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DIFFRACTION

DIFFRACTION

 

Sound waves propagate spherically. For smaller wavelengths and greater distance from the source, we can perceive them as waves with a relatively flat front and a direction perpendicular to it.

The ability of sound waves in meeting obstacles to go around them, penetrating into the area behind them, is called diffraction. Without a diffraction the sound would be audible only in a situation of direct sight to the sound source.By the diffraction we can hear sound behind barriers, penetrating through different holes etc.

Light diffraction also occurs, but due to much smaller wavelength of the light wave in normal conditions it does not result in circumvention of the objects. Here we observe other interesting side effects, such as contrast edges of the observed object, especially when backlit. Diffraction effects in the electromagnetic waves occur all the way in atomic and subatomic level.

We can locate a diffraction on several occasions – passing of a wave along an edge, passing of a wave through an obstacle or passing through an slit:
waves5

 

 

 

 

 

 

 

 

This is due to the Huygens principle, according to which each point, at which the wave reaches, becomes a new point source of spherical waves. For this reason, in the above example when the sound wave reaches the edge of the obstacle, it converts this edge to a source of a spherical wave and thus surrounds it.

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If it reaches  small aperture that is commensurate with the wavelength, this opening becomes a point source of the spherical wave.

 

 

 

 

At presence of a diffraction we have no change in the wavelength or the frequency or the speed of the wave. The direction of propagation is what only changes.

Here’s an illustration of the diffraction in nature:

Refraction

 

 

How the diffraction is connected to the wavelength?

 

 

diffisland1. If the wavelength is much larger from the the obstacle that meets, it surrounds it with hardly be affected by it and it changes the shape and the structure too little.
2. If the wavelength is comparable to the size of the object it surrounds partially, and we have a acoustic shadow just behind it.
3. If the wavelength is less than the size of the obstacle, we have an acoustic shadow and only a small portion of the sound enters behind the object .

As much as greater is the size of the obstacle relatively to the wavelength, the long and clear acoustic shadow we have behind it.

 

Therefore it can be said that in the area of high frequencies we have dissipation and reflection of the sound waves in meeting an obstacle, and in the area of the lower frequencies – diffraction and surrounding of the obstacle. In this situation, for example in a hall the objects that are in front of us (other people, balconies, columns, etc.). will carry frequency change of the sound while skipping with a priority the lower frequencies at the expense of the high ones.

The same situation occurs in acoustic barriers when needed to reduce the traffic noise from residents. Since the diffraction of the sound depends on the ratio between the wavelength and the size of the barrier, so with the same barrier we will have different efficiency at different frequencies due to the diffraction:

fig9

 

As we see, the high frequencies can be stopped successfully  from the barrier while the low turn around it by the effect of diffraction. In this situation, only the increase in the size of the barrier would help for its greater efficiency.

 

Diffraction when the sound is passing trough aperture

 

The diffraction when passing through aperture also depends on the ratio between the aperture and the wavelength.

If the opening is much larger than the wavelength, we virtually can not observe diffraction, the  wave passes freely. However, due to the fact that the wavefront is not virtually changed, are created zones of acoustic shadow caused by the barrier in which the aperture is located.

Upon aperture commensurate or smaller than the wavelength we observe diffraction processes on both its sides. In this situation, the aperture starts to react as a new point of spherical radiation spreading sound in the form

Screen Shot 2014-04-14 at 11.29.46 AM

of a hemisphere behind the barrier. Thus, the acoustic shadows disappear and the location of such a sound ceases to be from the point of its initial transmission, but is displaced from the point of the spherical secondary radiation, i.e. aperture:

 

 

 

 

 

Diffraction is an important phenomenon in acoustics because it affects many processes associated with the propagation of sound.

For example, in the manufacture of microphones and loudspeakers the diffraction processes can lead to a strong nonlinearity and to change of the direction of the sound at some frequencies relative to others.  In the construction and operation of halls and studios also the diffraction is an important factor.

Adam-F7_01

In the field of sound insulation and sound absorption also is important to know that even in well-executed work on isolation between  two  rooms or between room and ? sound source the  presence of even a small hole leads to a spot radiation from this hole,  which is able to compromise the rest of the process .

 

 

 

 

 

Here we can see the borders of the speaker, which are made in this way to avoid the diffraction.

 

 

 

Useful links:

http://www.scienceclarified.com/everyday/Real-Life-Physics-Vol-2/Diffraction-How-it-works.html

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html

http://www.physicsclassroom.com/class/sound/Lesson-3/Reflection,-Refraction,-and-Diffraction

http://study.com/academy/lesson/diffraction-relation-to-sound-light-and-effects-on-wavelength.html

http://www.acoustics.salford.ac.uk/feschools/waves/diffract2.php

Now, make this small test:

http://www.acoustics.salford.ac.uk/feschools/waves/diffract.php#diffraction

and an example how diffraction, reflection and the doppler effect are used in creating a virtual sound space:

Filed under: ESPOL, Sound theory

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