Grade 9

Waves and Sound

Six lessons that pin down what a wave is before saying anything about sound, and then use that one definition to explain pitch, loudness, echoes and resonance.

6 lessons

  1. 1If a wave travels towards the shore, why does a cork not travel with it?Because a wave carries energy, not material. The cork rises and falls on the spot, and so does the water around it: each particle vibrates about its resting position and passes the disturbance to its neighbour. What moves towards the shore is the pattern itself. If the water travelled, the sea would empty.30m
  2. 2What happens to the wavelength when the frequency doubles and the speed stays the same?It halves exactly. The speed of a wave equals its frequency times its wavelength, so if the speed is fixed, raising one factor forces the other down in the same proportion. The frequency is set by the source; the speed is set by the medium; the wavelength is simply what follows from the two.40m
  3. 3Why does a ringing bell fall silent when the air is pumped out of the jar?Because there is nothing left for the sound to move through. The hammer still strikes and the bell still vibrates — you can see it — but with no air molecules there is nothing to be squashed and stretched, so no wave forms at all. Let the air back in and the sound returns. This is the experiment that proves sound needs a medium.30m
  4. 4Is a high-pitched sound necessarily a loud one?No — these are two entirely separate quantities. Pitch is set by frequency: more cycles per second means a higher note. Loudness is set by amplitude: a bigger vibration means a louder sound. A child's whisper can be high and faint at once, and thunder can be low and deafening.35m
  5. 5How can you measure the distance to a cliff with a shout and a stopwatch?The sound reflects off the cliff and comes back, so the time you measure covers the journey there and back. Multiply the time by the speed of sound and divide by two. Five seconds gives 340 times 5 over 2, which is 850 metres. Sonar and medical ultrasound run on exactly the same idea.35m
  6. 6Why is a guitar string loud over a wooden box and almost silent without one?Because the string on its own pushes very little air. It passes its vibration through the bridge to the body of the guitar, and that wide panel moves a far larger area of air at the same frequency. The box adds no energy — it only delivers the energy to the air more efficiently, which is why the note is louder and dies away faster.40m