The absence of sound is the result of the particles remaining at rest and behaving as though there were no disturbance passing through it.
Now if a particular location along the medium repeatedly experiences the interference of a compression and rarefaction followed up by the interference of a rarefaction and a compression, then the two sound waves will continually cancel each other and no sound is heard. This is a form of destructive interference. The tendency of the compression to push particles together is canceled by the tendency of the rarefactions to pull particles apart the particles would remain at their rest position as though there wasn't even a disturbance passing through them.
The net effect of a compression (which pushes particles together) and a rarefaction (which pulls particles apart) upon the particles in a given region of the medium is to not even cause a displacement of the particles. Now if two sound waves interfere at a given location in such a way that the compression of one wave meets up with the rarefaction of a second wave, destructive interference results. Note that compressions are labeled with a C and rarefactions are labeled with an R. The animation below shows two sound waves interfering constructively in order to produce very large oscillations in pressure at a variety of anti-nodal locations. As mentioned in a previous unit, locations along the medium where constructive interference continually occurs are known as anti-nodes. The loudness of the sound is the result of the particles at that location of the medium undergoing oscillations from very high to very low pressures. Now if a particular location along the medium repeatedly experiences the interference of two compressions followed up by the interference of two rarefactions, then the two sound waves will continually reinforce each other and produce a very loud sound. This is also an example of constructive interference. If two rarefactions (two low-pressure disturbances) from two different sound waves meet up at the same location, then the net effect is that that particular location will experience an even lower pressure. This is a form of constructive interference. For example, if a compression (high pressure) of one wave meets up with a compression (high pressure) of a second wave at the same location in the medium, then the net effect is that that particular location will experience an even greater pressure. The interference of sound waves causes the particles of the medium to behave in a manner that reflects the net effect of the two individual waves upon the particles. And as a rarefaction passes through a section of a medium, it tends to push particles apart, thus creating a low-pressure region. As a compression passes through a section of a medium, it tends to pull particles together into a small region of space, thus creating a high-pressure region. In two cases (on the left and in the middle), constructive interference occurs and in the third case (on the far right, destructive interference occurs.īut how can sound waves that do not possess upward and downward displacements interfere constructively and destructively? Sound is a pressure wave that consists of compressions and rarefactions. The diagrams below show two waves - one is blue and the other is red - interfering in such a way to produce a resultant shape in a medium the resultant is shown in green. This type of interference is known as destructive interference. If an upward displaced pulse and a downward displaced pulse having the same shape meet up with one another while traveling in opposite directions along a medium, the two pulses will cancel each other's effect upon the displacement of the medium and the medium will assume the equilibrium position. This type of interference is known as constructive interference. As mentioned in a previous unit of The Physics Classroom Tutorial, if two upward displaced pulses having the same shape meet up with one another while traveling in opposite directions along a medium, the medium will take on the shape of an upward displaced pulse with twice the amplitude of the two interfering pulses. The interference of waves causes the medium to take on a shape that results from the net effect of the two individual waves upon the particles of the medium. Wave interference is the phenomenon that occurs when two waves meet while traveling along the same medium.
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