Sound waves - properties explained, uses of sound including ultrasound

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

This page will answer many questions e.g.

 Why is sound a longitudinal wave?

 Why can't sound travel through a vacuum?

 Know that and understand that sound waves and some mechanical waves are longitudinal, and cannot travel through a vacuum.

Describe an experiment to measure the speed of sound.

What are the uses of ultrasound?

The above diagram gives an idea of a longitudinal wave where the oscillations are in the direction the wave moves


eg sound waves, push and pulling on a slinky spring

Know and understand that longitudinal waves show areas of compression and rarefaction (diagram above illustrates longitudinal sound waves, wave B has twice the frequency and half the wavelength of wave A).

  • (a) Know and understand that sound waves are longitudinal waves and cause vibrations in a medium, which are detected as sound.

    • The oscillations (rarefactions <=> compressions) of longitudinal waves are in the same direction as the wave motion.

      • For transverse waves like water waves or electromagnetic radiation, the oscillations are at 90o to the direction of wave movement.

      • Unlike electromagnetic light waves, sound cannot travel through empty space (vacuum) because you need a material substance (gas, liquid or solid) which can be compressed and decompressed to transmit the wave vibration.

    • Longitudinal waves eg sound, show areas of compression and rarefaction (diagram above illustrates longitudinal sound waves, wave B has twice the frequency and half the wavelength of wave A).

      • The wavelength equals the distance between two compressions.

    • Sound waves are produced by mechanical vibrations and travel through any medium, gas, liquid or solid, but not vacuum, where there is nothing to vibrate!

      • In music, if a middle C tuning fork is struck, the two prongs vibrate from side to side 262 times every second ie middle C has a frequency or pitch of 262 Hz.

      • The more dense a material, the faster the sound wave travels. Typically at room temperature, the speed of sound is 340 m/s but in steel its 6000 m/s.

    • Sound is limited to human hearing and no details of the structure of the ear are required.

      • Your ear drum resonates with a sound wave hitting it and via some bones and nerve receptors, 'sound impulses' are transmitted to the brain.

  • (b) Know that the pitch of a sound is determined by its frequency and loudness by its amplitude.

    • The frequency equals the number of compressions passing a point per second, and is perceived as the pitch eg of a musical note.

      • The frequency of sound is the number of vibrations per second (unit hertz, Hz).

    • The amplitude is the maximum compression with respect to the 'rest line' and is perceived as loudness.

      • The 'rest line' is effectively the point of no disturbance, zero amplitude - neither compression or rarefaction.

      • The rest line is represented by the horizontal red line on the CRO diagrams below.

      • The four pictures could represent the sound waves of musical notes recorded by a microphone, converted to an electronic signal and displayed in wave form on an oscilloscope screen (CRO).

      • 1. has the smallest amplitude, the softest note

      • 2. has the largest amplitude, the loudest note

      • 3. has the longest wavelength, lowest frequency, lowest pitch eg a base note sung by a deep base singer.

      • 4. has the shortest wavelength, highest frequency, highest pitch, eg a treble note or a squeaky animal.

      • Note that ...

        • the higher the pitch or frequency, the smaller the wavelength, and,

        • the bigger the amplitude, the louder the sound (and conveying more energy).

  • (c) Know that echoes are reflections of sounds.

    • Sound waves are reflected of hard flat surfaces eg walls, but tend to be absorbed by rough soft surfaces eg like foam -used in ear protectors.

      • Note the difference in echoes between an empty bare room in a house and when it is carpeted and filled with furniture and curtains etc.

    • Echoes are heard when you shout towards a hard flat surface and you then hear the reflected sound waves impacting on your inner ear drum.

      • The further away a reflecting surface is, the longer the time interval between your shout and hearing the echo.

      • If the wall or side of a mountain or valley is 340 m away, its 2 seconds before you hear the echo (speed of sound 340 m/s).

      • If the reflecting surface is a km (1000 m) away, its about 6 seconds before hear the echo.

        • speed = total distance / total time, time = distance/speed, time = 2000/340 = 5.9 s

        • See formulae near the end of the page

    • You can hear sounds from some distance throughout a building or even a wide area outside because the sound waves are reflected and bounced around by all hard flat surfaces BUT sound waves are also diffracted and can therefore bend round corners into your ear!

      • The further away you are from the sound source, the fainter it will sound, because on every reflection some of the sound wave energy is absorbed - remember the louder the sound, the more energy is transmitted per second.

  • Diffraction is the effect of waves spreading out when passing through a gap or passing by a barrier. In effect, waves go round corners! and it doesn't matter if its sound, light or water waves - they all diffract and bend round corners! The effect is so small with light (tiny wavelength), you don't notice it, but you see water waves bending around objects at the seaside and you can hear sounds from round a corner.

  • You should appreciate that significant diffraction only occurs when the wavelength of the sound wave is of the same order of magnitude as the size of the gap or obstacle.

  • A: There is a relatively small diffraction effect when sound waves pass through a wide gap that is much bigger than the wavelength of the wave.

  • B: You get the maximum spreading or diffraction when the waves pass through a gap of similar size to the wavelength of the incident sound waves.

An experiment to measure the speed of sound

The experiment is performed by connecting a loud speaker to a signal generator to generate the sound to be picked up by the microphones.

You select a particular known specific frequency e.g. 250 Hz (f in Hz).

Two microphones are connected to an oscilloscope which pick up the sound from the speaker and which is converted to an electrical signal by the microphone and displayed as a trace on the cathode ray oscilloscope screen.

You can secure the speaker and two microphones with stands and clamps making sure they are aligned at the same height.

You set up the oscilloscope to detect the sound wave signals from both microphones - to give you two traces on the screen.

You start with the two microphones close together.

You then slowly move one microphone away from the other.

When the two microphones are first exactly one wavelength apart, the two signal traces on the oscilloscope are exactly aligned - synchronised, as in the diagram above.

You then measure the distance between the microphones and this gives you the wavelength of the sound.

This is because the sound waves are aligned so that they are just one wavelength apart.

speed of sound wave (m/s) = frequency of sound (Hz) x wavelength of sound wave (m)

in 'shorthand'    v = f x λ

you know the frequency in Hz from the signal generator setting

and the wavelength is the distance between the microphones in cm ==> m


You repeat the experiment to calculate the average wavelength to give statistically the best result.

You can then repeat the experiments with other frequencies from the signal generator.



Ultrasound is a very high frequency sound wave used in scanning pregnant women to monitor the progress of unborn baby.

The ultrasound waves enter the woman's body and the echoes-reflections are picked up by a microphone and converted into electronic signals from a which an internal picture can be constructed.

Tissues or fluids of different density give different intensities of reflection and so differentiation of the structure of the womb, foetus or baby can be seen.



Be able to use both the equations below, which apply to sound waves (and their rearrangements):

  • appropriate units used in ()

  • a) sound wave speed (metre/second, m/s) = frequency (hertz, Hz) x wavelength (metre, m)

    • in 'shorthand'    v = f x λ

      • rearrangements:  f = v / λ   and   λ = v / f

  • b) sound wave speed (metre/second, m/s) = distance (metre, m) / time (second, s)

    • in 'shorthand'    v = d / t

      • rearrangements:  d = v x t   and   t = d / v

    • This is the general formula for the speed or velocity of anything moving.

  • (a), (b)


  • Check out your practical work you did or teacher demonstrations you observed in Unit P1.5, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

    • demonstrating transverse and longitudinal waves with a slinky spring

    • demonstrating the Doppler effect for sound.

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