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Class 9 Science — Sound waves

Say why sound needs a medium, use v = λν and ν = 1/T, work out an echo distance, and place a frequency in the infrasonic, audible or ultrasonic band.

Name: ________________________

Before you start

Two sound waves, drawn one above the other.

12
  1. 1. The quieter wave — a small change in density, so less energy
  2. 2. The louder wave — a large change in density, so more energy

The two waves rise and fall the same number of times across the page, so they share a wavelength and a frequency. What differs is how far the density swings from its average — the amplitude.

  1. 1.Through which of these does sound travel fastest?

    • a) A steel rail
    • b) Air
    • c) It is the same speed in each
    • d) Sea water

  2. 2.In a large hall a sound seems to hang in the air after the speaker has stopped. What is that called, and what causes it?

    • a) Resonance, from the hall matching the speaker's frequency
    • b) An echo, from a single reflection arriving more than 0.1 s later
    • c) Reverberation, from repeated reflections arriving less than 0.05 s apart
    • d) Refraction, from the sound bending round the pillars

  3. 3.A point in a medium goes through 10 density oscillations in 4 s. What is the frequency of the wave, in Hz? Answer with a number.

  4. 4.A sound wave has a wavelength of 0.5 m and a frequency of 600 Hz. What is its speed, in m/s? Answer with a number.

  5. 5.A tuning fork is struck across the room. What reaches your ear?

    • a) Energy carried by the sound wave
    • b) A stream of compressed air from the fork
    • c) Small pieces of the fork itself
    • d) The air particles that were next to the fork

  6. 6.A wave has a time period of 0.02 s. What is its frequency, in Hz? Answer with a number.

  7. 7.Sort each description by the band of frequency it belongs to.

    Groups: Infrasonic · Audible to humans · Ultrasonic

    • Below 20 hertz
    • Between 20 hertz and 20 kilohertz
    • Above 20 kilohertz
    • Bursts a bat sends out to find its prey
    • Waves used to image internal organs without surgery
    • Waves used to detect earthquakes and volcanic eruptions

  8. 8.Sound travels at 340 m/s. What is the wavelength of a note of frequency 850 Hz, in m? Answer with a number.

Answer key — Class 9 Science — Sound waves

  1. 1. a) A steel rail
  2. 2. c) Reverberation, from repeated reflections arriving less than 0.05 s apart
  3. 3. 2.5
  4. 4. 300
  5. 5. a) Energy carried by the sound wave
  6. 6. 50
  7. 7. Below 20 hertz → Infrasonic; Waves used to detect earthquakes and volcanic eruptions → Infrasonic; Between 20 hertz and 20 kilohertz → Audible to humans; Above 20 kilohertz → Ultrasonic; Waves used to image internal organs without surgery → Ultrasonic; Bursts a bat sends out to find its prey → Ultrasonic
  8. 8. 0.4