Standing waves are a phenomenon that occurs in fluid (or air) media when a frequency and wavelength are matched in an interference pattern. These waves can be very small or very large, depending on the medium and frequency.
They are most often observed in water, as in the case of waterfalls, where the flow of water hits an obstacle and is forced to bounce back, meeting other flows of water at certain angles to create a repeated pattern.
With the right conditions, a standing wave can be created in a pipe that is filled with air at a specific frequency. This is what we will be discussing in this article!
You will need some materials to complete this experiment, so check out the supplies section below.
Make the pipe 7 times longer
You can create a standing wave in a pipe if the length of the pipe is 7 times longer than the diameter of the pipe. This is called an integer number ratio between length and diameter.
So if the diameter of the pipe is 5 centimeters, then the length of the pipe must be 35 centimeters. This is not easy to do!
The more difficulty in achieving this is why it is hard to get N=7 in a tube. The number seven represents the number of rings inside of the tube; thus, having a N=7 ring configuration is trying to have seven rings inside of the tube.
The more rings you have, the harder it is to make them all line up so that there are no gaps between them.
Choose the correct resonant frequency
Now that you know the required number of half-waves, the next step is to find the appropriate frequency of the standing wave.
To do this, you need to calculate how many times per second all of the strings will line up in relation to each other. Once again, you will use math to accomplish this!
You already know how many half-waves are in the length of your pipe, so now you need to figure out how many times per second each string will line up with the next. To do this, divide one number by the other.
If you have a seven-string guitar, for example, and your pipe is thirty feet long, then you are attempting to create a standing wave with N=7 in a pipe that is 30 ft long.
Hold the pipe at the end with one hand
Now, with your free hand, tap the pipe a few times at the same point as you held it. You will feel a vibration in your hand as you do so. This is called a resonance, or a standing wave created in the pipe.
You just created a fundamental frequency resonance in the pipe! At this point, if someone were to hold the other end of the pipe and tap it, they would receive feedback—their tapping would be returned to them.
There are many different frequencies that can create resonances in a tube, but the easiest ones to recognize are the first seven: 1, 2, 3, 4, 5, 6, and 7!
Try it out yourself and see what frequency you can create by holding the pipe at the appropriate length.
Use your other hand to block the open end of the pipe
Once you have the pipe parallel to the floor, with your other hand, pull the top end of the pipe away from you. You will have to keep pulling until you can no longer see the top end of the pipe.
At this point, your hands will be perpendicular to each other and you will have an empty space between your hands and the floor. This empty space is what creates the longitudinal wave in the pipe.
As you reinsert the top end of the pipe into this empty space, make sure that it is parallel to the floor. If it is not parallel, then you will not achieve n=7 and you will have to start over.
Blow a steady stream of air into the open end of the pipe
Once you have a steady stream of air flowing out of the pipe, you must now add more air to the stream to create a standing wave. Add enough air so that the stream of air bumps into the wall of the pipe and rebounds back towards you.
The tricky part is getting the length of the pipe (n) just right so that there is a constant flow of air bouncing back and forth. If the n is too short, there will not be a continuous flow of air bouncing back and forth. If it is too long, then there will be too much space for air to flow, which will result in no standing waves.
Move your hands along the length of the pipe to create a standing wave
You can also create a standing wave by moving your hands along the length of the pipe instead of in circles. This is known as linear motion and requires a longer pipe. you
The length of the pipe required for linear motion depends on how many waves you want to combine. You will need a pipe twice as long as the number of waves you want to combine!
For instance, to create a seven wave combination, you would need a thirty-seven foot long pipe. That is a really long flute!
You can also choose to have fewer waves in your standing wave set up.
