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Year 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.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

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

  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.You clap and hear the echo 0.6 s later. Taking the speed of sound as 340 m/s, how far away is the reflecting wall, in m? Answer with a number.

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

  6. 6.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.

  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) Reverberation, from repeated reflections arriving less than 0.05 s apart
    • c) Resonance, from the hall matching the speaker's frequency
    • d) Refraction, from the sound bending round the pillars

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

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

Answer key — Year 9 Science — Sound waves

  1. 1. 1 — The span from one crest across to the next crest
  2. 2. 50
  3. 3. 2.5
  4. 4. 102
  5. 5. 0.4
  6. 6. 300
  7. 7. b) Reverberation, from repeated reflections arriving less than 0.05 s apart
  8. 8. b) Its amplitude is larger, so it carries more energy