Physics · Class 9-10 · CBSE
Sound: Waves, Echo, Ultrasound and the Human Ear
Learn how sound propagates as longitudinal compression-rarefaction waves, speed equation v = fλ, minimum distance for clear echo, ultrasound medical & SONAR applications, and human ear anatomy.
1Nature and Propagation of Sound Waves
Sound is mechanical energy that requires a material medium (solid, liquid, or gas) to travel. It cannot travel through a vacuum. It propagates as aLongitudinal Waveconsisting of alternatingCompressions(regions of high pressure and density) andRarefactions(regions of low pressure and density).
Wave Speed Equation
- v = Wave velocity in metres/second (m/s)
- ν (or f) = Frequency in Hertz (Hz) = number of vibrations per second
- λ = Wavelength in metres (distance between two consecutive compressions)
- T = Time period of one complete wave cycle (T = 1/f)
Speed of Sound Across Media (at 20°C)
| Medium Phase | Typical Material | Speed of Sound (m/s) | Why Speed Differs? |
|---|---|---|---|
| Solid | Steel / Aluminium | 5,000 – 6,420 m/s | High elasticity and densely packed molecules transmit vibrations fastest |
| Liquid | Water (fresh / sea) | 1,480 – 1,530 m/s | Intermediate elasticity and density |
| Gas | Air (at 20°C) | 344 m/s | Molecules are far apart, transmission depends on thermal collision rates |
| Vacuum | Outer space | 0 m/s (Cannot travel) | Sound strictly requires a material medium to propagate |
Solid
- Typical Material
- Steel / Aluminium
- Speed of Sound (m/s)
- 5,000 – 6,420 m/s
- Why Speed Differs?
- High elasticity and densely packed molecules transmit vibrations fastest
Liquid
- Typical Material
- Water (fresh / sea)
- Speed of Sound (m/s)
- 1,480 – 1,530 m/s
- Why Speed Differs?
- Intermediate elasticity and density
Gas
- Typical Material
- Air (at 20°C)
- Speed of Sound (m/s)
- 344 m/s
- Why Speed Differs?
- Molecules are far apart, transmission depends on thermal collision rates
Vacuum
- Typical Material
- Outer space
- Speed of Sound (m/s)
- 0 m/s (Cannot travel)
- Why Speed Differs?
- Sound strictly requires a material medium to propagate
2Reflection of Sound, Echo and Reverberation
The repetition of sound caused by reflection from a large obstacle is called anEcho. The sensation of sound persists in our brain for about0.1 seconds(persistence of hearing).
Derivation of Minimum Distance for Echo in Air
- 1
1. Speed of sound in air at 22°C = 344 m/s.
- 2
2. Minimum time delay needed between original and reflected sound = 0.1 s.
- 3
3. Total distance traveled by sound (to obstacle and back) = Speed × Time = 344 × 0.1 = 34.4 m.
- 4
4. Minimum distance of obstacle from source = 34.4 / 2 = 17.2 metres.
Echo vs Reverberation
| Feature | Echo | Reverberation |
|---|---|---|
| Definition | Single clear repetition of sound from distant obstacle | Persistence of sound due to repeated reflections from walls/ceilings |
| Obstacle Distance | Obstacle must be at least 17.2 m away | Occurs in closed halls/auditoriums even when distance < 17 m |
| Clarity of Speech | Clean separation, words are distinct | Causes speech blurring; requires sound-absorbent panels |
Definition
- Echo
- Single clear repetition of sound from distant obstacle
- Reverberation
- Persistence of sound due to repeated reflections from walls/ceilings
Obstacle Distance
- Echo
- Obstacle must be at least 17.2 m away
- Reverberation
- Occurs in closed halls/auditoriums even when distance < 17 m
Clarity of Speech
- Echo
- Clean separation, words are distinct
- Reverberation
- Causes speech blurring; requires sound-absorbent panels
3Ultrasound Applications and Human Ear Anatomy
Sound Frequency Spectrum
| Range Name | Frequency Band | Detectors / Real-world Examples |
|---|---|---|
| Infrasound | < 20 Hz | Elephants, rhinoceros, earthquakes, volcanic eruptions |
| Audible Sound | 20 Hz – 20,000 Hz | Normal human hearing range |
| Ultrasound | > 20,000 Hz (20 kHz) | Bats, dolphins, echocardiography, SONAR, kidney stone lithotripsy |
Infrasound
- Frequency Band
- < 20 Hz
- Detectors / Real-world Examples
- Elephants, rhinoceros, earthquakes, volcanic eruptions
Audible Sound
- Frequency Band
- 20 Hz – 20,000 Hz
- Detectors / Real-world Examples
- Normal human hearing range
Ultrasound
- Frequency Band
- > 20,000 Hz (20 kHz)
- Detectors / Real-world Examples
- Bats, dolphins, echocardiography, SONAR, kidney stone lithotripsy
How the Human Ear Converts Waves to Nerve Impulses
- 1
1. Pinna collects sound waves and channels them down the auditory canal.
- 2
2. Eardrum (tympanum) vibrates upon encountering sound pressure waves.
- 3
3. Three middle ear bones (Hammer, Anvil, Stirrup) amplify the mechanical vibrations by ~20 times.
- 4
4. Cochlea fluid converts mechanical pressure into electrical nerve impulses via hair sensory cells.
- 5
5. Auditory nerve transmits electrical signals to the brain for sound recognition.
4Exam Corner: Tips, Mistakes & Board Questions
High-Frequency Exam Takeaways
- SONAR (Sound Navigation and Ranging) uses ultrasound pulse echo: 2d = v × t to measure sea depth.
- Pitch is determined by frequency; Loudness is determined by amplitude of vibration.
- Reverberation in auditoriums is minimized using sound-absorbing drapes, compressed fibreboards, and curved ceilings.
Exam question · CBSE Class 9/10 Board
- A.4896 m
- B.2448 m
- C.1530 m
- D.3060 m
- Total round trip distance = 2d = v × t.
- 2d = 1530 m/s × 3.2 s = 4896 m.
- Correct answer is option B.
Answer: B. 2448 m
Remember
- Sound is a longitudinal wave needing a material medium; fastest in solids, zero in vacuum.
- Wave speed equation: ==v = f · λ==.
- Echo requires a minimum reflection distance of17.2 metresin air.
- Ultrasound (> 20 kHz) powersSONARand non-invasive medical diagnostics.
Check yourself
Question 1
Which characteristic of sound allows us to distinguish between two different singers singing the same note at equal loudness?