Add to collection. Related content This article is part of an article series : Sound — understanding standing waves Sound — visualising sound waves Sound — resonance Sound — beats, the Doppler effect and sonic booms with accompanying investigations: Measuring the speed of sound Investigating sound wave resonance Visit the sound topic for additional resources. Useful link Visit this website to view an excellent simulation of a ripple tank for demonstrating wave-related phenomena.
Go to full glossary Add 0 items to collection. Download 0 items. Twitter Pinterest Facebook Instagram. Email Us. See our newsletters here. Would you like to take a short survey? This survey will open in a new tab and you can fill it out after your visit to the site. Yes No. Sound is a pressure wave that consists of compressions and rarefactions. 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.
And as a rarefaction passes through a section of a medium, it tends to push particles apart, thus creating a low-pressure region. 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. 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.
This is a form 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 also an example of constructive interference. 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.
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. As mentioned in a previous unit , locations along the medium where constructive interference continually occurs are known as anti-nodes. 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.
Note that compressions are labeled with a C and rarefactions are labeled with an R. 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.
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.
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.
This is a form of destructive interference. 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.
The absence of sound is the result of the particles remaining at rest and behaving as though there were no disturbance passing through it.
Amazingly, in a situation such as this, two sound waves would combine to produce no sound. As mentioned in a previous unit , locations along the medium where destructive interference continually occurs are known as nodes.
A popular Physics demonstration involves the interference of two sound waves from two speakers. The speakers are set approximately 1-meter apart and produced identical tones. The two sound waves traveled through the air in front of the speakers, spreading out through the room in spherical fashion.
A snapshot in time of the appearance of these waves is shown in the diagram below. In the diagram, the compressions of a wavefront are represented by a thick line and the rarefactions are represented by thin lines.
These two waves interfere in such a manner as to produce locations of some loud sounds and other locations of no sound. Of course the loud sounds are heard at locations where compressions meet compressions or rarefactions meet rarefactions and the "no sound" locations appear wherever the compressions of one of the waves meet the rarefactions of the other wave.
If you were to plug one ear and turn the other ear towards the place of the speakers and then slowly walk across the room parallel to the plane of the speakers, then you would encounter an amazing phenomenon.
You would alternatively hear loud sounds as you approached anti-nodal locations and virtually no sound as you approached nodal locations. As would commonly be observed, the nodal locations are not true nodal locations due to reflections of sound waves off the walls. These reflections tend to fill the entire room with reflected sound. Even though the sound waves that reach the nodal locations directly from the speakers destructively interfere, other waves reflecting off the walls tend to reach that same location to produce a pressure disturbance.
Destructive interference of sound waves becomes an important issue in the design of concert halls and auditoriums. The idea that interference is caused by superposition means that when two waves meet their two amplitudes their maximum absolute value combine together. Interference : Two overlapping waves exhibit interference. Interference can be constructive or destructive.
In constructive interference, the two amplitudes of the waves add together and result in a higher displacement than would have been the case if there were only one wave. An example of constructive interference may be seen in. Constructive Interference : Pure constructive interference of two identical waves produces one with twice the amplitude, but the same wavelength.
Destructive interference is when two waves add together and the result is a smaller displacement than would have been the case. An example of destructive interference can be seen in. When the waves have opposite amplitudes at the point they meet they can destructively interfere, resulting in no amplitude at that point.
For example, this is how noise cancelling headphones work. By playing a sound with the opposite amplitude as the incoming sound, the two sound waves destructively interfere and this cancel each other out.
The superposition of two waves of similar but not identical frequencies produces a pulsing known as a beat. Striking two adjacent keys on a piano produces a warbling combination usually considered unpleasant to the ear. The culprit is the superposition of two waves of similar but not identical frequencies.
When two waves of similar frequency arrive at the same point and superimpose, they alternately constructively and destructively interfere. This alternating is known as a beat because it produces an unpleasant pulsing sound.
Another example is often noticeable in a taxiing jet aircraft particularly the two-engine variety. The loudness of the combined sound of the engines increases and decreases.
This varying loudness occurs because the sound waves have similar but not identical frequencies. The discordant warbling of the piano and the fluctuating loudness of the jet engine noise are both due to alternately constructive and destructive interference as the two waves go in and out of phase. Beat Frequency : Beats are produced by the superposition of two waves of slightly different frequencies but identical amplitudes. The waves alternate in time between constructive interference and destructive interference, giving the resulting wave a time-varying amplitude.
The wave resulting from the superposition of two similar-frequency waves has a frequency that is the average of the two. This wave fluctuates in amplitude, or beats, with a frequency called the beat frequency. We can determine the beat frequency mathematically by adding two waves together. One can also measure the beat frequency directly.
When you hear a beat coming from two discordant sounds say, two notes on a piano you can count the number of beats per second. The number of beats per second, or the beat frequency, shows the difference in frequency between the two notes. Musicians often use this phenomena to ensure that two notes are in tune if they are in tune then there are no beats. The ear is the sensory organ that picks up sound waves from the air and turns them into nerve impulses that can be sent to the brain. Sound waves are vibrations in the air.
The ear is the sensory organ that picks up sound waves from the surrounding air and turns them into nerve impulses, which are then sent to the brain. When the peaks of the waves line up, there is constructive interference.
Often, this is describe by saying the waves are "in-phase". Although this phrase is not so important for this course, it is so commonly used that I might use it without thinking and you may hear it used in other settings.
Similarly, when the peaks of one wave line up with the valleys of the other, the waves are said to be "out-of-phase". Phase, itself, is an important aspect of waves, but we will not use this concept in this course. How could we observe this difference between constructive and destructive interference. Given the fact that in one case we get a bigger or louder wave, and in the other case we get nothing, there should be a pretty big difference between the two.
We will explore how to hear this difference in detail in Lab 7. The most important requirement for interference is to have at least two waves. One wave alone behaves just as we have been discussing. We shall see that there are many ways to create a pair of waves to demonstrate interference.
The simplest way to create two sound waves is to use two speakers. If we place them side-by-side, point them in the same direction and play the same frequency, we have just the situation described above to produce constructive interference:. If we stand in front of the two speakers, we will hear a tone louder than the individual speakers would produce.
The two waves are in phase. Now imagine that we start moving on of the speakers back:. At some point, the two waves will be out of phase — that is, the peaks of one line up with the valleys of the other creating the conditions for destructive interference. If we stand in front of the speakers right now, we will not hear anything!
This must be experienced to really appreciate. Equally as strange, if you now block one speaker, the destructive interference goes away and you hear the unblocked speaker.
In other words, the sound gets louder as you block one speaker! How far back must we move the speaker to go from constructive to destructive interference? We know that the distance between peaks in a wave is equal to the wavelength. If we look back at the first two figures in this section, we see that the waves are shifted by half of a wavelength.
So, in the example with the speakers, we must move the speaker back by one half of a wavelength. What happens if we keep moving the speaker back? At some point the peaks of the two waves will again line up:. At this position, we will again have constructive interference!
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