Why do waves need to be in phase for constructive interference to occur?

In summary, constructive interference occurs when in phase waves are separated by a distance that is an integer multiple of the wavelength. This means that the crests of the waves are aligned and add together, while troughs will cancel out. However, if the waves are not perfectly aligned and have a slight difference in path length, there will be partial addition or subtraction resulting in a range of amplitudes. This can be seen in the frequency plot of the waves, where some frequencies will be in phase and others will be in antiphase. In the case of two speakers facing each other, the path lengths from the two speakers will constantly change as a person walks between them, resulting in a range of amplitudes and the creation of a standing wave
  • #1
maccha
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So I know that for constructive interference to occur, in phase waves must be separated by a distance 2 pi. What I'm confused about, though, is what if the point they interfere at is separated by a distance 2 pi, but isn't where their crests meet? Like what if one speaker is one wavelength behind the other, but they both start off at zero amplitude, so wouldn't that point where they meet still be zero? Thanks for the help.
 
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  • #2
I think you mean to say that in order for constructie...the differences in path length must be an integer multiple of the wavelength, could be 1, or 100 so long as they arrive in phase.

And yes zero plus zero is zero, just as the point of maximum will be two times the individual. Is this any help?
 
  • #3
Yes sorry that is what I meant. But don't they just interfere at one point, so if it's not at the troughs wouldn't maximum interference not always occur for a distance of one wavelength?
 
  • #4
When they are exactly in phase (lined up so the crests are arriving simultaneously), all points add and only the zeros (crossing point) don't get bigger. When exactly antiphase, 180 degrees apart, there will be no sound heard at the listening point--imagine a point size microphone. Anywhere else you will hear sound. Bass is especially prone to these issues, but if you were to do an exact frequency plot with a high quality mike using a signal generator, the graph looks like the teeth of a comb for just this reason: at any distance some frequencies will be perfectly in phase, others antiphase and most in between.
 
  • #5
Perhaps it may help you understand if you think about what happens as time continues.

The instantaneous wave amplitude may be zero at the point in time and space you are considering, but as time goes on it will continually vary between its crest values.
 
  • #6
Perhaps it may help you understand if you think about what happens as time continues.

The instantaneous wave amplitude may be zero at the point in time and space you are considering, but as time goes on it will continually vary between its crest values.
 
  • #7
Hm, I understand it when you think of it over time for two speakers on the same axis facing the same direction. But I don't understand when it's two speakers facing each other. I have an example that asks to find loud spots a person will hear when walking between two speakers facing each other (in phase). Why don't they constructively interfere the entire time? I don't know, I think I'm thinking about this completely wrong but I'm imagining two waves starting off at the same point and overlapping the entire time.
 
  • #8
Because as you walk towards one speaker and away from the other the path length from the two changes. It is all about the difference in path lengths. If the difference in pathlengths is off by a 1/2 wavelength, you have destructive interference, off by one complete wavelength, the sounds add. In between is in between.
 
  • #9
Okay so if it's a complete wavelength they'll add- but how do we know they'll interfere at a point of amplitude A and not a point 0 on their wave? Sorry I know I'm going in circles.
 
  • #10
You really should just google the question and see with your own eyes how it works. We are talking about a collision of two "forces"--if the surges coincide you have constructive interference (additive) or if a trough and crest collide, you have zilch, but everything between these extremes is partial addition or subtraction.
 
  • #11
Okay thanks for the help. One last quick question. Don't two waves of the same frequency traveling in opposite directions produce a standing wave? Why wouldn't this produce a standing wave?
 
  • #12
Damn straight. Put two big facing woofers in a big room and walk slowly from one to the other. This isn't some BS effect. In most rooms you can feel a huge difference as you go from null to peak (destructive interference vs constructive), It is not sudden.
 

FAQ: Why do waves need to be in phase for constructive interference to occur?

1. What is interference confusion?

Interference confusion is a phenomenon that occurs when multiple signals or stimuli are present, causing difficulty in accurately perceiving or interpreting a specific signal or stimulus. This can happen in various fields such as psychology, electronics, and communication.

2. How does interference confusion affect experiments?

In experiments, interference confusion can lead to inaccurate results and conclusions. It can occur when there are unintended variables or distractions that interfere with the intended manipulation or observation, making it difficult to isolate the effect of the independent variable.

3. What are some examples of interference confusion in everyday life?

Interference confusion can occur in many situations, such as when trying to listen to someone in a noisy environment or when multiple devices are emitting signals in close proximity, causing interference. It can also occur in the workplace, where conflicting instructions or tasks can cause confusion and hinder productivity.

4. How can interference confusion be minimized or avoided?

In experiments, researchers can minimize interference confusion by carefully controlling the environment and limiting potential distractions. In everyday life, individuals can reduce interference confusion by focusing on one task at a time, avoiding multitasking, and finding a quiet and distraction-free environment.

5. Can interference confusion have positive effects?

In certain cases, interference confusion can lead to novel ideas and solutions. In the field of creativity and innovation, some experts suggest that exposing oneself to a diverse range of stimuli can stimulate new ideas and break through mental blocks caused by interference confusion.

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