Goal
Understand what diffraction is.
Diffraction
Picture this, your sitting in a waiting room of a doctor’s office. Suddenly you hear yelling from one of the rooms, and this stresses you out. Yet how can you hear the yelling? The door is closed, and the chance of reflections reaching you aren’t that great because they are a ways back in the building, yet you hear them! It can’t be transmission through the walls because there are simply to many of them. The answer is diffraction. The sound is bending around objects, specifically under doors in this case, and that is why you can hear them.
Diffraction is how a wavefront is spread out as a result of passing an edge or going through a narrow aperture (hole). Waves are always seeking the best way to spread out their energy and so when they pass through a hole to a new space the wave will encounter a bunch of space. It would be inefficient to spread out in only one direction, so it spreads out in all directions. This occurs at holes, edges and corners. As a result these effects are sometimes called edge diffraction and at the spaces they act like a new point source.
This principle is called Huygens’ Principle which is: Every point on the wavefront of sound that has passed through an aperture or passed a diffracting edge is considered a new point source radiating energy into the shadow zone. Where the shadow zone is the area the sound would not have gone if it didn't diffract.
A major property of diffraction is that diffraction is frequency dependent as a function of size. This size dependency means that if it’s a low frequency, which has a large wavelength, then it will diffract more around objects that are smaller than its wavelength.
In order to avoid diffraction you must have a very large object so that it cannot bend around it and must instead reflect of it, transmit through it, or get absorbed. Because of this, higher frequencies tend to be more directional while lower frequencies tend to go everywhere since they just diffract around most things. Leveraging this has led to many technologies such as sonar and ultrasound.
Diffraction is a straightforward problem. It boils down to this: slits and edges = bad. The bigger the slit the more noise can get through. If you have a doorway with a gap at the bottom you can bet sound will diffract around that. It’s easy enough to solve, just plug it up! The material you use to do this will be important. Dense rubber will fare better than just cramming paper into the hole because now you’re dealing with transmission and reflection.
Here is the simulation of a small gap and a large one. Note how much more bending happens for the small gap creating a "shadow zone". Many structures such as highway walls try to leverage this shadow zone.
In general, a wide gab will actually cause less diffraction. Keep in mind “wide” is a relative term since it may be wide to high frequencies but not to low frequencies!
So, if you want more diffraction simply close the gap a bit and boom! More diffraction. Notice that each aperture acts like a new sound source. This can lead to some complex interference patterns:
If a barrier is placed, it will form an acoustic shadow. This is an area that the sound cannot reach due to the barrier in the way, however, some frequencies may be so large that they simply diffract around the barrier and spill into the acoustic shadow. This can be a problem with walls that are constructed for the very purpose to stopping noise. If the wall isn’t tall enough low frequencies will simply diffract around it! If the wall isn’t long enough low frequencies again will just diffract around it!
Diffraction Cases
We can take advantage of the fact that diffraction bends around edges and corners. Absorbers placed in a room will have 2 ways to absorb the sound! First on the initial sound that hits the absorber and then again on the sound that diffracts at the edge. That is why when placing absorbers, it is a good idea to place them spaced out, taking full advantage of diffraction and getting the most absorption for your money!
Another case in which diffraction is an issue is in designing speakers, or even sound measurement devices. If there are many sharp corners and edges so, the sound will diffract around those and can alter the frequency response and pressure in those places. For this reason, most speakers are designed with curved corners to deal with the diffraction. Sound meters are designed with rounded smooth bodies and a stalk that the microphone is mounted on the keep it away from the body of the device this way its readings are more accurate.
Loudspeaker Diffraction
Loudspeakers that are placed near walls will have significant diffraction effects. The design of how the cabinets loudspeaker is placed also has a large influence.
Here is a great article on Cabinet edge diffraction and why it is an issue and how to possibly solve it.
Here is another one on baffle design.
Great video explaining the importance of flush mounting in speaker baffles:
Questions
Given we have 8 square absorbers to place on a wall, what placement will provide the most absorption? Assume there are no issues with early reflections or other acoustic problems.
Given we have an gobo that is 4 feet tall by 3 feet wide what frequency range do we expect it to effect in each dimension? Assume the gobo is totally absorptive so the depth will not matter. Estimate which frequencies will start diffracting and which is be absorbed.
You are recording a vocal but there is a bass sound coming from around a corner. You shield the mic by placing a 6in by 6in acoustic foam barrier between the mic and the corner. Will it improve the clarity noticeably? What if the sound from the around the corner was a flute instead?
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