• Sound = longitudinal mechanical wave; needs a medium; travels as alternate compressions & rarefactions.
• Graph view: Pressure/density vs distance shows crests (high pressure) and troughs (low pressure). Symbols: \( \lambda, \nu, A, T \).
• Pitch ↔ frequency; Loudness ↔ amplitude.
• Speed relation: \(v=\lambda\,\nu\). In gases \(v\propto\sqrt{T}\), \(v\propto\dfrac{1}{\sqrt{\rho}}\), \(v\propto\dfrac{1}{\sqrt{M}}\).
• Speed trend: highest in solids, then liquids, least in gases; increases with temperature.
• Audible range: 20 Hz–20 kHz; Infra < 20 Hz; Ultra > 20 kHz. Bats/dolphins can produce & detect ultrasound.
• Reflection of sound: Follows laws like light. Hard, flat surfaces = good reflectors; soft/porous = absorbers.
• Echo: distinct only if reflected sound returns after ≥ 0.1 s. At ~\(22^\circ\text{C}\), min reflector distance ≈ \(17.2\) m.
• Reverberation: multiple reflections overlap—reduce by curtains, carpets, acoustic panels, irregular surfaces.
• SONAR: uses ultrasound; measures time gap between pulse & echo: \( \text{distance}=\dfrac{vt}{2} \).
• Sonography: medical imaging using ultrasound; safe and painless; (use ethically per PNDT Act).
• Human Ear: Outer (pinna) → Middle (eardrum & ossicles) → Inner (cochlea → auditory nerve → brain).

1) What kind of wave is sound in air?

Mechanical longitudinal wave.

2) What is the relation between speed, wavelength and frequency?

\(v=\lambda\,\nu\).

3) Which property decides loudness?

Amplitude.

4) Which property decides pitch?

Frequency.

5) Can sound travel in vacuum?

No.

6) Speed of sound is maximum in which state?

Solids.

7) Audible range for humans?

20 Hz to 20 kHz.

8) Frequencies < 20 Hz are called?

Infrasound.

9) Frequencies > 20 kHz are called?

Ultrasound.

10) What is echo?

Reflected sound heard distinctly after the original.

11) Minimum distance for distinct echo at ~\(22^\circ\)C?

\(\,\approx 17.2\) m.

12) Which surfaces are good sound reflectors?

Hard, flat surfaces (walls, rock, metal).

13) Full form of SONAR?

Sound Navigation And Ranging.

14) Device using ultrasound in medicine?

Sonography (Ultrasound imaging).

15) Organ in inner ear converting vibrations to signals?

Cochlea.

16) Which animals navigate with ultrasound?

Bats and dolphins.

17) On what does reverberation depend?

Multiple reflections and poor absorption in a hall/room.

18) What is the typical brain persistence time for sound?

About 0.1 s.

19) Unit of frequency?

Hertz (Hz).

20) Does sound speed depend on gas pressure (at fixed T)?

No, essentially independent of pressure at fixed temperature.

1) Define compression and rarefaction.

Compression: high pressure/density region; Rarefaction: low pressure/density region.

2) What is wavelength (λ)?

Distance between successive compressions (or rarefactions).

3) What is time period (T)?

Time for one complete oscillation.

4) Relation between \(T\) and \(\nu\)?

\(\nu=\dfrac{1}{T}\).

5) How does speed vary with temperature in gases?

\(v\propto\sqrt{T}\) (increases with \(T\)).

6) How does speed vary with density?

\(v\propto \dfrac{1}{\sqrt{\rho}}\).

7) What is reverberation?

Overlapping echoes due to multiple reflections causing persistence of sound.

8) Give one way to reduce reverberation.

Use curtains, carpets, acoustic panels, cushioned seats.

9) State the law of reflection for sound.

Angle of incidence = angle of reflection; all in the same plane.

10) Why are soft materials poor reflectors?

They absorb sound energy, converting it to heat.

11) What does SONAR measure?

Direction, distance, sometimes speed of underwater objects.

12) Echo time gap needed for distinct echo?

\(\ge 0.1\) s.

13) Write distance formula with echo time \(t\).

\(d=\dfrac{vt}{2}\).

14) Audible limit of a small child can extend up to?

≈ 25 kHz.

15) Which ear part vibrates first?

Eardrum (tympanic membrane).

16) Which three tiny bones are in the middle ear?

Malleus, Incus, Stapes (ossicles).

17) What carries signals from ear to brain?

Auditory nerve.

18) Why dangerous to play loud music in earphones?

May damage eardrum and hearing.

19) Example of infrasound in nature?

Pre-earthquake crust vibrations; pendulum swish.

20) Example of ultrasound device at home/hospital?

Ultrasonic cleaner; medical ultrasound (sonography).

1) Explain why sound cannot travel in vacuum.

Sound needs particle-to-particle vibration transfer; vacuum has no particles, so no propagation.

2) Compare longitudinal and transverse waves with examples.

Longitudinal: oscillations ∥ propagation (sound in air). Transverse: oscillations ⟂ propagation (water surface ripples).

3) Derive \(v=\lambda\nu\) briefly.

One wavelength travels in one time period \(T\): \(v=\dfrac{\lambda}{T}=\lambda\nu\).

4) Why is speed of sound highest in solids?

Particles are closely packed and strongly coupled, allowing faster disturbance transmission.

5) State two everyday evidences that light travels faster than sound.

Lightning seen before thunder; fireworks flash before bang.

6) Why are hard walls good echo surfaces?

They reflect most incident sound with little absorption.

7) Conditions for distinct echo.

Reflector at distance ≥ \(17.2\) m (≈ at \(22^\circ\)C), large hard surface, quiet surroundings.

8) What is reverberation and one harmful effect?

Persistence due to multiple reflections; causes poor speech intelligibility.

9) List two uses of ultrasound.

Medical imaging (sonography); industrial flaw detection/cleaning.

10) How does SONAR find depth?

Sends pulse, measures round-trip time \(t\); depth \(=\dfrac{vt}{2}\).

11) Why are theatre ceilings curved?

To reflect and distribute sound towards the audience (with acoustic treatment to control echoes).

12) Why empty rooms feel ‘echoey’?

Lack of absorbers (curtains/furniture) increases multiple reflections → higher reverberation.

13) Name ear parts and their roles.

Outer: collects sound; Middle: transmits via ossicles; Inner: cochlea converts to nerve signals.

14) Why can bats fly in dark?

Ultrasound echolocation—detect obstacles from echoes.

15) How does speed vary with molecular weight?

\(v\propto \dfrac{1}{\sqrt{M}}\): lighter gases (H₂) conduct sound faster than heavier (O₂).

16) Write temperature dependence of sound speed.

\(v\propto\sqrt{T}\) (Kelvin). Approx. \(v\) increases ~\(0.6\,\text{m/s}\) per \(1^\circ\)C in air.

17) Why carpets and panels in studios?

To absorb reflections and reduce reverberation for clear recording.

18) What precautions while using ultrasound?

Use medically justified procedures; follow safety/ethical norms (e.g., PNDT Act).

19) Show that in hydrogen \(v\) is four times oxygen at same \(T\).

\(v\propto 1/\sqrt{M}\). \(M_{\text{O}_2}=32\), \(M_{\text{H}_2}=2\) → \(v_H/v_O=\sqrt{32/2}=4\).

20) Why clocks sound ‘sharper’ through metal rail than through air?

Higher speed and lower loss in solids; vibrations reach earlier and stronger.

1) Fill in the blanks and explain.

a) Sound does not travel through vacuum (needs a medium).
b) Speed of sound in steel is greater than the speed of sound in water.
c) We see lightning before hearing thunder → \(v_{\text{sound}} \ll v_{\text{light}}\).
d) To discover objects deep in sea, SONAR technology is used.

2) Explain giving scientific reasons.

a) Theatre/conference hall roofs are often curved so reflected sound can be directed towards the audience; combined with acoustic treatment to avoid harsh echoes.
b) In a closed, empty house, there are fewer absorbers (curtains/furniture), so reverberation intensity is higher.
c) In a classroom, typical distances are small; reflected sound returns within \(\lt 0.1\) s, so no distinct echo (it merges with the original).

3) Answer the following.

a) Echo: distinct repetition of sound due to reflection. For a clear echo: hard large surface, sufficient distance (≈ \(\ge 17.2\) m at \(22^\circ\)C), quiet surroundings, appropriate temperature/humidity.

b) Gol Gumbaz, Vijayapura (Vijapur): Its vast dome and smooth surfaces cause multiple reflections; echoes can be heard several times due to long path lengths and low absorption.

c-1) Nita hears thunder 4 s after lightning. Distance \(= v\times t = 340\,\text{m/s}\times 4\,\text{s} = \boxed{1360\ \text{m}}\).

c-2) Sunil between two walls; first echo after 4 s, second after 6 s (another 2 s later). Using textbook’s results: speed in air \(\boxed{330\ \text{m/s}}\), and distance between walls \(\boxed{1650\ \text{m}}\). (Method: \(d=\tfrac{vt}{2}\); first echo gives nearer-wall distance; second echo gives farther-wall distance; sum gives separation.)

4) What classroom dimensions/shape avoid echoes? Where are absorbing materials used?

Use moderate room volumes with non-parallel, irregular or acoustically treated surfaces; add curtains, notice boards, soft boards, pin-ups to absorb sound. Absorbing materials are used in studios, theatres, lecture halls, libraries to reduce reverberation and improve clarity.

5) Solve the following.

a) Speed in air at \(0^\circ\)C is 332 m/s; increases \(0.6\,\text{m/s}/^\circ\)C. For 344 m/s, increase \(=12\) m/s → \(T=\dfrac{12}{0.6}=\boxed{20^\circ\text{C}}\).

b) Helium in bottles A (10 g) and B (40 g) with same speed of sound. Since \(v\propto \sqrt{T/M}\) and gas is same (\(M\) equal), equal speeds imply \(T_B = 4T_A\). Conclusion: \(\boxed{\text{Temperature of B is 4 times that of A}}\).

• Speed–wavelength–frequency: \( \displaystyle v=\lambda\,\nu \)
• Temperature dependence (gas): \( \displaystyle v\propto\sqrt{T} \)
• Density / molecular weight: \( \displaystyle v\propto \frac{1}{\sqrt{\rho}},\quad v\propto \frac{1}{\sqrt{M}} \)
• Echo distance: \( \displaystyle d=\frac{vt}{2} \)