Sound
Concept-first revision with scientific reasoning, everyday links, diagrams, worked examples and exam-focused practice.
Chapter Snapshot
- Sound is produced by vibrating objects and usually requires a material medium.
- In air, sound travels mainly as longitudinal pressure variations called compressions and rarefactions.
- Frequency determines pitch; amplitude is related to intensity and perceived loudness.
- Wave speed equals frequency multiplied by wavelength.
- Sound reflects and can produce echoes and reverberation.
- Humans typically hear roughly 20 Hz to 20 kHz, though sensitivity varies.
What You Will Learn
- Explain how vibrating sources produce sound
- Describe longitudinal waves using compressions and rarefactions
- Relate amplitude frequency time period and wavelength to sound
- Use v=fλ
- Explain reflection echo and reverberation
- Describe audible infrasonic and ultrasonic ranges and basic ear function
Why This Chapter Matters
A vibrating tuning fork does not travel across the room to your ear. Instead, it disturbs nearby air, which disturbs neighbouring air, so the disturbance travels. This distinction between motion of the medium and propagation of the wave is the key idea. Once it is clear, compressions, frequency, wavelength, pitch, echo and hearing fit into one connected story.
Bridge from Earlier Learning
- Vibrations
- Motion graphs
- Speed relation
- Basic pressure idea
These ideas are reused and deepened in this chapter.
1. Producing Sound
2. Sound Needs a Medium
Sound in ordinary school physics is a mechanical wave. It needs particles that can interact and pass the disturbance onward. It travels through gases, liquids and solids at different speeds because spacing and elastic properties differ. It cannot travel through a perfect vacuum, unlike light.
3. Compressions and Rarefactions
As a source moves forward, nearby air is compressed into a high-pressure, high-density region. As it moves backward, a lower-pressure rarefaction forms. Alternating compressions and rarefactions travel outward. Air particles oscillate roughly parallel to the direction of wave propagation, so sound in air is longitudinal.
4. Amplitude, Frequency and Time Period
Amplitude is the maximum disturbance from equilibrium and is related to intensity and perceived loudness. Frequency is oscillations per second and controls pitch. Time period is time for one oscillation, so frequency and period are reciprocals. Loudness and pitch are therefore different perceptual properties linked to different wave quantities.
5. Wavelength and Wave Speed
Wavelength is the distance between successive compressions, successive rarefactions or other corresponding phase points. Wave speed equals frequency multiplied by wavelength. In a given medium under fixed conditions, if frequency rises, wavelength decreases so that their product remains the wave speed.
6. Reflection, Echo and Reverberation
Sound reflects from surfaces. An echo is a distinct reflected sound heard after the original. Reverberation is persistence caused by many reflections in an enclosure. Acoustic materials and room design reduce excessive reverberation by absorbing or redirecting sound.
7. Multiple Reflection and Ultrasound
Megaphones, horns and some devices guide sound using reflection. Ultrasound has frequency above the upper human audible range and is used in imaging, cleaning, industrial inspection and ranging. SONAR sends sound pulses in water and uses returning echoes to estimate distance or depth.
8. Human Ear
The outer ear collects sound, the eardrum vibrates, middle-ear bones transmit the vibration and the inner ear converts mechanical disturbance into nerve signals. The brain interprets those signals as sound. Hearing range and sensitivity vary, and prolonged very loud sound can damage hearing.
Compare the Ideas
Wave Quantities
| Quantity | Meaning | Unit |
|---|---|---|
| Amplitude | Maximum disturbance | Depends on variable |
| Frequency | Oscillations per second | Hz |
| Time period | Time per oscillation | s |
| Wavelength | Distance between repeated phase points | m |
| Speed | Propagation rate | m/s |
Visualise the Concept
Sound links source vibration, propagation through a medium and detection by the ear.
Formula / Relationship Sheet
Formula / Key Relations
Wave speed
Speed = frequency × wavelength.
Frequency-period relation
T is time period.
Echo distance
Divide by 2 for out-and-back travel.
Worked Examples & Scientific Reasoning
Observe Like a Scientist
Exam Thinking Strategy
Trace source vibration → medium disturbance → wave quantity → reflection or detection. Keep frequency, amplitude and wavelength roles distinct.
Common Mistakes
Key Terms
Quick Revision
Quick Revision
- Sound is produced by vibrating objects and usually requires a material medium.
- In air, sound travels mainly as longitudinal pressure variations called compressions and rarefactions.
- Frequency determines pitch; amplitude is related to intensity and perceived loudness.
- Wave speed equals frequency multiplied by wavelength.
- Sound reflects and can produce echoes and reverberation.
- Humans typically hear roughly 20 Hz to 20 kHz, though sensitivity varies.
- Revise the explanation behind each definition or formula, not only the wording.
Focused Practice
Which property mainly determines pitch?
View Solution
Frequency
Why can sound not travel through a perfect vacuum?
View Solution
There are no material particles to transmit the mechanical disturbance.
Find wavelength for v=330 m/s and f=110 Hz.
View Solution
3 m
Echo time is 0.4 s, v=340 m/s. Find reflector distance.
View Solution
68 m
If wave speed stays fixed and frequency doubles, what happens to wavelength?
View Solution
It halves because v=fλ.
Explain how a vibrating source creates a longitudinal sound wave in air.
View Solution
Forward/back motion creates alternating compressions and rarefactions that propagate while air particles oscillate parallel to wave direction.
Final Chapter Summary
Chapter Summary
- Sound is produced by vibrating objects and usually requires a material medium.
- In air, sound travels mainly as longitudinal pressure variations called compressions and rarefactions.
- Frequency determines pitch; amplitude is related to intensity and perceived loudness.
- Wave speed equals frequency multiplied by wavelength.
- Sound reflects and can produce echoes and reverberation.
- Humans typically hear roughly 20 Hz to 20 kHz, though sensitivity varies.
- A strong answer explains the mechanism or reason, then uses the correct scientific term, relation or example.