I had a Physics lab with the following setting: A signal generator is connected

Question

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!

Explanation / Answer

It is easy - an amplitude node is pressure anti node and amplitude anti node is pressure node.
In the first case u opened the pressure anti node so u were hearing louder sound (pressure is what causes the sensation of hearing) in the other case it's just the opposite
I hope this helps....:)

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