Mechanical waves

Diffraction is not as common a wave behaviour as reflection and refraction because it involves a specific set of conditions.

The amount of diffraction depends on the wavelength of a wave, and the width of an obstacle or opening with which the wave, interacts.

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To discover the relationship try this activity.

• Select the wavelength of the waves using the wavelength dial.
• Select the width of the opening using the width dial.

1

When the width of the opening is fixed and the wavelength of the waves is decreased, describe what happens to the amount of diffraction. The diffraction is: a) decreased b) constant c) increased Correct. The diffraction decreases as the wavelength decreases.Incorrect. The diffraction varies with wavelength. Try again.Your answer has been saved.

2

The diffraction is most noticeable when the width of the opening is: a) much smaller than the wavelength of the waves b) the same length as the wavelength of the waves c) much larger than the wavelength of the waves Correct. The diffraction increases when the opening is made larger.Incorrect. There is some diffraction. Try again.Your answer has been saved.

3

High frequency sound waves have a smaller wavelength than low frequency sounds. If you listen to traffic sounds from vehicles on a nearby road, how does this compare to the noise level when you are positioned along a side street, next to a tall building facing the road? a) The sound has much less high-frequency noise. b) The frequencies of the sound are identical, but they have smaller amplitudes. c) The sound has much less low-frequency noise. Correct. The sound lacks 'sharpness'.Incorrect. The sound is not as loud but there is a difference in the frequencies diffracted. Try again.Your answer has been saved.

Noise from vehicle traffic along freeways and highways through urban centres is minimised thanks to constructed sound barriers. Although these barriers provide a direct blocking action to road noise, they are principally designed to minimise diffracted sound. Road noise peaks in intensity at a frequency of about 1000 Hz ( ≈ 0.34 m) mainly due to the contribution of tyres rolling over the road surface. Barriers with heights much greater than this wavelength can markedly reduce any diffracted sound at this or smaller wavelengths. Larger wavelength noise diffracted over the barriers is minimised by incorporating design features on the top edge of barriers, such as angled extensions.

Your ability to locate someone talking in an open space, without first seeing him or her, is somewhat dependent on diffraction. Try placing your hand over one ear, closing your eyes and mentally locating the source of a sound. Now keep your eyes closed and remove your hand from your ear. Your head acts as an obstacle around which sound waves with wavelengths much longer than the width of your head diffract around.

This allows your brain to receive sound signals separated in time, and different in amplitude and in their range of frequencies. Your brain detects the lack of higher frequencies received by your ear further away from the sound and uses this information, and particularly the amplitude and time differences, to estimate the position of the sound source.