Sound Waves and Acoustics

~8 min read

In 30 seconds

Sound: Longitudinal mechanical wave. Needs medium. Speed in air ≈ 343 m/s at 20°C.
Frequency: Audible: 20 Hz to 20,000 Hz. Below: infrasonic. Above: ultrasonic.
Doppler: Apparent frequency change due to source/observer motion. Higher when approaching, lower when receding.

Sound waves are pressure variations propagating through a medium. NDA tests longitudinal vs transverse, speed of sound, Doppler effect, and applications.

Nature of Sound

Sound is a longitudinal mechanical wave (vibration along direction of propagation).
Needs a material medium (solid, liquid, or gas) to travel. Cannot travel in vacuum.
Speed in various media (at 20°C):
- Air: ~343 m/s
- Water: ~1,500 m/s
- Steel: ~5,000-6,000 m/s
Speed increases with temperature in air (by ~0.6 m/s per °C).
Speed greater in denser/stiffer materials. Solids fastest; gases slowest.

Properties — Frequency, Pitch, Loudness

Frequency (Hz): Cycles per second. Determines pitch.
- Infrasonic: below 20 Hz (whales, earthquakes).
- Audible: 20 Hz - 20,000 Hz (~20 kHz).
- Ultrasonic: above 20 kHz (bats use ~20-100 kHz; medical ultrasound 1-15 MHz).
Amplitude: Determines loudness. Measured in decibels (dB).
Pitch: Subjective perception of frequency. High frequency = high pitch.
Timbre: Quality of sound. Lets us distinguish flute from violin even at same pitch and loudness.

Reflection — Echo and Reverberation

Echo: Distinct reflection of sound from a distant surface. To hear an echo, reflecting surface must be at least ~17 m away (so reflection arrives at least 0.1 s after original — minimum gap our ears can distinguish).
Reverberation: Multiple short overlapping reflections. Long reverberation in halls makes sound muddy.
SONAR (Sound Navigation and Ranging): Uses ultrasonic sound reflection underwater to detect depth/objects.
Bats: Echo-location to navigate and hunt. Dolphins similarly.

Doppler Effect

Apparent change in frequency due to relative motion between source and observer.
Source approaching: apparent frequency higher.
Source receding: apparent frequency lower.
Examples: Train whistle — higher when approaching, lower when receding. Police speed-radar uses Doppler. Astronomers use Doppler shift of light to measure star/galaxy velocities (redshift = receding; blueshift = approaching).

Resonance, Beats, Musical Instruments

Resonance: When driving frequency matches natural frequency. Why singing in a shower amplifies voice (bathroom resonates).
Beats: When two waves of slightly different frequencies superpose, the resultant amplitude varies — heard as periodic loud-soft pulsation. Beat frequency = |f₁ - f₂|.
String instruments (sitar, violin): wave on stretched string. Frequency depends on length, tension, mass per unit length.
Wind instruments (flute, clarinet): air column resonance. Frequency depends on length, end conditions (open/closed).

NDA PYQ Examples

Q: Sound cannot travel through:

(a) Air (b) Water (c) Iron (d) Vacuum

Answer: (d) Vacuum — needs a material medium.

Q: Audible range for normal humans is:

(a) 0-100 Hz (b) 20-20,000 Hz (c) 1-1000 Hz (d) 1 Hz - 1 MHz

Answer: (b) 20-20,000 Hz.

Q: Speed of sound is fastest in:

(a) Vacuum (b) Gas (c) Liquid (d) Solid

Answer: (d) Solid — closely packed molecules transmit vibrations faster.

Q: Bats navigate using:

(a) Infrasonic sound (b) Ultrasonic sound (c) Visible light (d) Smell

Answer: (b) Ultrasonic sound — echolocation.

Drill Sound Waves and Acoustics for NDA

NDA-pattern items on Sound Waves and Acoustics with answer keys and explanations.

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Frequently Asked Questions

Why is there no sound in space?

Sound is a mechanical wave that needs a material medium (solid, liquid, gas). Outer space is near-vacuum — no molecules to vibrate, so no sound propagation. Hollywood explosions in space are dramatic but unphysical.

Why does a falling water drop sound louder in a quiet room?

Sound intensity is the same, but the background noise level is lower. Our perception is relative — same sound seems louder against quieter background.

How does SONAR work?

Sound Navigation and Ranging. A ship/submarine sends ultrasonic pulses underwater; measures time for echoes to return; calculates distance to obstacles or sea floor. Used for ocean mapping, submarine warfare, fishing.

Why do thunder and lightning seem to come at different times?

Light travels much faster (~300,000 km/s) than sound (~0.34 km/s). You see lightning almost instantly; sound takes ~3 seconds per km. Counting seconds between flash and thunder gives the distance in km divided by 3.

What is an ultrasound scan?

Medical imaging using ultrasonic sound (1-15 MHz). Sound reflects differently off different tissues; computer reconstructs image. Safe (no ionising radiation), real-time, widely used in pregnancy, cardiology, abdominal scans.