Sound — Set 4

Correct Answer: A. Amplitude squared

• **Amplitude squared** = sound intensity I = ½ρvω²A², directly proportional to A²; doubling amplitude quadruples intensity. • **+6 dB level increase** corresponds to doubling amplitude (since 20 log₁₀(2) ≈ 6 dB for amplitude ratios). • 💡 Wrong-option analysis: Wavelength only: wavelength is inversely proportional to frequency for a fixed speed — not a determinant of intensity; Time period only: inverse of frequency — affects pitch, not intensity; Frequency squared only: I is proportional to ω² = (2πf)² and A² — not just f² alone.

Correct Answer: D. Intensity is physical, loudness is perceived

• **Intensity is physical, loudness is perceived** = intensity (W/m²) is an objective measurable quantity; loudness is the subjective psychological response of the human auditory system to that intensity. • **Equal-loudness contours (Fletcher–Munson)** show that the same intensity at different frequencies produces different loudness — the ear is not equally sensitive at all frequencies. • 💡 Wrong-option analysis: Loudness is physical, intensity is psychological: this is the reverse — intensity is objective/physical; Both are the same always: they differ because human hearing is frequency-dependent; Intensity is subjective, loudness is objective: reversed again.

Correct Answer: C. Difference in timbre

• **Difference in timbre** = two instruments playing the same fundamental frequency at the same loudness differ in the harmonic content (overtones) and envelope — these together define timbre. • **Waveform analysis**: a violin produces rich odd/even harmonics; a flute produces predominantly the fundamental — different timbres despite same pitch. • 💡 Wrong-option analysis: Difference in vacuum level: instruments play in air — vacuum is irrelevant; Difference in gravitational force: gravity does not affect the harmonic content of sound; Difference in electric charge: acoustic instruments are not driven by electric charge differences.

Correct Answer: B. Unwanted or unpleasant sound

• **Unwanted or unpleasant sound** = noise is defined contextually as sound that is undesired, disturbing, or harmful — it can be any frequency, not just low or high. • **Noise-induced hearing loss (NIHL)** occurs at exposures > 85 dB(A) over 8 hours; the frequency range causing damage is 2–8 kHz. • 💡 Wrong-option analysis: Sound that travels in vacuum: sound cannot travel in vacuum; Any high-frequency sound only: noise is context-dependent, not defined by high frequency alone; Any low-frequency sound only: infrasound can also be noise, but noise is not defined as low-frequency only.

Correct Answer: A. Supersonic motion creating shock waves

• **Supersonic motion creating shock waves** = when an object exceeds the speed of sound, pressure waves pile up into a conical shock wave (Mach cone) — as this cone sweeps past an observer, the sudden pressure change is heard as a sonic boom. • **Mach angle**: sin θ = v_sound / v_object — a sharp crack rather than a gradual sound change. • 💡 Wrong-option analysis: Only resonance in a pipe: organ pipe resonance produces standing waves — not shock waves; Subsonic motion: below the speed of sound, no shock front forms; Only echo in a hall: an echo is a simple reflection — not the pressure-wave pile-up of a sonic boom.

Correct Answer: A. Sending sounds and analyzing reflected echoes

• **Sending sounds and analyzing reflected echoes** = bats emit ultrasonic pulses (20–100 kHz) and process the returning echoes to compute distance, size, and velocity of objects — a biological sonar. • **Time resolution**: bats can detect echo delays as small as ~1 µs, allowing location of objects to within millimetres. • 💡 Wrong-option analysis: Creating only infrasound always: bats use ultrasound (above 20 kHz), not infrasound (below 20 Hz); Measuring gravity changes: gravitational sensing is not how bats navigate; Producing light and measuring reflection: bats are essentially blind in the sense that they do not use vision for echolocation.

Correct Answer: B. Reflection from tissue boundaries

• **Reflection from tissue boundaries** = when ultrasound encounters an interface between tissues with different acoustic impedances (Z = ρv), a fraction reflects back — the transducer detects these echoes to reconstruct a 2D image. • **Acoustic impedance mismatch**: the larger the impedance difference, the stronger the echo — soft tissue/bone interfaces give strong reflections. • 💡 Wrong-option analysis: Polarization of sound: ultrasound is longitudinal in tissue — cannot be polarized; Absorption of sound into bone only: absorption does occur but image formation relies on reflection at all tissue boundaries, not just bone; Sound traveling in vacuum: soft tissue and organs are not vacuum.

Correct Answer: D. Blood flow speed by frequency shift

• **Blood flow speed by frequency shift** = moving blood cells reflect ultrasound at a shifted frequency (Doppler shift Δf = 2fv cos θ / c); the shift magnitude and sign reveal flow speed and direction. • **Clinical use**: Doppler ultrasound detects stenoses (narrowing causes higher velocity) and valve regurgitation in cardiac imaging. • 💡 Wrong-option analysis: Temperature of organs: temperature is not measured by ultrasound Doppler — infrared or MRI is used; Electrical resistance of skin: this is measured by impedance plethysmography, not Doppler ultrasound; Light intensity in veins: optical methods like pulse oximetry do this — not Doppler ultrasound.

Correct Answer: A. Add airtight seals and mass

• **Add airtight seals and mass** = sound transmission through a door is reduced by (a) mass — heavier doors vibrate less (mass law: each doubling of mass reduces transmission by ~6 dB) and (b) sealing air gaps that act as acoustic short circuits. • **STC (Sound Transmission Class)** of a well-sealed heavy door can reach 45–55, compared to ~20 for an unsealed hollow door. • 💡 Wrong-option analysis: Increase room temperature greatly: temperature changes sound speed slightly but does not block transmission; Use lighter materials only: lighter materials vibrate more easily — worse insulation; Remove all curtains from the room: curtains are absorbers, not insulators — removing them would increase reverberation.

Correct Answer: A. Malleus, incus, stapes

• **Malleus, incus, stapes** = the three ossicles of the middle ear form a mechanical lever system that transmits and amplifies vibrations from the eardrum to the oval window of the cochlea. • **Mechanical advantage** of the ossicular chain ≈ 1.3× force; combined with the eardrum-to-oval-window area ratio (~17:1), the pressure gain is ~25 dB. • 💡 Wrong-option analysis: Ribs, vertebrae, pelvis: these are bones of the trunk — no role in hearing; Radius, ulna, humerus: these are bones of the arm — completely unrelated to the ear; Femur, tibia, fibula: these are bones of the leg — not in the ear.