Waves - Sound

This GCSE Physics quiz on waves looks at sound. Sound energy travels as a longitudinal wave. Sound waves travel through a medium in a different way to electromagnetic waves, they require particles to travel across a distance. The particles vibrate in a series of compressions and rarefactions as the wave passes. A compression is where the particles of the medium are pushed closer together by the wave and rarefactions are the opposite. The vibrations occur in the same direction as the wave is travelling. As the wave travels through the medium, the particles are displaced, but after the wave has passed, they are in the same position as they started. It is the energy carried by the wave that moves, not the particles. Sound is therefore a mechanical wave and cannot travel through a vacuum. This is why we can see the Sun but we can't hear it and is also partly why double and triple glazing in homes and offices reduces the sound coming from outside.

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Sound waves have a frequency, wavelength, amplitude and period just like all waves. The frequency is the number of compressions (or rarefactions) that pass a given point in one second, the SI units are hertz (Hz). The wavelength is the distance from one point of a compression (or rarefaction) to the equivalent point on the next. The SI units are metres (m). The amplitude is really a measure of how much energy the wave is carrying, in other words its loudness. The period is the reciprocal of the frequency. Louder sounds carry more energy than quiet ones. There is another important term that you need to know about sound waves - pitch. This is how high or how low the note sounds and is determined by the frequency. A high-pitched note has a high frequency and vice versa. Sound whose pitch is so high that it cannot be heard by humans (above about 20 kHz) is called ultrasound and has several uses such as scanning developing babies in the womb and breaking up kidney stones. Sound that is too low-pitched to hear (below about 20 Hz) is called infrasound and is used for studying the heart and the structure of the Earth.

Sound waves can be reflected and diffracted quite easily. When sound is reflected, we call it an echo. One of the favourite echo questions in the GCSE is to get you to calculate the distance from an object using data about an echo. What you must remember is that to hear an echo, the sound has travelled there and back. The biggest mistake that students make is not to divide the distance by two - but do pay attention to the wording of the question as sometimes you may be asked to calculate the distance the sound has travelled, there and back. The speed of sound in air is normally about 330 m/s but is much faster in denser media like water and solids.

Diffraction is one reason why you can hear sound from round a corner. When any wave touches an object that is about the same size at its wavelength, or passes through a gap that is about as wide as its wavelength, it diffracts - spreads out in a circular pattern from the edge of the object.

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  1. What is the pitch of a sound determined by?
    On the tonic sol-fa (doh, ray, mi etc) the first doh has a lower pitch than ray, which has a lower pitch than mi and so on
  2. A car horn produces a sound wave of frequency 680 Hz. When the sound wave is travelling through air, it has a wavelength of 0.5 m. What is the speed of the sound wave?
    Light travels much faster so during a thunderstorm you see the lightning before hearing the thunder. If you count the time taken for the thunder to arrive after the flash, for every 3 seconds you count, the lightning was about one kilometre away
  3. What kind of a wave is a sound wave?
    The energy and vibrations are in the same direction
  4. If a sound wave is made louder, what happens to the amount of energy the wave has?
    The pressure difference between compressions and rarefactions is greater
  5. The car horn and car unfortunately find themselves under water in a river. The car horn still produces a sound wave with a frequency of 680 Hz, however the speed of the sound is dramatically increased to 1485 m/s. What is the wavelength of the sound wave in water?
    The denser the medium, the faster a sound wave will travel
  6. What is an ultrasound wave?
    20 kHz is the average upper limit for human hearing. Many animals including cats, dogs and bats can hear ultrasound
  7. What is the loudness of a sound wave determined by?
    A larger amplitude means the energy carried by the waves is bigger, but the frequency and wavelength remain the same
  8. What are echoes?
    Common examples used in the GCSE are sonar from ships and submarines to measure depth, bats using echo location to find their prey and echoes in the mountains
  9. How do sound waves travel?
    They are waves of alternating higher and lower pressure
  10. If the pitch of a sound wave is increased, does the time period between waveforms increase or decrease?
    Increasing the pitch means the sound wave vibrates more rapidly therefore there will be less time between the arrival of each successive compression or rarefaction

Author: Martin Moore

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