AQA · GCSE · Physics

    Course version: 8463 · Version from Sept 2016

    The conversion of sound waves to vibrations of solids works over a limited…

    Full specification wording

    Sound waves (physics only) (HT only): The conversion of sound waves to vibrations of solids works over a limited frequency range.

    A solid part such as the ear drum has its own mass and stiffness, so it responds well only to a band of driving frequencies.

    Checked against the AQA 8463 specification (opens in a new tab) on .

    • Point 3 of 6
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A solid part such as the ear drum has its own mass and stiffness, so it responds well only to a band of driving frequencies. If the wave oscillates far faster than the drum can follow, the drum barely moves; if it oscillates very slowly, the pressure change is too gradual to build a useful vibration. So converting a sound wave into vibration of a solid, and back again, is efficient only over a limited frequency range. The same limit applies to every device that does this job: a microphone diaphragm, a loudspeaker cone and the ossicles of the ear all have a working range set by their size, mass and stiffness. Outside that band the transfer becomes inefficient, so waves outside it produce little or no vibration and therefore little or no signal.

    What earns the marks

    • Stating that the vibrating solid has a mass and stiffness that limit how fast it can be made to vibrate.
    • Saying the conversion becomes inefficient outside a particular frequency band.
    • Linking the limited range of the ear drum to the limits of human hearing.
    • Applying the idea to another converter, such as a microphone diaphragm or a loudspeaker cone.

    Common mistakes and how to put them right

    • Saying high frequency sounds are too quiet to hear, rather than too fast for the solid part to follow.

    • Assuming the limit is set by the air rather than by the properties of the vibrating solid.

    • Treating the range as identical for every person and every device.

    • Confusing the frequency range with the loudness needed before a sound is heard.

    Exam tips

    • Answer in terms of the physical properties of the vibrating part: mass, stiffness and how fast it can respond.
    • Link the limit straight to numbers, 20 Hz to 20 kHz, when the question is about human hearing.
    • If a graph of minimum sound level against frequency is given, describe the overall trend before quoting individual values.

    Try the lesson

    The start of the Sound waves (physics only) (HT only): The conversion of sound waves to vibrations of solids works over a limited frequency range. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A microphone converts sound waves into vibrations of a solid. What happens to this conversion for sounds with frequencies outside the microphone's limited frequency range?

    Listen

    A solid part such as the ear drum has its own mass and stiffness, so it responds well only to a band of driving frequencies. If the wave oscillates far faster than the drum can follow, the drum barely moves; if it oscillates very slowly, the pressure change is too gradual to build a useful vibration. So converting a sound wave into vibration of a solid, and back again, is efficient only over a limited frequency range. The same limit applies to every device that does this job: a microphone diaphragm, a loudspeaker cone and the ossicles of the ear all have a working range set by their size, mass and stiffness. Outside that band the transfer becomes inefficient, so waves outside it produce little or no vibration and therefore little or no signal. A strong answer earns marks for these points. stating that the vibrating solid has a mass and stiffness that limit how fast it can be made to vibrate. saying the conversion becomes inefficient outside a particular frequency band. linking the limited range of the ear drum to the limits of human hearing. applying the idea to another converter, such as a microphone diaphragm or a loudspeaker cone. Exam tip: Answer in terms of the physical properties of the vibrating part: mass, stiffness and how fast it can respond.

    See it

    A top-down flowchart. The first box reads 'Sound wave drives a solid part' and points to 'Solid has its own mass and stiffness', which points to 'Responds well only to a band of driving frequencies'. That box splits into two branches: 'Wave far faster than the solid can follow', leading to 'Solid barely moves', and 'Wave oscillates very slowly', leading to 'Pressure change too gradual to build a useful vibration'. Both of these boxes lead to 'Conversion inefficient outside the band', which leads to the final box 'Little or no vibration and little or no signal'.
    It shows how the mass and stiffness of a solid part limit the band of frequencies it converts efficiently, and what happens above and below that band.

    Key words

    Say what each word means before you turn the card.

    limited frequency range

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    mass and stiffness

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    efficiency of conversion

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    diaphragms and drums

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    See every definition
    limited frequency range
    The conversion of sound waves to vibrations of solids works only over a band of driving frequencies.
    mass and stiffness
    Properties of a solid part such as the ear drum that set its working range.
    efficiency of conversion
    Converting a sound wave into vibration of a solid, and back again, is efficient only over a limited frequency range.
    diaphragms and drums
    A microphone diaphragm, a loudspeaker cone and the ear drum all have a working range set by their size, mass and stiffness.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box reads 'Sound wave drives a solid part' and points to 'Solid has its own mass and stiffness', which points to 'Responds well only to a band of driving frequencies'. That box splits into two branches: 'Wave far faster than the solid can follow', leading to 'Solid barely moves', and 'Wave oscillates very slowly', leading to 'Pressure change too gradual to build a useful vibration'. Both of these boxes lead to 'Conversion inefficient outside the band', which leads to the final box 'Little or no vibration and little or no signal'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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