Sound

• Sound is a form of energy that makes us hear
• Law of conservation of energy applies to sound
• Sound travels in the form of waves
• Sound is produced when an object vibrates or by vibrating objects
• Examples of sound production methods:

  • Vocal cords vibrating in the throat for voice
  • Membrane vibration in drum or tabla
  • Vibrating tuning fork in laboratory experiments
• Sound propagation:

  • Substance through which sound travels is called a medium
  • Medium can be solid, liquid, or gas
  • Vibrations in the medium cause air particles to vibrate and propagate sound waves
• Sound waves are longitudinal waves:

  • Particles vibrate back and forth in the same direction as the wave
  • Compression and rarefaction are formed in the medium
• Characteristics of sound waves:

  • Wavelength, frequency, amplitude, time period, and velocity
  • Wavelength is the combined length of compression and rarefaction
  • Frequency is the number of vibrations per second
  • Amplitude is the maximum displacement of particles from their original position
• Pitch, loudness, and quality of sound:

  • Pitch depends on frequency, higher frequency means higher pitch
  • Loudness depends on amplitude, greater amplitude means louder sound
  • Timbre distinguishes between sounds of the same pitch and loudness
• Velocity of sound:

  • Distance traveled by a wave in one second
  • Velocity = Wavelength × Frequency
• Speed of sound in various mediums:

  • Slowest in gases, faster in liquids, fastest in solids
  • Increases with temperature and humidity
  • In air, speed of sound is 344 m/s at 22ºC
• Sonic boom:

  • Produced by objects moving faster than the speed of sound
  • Causes shock waves and explosive noise
• Reflection of sound:

  • Sound waves can reflect like light waves
• Sonic boom is an explosive noise caused by shock waves
• It emits tremendous sound energy which can shatter the glass panes of windows
• Reflection of Sound:

  • Sound bounces back when it falls on a hard surface, known as reflection of sound
  • The laws of reflection of light are obeyed during reflection of sound
  • The incident sound wave, the reflected sound wave and normal at the point of incidence lie in the same plane
  • Angle of reflection of sound is always equal to the angle of incidence of sound
• Echo:

  • Repetition of sound caused by the reflection of sound waves
  • Echo is heard when there is a time gap of 0.1 second in original sound and echo
  • Produced when sound is reflected from a hard surface like a brick wall or mountain
• Minimum distance to hear an echo:

  • Speed of sound in air = 344 m/s at 22ºC
  • Distance = Speed x Time = 344 x 0.1 = 34.4 m
  • Distance between reflecting surface and audience = 34.4/2 = 17.2 m at 22ºC
• Reverberation:

  • Persistence of sound in a big hall due to repeated reflection of sound from walls, ceiling, and floor
  • If reverberation is too long, sound becomes blurred, distorted, and confusing
  • Methods to reduce reverberation in big halls or auditoriums:
  • Panels made of felt or compressed fibre board on walls and ceiling to absorb sound
  • Heavy curtains on doors and windows
  • Carpets on the floor
  • Seats made of material with sound-absorbing properties
• Applications of Reflection of Sound:

  • Megaphones, loudspeakers, bulb horns, trumpets, shehnai designed to send sound in a particular direction without spreading all around
  • Stethoscope used for listening to sounds in the human body
  • Soundboard placed behind speakers in big halls for clear hearing by audiences
  • Ceiling of concert halls made curved to reflect sound to all parts of the hall
• Range of Hearing:

  • Human range of hearing is 20 Hz to 20000 Hz
  • Sounds below 20 Hz are infrasonic, above 20 KHz are ultrasonic
  • Infrasonic sounds produced by vibrating simple pendulum, used by animals like rhinoceroses, elephants, and whales
  • Ultrasonic sounds heard by animals like dogs, parpoises, dolphins, bats, and rats
• Hearing Aid:

  • Battery-operated electronic device for persons who are hard of hearing
  • Microphone converts sound into electrical signals, amplified by amplifier, and sent to speaker for clear hearing
• Applications of Ultrasound:

  • Used to detect cracks in metal blocks without damaging them
  • Used in industries to clean 'hard to reach' parts of objects
  • Used to investigate internal organs of human body like liver, gall bladder, kidneys, uterus, and heart
  • Ecocardiography and Ultrasonography techniques for imaging internal organs
  • Used to split tiny stones in kidneys into fine grains
• SONAR:

  • Stands for 'Sound Navigation And Ranging'
  • Device used to find distance, direction, and speed of underwater objects
  • Consists of a transmitter and a receptor installed at the bottom of a ship
  • Transmitter produces and transmits ultrasonic waves, reflected waves are received by detector
  • Used to find depth of sea, locate underwater hills, valleys, submarines, icebergs, and sunken ships
• Structure of Human Ear:

  • Consists of outer ear, middle ear, and inner ear
  • Outer ear collects sound, passes through auditory canal to ear drum
  • Middle ear contains hammer, anvil, stirrup bones linked together
  • Inner ear has cochlea connected to oval window, filled with nerve cells
  • Cochlea connected to auditory nerve which goes to brain
• Working of Human Ear:

  • Compression of sound wave pushes ear drum inwards, rarefaction moves ear drum outwards causing vibrations
  • Vibrations amplified by three bones in middle ear and transmitted to inner ear
  • Pressure variations in inner ear turned into electric signals by cochlea
  • Electric signals sent to brain via auditory nerve and interpreted as sound