Sound is a form of energy produced by vibrating objects and propagated as longitudinal waves in a medium.

Sound is a longitudinal mechanical wave (particles oscillate parallel to propagation direction).

No. Sound requires a material medium (solid, liquid or gas) and cannot travel through vacuum.

Wavelength λ is the distance between two successive compressions or rarefactions in a sound wave.

Frequency f is the number of oscillations (or cycles) per second measured in hertz (Hz).

v = f × λ (wave speed equals frequency times wavelength).

Amplitude is the maximum displacement of particles from their mean position; larger amplitude ⇒ louder sound.

Hertz (Hz), which is s⁻¹.

Approximately 20 Hz to 20,000 Hz (20 kHz).

Compressions are regions of high pressure; rarefactions are regions of low pressure in a longitudinal wave.

Sound travels fastest in solids, slower in liquids, and slowest in gases.

v ≈ 331 + 0.6 T (m·s⁻¹), where T is temperature in °C.

About 343 m·s⁻¹ (approx.).

Yes — humid air is slightly less dense, causing a small increase in speed of sound compared to dry air.

Speed increases with temperature because molecules move faster, facilitating quicker propagation of pressure disturbances.

No; because vacuum has no particles to oscillate and transfer mechanical energy.

In a given medium, all frequencies travel at nearly the same speed (dispersion negligible for air at audible range).

Direction is set by the initial vibration direction and geometry; reflections and diffraction also alter propagation direction.

Reverberation is the persistence of sound in a closed space caused by multiple reflections arriving within a short time so they blend with the original sound.

Echo is a reflected sound heard distinctly if the reflected sound returns after a perceptible delay (≈0.1 s or more).

Angle of incidence equals angle of reflection (measured with respect to normal to reflecting surface).

About 0.1 second or more between original and reflected sound.

Minimum one-way distance ≈ v × 0.05 ≈ 17 m (so round-trip ~34 m).

A large, hard, and distant reflecting surface returns sound waves with little absorption, producing a distinct echo.

Regions where sound intensity is significantly reduced due to obstacles or wave diffraction, analogous to light shadows.

Use sound-absorbing materials (curtains, carpets, acoustic panels) to reduce multiple reflections and reverberation time.

Intensity is power transmitted per unit area perpendicular to wave propagation (I = P/A).

For a point source, intensity falls approximately as 1/r² (inverse square law) in free space.

Decibel (dB) is a logarithmic unit to express sound intensity levels relative to a reference intensity.

Minimum intensity a typical human ear can detect, about 10⁻¹² W·m⁻² at 1 kHz.

Doppler effect: observed frequency changes when there is relative motion between source and observer; approaching → higher frequency, receding → lower frequency.

The change in pitch of a passing vehicle’s horn as it approaches and then moves away.

Beats are periodic variations in loudness heard when two close frequencies interfere; beat frequency = |f₁ − f₂|.

Beats help in tuning instruments by indicating small frequency differences between notes.

Ultrasonic sound has frequency above 20 kHz, beyond human hearing range.

Medical imaging (ultrasound), cleaning, and non-destructive testing are common applications.

Infrasonic sound has frequency below 20 Hz, below human hearing range.

Earthquakes, volcanic eruptions and large storms can generate infrasonic waves.

Pitch depends on string frequency determined by length, tension and mass per unit length; higher tension or shorter length → higher pitch.

Timbre is the characteristic quality of sound depending on harmonic content, allowing us to distinguish instruments even at same pitch and loudness.

Outer ear (pinna & ear canal), middle ear (ossicles: malleus, incus, stapes), and inner ear (cochlea).

Cochlea converts mechanical vibrations into nerve impulses which the brain interprets as sound; different parts respond to different frequencies.

Ossicles amplify and transmit vibrations from eardrum to the inner ear (cochlea).

Hearing loss is reduced ability to perceive sounds; caused by damage to ear structures, prolonged loud noise exposure, infections or aging.

Avoid prolonged exposure to loud sounds; use ear protection (earplugs) in noisy environments and keep volume moderate when using headphones.

If obstacle size is comparable to or smaller than wavelength, diffraction is significant; large obstacle relative to wavelength produces sharp shadow.

Low frequencies suffer less atmospheric absorption and diffract more around obstacles, so they propagate farther.

Resonance occurs when an object vibrates with maximum amplitude at its natural frequency due to a matching external frequency.

A tuning fork coupled to a resonating box or the body of a guitar amplifies sound at the fork’s frequency (resonance).

Ultrasonic imaging in medicine (sonography) is used to visualize internal body structures; ultrasonic cleaning is used in industry.