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

    Groups: Infrasonic · Audible to humans · Ultrasonic

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

  2. 2.A metal plate is struck harder than before. What changes in the sound wave it sends out?

    • a) Its amplitude is larger, so it carries more energy
    • b) Its time period is longer, so it carries more energy
    • c) Its wavelength is larger, so it carries more energy
    • d) Its speed is greater, so it carries more energy

  3. 3.Match each quantity to the unit it is measured in.

    • Wavelength
    • Frequency
    • Time period
    • Speed of sound
    • meter per second
    • hertz
    • second
    • meter

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

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

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

  6. 6.A sound wave is drawn as density against distance, with the middle line marking the average density. Tap the span that is one wavelength.

    Write the number of the part.

    123

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

  8. 8.Two astronauts float side by side in the near-vacuum of space and clank two metal tools together. What happens?

    • a) Both hear it, but faintly and after a long delay
    • b) Just the astronaut holding the tools hears it, through her gloves
    • c) Both hear it, because a vacuum lets sound through freely
    • d) Neither hears it directly, because sound needs a medium

Answer key — Grade 9 Science — Sound waves

  1. 1. 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
  2. 2. a) Its amplitude is larger, so it carries more energy
  3. 3. Wavelength → meter; Frequency → hertz; Time period → second; Speed of sound → meter per second
  4. 4. 50
  5. 5. d) Energy carried by the sound wave
  6. 6. 1 — The span from one crest across to the next crest
  7. 7. d) Reverberation, from repeated reflections arriving less than 0.05 s apart
  8. 8. d) Neither hears it directly, because sound needs a medium