Sound — Set 2

Correct Answer: B. Frequency

• **Frequency** = pitch is the subjective perception of how high or low a sound is, and it correlates directly with the frequency of the sound wave. • **Higher frequency → higher pitch**: a 440 Hz tone (A4) sounds higher than a 220 Hz tone (A3). • 💡 Wrong-option analysis: Amplitude: controls loudness, not pitch; Speed: a property of the medium — does not directly determine pitch; Intensity only: a physical measure of energy flow per unit area — related to loudness, not pitch.

Correct Answer: D. Longitudinal waves with compressions and rarefactions

• **Longitudinal waves with compressions and rarefactions** = in air, sound causes particles to vibrate parallel to the direction of propagation, creating alternating regions of high pressure (compressions) and low pressure (rarefactions). • **Wavelength in air at 343 m/s, 1 kHz** = 0.343 m — each compression–rarefaction cycle spans this distance. • 💡 Wrong-option analysis: Stationary electric waves: these are electromagnetic, not mechanical sound waves; Light waves only: light is transverse electromagnetic — entirely different from sound; Transverse waves with crests and troughs: transverse waves (e.g., on strings) have crests/troughs but sound in air does not — particles move longitudinally.

Correct Answer: D. A nearly pure tone with few overtones

• **A nearly pure tone with few overtones** = a tuning fork vibrates predominantly at its fundamental frequency with almost no harmonics, making it ideal as a reference tone. • **Fork frequency** is stamped on it (e.g., 440 Hz for A4) — its highly symmetric tines suppress overtones. • 💡 Wrong-option analysis: Only ultrasonic waves: standard tuning forks produce audible frequencies (100–5000 Hz range); Many random frequencies together: that describes noise — the opposite of a tuning fork; Only infrasonic waves: tuning fork frequencies are well within the audible range.

Correct Answer: C. Persistence of sound due to multiple reflections

• **Persistence of sound due to multiple reflections** = reverberation is the prolongation of sound after the source stops, caused by repeated reflections from walls, ceiling, and floor. • **Reverberation time (RT60)**: time for sound to decay by 60 dB — concert halls target RT60 ≈ 1.8–2.2 s for music. • 💡 Wrong-option analysis: Only refraction of sound: refraction changes direction in different media — not sustained persistence; Only Doppler shift: frequency change due to motion — not persistence after source stops; Complete absorption of sound: absorption ends sound rather than prolonging it.

Correct Answer: A. Soft porous materials like curtains

• **Soft porous materials like curtains** = fibrous, open-cell materials trap air, cause multiple reflections within the material, and convert sound energy to heat through viscous losses. • **Absorption coefficient** of heavy curtains ≈ 0.35–0.50 at mid-frequencies — far higher than smooth hard surfaces (~0.01–0.05). • 💡 Wrong-option analysis: Smooth glass sheet: specular surface — reflects almost all sound; Bare metal plate: rigid and smooth — excellent reflector, very poor absorber; Highly polished marble: hard, dense, smooth — near-zero absorption.

Correct Answer: B. Guides body sounds through tubes with less loss

• **Guides body sounds through tubes with less loss** = a stethoscope uses enclosed air columns and flexible tubing to channel faint sounds (heart, lungs) from the patient's body to the doctor's ears with minimal energy loss. • **Acoustic channelling**: the bell/diaphragm chest piece collects vibrations and the tubing preserves them — no ultrasound is involved. • 💡 Wrong-option analysis: Works only by producing ultrasound: a stethoscope amplifies ordinary audible sounds (~20–1000 Hz) — ultrasound is not generated; Converts sound into light: no such transduction occurs; Stops all sound reflections completely: the tubing actually manages reflections to preserve signal.

Correct Answer: A. Cavitation produced by high-frequency waves

• **Cavitation produced by high-frequency waves** = ultrasound at ~20–40 kHz creates rapid pressure oscillations in a liquid, nucleating and violently collapsing microscopic bubbles (cavitation) that produce intense local jets and shock waves to dislodge contaminants. • **Cleaning frequency** typically 20–400 kHz — used for jewellery, surgical instruments, and circuit boards. • 💡 Wrong-option analysis: Heating by visible light: visible light is not produced by ultrasound transducers; Reflection from a mirror surface only: reflection is a secondary effect — not the cleaning mechanism; Polarization of sound waves: sound is longitudinal in liquids — cannot be polarized.

Correct Answer: D. Time delay of echoes

• **Time delay of echoes** = SONAR sends an acoustic pulse and measures the time t for the echo to return; depth = (v × t) / 2, where v is the speed of sound in water (~1500 m/s). • **Round-trip timing**: a 1-second round trip implies depth of ~750 m — the factor of 2 accounts for the return journey. • 💡 Wrong-option analysis: Electrolysis: decomposition of water by electricity — completely unrelated to depth sounding; Thermal conduction: heat flow through a medium — no role in echo ranging; Magnetic induction: electromagnetic phenomenon — not used in acoustic depth measurement.

Correct Answer: A. Sound waves in air are longitudinal

• **Sound waves in air are longitudinal** = air molecules cannot sustain shear stresses, so sound waves in air must be longitudinal — particles vibrate along the direction of propagation. • **Contrast**: transverse waves (light, water surface ripples) have particle displacement perpendicular to propagation direction and can be polarized; sound in air cannot. • 💡 Wrong-option analysis: Sound waves in air can be polarized easily: polarization requires transverse waves — sound in air is longitudinal, so it cannot be polarized; Sound waves in air are transverse: incorrect — they are longitudinal; Sound waves in air show only crests: crests/troughs describe transverse waves — sound in air has compressions/rarefactions.

Correct Answer: B. Its elasticity and density

• **Its elasticity and density** = sound speed v = √(E/ρ), where E is the elastic modulus and ρ is density; higher elasticity → faster speed, higher density → slower speed. • **Newton–Laplace formula**: v = √(γP/ρ) for ideal gases — pressure and elasticity are interlinked, but the container shape is irrelevant. • 💡 Wrong-option analysis: Its color and transparency: optical properties — no effect on sound speed; Only its shape of container: container geometry can affect standing waves but not the intrinsic speed; Only its temperature regardless of medium: temperature affects speed in gases but not the fundamental determinants (E and ρ).