Sound Class 9 Short Notes| Important Points

Nature of Sound

• Sound is produced by vibrating objects.
• Requires a medium (solid, liquid, or gas) for propagation.
• Does not travel through vacuum.

Propagation of Sound

• When an object vibrates, it disturbs nearby particles of the medium.
• Particles do not travel from source to ear; only the disturbance moves.
• Each particle:
  • Gets displaced from its equilibrium position.
  • Exerts force on adjacent particle.
  • Returns to original position after displacement.
• This sequential disturbance propagates as a wave.

Sound as a Mechanical Wave

• Sound is a mechanical wave.
• Requires particles of a medium to propagate.
• Particles oscillate but do not move forward with the wave.
• Disturbance (energy) travels through the medium.

Pressure and Density Variations

• Compression = high particle density → high pressure.
• Rarefaction = low particle density → low pressure.
• Sound propagation = propagation of pressure or density variations in the medium.

Longitudinal Waves

• Sound travels as longitudinal waves.
• Particles oscillate parallel to wave direction → creates compressions (high pressure) and rarefactions (low pressure).
• Compression = high particle density; Rarefaction = low particle density.

Transverse Waves (Contrast)

• Particles oscillate perpendicular to wave direction (e.g., water waves).
• Light is transverse but not mechanical (no medium needed).
• Sound is NOT transverse — it is longitudinal in air and liquids.

Wave Parameters

• Wavelength (λ): Distance between two consecutive compressions or rarefactions (unit: metre, m).
• Frequency (ν): Number of oscillations (or compressions/rarefactions passing a point) per second (unit: hertz, Hz).
• Time period (T): Time for one complete oscillation (unit: second, s).
• Relation: ν = 1/T
• Speed of sound (v): Distance travelled by a compression/rarefaction per unit time.
  • Formula: v = λν
  • Same for all frequencies in a given medium under same conditions.

1. Pitch

• Determined by frequency.
• Higher frequency → higher pitch.
• Depends on vibration rate of source.

2. Loudness

• Determined by amplitude (A) of wave.
• Greater amplitude → louder sound.
• Amplitude depends on force of vibration.
• Decreases with distance from source.
• Not the same as intensity – loudness is subjective (ear’s response); intensity is objective (energy per unit area per second).

3. Quality (Timbre)

• Enables distinction between sounds of same pitch and loudness.
• Tone: Sound of single frequency.
• Note: Pleasing sound with mixture of frequencies.
• Noise: Unpleasant sound.
• Music: Pleasant, rich-quality sound.

General Facts

• Sound travels at a finite speed — much slower than light (e.g., thunder heard after lightning flash).
• Speed depends on:
  • Nature of the medium (solid, liquid, gas)
  • Temperature of the medium

Dependence on Medium

• Solids > Liquids > Gases
  • Speed decreases from solid to gas due to particle spacing and elasticity.

Dependence on Temperature

• Higher temperature → higher speed (in the same medium).
• In air:
  • 331 m/s at 0°C
  • 344 m/s at 22°C

Laws of Reflection

• Sound reflects off solid or liquid surfaces (like light).
• Angle of incidence = Angle of reflection.
• Incident sound, reflected sound, and normal all lie in the same plane.
• Requires a large obstacle (polished or rough).

Echo

• Reflected sound heard after original sound (e.g., from a building or mountain).
• Human ear retains sound for 0.1 s.
• To hear a distinct echo, time gap ≥ 0.1 s.
• At 22°C (speed of sound = 344 m/s):
  • Total distance (to obstacle + back) ≥ 34.4 m
  • Minimum distance to obstacle = 17.2 m
• Distance varies with temperature.
• Multiple echoes possible due to successive reflections (e.g., rolling thunder).

Reverberation

• Persistence of sound in a large hall due to repeated reflections.
• Excessive reverberation is undesirable (e.g., in auditoriums).
• Reduced by using sound-absorbing materials:
  • Compressed fibreboard
  • Rough plaster
  • Draperies
  • Sound-absorbent seating

Megaphones, Loudhailers & Musical Instruments (e.g., trumpet, shehnai)

• Use conical tubes to reflect sound successively.
• Direct sound forward without spreading in all directions.

Stethoscope

• Allows doctor to hear internal body sounds (heart, lungs).
• Sound reaches ears through multiple reflections inside the tube.

Curved Ceilings & Soundboards (in concert halls, cinema halls, auditoriums)

• Curved ceilings reflect sound to all corners of the hall.
• Soundboards behind stage reflect sound evenly across the audience.

Human Audible Range

• 20 Hz to 20,000 Hz (20 kHz)
• Children (< 5 years) can hear up to 25 kHz.
• Sensitivity to high frequencies decreases with age.

Infrasound

• Below 20 Hz
• Examples:
  • Pendulum vibrations
  • Rhinoceroses (as low as 5 Hz)
  • Elephants, whales
• Earthquakes produce infrasound before main shock → may alert animals.

Ultrasound

• Above 20 kHz
• Produced/used by:
  • Bats, dolphins, porpoises (for echolocation)
  • Moths: detect bat squeaks → escape predation
  • Rats: produce ultrasound during play

1. Cleaning

• Used to clean hard-to-reach parts:
  spiral tubes, electronic components, odd-shaped objects.
• Objects placed in cleaning solution;
  ultrasonic waves detach dust, grease, and dirt.

2. Detecting Flaws in Metals

• Used in buildings, bridges, machines, scientific equipment.
• Ultrasound passed through metal blocks; detectors check for transmitted/reflected waves.
• Defects (cracks/holes) reflect ultrasound → reveal flaws.
• Ordinary sound not suitable – longer wavelengths diffract around defects

3. Medical Imaging

• Echocardiography:
  • Ultrasound reflects from heart parts → forms image of the heart.
• Ultrasonography:
  • Produces images of internal organs (liver, kidney, gall bladder, uterus, etc.).
  • Detects stones, tumours, tissue abnormalities.
  • Also used in prenatal care to check foetal development and detect congenital defects.
  • Works by detecting reflections at tissue density boundaries → converted to images on monitor or film.