Sound – Case-based Questions with Answers
Physics — Chapter 12: Sound
An artist stands near a canyon and shouts. After 0.22 s she hears a reflected sound distinct from her shout.
1. Distance = v × time = 340 × 0.22 = 74.8 m (round-trip). So one-way distance to reflecting surface ≈ 37.4 m.
2. Yes — since delay 0.22 s > 0.1 s threshold, the reflected sound is perceived as an echo. NCERT states that reflections heard after ≈0.1 s are distinct echoes; shorter delays cause reverberation.
Two tuning forks of frequencies 256 Hz and 259 Hz are struck near each other. A listener notices periodic variation in loudness.
1. Beat frequency = |259 − 256| = 3 Hz, so loudness varies 3 times per second.
2. To tune, adjust one fork until beats disappear (beat frequency → 0), indicating matching frequencies. This uses NCERT beat concept: beats indicate small frequency differences.
In a dome-shaped room a whisper at one wall can be heard clearly on the opposite side. The room has curved, smooth surfaces.
1. Curved smooth surfaces act as reflectors focusing sound along predictable paths (law of reflection). The geometry guides reflected waves to distant points, creating a 'whispering gallery' effect.
2. To reduce effect, add sound-absorbing panels or irregular surfaces to scatter and absorb reflections, preventing focused transmission—consistent with NCERT on controlling reverberation using absorbents.
A diver taps a metal rod; a friend listening underwater perceives sound much sooner than someone in air nearby.
1. Speed of sound in water (~1480 m·s−1) is much greater than in air (~340 m·s−1), so underwater listener receives waves earlier. Sound transmits faster in denser, more elastic media.
2. Steel (solid) transmits fastest among the three (≈ 5000 m·s−1) because solids have higher elasticity and closely packed particles, allowing rapid transfer of disturbances (NCERT principle: solids > liquids > gases).
A lecture hall has poor speech clarity due to long reverberation. Students complain they cannot understand the speaker.
1. Causes: many hard reflective surfaces and large enclosed volume causing multiple reflections that blend into prolonged sound.
2. Remedies: install sound-absorbing materials (curtains, carpets, acoustic panels) and add diffusers or irregular surfaces to scatter sound, reducing reverberation time and improving clarity.
While standing by the road, Priya notices the pitch of an ambulance siren rise as it approaches and fall as it moves away.
1. As the ambulance approaches, successive wavefronts are emitted from positions closer to the observer, shortening the wavelength and increasing observed frequency (higher pitch). As it recedes, wavelength lengthens, lowering observed frequency (lower pitch).
2. Doppler effect changes perceived frequency (pitch), not the actual speed of sound in the medium — speed remains determined by medium properties (NCERT concept).
Two sections of an orchestra play the same note but slightly out of tune, producing a throbbing sound.
1. Phenomenon: beats; beat frequency = |f1 − f2|. The throbbing rate equals this difference.
2. Players adjust tuning (tightening/loosening strings, changing breath/embouchure) to reduce frequency difference; minimizing beat frequency yields a steady tone.
A research vessel emits a sonar pulse; echo returns after 2.5 s. Speed of sound in seawater is 1500 m·s−1.
1. Round-trip time Δt = 2.5 s ⇒ distance = c × Δt = 1500 × 2.5 = 3750 m round-trip. Depth = 3750/2 = 1875 m.
2. Accuracy affected by variations in sound speed due to temperature, salinity and pressure (depth); also by oblique paths and seabed slope.
Students use a resonance tube and a tuning fork of frequency 512 Hz to find air column length for resonance. They find first resonance at 0.167 m (closed tube).
1. λ = 4L = 4 × 0.167 = 0.668 m. v = f λ = 512 × 0.668 ≈ 341.0 m·s−1.
2. Speed of sound v changes with temperature (v ≈ 331 + 0.6T). Measured resonance length L for same frequency will change slightly because λ and hence L depend on v.
Workers in a factory are exposed to continuous loud noise (~95 dB). Management wants to protect hearing.
1. 95 dB corresponds to high intensity that can damage hair cells in the cochlea with prolonged exposure, leading to permanent hearing loss (NCERT: loud sounds damage hearing).
2. Measures: provide ear protection (earplugs/muffs) and implement engineering controls (noise-damping materials, quieter machinery, sound enclosures). Also limit exposure time and rotate shifts.
An architect must design a music hall for orchestral performance which requires richer reverberation than a lecture hall.
1. Music benefits from longer reverberation (to enrich tone), while speech requires shorter reverberation for clarity. Design balances diffusion and selective absorption.
2. To increase reverberation: use reflective hard surfaces and wooden panels. To decrease reverberation: use absorbent materials like heavy curtains, carpets and acoustic foam.
An engineer uses ultrasound pulses to detect cracks inside a metal component. Reflected pulses indicate defects.
1. Ultrasound has short wavelengths at high frequencies, enabling detection of small defects whose size is comparable to wavelength; reflections from interfaces reveal discontinuities.
2. Limitation: poor coupling through rough surfaces or complex geometries; requires skill and suitable transducers, and may not work well near highly attenuating materials.
A student claps at a wall and measures time between clap and echo as 0.3 s. Using v = 340 m·s−1, student estimates distance to wall.
1. Round-trip distance = v × t = 340 × 0.3 = 102 m ⇒ one-way distance = 51 m.
2. Time uncertainty ±0.02 s ⇒ distance uncertainty (one-way) = (340 × 0.02)/2 = 3.4 m. So distance = 51 ± 3.4 m.
A tuning fork of frequency 512 Hz is held over an empty resonator box and the sound becomes louder.
1. The box acts as a resonant cavity; it couples efficiently to the tuning fork's frequency, increases radiating area and amplifies the sound by constructive interference and energy transfer.
2. Resonance allows the air column or cavity modes to vibrate at the fork’s natural frequency producing larger amplitude (loudness) than the fork alone.
A school conducts hearing tests and finds some students require louder tones at high frequencies.
1. Higher threshold at high frequencies suggests partial hearing loss, often due to exposure to loud sounds or age-related damage affecting hair cells sensitive to high frequencies.
2. Preventive measures: reduce exposure to loud noises (limit school loud events, control volume of PA systems), and teach safe headphone use; provide periodic hearing screenings.
A music teacher wants to soundproof a practice room to prevent leakage of sound to neighbouring rooms.
1. Use dense mass-loaded barriers (thicker walls, double glazing) and add absorptive linings (acoustic foam, carpets). Decouple surfaces to reduce structural transmission.
2. Absorption converts sound energy to heat inside a room (reducing reverberation), while insulation (soundproofing) blocks transmission of sound to other spaces (reducing leakage).
Students measure frequency 256 Hz and wavelength 1.33 m in an experiment.
1. v = f λ = 256 × 1.33 ≈ 340.48 m·s−1.
2. Percentage errors add for multiplication: ≈ 2% + 1% = 3% ⇒ v error ≈ 3%.
An ultrasonic cleaner uses high-frequency sound to clean jewellery by cavitation.
1. Ultrasound generates rapid pressure variations causing microscopic cavitation bubbles in liquid; their collapse produces micro-jets that dislodge dirt from surfaces, even in crevices.
2. Advantage: reaches small crevices and cleans uniformly without abrasive action, reducing damage to delicate items.
A pedestrian hears a speaker around a corner while the source is out of direct sight.
1. Sound waves bend (diffract) around edges; if wavelength is comparable to obstacle size, significant diffraction allows waves to reach shadowed regions.
2. Low frequencies (long wavelengths) diffract more and are more likely to be heard around obstacles; high frequencies are more easily blocked or absorbed.
An engineer must design a siren audible over a wide area with minimal distortion.
1. Prioritise mid-to-low frequencies for range (low frequencies travel farther) and sufficiently high amplitude (intensity) for audibility. A range of frequencies (wideband) helps penetrate different environments and listeners.
2. Use intermittent or frequency-modulated signals that stand out from background noise, and select frequencies that avoid common urban noise bands; also increase peak amplitude while respecting hearing safety for nearby people.