Sound Waves: Characteristics and Applications

• Activity 10.1: a plucked stretched rubber band over a box produces sound while it vibrates; changing the tension changes the sound.
• Sources: vibrating strings, membranes, air columns (bansuri), struck metal objects. Most instruments use more than one vibrating part.
• Tuning fork: U-shaped steel or aluminium bar with a stem; its sides are prongs (tines). Struck on a rubber pad (never a hard surface), it vibrates; a prong touching water makes ripples, showing it is vibrating.
• Humans: sound from vibrating vocal cords (tightly stretched muscular flaps in the larynx); tongue, lips, mouth and nasal cavity shape speech or music. Crickets and grasshoppers rub wings or legs.
• Kongthong (Meghalaya): the 'Whistling Village' where each person has a tune name (Jingrwai Iawbei).

• Sound travels through solids (ear on a desk; knocks heard through the desk), liquids (submerged spoons tapped under water) and gases. The material through which it travels is the medium. Vacuum is a space with no matter.

• Space: near vacuum, so astronauts on a spacewalk cannot hear each other or clanking metal directly; they use special communication devices in their suits.

• Slinky analogy (Activity 10.5): pushing and pulling one end creates regions where turns are closer and regions more spread out that travel along the slinky, while each turn (a marked point) only oscillates about its own position parallel to the disturbance.
• Piston in an air-filled tube: the forward stroke pushes particles together; the backward stroke leaves them apart; collisions pass each disturbance forward.

• Longitudinal wave: particles vibrate parallel to the direction of propagation. Sound is a longitudinal wave.
• Mechanical wave: needs a material medium. Sound is a longitudinal mechanical wave.
• Beyond: in a transverse wave particles vibrate perpendicular to propagation. Light is a transverse wave that can travel in vacuum (so sunlight reaches Earth). Longitudinal seismic waves are the first detected by seismographs.
• Spherical waves: a small source in an open medium sends waves in all directions.
• Sudden loud sounds (firecrackers, thunder): rapid heating makes gases expand suddenly and a density disturbance travels outward. A supersonic aircraft (faster than sound) creates a sonic boom.

• Activity 10.6: a loud sound near a stretched sheet makes grains on it jump: the sound vibrates the sheet. Sound is a form of energy; it transfers energy through collisions of particles. In propagation, energy is transferred, not particles.
• Particles in a medium are always randomly vibrating (thermal energy); a sound wave temporarily increases this vibration.
• Microphone: sound waves vibrate a diaphragm; it is converted to an electrical signal (sound → electrical). Speaker: an electrical signal vibrates a cone or diaphragm to produce sound (electrical → sound).

• At a given instant, density varies periodically with distance. Plot distance on the x-axis and density on the y-axis, with a dashed line for the average density.
• Crest (highest point): maximum density, in a compression. Trough (lowest point): minimum density, in a rarefaction.
• Another representation plots density against time at one position.

• Shorter time period means higher frequency. A tuning fork or a whistle produces nearly a single frequency; everyday sounds mix many frequencies.
• Musical notes (Sa, Re, Ga …): each note has a distinct frequency, lowest for Sa and rising along the scale.
• Amplitude and energy: larger amplitude carries more energy (struck harder, grains jump higher).
• Intensity falls with distance: energy spreads over a larger area (a larger-amplitude sound travels farther before intensity becomes zero).
• Speed depends on the medium (not on source or frequency in most media): fastest in solids, slower in liquids, slowest in gases. If the frequency changes, the wavelength changes and the speed stays constant.
• In air the speed rises with temperature and humidity: about 331 m s⁻¹ at 0°C and 344 m s⁻¹ at 22°C.

• Sound is about 4–5 times faster in water and typically 15–20 times faster in solids than in air.

Physical properties (time period, wavelength, frequency, amplitude, speed) can be measured; pitch and loudness are subjective and not simply related to them.

• Voices differ in frequency and in how the throat, mouth and nasal cavities shape sound. In adolescence, boys' vocal cords lengthen and thicken, vibrate less often, and the voice deepens.
• Decibel (dB): unit of loudness: rustling leaves a few dB, conversation about 60 dB, firecrackers over 100 dB. A small rise in dB means a large rise in intensity.
• Noise = unwanted or harmful sound; noise pollution affects health, sleep and hearing. Prolonged loud sound can cause hearing loss (tested by audiograms); hearing aids contain a microphone, amplifier and speaker.
• Human ear: sound makes the eardrum vibrate; tiny bones amplify the vibrations; the cochlea converts them to electrical signals for the brain. Two ears help find the direction of a sound from the tiny time gap between ears.
• Tone vs note: a tone has a single frequency (tuning fork, whistle). A musical note is a lowest fundamental plus higher overtones. An octave is the interval between notes where one has double the frequency (eg: 200 and 400 Hz).
• C. V. Raman: India's first science Nobel Prize (Raman Effect in light); also studied acoustics of the tabla and mridangam. The black syaahi patch on these drums alters the membrane's vibration for rich sounds.

• Reflection: sound bounces off solids or liquids and follows the laws of reflection (angle of incidence = angle of reflection; incident ray, reflected ray and normal in the same plane).
• Echo: hearing your voice again after reflection from a hard surface (cliff, long corridor). Echoes need the reflected sound to arrive at least 0.1 s after the original.

• Hard, smooth surfaces give strong echoes; soft surfaces (curtains) absorb sound; rough surfaces scatter it.
• Example 10.5: echo after 0.5 s: distance $= 340\times0.5/2 = 85$ m.
• Reverberation: multiple reflections in a large hall make sound persist after the source stops; reflections arrive within 0.05 s. Auditoriums use sound-absorbing panels, upholstered chairs and curtains to control unwanted reverberation.
• Whispering Gallery of Gol Gumbaz (Bijapur, Karnataka): a faint whisper is heard multiple times across the dome.
• Distinguishing two sounds: a gap of at least 0.1 s is needed to hear them separately.

• Echolocation: locating objects with reflected sound. Most bats emit short bursts of ultrasound and sense the echoes to find obstacles and prey in the dark; dolphins, whales and some birds also use it.
• Sonar (sound navigation and ranging): ultrasonic waves sent into water; the reflected waves give the distance, direction and speed of submarines, shipwrecks, etc. Depth or distance $= \dfrac{v\times t}{2}$.
• Example 10.6: sonar signal returns after 0.90 s in seawater ($v = 1530$): $d = 1530\times0.45 = 688.5$ m.
• Sound in water: sound travels much farther than light in water, so it is used for underwater applications.
• Audio surveillance: the low-frequency humming of drones and aircraft is detected by sensitive sound sensors to monitor airspace.
• Sound as a tool: recording sounds on Mars, timing distant earthquake sounds to study ocean temperature, mosquito buzz to identify disease carriers, and soil crackles to study soil health.