that sets the wave speed, which is why stiff solids are fast and air is slow.
Q10. For a gas, the speed of sound varies with absolute temperature as:
Warmer air raises molecular speeds, so ##v \propto \sqrt{T}##. This is why the speed rises from about 331 m/s at 0 °C to about 343 m/s at 20 °C.
Q11. The approximate speed of sound in air at 0 °C is:
At 0 °C the speed is approximately 331 m/s, rising to about 343 m/s at 20 °C, a difference of roughly 3.5 per cent.
Q12. The number of complete oscillations a particle performs per second is called the:
Frequency is the number of complete cycles per second, measured in hertz. Wavelength is the distance between successive corresponding points, and amplitude is the maximum displacement.
Q13. The normal human hearing range is approximately:
Human hearing extends from roughly 20 Hz to 20 kHz. Below this range lies the infrasonic region and above it the ultrasonic region.
Q14. Sound with frequency above the upper limit of human hearing is called:
Ultrasonic waves have frequencies above about 20 kHz. Infrasonic waves lie below about 20 Hz. Subsonic and supersonic describe speeds, not frequencies.
Q15. Which everyday example best illustrates sound as a mechanical wave requiring a medium?
The evacuated jar removes the medium, so the ringing bell becomes inaudible. This demonstrates that sound, unlike light, cannot cross a vacuum.