The parallel lines are actually diffraction patterns. As the fingers approach each other and come very close together, you begin to see a series of dark lines parallel to the fingers. We’ve been using sound as an example since it has a much longer wavelength (from a few centimetres to a few metres depending on the frequency) and so objects such as the edges of walls will cause diffraction and enable sound to travel round corners.A very simple demonstration of diffraction of waves can be conducted by holding your hand in front of a light source and slowly closing two fingers while observing the light transmitted between them. We can set up specialised experiments in the lab to demonstrate light diffraction, but if you’re in the shadows on a sunny day, diffraction around the obstruction to the direct sunlight is not going to get you a tan. Light waves have a very small wavelength (typically 500 nm, although of course it changes with colour) and diffract noticeably in everyday life. Seats in a theatre or stadium are on a slope so you can see and hear well. If you’re behind someone tall, then not only do you not get to see the musicians, you also get less direct sound from the stage because it has to diffract around the tall person’s head. Going back to acoustics, you’ll want to avoid this shadowing effect when you go to see a band or orchestra play. This is another way of picturing the bending that diffraction describes. This can’t happen, some of the waves must get into the flat water. If the water inside the harbour stayed dead calm, then somewhere near the harbour mouth you would see completely flat water immediately next to big waves. Some of the energy will reflect, but at the end of the barrier (near the opening of the harbour) the waves will bend around and come inside. If you live near the sea, have a look at waves on a windy day hitting a harbour wall. Acoustic Consultant, Vicky StewartĬareer case study: Why Vicky chose to study acoustical engineeringĪ really good example of diffraction can be seen with another type of wave barrier – a harbour or dock wall. One mitigation measure I proposed was to use the villa’s own infinity swimming pool to screen the traffic sound, acting like a noise barrier. The developer was worried about the noise from a road. One of my favourite projects was for the developer of luxury holiday villas in the hills just outside Paphos, Cyprus. Photo © Robin Stott ( cc-by-sa/2.0) Case study: an unusual noise barrier Sometimes the barriers are made of earth, in which case they’re called a bund. Look out for heavily-built fences along the side of motorways – these are noise barriers. But low frequencies are less attenuated by the barriers because they can diffract (bend) over the top. Noise barriers alongside roads reduce the traffic sound for houses in the shadow zone. Because if you stand there you are in an acoustic shadow (just like the optical shadows we see) and the sound is quieter.Īcoustic shadow zones are exploited by acoustical engineers to reduce noise. In acoustics, we use the term shadow zone to describe the area behind the object. At high frequency, when the wavelength is small compared to the object size, then the sound does not diffract very effectively.
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