Wave Diffraction:

Important parts of our experience with sound involve diffraction. The fact that you can hear sounds around corners and around barriers involves both diffraction and reflection of sound. Diffraction in such cases helps the sound to "bend around" the obstacles. The fact that diffraction is more pronounced with longer wavelengths implies that you can hear low frequencies around obstacles better than high frequencies, as illustrated by the example of a marching band on the street. Another common example of diffraction is the contrast in sound from a close lightning strike and a distant one. The thunder from a close bolt of lightning will be experienced as a sharp crack, indicating the presence of a lot of high frequency sound. The thunder from a distant strike will be experienced as a low rumble since it is the long wavelengths which can bend around obstacles to get to you. There are other factors such as the higher air absorption of high frequencies involved, but diffraction plays a part in the experience.

You may perceive diffraction to have a dual nature, since the same phenomenon which causes waves to bend around obstacles causes them to spread out past small openings. This aspect of diffraction also has many implications. Besides being able to hear the sound when you are outside the door as in the illustration above, this spreading out of sound waves has consequences when you are trying to soundproof a room. Good soundproofing requires that a room be well sealed, because any openings will allow sound from the outside to spread out in the room - it is surprising how much sound can get in through a small opening. Good sealing of loudspeaker cabinets is required for similar reasons.

Another implication of diffraction is the fact that a wave which is much longer than the size of an obstacle, like the post in the auditorium above, cannot give you information about that obstacle. A fundamental principle of imaging is that you cannot see an object which is smaller than the wavelength of the wave with which you view it. The reason for that limitation can be visualized with the auditorium example: the sound waves bend in and reconstruct the wavefront past the post, having a shadowing effect.

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.

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.

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?

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).

Credits to

[http://www.acoustics.salford.ac.uk/feschools/waves/diffract2.htm]

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