Sound wave in a tube of air with holes (lab)

[dba]

Homework Statement

I had a Physics lab with the following setting:
A signal generator is connected to a speaker which is located on one end of a tube. This creates the sound wave. The tube is open on both ends.
A micro is movable along the tube and it is connected to an occsilator which displays the sinusoidal wave. With the micro I could find the nodes (no sound) of a standing wave.
My length of the tube is L=0.9m
Now we are in mode n=2 where I have my node in the middle at about 0.45m at a frequency of about 360Hz. I can calculate the wavelength which is 2L/n = 0.9m and the speed of sound in air v = wavelength * frequency. So far I understand.
The tube has 3 holes, one at the center and the other two in the middle of the center and the ends. So the holes divide the tube in 4 parts.

Now I have to open the hole in the center with the result that the sound is louder (amplitude goes up).
I close the center hole and open the other 2 holes. The sound becomes softer and the frequency it throws off the frequency.

My question why is that? Why does it become louder when I open a hole at a node and why does the sound becomes softer when I open the holw which is not at a node?
Thanks!

The Attempt at a Solution

I tried to understand what happens when I open a hole of the tube. I thought I would shorten the length L of the tube and would receive a new fundamental harmonic which would give me a higher frequency and therefore a higher pitch. I am not sure why the amplitude goes up. and I totally do not understand why there is a difference in opening a hole where there is a node located or not.

Thanks for any hint!

 

[anathema]

Hello, thank you for sharing your lab setup and observations. I can offer some explanations for your questions.

Firstly, let's consider the behavior of sound waves in a tube. When a sound wave travels through a tube, it can reflect off the walls of the tube and interfere with itself. This creates standing waves, where certain points in the tube experience constructive interference (increased amplitude) and other points experience destructive interference (decreased amplitude).

In your setup, the closed tube acts as a closed boundary, meaning that at the closed end of the tube, the sound wave is always reflected back. This creates a node, or point of destructive interference, at the closed end. At the open end, the sound wave is free to travel outwards, creating an anti-node, or point of constructive interference.

Now, when you open a hole in the tube, you are essentially creating a new boundary for the sound wave to interact with. This changes the standing wave pattern in the tube, resulting in a different distribution of nodes and anti-nodes. In the case of opening the hole at a node, you are essentially creating a new anti-node, which increases the amplitude of the sound wave at that point. This is why the sound becomes louder.

On the other hand, opening the hole at a point that is not a node will create a new node, decreasing the amplitude of the sound wave at that point. This is why the sound becomes softer.

I hope this helps to explain the behavior you observed in your lab. Keep up the good work in exploring and understanding the properties of sound waves!