Possible behaviors include reflection off the obstacle, diffraction around the obstacle, and transmission accompanied by refraction into the obstacle or into a new space. When a sound wave reaches the boundary between one space and another, a portion of the wave undergoes reflection and a portion of the wave undergoes transmission across the boundary.
The amount of reflection is dependent upon the dissimilarity of the two spaces. For this reason, acoustically minded builders of auditoriums and concert halls avoid the use of hard, smooth materials in the construction of their inside halls.
A hard material such as concrete is as dissimilar as can be to the air through which the sound moves; subsequently, most of the sound wave is reflected by the walls and little is absorbed. Walls and ceilings of concert halls are made of softer materials such as fiberglass and acoustic tiles. These materials are more like air than concrete and thus have a greater ability to absorb sound. This gives the room more pleasing acoustic properties. Reflection of sound waves off surfaces can lead to one of two phenomena - an echo or a reverberation.
A reverberation often occurs in a small room with height, width, and length dimensions of approximately 17 meters Why the magical 17 meters? The effect of a sound wave upon the brain endures for more than a tiny fraction of a second; the human brain keeps a sound in memory for up to 0. If a reflected sound wave reaches the ear within 0. The reception of multiple reflections off walls and ceilings within 0. Perhaps you have observed reverberations when talking in an empty room, when honking the horn while driving through a highway tunnel or underpass, or when singing in the shower.
In auditoriums and concert halls, reverberations occasionally occur and lead to the displeasing garbling of a sound. Reflection of sound waves in auditoriums and concert halls do not always lead to displeasing results, especially if the reflections are designed correctly.
Smooth walls tend to direct sound waves in a specific direction. Subsequently the use of smooth walls in an auditorium will cause spectators to receive a large amount of sound from one location along the wall; there would be only one possible path by which sound waves could travel from the speakers to the listener. The auditorium would not seem to be as lively and full of sound.
Rough walls tend to diffuse sound, reflecting it in a variety of directions. This allows a spectator to perceive sounds from every part of the room, making it seem lively and full. For this reason, auditorium and concert hall designers prefer construction materials that are rough rather than smooth. Reflection of sound waves also leads to echoes. Echoes are different than reverberations. Echoes occur when a reflected sound wave reaches the ear more than 0.
This law will be discussed in more detail in Unit 13 of The Physics Classroom. The discussion above pertains to the reflection of waves off of straight surfaces. But what if the surface is curved, perhaps in the shape of a parabola? What generalizations can be made for the reflection of water waves off parabolic surfaces?
Suppose that a rubber tube having the shape of a parabola is placed within the water. The diagram at the right depicts such a parabolic barrier in the ripple tank. Several wavefronts are approaching the barrier; the ray is drawn for these wavefronts. Upon reflection off the parabolic barrier, the water waves will change direction and head towards a point.
This is depicted in the diagram below. It is as though all the energy being carried by the water waves is converged at a single point - the point is known as the focal point. After passing through the focal point, the waves spread out through the water.
Reflection of waves off of curved surfaces will be discussed in more detail in Unit 13 of The Physics Classroom. Reflection involves a change in direction of waves when they bounce off a barrier. Refraction of waves involves a change in the direction of waves as they pass from one medium to another. Refraction, or the bending of the path of the waves, is accompanied by a change in speed and wavelength of the waves.
In Lesson 2 , it was mentioned that the speed of a wave is dependent upon the properties of the medium through which the waves travel. So if the medium and its properties is changed, the speed of the waves is changed. The most significant property of water that would affect the speed of waves traveling on its surface is the depth of the water. Water waves travel fastest when the medium is the deepest. Thus, if water waves are passing from deep water into shallow water, they will slow down.
And as mentioned in the previous section of Lesson 3 , this decrease in speed will also be accompanied by a decrease in wavelength. So as water waves are transmitted from deep water into shallow water, the speed decreases, the wavelength decreases, and the direction changes. This boundary behavior of water waves can be observed in a ripple tank if the tank is partitioned into a deep and a shallow section. If a pane of glass is placed in the bottom of the tank, one part of the tank will be deep and the other part of the tank will be shallow.
Waves traveling from the deep end to the shallow end can be seen to refract i. When traveling from deep water to shallow water, the waves are seen to bend in such a manner that they seem to be traveling more perpendicular to the surface.
If traveling from shallow water to deep water, the waves bend in the opposite direction. At the interface between two media, part of the light is reflected, and the other part passes through the interface with a modified propagation direction: this is called refraction. This process is observed in a prism like the one in the vignette of the focus Deviation of light by a prism link.
The different wavelengths of white light have different refractive angles, which leads to colour separation. Water drops or ice crystals can act as prisms, which leads to rainbows Spectacular Rainbows or other atmospheric halos phenomena Atmospheric Halos. In clouds, reflections and refractions are multiple, which blurs the separation of colors, and restores the white color of sunlight. Figure 3. Diffraction of polychromatic light through a circular orifice.
The central spot is white due to the superposition of all wavelengths. Peripheral spots represent the entire spectrum of visible colours, with the shift in various wavelengths increasing as we move away from the centre.
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