# Sound pressures at the open end

1. Dec 1, 2005

### ivanhooace

Hi guys
I am a A level student. Lately, i have done a practical investigation on pressure difference of the sounds in the air (Inside a both opened end tube). There is some question that i would like to know and I am wondering If anyone could help me please?
The experiment is carried out by varying the distance of the microphone to the speaker inside the tube. Before the experiment was carried out, I have expected to see a sine graph. At both the open end, the pressure variation is at its minimum; therefore the EMF generated by microphone will be the smallest.
(Note: the reasons saying it is a sine graph is that a microphone is detecing the pressure.... not the displacement of air molecules)
Well, the experiment did verify my hypothesis; nevertheless, a bizarre data occurred to me that the EMF dropped from 100mv to 49mv upon reaching the end. (the 100mv is obtained at a distance 1 or 2 cm away from the open end). Before that, the EMF varied gradually, but it decreases rapidly as the microphone approaching the open end.
The only explanation I could think of is that the air molecule moves freely at the opened end ( As it is just beside the “outside of the tube”). However, since there will be some reflection as a result of the collision of the air molecules at the opened end, the pressure variation should not be very low, at least not decreased rapidly…
the picture attached shows the setup of my instruments.

The sequence of the EMF from one open end A to another B is
49, 160, 190, 210, 250, 280,320, 360, 400, 440, 487, 520, 487, 440,400, 360,320, 280, 250, 210, 190, 160, 49

Notice that the EMF changes rapidly upon reaching A and B.

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Last edited: Dec 2, 2005
2. Dec 3, 2005

### ivanhooace

why no one reply my question??? poor me....

3. Dec 3, 2005

### emptymaximum

(Note: the reasons saying it is a sine graph is that a microphone is detecing the pressure.... not the displacement of air molecules)

this is not entirely correct.
it is detecting both, as they are related.
when the pressure is at a maximum, the displacement of the air molecules are at a minimum. when the pressure is at a minimum, the displacement of the air molecules is at a maximum.
the nodal points occur at maximum pressure.
maximum amplitude occurs when the pressure is at a minimum.

how are you measuring the emf of the microphone?

4. Dec 3, 2005

### lightgrav

If the "active" surface Area of the microphone is not exactly parallel to the tube wall surface Area, some of the momentum of the moving air molecules will lead to microphone EMF, which would make interpretation of the EMF ambiguous. You should have been VERY careful to not change the orientation of the microphone, or its distance from the axis of the tube, even when it was outside the tube.

If you were driving the speaker at EXACTLY the resonance frequency of the tube, the pressure amplitude should've been quite a bit LESS than 49 mV , just a little bit outside the tube (a pressure node is usually about pi R/2 outside of the tube end - but exactly how far depends on the stiffness of the tube material).

Putting the microphone in the end of the tube will
(1) change the resonant frequency of the tube
(2) change the reflection properties at that end of the tube
(3) change the pressure amplitude all thru the tube, mostly near that end.