Acoustical Diffraction Near The Earth's Surface
In the case of sound waves travelling near the Earth's surface, the waves are diffracted or bent as they traverse by a geometric edge, such as a wall or building. This phenomenon leads to a very important practical effect: that we can hear "around corners". Because of the frequencies involved considerable amount of the sound energy (on the order of ten percent) actually travels into this would be sound "shadow zone". Visible light exhibits a similar effect, but, due to its much higher frequency, only a minute amount of light energy travels around a corner.
A useful branch of acoustics dealing with the design of noise barriers examines this acoustical diffraction phenomenon in quantitative detail to calculate the optimum height and placement of a soundwall or berm adjacent to a highway.
This phenomenon is also inherent in calculating the sound levels from aircraft noise, so that an accurate determination of topographic features may be understood. In that way one can produce sound level isopleths, or contour maps, which faithfully depict outcomes over variable terrain.
Read more about this topic: Atmospheric Diffraction
Famous quotes containing the words earth and/or surface:
“till seven years were gane and past,
True Thomas on earth was never seen.”
—Unknown. Thomas the Rhymer (l. 79–80)
“See how peaceful it is here. The sea is everything. An immense reservoir of nature where I roam at will.... Think of it. On the surface there is hunger and fear. Men still exercise unjust laws. They fight, tear one another to pieces. A mere few feet beneath the waves their reign ceases, their evil drowns. Here on the ocean floor is the only independence. Here I am free.”
—Earl Felton, and Richard Fleischer. Captain Nemo (James Mason)