Sound — Set 1

Correct Answer: A. Sound cannot travel through a vacuum.

• **Sound cannot travel through a vacuum** = sound is a mechanical wave that needs particles to vibrate and transmit energy; no particles means no propagation. • **Speed in air** ≈ 343 m/s at 20°C — compared to light at 3×10⁸ m/s, showing sound is a slow mechanical disturbance. • 💡 Wrong-option analysis: Sound is produced only in a vacuum: production requires a vibrating source in a medium, not vacuum; Sound travels fastest in a vacuum: it cannot travel at all in vacuum; Sound becomes light in a vacuum: two entirely different phenomena — no conversion occurs.

Correct Answer: C. Hertz

• **Hertz (Hz)** = the SI unit of frequency, defined as one complete oscillation per second (1 Hz = 1 s⁻¹). • **1 kHz = 1000 Hz** — the human audible range spans 20 Hz to 20,000 Hz (20 kHz), covering a factor of 1000. • 💡 Wrong-option analysis: Joule: unit of energy — not frequency; Watt: unit of power (energy per second) — not frequency; Pascal: unit of pressure — not frequency.

Correct Answer: A. Amplitude of vibration

• **Amplitude of vibration** = loudness (a subjective sensation) correlates with the amplitude of the sound wave; larger amplitude means more energy. • **Intensity ∝ A²** — doubling amplitude quadruples intensity, noticeably increasing perceived loudness. • 💡 Wrong-option analysis: Wavelength: determines pitch only when frequency changes — not directly loudness; Speed of sound: a property of the medium, does not determine loudness; Time period: inverse of frequency — affects pitch, not loudness.

Correct Answer: A. Slightly different frequencies

• **Slightly different frequencies** = when two waves of closely-spaced frequencies f₁ and f₂ superpose, they alternately reinforce and cancel — producing periodic loudness variations called beats. • **Beat frequency = |f₁ − f₂|** — musicians use this to tune instruments: when beats disappear, the two sources are at the same frequency. • 💡 Wrong-option analysis: Exactly the same frequency: produces a steady sound with no beats; Exactly opposite phases always: constant destructive interference — silence, not beats; Very high frequencies only: beat production depends on frequency difference, not absolute value.

Correct Answer: A. Doppler effect

• **Doppler effect** = when a source or observer moves relative to the other, the observed frequency differs from the emitted frequency. • **Formula (source moving)**: f_obs = f × v / (v ∓ v_s) — a source moving toward the observer raises the observed pitch. • 💡 Wrong-option analysis: Diffraction: bending of waves around obstacles — does not change pitch; Refraction: change in direction when entering a new medium — not a pitch change; Resonance: vibration at natural frequency — unrelated to relative motion.

Correct Answer: C. Temperature increases

• **Temperature increases** = higher temperature means faster-moving gas molecules, so disturbances propagate more rapidly. • **Speed in air** ≈ 331 + 0.6T m/s (T in °C) — at 20°C it is ~343 m/s; at 0°C it is ~331 m/s. • 💡 Wrong-option analysis: Gas is replaced by vacuum: no medium means no sound propagation at all; Temperature decreases: slower molecules means lower sound speed; Pressure reduced at constant temperature: speed depends on √(γRT/M) — pressure cancels out, so no change.

Correct Answer: A. Solids

• **Solids** = sound speed depends on the medium's elastic modulus and density; solids have much higher elastic moduli than liquids or gases. • **Typical speeds**: steel ~5000 m/s, water ~1500 m/s, air ~343 m/s — illustrating the solid > liquid > gas order. • 💡 Wrong-option analysis: Gases: low elastic modulus and compressible — slowest; Liquids: intermediate elasticity and density — faster than gases; Vacuum: no particles — sound cannot travel at all.

Correct Answer: A. Reflection of sound

• **Reflection of sound** = an echo occurs when sound waves hit a large, hard surface and bounce back to the listener with a perceptible time delay. • **Minimum distance** for a distinct echo: ~17 m at 20°C (so the round-trip delay ≥ 0.1 s, the ear's resolution limit). • 💡 Wrong-option analysis: Absorption of sound: converts sound energy to heat — reduces sound, does not create an echo; Diffusion of sound: scattering in many directions — reduces echo clarity; Polarization of sound: sound is longitudinal in air — polarization is a transverse-wave concept.

Correct Answer: B. Equal to the natural frequency of the system

• **Equal to the natural frequency of the system** = resonance occurs when the driving frequency matches the system's natural (resonant) frequency, enabling maximum energy transfer. • **Amplitude** at resonance can be very large (limited only by damping) — this is exploited in musical instrument bodies. • 💡 Wrong-option analysis: Always twice the natural frequency: doubling the driving frequency does not generally produce resonance; Zero for the system: zero frequency means no oscillation — no resonance; Independent of the natural frequency: resonance is defined by frequency matching.

Correct Answer: A. Its waveform and overtones

• **Its waveform and overtones** = timbre (tone quality) is determined by the mixture of harmonics and the shape of the sound waveform. • **Fourier analysis** shows that every periodic sound is a sum of sine waves at the fundamental and its integer multiples — their relative amplitudes define the timbre. • 💡 Wrong-option analysis: Only its speed in air: sound speed is a medium property, same for all instruments — does not differentiate timbre; Only its wavelength: depends on speed and frequency — not the full picture; Only its amplitude: determines loudness, not tone quality.