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School Physics Notes: SOUND 2. Reflection, refraction and diffraction

SOUND  2. The reflection, refraction and diffraction of sound waves explained using a longitudinal sound wave model

Doc Brown's Physics exam study revision notes

2. The reflection, refraction and diffraction of sound waves and a sound wave model

(a) Reflection of sound waves

When sound waves meet a barrier between two different media they can be reflected just like any other wave (diagram of wavefronts on the right). The angle of incidence will equal the angle of reflection (with respect to the normal at 90o to the boundary). Any solid surface will reflect sound, though soft material will tend to absorb the sound wave energy.

A flat hard smooth surface is the best reflector of sound waves - think of echoes (reflected sounds) that permeate an empty house with no carpets laid down.

A soft rough surface is the best absorber of sound wave energy - the idea is used in recording studios to minimise unwanted sounds and in ear muffs to protect you from ear damage due to very loud sounds.

In reflection there is no change in speed, frequency or wavelength, only the direction of movement of the wave changes.

(b) Refraction of sound waves

(above right diagram, same diagram as wavefronts in reflection)

When waves, including sound, meet a boundary between two mediums, through which they can pass, there is a change in speed related to the difference in density.

The frequency stays the same, but both the speed and wavelength of the sound wave changes.

If the incident wave fronts are parallel to the boundary surface, there is no change in direction

→ IIIIII│IIIIII→

BUT, if the incident angle is NOT 90o, at the same time as reflection, and the sound waves can penetrate the 2nd medium of different density at a boundary, the wave will change speed, wavelength and direction, but no change in frequency.

Therefore if the sound waves have changed direction, the sound waves are refracted.

The number of waves passing through each medium per second is the same.

Since speed = wavelength x frequency (and frequency is constant),

if speed increases, wavelength must get larger,

if speed decreases, wavelength must get smaller.

Although they are longitudinal sound waves, this is normal wave behaviour just as you see with experiments with transverse light waves or water waves.

(c) Diffraction of sound waves

Diffraction is the effect of waves spreading out when passing through a gap or passing by a barrier. In effect, waves bend round corners into the 'shadow zone'! and it doesn't matter if its sound, light or water waves - they all diffract and bend round corners!

You should appreciate that significant diffraction of sound waves 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 sound wave - but the sound waves still bend round the 'corners' into the 'shadow zones'.

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.

Keywords, phrases and learning objectives for sound waves

Know that sound waves can be reflected, refracted and diffracted and the behaviour of sound waves can be explained using a longitudinal sound wave model.

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