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PHYSICS · Stage 9 · Chapter 13: Sound Waves

Physics 13.1 Loudness and amplitude — Oscilloscope Traces and Wave Amplitude

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Physics 13.1 Loudness and amplitude — Oscilloscope Traces and Wave Amplitude — Alhussan International Academy — Physics (Stages 7–9) notes: Define wavelength, amplitude and frequency, describe what affects the loudness of a sound and interpret waveforms.Wavelength = the distance …

  • Compressions and rarefactions
  • Longitudinal
  • Soft and loud
  • Spreading out
  • Dissipation
  • Not weaker waves

Section 1 of 6Properties of waves

  • Wavelength the distance from one point on a wave to the same point on the next wave, such as peak to peak.
  • Amplitude the maximum displacement from the centre (rest) line to a crest or trough, which is half the peak-to-trough height.
  • Frequency the number of waves passing a point per second.

Section 2 of 6Sound as a pressure wave

  • Source
    • A sound wave is a pressure wave made by a vibrating object.
  • Compressions and rarefactions
    • Air molecules move backwards and forwards, producing regions of higher and lower pressure.
  • Longitudinal
    • The particles vibrate parallel to the direction the wave moves.
  • Wavelength
    • It is the distance from one compression to the next compression, or rarefaction to the next rarefaction.

Section 3 of 6Using an oscilloscope

  1. Connect a microphone to an oscilloscope.
  2. The microphone produces an electrical signal that represents the sound.
  3. The signal is displayed on the screen with time on the horizontal axis.
  4. The line shows how the pressure varies with time.
  5. Where the line goes up the particles are close together, and where it goes down they are far apart.

Section 4 of 6Amplitude and loudness

  • Loudness
    • The loudness of a sound depends on the amplitude.
  • Soft and loud
    • A soft sound has a small amplitude and a loud sound has a large amplitude.
    • The same note sung louder has the same number of waves in the same time, so the frequency and pitch stay the same.
    • The waves are taller because the wave carries more energy.
Trace featureMeaning
Height of the waveAmplitude, so loudness
Waves per unit of timeFrequency, so pitch

Section 5 of 6Why sound gets quieter

  • Spreading out
    • Sound spreads out as it travels away from the source, so the energy covers a larger area.
  • Dissipation
    • Some energy is transferred to the medium, such as air, so the sound becomes softer.
  • Not weaker waves
    • Sound does not simply die away; its energy is spread out and dissipated.
  • Vacuum
    • Sound cannot travel through a vacuum because it needs particles to pass on the vibration.

Section 6 of 6Amplifiers

  • Concert halls
    • Singers cannot sing loudly enough for everyone, so they use a microphone connected to an amplifier.
  • Amplifier = a device that increases the amplitude of the electrical signal from a sound wave.
  • Loudspeaker
    • The larger signal makes the broadcast sound louder.
  • Pitch unchanged
    • The frequency stays the same, so the pitch is unchanged.

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