# Closed end tube with anti-node

The experiment was about creating a sound wave inside a close ended resonance tube and finding the locations of maximum and minimum amplitude. (after adjusting the tube length that makes a standing wave). The experiment used an oscilloscope and at the closed end there was a maximum amplitude. Why would this happen? Something wrong with the setup?

Is is possible for a closed ended tube to have an anti-node instead of a node?

The speaker is at the left end side and the right side is the close end. The mic (the device inside the tube with the wire attached) measures the amplitude of the standing wave. The mic is right next to the close end. The function at the top is the input sound wave and the bottom function is the amplitude of the standing wave at the location (where mic is at).

## 2 Answers

One complication is that a sound wave can be described as a pressure wave or a displacement wave and they are $$90^\circ$$ out of phase with one another.

The function at the top is the input sound wave

I suspect it is the voltage which is applied to the loudspeaker which is producing the sound waves, so which description of a sound wave is this voltage in phase with?
(The position of the loudspeaker is most likely not a node or an antinode.)

the bottom function is the amplitude of the standing wave at the location (where mic is at).

which is close to(?) but not at the fixed end so not necessarily at a node or antinode.

What is the microphone measuring, the pressure wave or the displacement wave and what is the phase relation with the output of the microphone?

The resolution of your image is not good enough to assess the phase between the two voltage traces and I suspect it is not $$90^\circ$$.

To get an accurate value for the phase feed one voltage signal into the X-input of the oscilloscope and the other voltage signal into the Y-input to get a Lissajous figure from which the phase relationship between the two voltage can be found.

All is OK. Remember that the mic responds to pressure and finding a signal (pressure) maximum at the closed end is exactly what you would expect.

• The mic also had a maximum amplitude at the left side of the tube (near the open end). Does this mean that a node is formed in the open end as well? – user242520 Sep 16 at 4:14
• it depends on the diameter of the mic- how big was it, and was it pointed in a specific direction? also, what type of mic was it- ribbon, or dynamic, or piezo? – niels nielsen Sep 16 at 16:42