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Physics - Sound: diffraction

Diffraction

The diffraction [1] of sound makes it possible for us to be heard around
the corners of a building, or from one room in a house to another. Sound
waves will bend, that is diffract, as they travel through an aperture or
past an obstacle. You may have noticed that in situations such as these the
deeper tones are more likely to be heard by the receiver than are the high
frequency sounds.

Water waves through a small opening video.

Significant diffraction occurs when the wavelength of the sound is at
least as large as the width of the aperture or obstacle. Our knowledge of
diffraction explains why the musicians playing higher-pitched tones such as
the flautist are placed at the front of an orchestra and bass instruments
at the back. The deep notes have longer wavelengths and are able to
diffract around the musicians sitting in front of them. Recall that in a
uniform medium the velocity of sound is constant and so the wavelength of
the sound is inversely proportional to the frequency of the sound. Hence
deeper notes have longer wavelengths. You must be aware that it is the
wavelength that determines the degree of diffraction caused by a given
aperture or obstacle. Explanations should not be stated purely in terms of
frequency.

Video of long and short water waves around an object.

If high frequency sound waves pass an obstacle they diffract less due to
their shorter wavelength. They are more likely to leave a 'shadow' region
behind the obstacle in which no sound would be heard. Longer wavelengths
diffract more and hence fill the region behind the obstacle.

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