Chapter in a nutshell: Sound is energy from vibrating bodies, travelling as a longitudinal wave that needs a material medium. Its three characteristics are loudness (amplitude), pitch (frequency) and quality (waveform). Reflected sound gives echoes and reverberation; matching frequencies cause resonance. Humans hear 20 Hz – 20 kHz.
1. Nature & Production of Sound
- Sound is produced by a vibrating body (vocal cords, tuning fork, string, drum).
- It propagates as a longitudinal wave — particles of the medium vibrate along the direction of travel, forming compressions (high pressure) and rarefactions (low pressure).
- Sound needs a material medium (solid/liquid/gas); it cannot travel through vacuum (a ringing bell in an evacuated jar is not heard).
- Speed of sound is greatest in solids, less in liquids, least in gases (≈ 332 m s⁻¹ in air at 0 °C). It increases with temperature and humidity.
2. Wave Terms
| Term | Meaning | Unit |
|---|---|---|
| Amplitude (a) | maximum displacement from mean position | m |
| Frequency (f) | number of vibrations per second | hertz (Hz) |
| Time period (T) | time for one vibration; $T=1/f$ | s |
| Wavelength (λ) | distance between two consecutive compressions | m |
| Wave speed (v) | $v=f\lambda$ | m s⁻¹ |
3. Characteristics of a Musical Sound
| Characteristic | Depends on | Description |
|---|---|---|
| Loudness | amplitude (also area of source, distance, density of medium) | larger amplitude → louder; measured in decibel (dB) |
| Pitch (shrillness) | frequency | higher frequency → higher pitch (a woman's/child's voice is shriller than a man's) |
| Quality (timbre) | waveform (number & strength of overtones) | lets us distinguish two instruments playing the same note at the same loudness |
- Musical sound is pleasant, regular and periodic; noise is unpleasant and irregular.
- Loudness is a subjective sensation; intensity (sound energy per unit area per second) is the related physical quantity.
4. Reflection of Sound
Sound obeys the laws of reflection (angle of incidence = angle of reflection; both rays and normal in one plane). A hard, large surface reflects sound well.Echo: a distinct reflected sound heard after the original.
- Heard only if the reflector is far enough that the reflected sound returns after ≥ 0.1 s (the persistence of hearing). With sound speed ≈ 340 m s⁻¹, the minimum distance is
- Echo / depth (SONAR) formula: $\;\text{distance}=\dfrac{v\times t}{2}\;$ (sound travels to and back).
Reverberation: persistence of sound due to repeated reflections in a closed hall (sound heard even after the source stops). Excessive reverberation is reduced using sound-absorbing materials (curtains, carpets, perforated boards, false ceilings).
Uses of reflection: megaphone/speaking tube, ear trumpet, stethoscope, SONAR (depth of sea, locating submarines/shoals), ultrasound scanning.
5. Vibrations & Resonance
- Free (natural) vibration: a body vibrating at its own natural frequency with constant amplitude (ideal, only in vacuum).
- Damped vibration: amplitude decreases with time due to friction/air resistance.
- Forced vibration: a body vibrates under an external periodic force at the forcing frequency.
- Resonance: a special case of forced vibration where the applied frequency equals the natural frequency, giving a large-amplitude response.
- Examples of resonance: a vibrating tuning fork setting another identical fork ringing; resonance in air columns; pushing a swing in rhythm; soldiers breaking step on a bridge; tuning a radio receiver.
6. Range of Hearing & Ultrasonics
- Audible range: 20 Hz to 20 000 Hz for humans.
- Infrasonic (< 20 Hz): produced by large vibrating bodies (earthquakes); sensed by some animals (elephants, whales).
- Ultrasonic (> 20 kHz): produced/heard by bats, dogs, dolphins. Uses: SONAR, echo-depth sounding, medical scanning, breaking kidney stones, cleaning fine machinery, detecting flaws (cracks) in metals, ultrasonic welding.
7. Worked Numerical Examples (ICSE pattern)
Q1. A person claps and hears the echo from a cliff after 3 s. Speed of sound = 340 m s⁻¹. How far is the cliff? Solution: $d=\dfrac{v\,t}{2}=\dfrac{340\times3}{2}=\mathbf{510\ m}$.Q2. A SONAR pulse returns from the sea-bed in 4 s. Speed of sound in water = 1500 m s⁻¹. Find the depth. Solution: $d=\dfrac{1500\times4}{2}=\mathbf{3000\ m}$.
Q3. What is the minimum distance from a wall to hear a distinct echo (v = 340 m s⁻¹)? Solution: $d=\dfrac{340\times0.1}{2}=\mathbf{17\ m}$.
Q4. A tuning fork of frequency 480 Hz produces sound of wavelength 0.7 m. Find the speed of sound. Solution: $v=f\lambda=480\times0.7=\mathbf{336\ m\,s^{-1}}$.
Q5. A sound wave travels 660 m in 2 s. If its frequency is 330 Hz, find its wavelength. Solution: $v=660/2=330$ m s⁻¹; $\lambda=v/f=330/330=\mathbf{1\ m}$.
8. Key Terms — Quick Glossary
| Term | One-line definition |
|---|---|
| Sound | energy from a vibrating body; a longitudinal wave. |
| Amplitude | maximum displacement of a particle from its mean position. |
| Frequency | vibrations per second (Hz). |
| Pitch | sensation of shrillness; depends on frequency. |
| Loudness | sensation of intensity; depends on amplitude (dB). |
| Quality | property distinguishing two sounds of same pitch/loudness; depends on waveform. |
| Echo | a distinct reflected sound (reflector ≥ 17 m away). |
| Reverberation | persistence of sound by repeated reflections in a hall. |
| Resonance | large vibration when forcing frequency = natural frequency. |
| Ultrasonic | sound above 20 kHz. |
9. Common Mistakes to Avoid
- Saying sound can travel through vacuum — it cannot (needs a medium).
- Mixing up the characteristics: pitch↔frequency, loudness↔amplitude, quality↔waveform.
- Forgetting the ÷2 in echo/SONAR problems (sound goes there and back).
- Confusing echo (one distinct reflection) with reverberation (overlapping reflections).
- Calling sound a transverse wave — it is longitudinal.
10. Likely Exam Questions (with crisp answers)
- How is sound produced and propagated? → By a vibrating body; as a longitudinal wave through a material medium.
- Why can't sound travel through vacuum? → There are no particles to transmit compressions/rarefactions.
- In which medium does sound travel fastest? → Solids.
- Name the three characteristics of a musical sound and what each depends on. → Loudness (amplitude), pitch (frequency), quality (waveform).
- Distinguish a musical sound from noise. → Musical sound is regular/pleasant; noise is irregular/unpleasant.
- State the condition for hearing a distinct echo. → Reflector ≥ 17 m away (time gap ≥ 0.1 s).
- What is reverberation; how is it reduced? → Persistence by repeated reflections; reduced with sound-absorbing materials.
- Define resonance with one example. → Forced vibration when applied frequency = natural frequency; e.g. two identical tuning forks.
- Why do soldiers break step while crossing a bridge? → To avoid resonance that could build dangerous oscillations.
- State the audible range of frequency. → 20 Hz to 20 000 Hz.
- Define ultrasonic waves; give two uses. → Sound above 20 kHz; SONAR and medical scanning.
- Why does the same note sound different on a flute and a violin? → Different waveform (quality/timbre).
- Write the relation between speed, frequency and wavelength. → $v=f\lambda$.
11. Loudness — factors in detail
Loudness of a sound increases with: (i) larger amplitude of vibration, (ii) larger surface area of the vibrating body (a tuning fork sounds louder on a sound-box/table), (iii) shorter distance from the source, (iv) higher density of the medium, and (v) wind blowing from source to listener. The unit of loudness level is the decibel (dB); sounds above ~120 dB are painful.12. Difference Tables
| Echo | Reverberation |
|---|---|
| distinct, separately-heard reflection | overlapping, prolonged reflections |
| reflector ≥ 17 m away | reflections within a closed hall |
| gap ≥ 0.1 s | continuous, no distinct gap |
| Music | Noise |
|---|---|
| regular, periodic waveform | irregular, aperiodic |
| pleasant | unpleasant |
| e.g. flute, veena | e.g. traffic, machinery |
13. More Worked Numericals
Q6. A boy fires a gun near a cliff and hears the echo after 1.5 s. If the cliff is 255 m away, find the speed of sound. Solution: $v=\dfrac{2d}{t}=\dfrac{2\times255}{1.5}=\mathbf{340\ m\,s^{-1}}$.Q7. A tuning fork has a time period of 0.0025 s. Find its frequency. Is it audible? Solution: $f=1/T=1/0.0025=\mathbf{400\ Hz}$ — yes, within 20 Hz–20 kHz.
Q8. A ship sends an ultrasonic pulse; the echo from the sea-bed returns in 0.8 s (v = 1500 m s⁻¹). Find the depth. Solution: $d=\dfrac{1500\times0.8}{2}=\mathbf{600\ m}$.
Q9. Sound of frequency 170 Hz has wavelength 2 m in air. Find the speed of sound in air. Solution: $v=f\lambda=170\times2=\mathbf{340\ m\,s^{-1}}$.
14. More Exam Questions (with crisp answers)
- On what factors does the loudness of sound depend? → Amplitude, area of source, distance, density of medium.
- Why is the sound of a tuning fork louder when held on a table? → The large surface forces more air to vibrate (forced vibration), increasing loudness.
- Define time period and frequency; relate them. → Time for one vibration; vibrations per second; $f=1/T$.
- Why is an echo not heard in a small room? → The wall is < 17 m away, so the reflected sound returns within 0.1 s and merges with the original.
- What type of wave is a sound wave? Name its parts. → Longitudinal; compressions and rarefactions.
- State two medical uses of ultrasound. → Imaging/scanning (sonography); breaking kidney stones.
- Why does sound travel faster in summer than in winter? → Speed of sound increases with temperature.
15. Speed of Sound in Different Media (approx, room temp)
| Medium | Speed (m s⁻¹) |
|---|---|
| Air (0 °C) | 332 |
| Air (20 °C) | 343 |
| Water | ~1500 |
| Steel / iron | ~5000 |
16. Why Sound Needs a Medium — the bell-jar experiment
An electric bell ringing inside a glass jar is heard normally. As the air is pumped out, the sound fades; in near-vacuum it is almost inaudible even though the hammer is seen striking. This proves sound cannot travel through vacuum — it needs particles to carry the compressions and rarefactions.17. Final Quick-Revision Q&A
- Why is an echo heard in a big empty hall but not a furnished one? → Furnishings absorb sound, reducing reflection/reverberation.
- Two sounds have the same loudness and pitch but sound different — which property differs? → Quality (timbre)/waveform.
- Can two astronauts on the Moon talk directly? → No — no atmosphere (vacuum), so sound cannot travel; they use radio.