Diffraction Effects

At high frequency, when the wavelength is small compared to the object size, then the sound does not diffract very effectively. In acoustics, we use the term "shadow zone" to describe the area behind the object, because if you stand there, you are in an 'acoustic shadow' (just like the 'optical shadows' we see on those rare occasions when the sun comes out) and the sound is quieter than elsewhere. This is exploited in acoustics to reduce noise levels. For instance, noise barriers can be put up alongside major roads - houses behind the barriers are exposed to less noise if they are in the shadow zone (but remember - low frequencies are unaffected by the barriers and can diffract over the top). Look out for heavily-built fences along the side of motorways in built-up areas - these are noise barriers. Sometimes the barriers are made of earth, in which case they're called a 'bund' (really, they are!).

A really good example of diffraction can be seen with another type of wave barrier - a harbour or dock wall. If you live near the sea, have a look at waves on a windy day hitting a harbour wall. 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. Think about it...if this did not happen, and the water inside the harbour stayed dead calm, then somewhere near the harbour mouth you would see completely 'flat' water immediately next to very 'wavy' water. This can't happen - the wavy water has to transmit its energy into the flat water - and this is another way of picturing the 'bending' which diffraction describes.

 

Man with a large hat watching a band play music on stage. His hat is blocking the view of the band.

Going back to acoustics - you might want to avoid this shadowing effect when you go to see a band or orchestra play...especially if it's a 'standing' gig and you're not so tall. If you're behind someone taller, 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 head (and perhaps hat) in front. Seats in theatres and stadia are 'raked' not only because it gives you a good line of sight, but also because it improves the sound quality.

Light waves have a very small wavelength (typically 500nm, although of course it changes with colour) and so do not diffract noticeably. We can set up specialised experiments in the lab to demonstrate light diffraction, but if you're on the beach and someone is standing in your sun, diffraction around the 'obstruction' is not going to get you a tan. 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.

Diffraction also plays an important role in allowing us to locate sources of sound. If you close your eyes, you can tell which direction sound is coming from. How does this work?

If sound travels to someone's head from straight ahead the sound reaches both ears together. If the sound comes from the side it reaches one ear before the other and has to travel around the head to reach the other ear.

When sound reaches you from straight ahead, the same sound signal is received at both ears. This is because the head is more-or-less symmetrical and the sound to both ears travels an identical path length. Your brain uses this information to locate the sound in front of you.

When sound comes from the side (directly, or via a reflection as shown above), the sound at each ear is different. Sound to the furthest ear has to diffract (bend) around the head. This means the sound wave arrives slightly later and is altered in terms of the balance of high and low frequencies it contains (we could call this a spectral alteration). As we have seen, sounds with short wavelength (high frequency) don't diffract as well, so the furthest ear hears less high frequencies. The brain senses this difference in arrival time and frequency content, and uses it to locate sound.

Try locating sound sources with a finger in one ear and your eyes shut. Make sure no-one is around to watch you do this, unless you have previously warned them what is about to happen...

Radio and TV Broadcasts

Diffraction also affects the way in which electromagnetic (radio) waves are broadcast and recieved for radio and TV signals.

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TV and VHF radio signals have wavelengths of around a few metres. This means they cannot diffract over hills or large buildings. The receiver must be in direct line-of-sight with the transmitter. Repeater stations are often positioned at the top of hills to reach all the houses in the valley that would otherwise be in the 'shadow' of the hill.

Long-wave radio is sent using waves with a much larger wavelength of around 1km. This means they can diffract around objects including hills and buildings; they can reach places that short-wave radio cannot. This is why it is often possible to listen to long wave radio stations such as radio 4, even when FM reception is poor. It's also why stations on long wave (BBC Radio 4 - 198LW) are tuned to the same frequency wherever you go - there's only one transmitter for the whole of England, Wales and Ireland (at Droitwich).

By contrast, FM transmitters only cover a small region - to see what frequencies BBC stations are broadcast on in your area, check out http://www.bbc.co.uk/radio/frequencies/ . This limited coverage is why you have to continually re-tune a car radio when listening to FM on a long journey...although if you have an 'rds' radio, it does this for you.

Question: Why can't BBC Radio 1 be broadcast on 98.9 FM over the whole country, using a large number of local transmitters all tuned to the same frequency? (Hint - think about superposition, and constructive and destructive interference).

University of Salford, Salford, Greater Manchester M5 4WT, UK. Telephone: +44 (0)161 295 5000 | Fax: +44 (0) 161 295 5999
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