Sound

Sound is a longitudinal wave that travels through a material medium (solid, liquid, or gas). It cannot travel in vacuum — so space is silent.

Wave parameters:

• Frequency (f): number of vibrations per second. Unit: hertz (Hz). 1 kHz = 1000 Hz.
• Wavelength (λ): distance between two successive crests or compressions.
• Speed (v): v = f × λ.
• Amplitude: maximum displacement; determines loudness.

Speed of sound in different media (at 20°C):

• Air: ~343 m/s (varies with temperature; faster in warm air).
• Water: ~1500 m/s.
• Steel: ~5000 m/s.
• Sound travels faster in denser, more elastic media (solid > liquid > gas).

Audible range:

• Human ear: ~20 Hz to 20,000 Hz (20 kHz).
• Infrasound: below 20 Hz (elephants, whales communicate; earthquakes produce).
• Ultrasound: above 20 kHz (bats, dolphins use; medical imaging).

Characteristics of sound:

• Pitch: how high or low; determined by frequency (higher freq → higher pitch).
• Loudness: determined by amplitude; measured in decibels (dB).
• Quality (timbre): distinguishes two sounds of same pitch and loudness (a violin vs a piano playing the same note).

Reflection of sound (echo):

• An echo is heard when reflected sound returns after ~0.1 seconds (the persistence of hearing).
• Minimum distance for an echo: (343 × 0.1)/2 ≈ 17 m from the reflector.
• SONAR: uses ultrasonic waves to measure ocean depths, locate submarines.

Resonance: when an object vibrates at its natural frequency due to another vibrating object. A glass shattering when a singer hits the right pitch — that's resonance. Used in musical instruments.

Doppler effect: when source and observer move relative to each other, frequency changes. A train horn sounds higher-pitched when approaching, lower when receding. Used in police radar guns and astronomy (red-shift of galaxies).

Example 1 — Speed and frequency:
A tuning fork of frequency 256 Hz produces sound waves of wavelength 1.34 m. Speed of sound?
Method: v = f × λ = 256 × 1.34 = 343 m/s (≈ standard speed of sound in air).

Example 2 — Echo distance:
You shout in a canyon and hear an echo 2 seconds later. How far is the canyon wall?
Method: Sound travels to the wall and back. Total distance = 343 × 2 = 686 m. One-way = 343 m.

Example 3 — SONAR depth:
A SONAR signal sent down into the sea returns after 4 seconds. Speed of sound in sea water ≈ 1500 m/s. Depth?
Method: Total distance = 1500 × 4 = 6000 m. One-way = 3000 m = 3 km.

Decibel reference points (loudness):

• 0 dB: threshold of hearing.
• 30 dB: whisper.
• 60 dB: normal conversation.
• 80 dB: heavy traffic.
• 100 dB: lawn mower.
• 120 dB: rock concert (pain threshold).
• 140 dB: jet engine at takeoff (can cause permanent damage).

Ultrasound applications:

• Pregnancy scans (foetal imaging).
• Cleaning delicate items (ultrasonic baths).
• Drilling and cutting (ultrasonic machining).
• Echo-location in bats and dolphins.

Infrasound applications:

• Earthquake detection (seismic waves include infrasonic).
• Elephant and whale long-distance communication.

Why sound doesn't travel in space: there are no molecules in vacuum to vibrate.

Common facts:

• Lightning is seen before thunder is heard because light travels much faster than sound.
• A "sonic boom" is heard when an object moves faster than the speed of sound (Mach 1).
• The pitch of an aircraft's engine doesn't actually change as it passes; you perceive it changing (Doppler effect).
• Music notation: A4 (the A above middle C) = 440 Hz. An octave up doubles the frequency.

Speed-of-sound rule for distance: in air, sound takes about 3 seconds to travel 1 km. After a lightning flash, count seconds until thunder, divide by 3, that's roughly the distance to the storm in kilometres.