Sound: Production and Propagation

Sound Production

• It is produced by vibrations in a body.
• Each source of sound is a vibrating body.
• The to and fro motion of a body from its rest position is oscillatory motion.
• Vibratory motion is where a part of the body oscillates while keeping the rest fixed.
• Key example is when you blow a whistle, vibrations of particles in air produce it.
• Another key example is when speaking, vocal cords vibrate to produce it in the larynx (voice box).

Sound Requirements

• Sound requires a medium for propagation, it cannot travel through a vacuum.
• Sound can travel through solids, liquids, and gases.

Demonstrations

• Humming sound is produced by vibrating a ruler.
• Vibrations in a string produce sound.
• Vibrating membrane of a drum produces sound.
• Plucking a sitar or guitar string produces sound.
• Pressing your ear to a table to hear a clock demonstrates sound traveling in a solid.
• A toy telephone demonstrates sound traveling in a solid via a string.
• Filling a balloon with water and holding it to your ear while someone taps it demonstrates sound traveling in a liquid.
• Ringing a bell demonstrates sound traveling in air.

Sources of Sound

• Tuning forks are U-shaped metallic pieces.
• Their arms are prongs, which vibrate when struck to produce sound.
• Wind instruments (flute, clarinet) produce sound when air is blown into them, vibrating the air column inside.
• Reed instruments (harmonium, mouth organ) contain metal reeds that vibrate when air is blown through them.
• Stringed instruments (sitar, guitar, piano, violin) produce sound when strings are plucked, struck, or bowed.
• Percussion instruments (drum, tabla, dholak) produce sound when the leather membrane vibrates from being struck.

Human Sound Production

• Humans produce sound when vocal cords vibrate as air is blown through them.
• This occurs when we talk, shout, or sing.
• The larynx, or voice box, is designed to produce voice.
  • It contains vocal cords supported by tissues.

Sound as Longitudinal Waves

• Sound travels in air as longitudinal waves.
• Vibrating air particles transfer energy to nearby particles.
• Compressions are points where particles move toward each other.
• Rarefactions are points where particles move away from each other.
• Important wave parameters include:
  • Displacement.
  • Wavelength (distance of one wave).
  • Time period of the wave.

Types of Sound

• Audible sound frequency range is 20 Hz to 20,000 Hz.
• Ultrasonic sound frequency is higher than 20,000 Hz.
• Infrasonic sound frequency is lower than 20 Hz.
• Some animals can hear ultrasonic sounds.
• Example, bats use ultrasonics to navigate.

Characteristics of Sound

• Loudness depends on the amplitude of vibration.
  • The greater the amplitude, the louder the sound.
• It depends on the area of the vibrating body.
• Greater area leads to louder sound.
• Pitch is the high or low frequency of a sound.
  • Higher frequency means higher pitch and a shriller sound.

Musical Instruments

• The pitch of sound is changed by:
  • Varying the vibrating air column length.
  • This is achieved by opening or closing holes when playing flute/clarinet.
  • Changing the place of plucking or changing string tension.
    • In stringed instruments, strings are varied by tension to create varied pitches.

Sound Speed

• Sound travels at different speeds depending on the medium.
• Steel is the fastest at 5960 m/s.
• Water is in the middle at 1500 m/s.
• Air is the slowest at 330 m/s.

Key Equation

• Speed of sound (V) = Distance (d) / Time (t).

Echoes and Reflection

• Echoes are distinct sounds heard after reflection from a rigid surface.
• The minimum distance to hear an echo in air is 16.5 meters.

Key Equation

• Speed of sound (V) = 2d/t, where d is the distance to the reflecting surface.

Absorption of Sound

• Substances like cloth, wood, paper, and thermocol absorb sound.
• Sound proofing involves using materials with good absorption properties to reduce sound transmission.