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Study of Sound — Class 9 Maharashtra SSC Science revision notes

A 3-page visual revision sheet for Study of Sound, written for Class 9 Maharashtra SSC Science. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.

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Study of Sound in one look

a wave that needs a medium, the quantities that describe it, and the echo it can produce

How sound travels

  • sound is produced by a vibrating body and travels as a wave through a material medium
  • sound cannot travel through a vacuum, since there are no particles to pass the disturbance along
  • it is a longitudinal wave: the particles vibrate along the same direction the wave travels, not across it
  • the wave is a series of compressions, where particles are crowded together, and rarefactions, where they are spread apart
  • the particles do not travel with the wave. Each vibrates about its own position and passes the disturbance on
  • so energy travels, matter does not, which is true of every wave

The quantities that describe a wave

  • the wave relation v = fλ
  • frequency and time period f = 1/T
  • distance for an echo, there and back 2d = vt
  • so the distance to the reflector d = vt/2

What each quantity means to the ear

  • frequency is the number of vibrations per second, in hertz, and it decides the pitch
  • higher frequency means a higher pitch, so a whistle is high and a drum is low
  • amplitude is the greatest displacement from the rest position, and it decides the loudness
  • larger amplitude means louder, and loudness is measured in decibels
  • quality or timbre is what makes the same note sound different on a flute and on a violin
  • speed depends on the medium, not on the source: sound travels fastest in solids, slower in liquids and slowest in gases

Echo and reverberation

  • an echo is a reflected sound heard as a separate sound from the original
  • the ear retains a sound for about 0.1 second, which is the persistence of hearing
  • so the reflected sound must arrive at least 0.1 second later to be heard separately
  • at 340 m/s that means the reflector must be at least 17 metres away, since the sound covers 34 m there and back
  • reverberation is the repeated reflection that makes a sound persist in a hall, and too much of it blurs speech
  • it is reduced by curtains, carpets, cushioned seats and rough ceilings, which absorb rather than reflect

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