Sound pressures at the open end

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In summary: The reason the pressure amplitude varies so much as the microphone approaches the open end is that the air in the tube is not in equilibrium: momentum is constantly being exchanged between the air molecules and the microphone. When the microphone is very close to the open end, the momentum of the air molecules is so great that it causes the pressure to drop very quickly.
  • #1
ivanhooace
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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.
:smile:
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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  • #2
why no one reply my question? poor me...
 
  • #3
(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
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.
 

1. What is meant by "sound pressure at the open end"?

The sound pressure at the open end refers to the level of pressure exerted by sound waves at the opening of a pipe or other sound-producing device.

2. How is sound pressure at the open end measured?

Sound pressure at the open end is typically measured using a microphone or other sound level measuring device placed at the opening of the sound source. The resulting pressure readings are then converted into decibels (dB) to quantify the level of sound pressure.

3. What factors affect sound pressure at the open end?

The sound pressure at the open end can be affected by various factors such as the size and shape of the opening, the strength and frequency of the sound source, and the distance between the sound source and the opening.

4. How does sound pressure at the open end relate to sound intensity?

Sound pressure at the open end is directly related to sound intensity, as it is a measure of the force exerted by sound waves. However, sound intensity takes into account the area over which the sound is distributed, while sound pressure only measures the force at a specific point at the open end.

5. What are some applications of understanding sound pressure at the open end?

Understanding sound pressure at the open end is important in various fields such as acoustics, engineering, and music. It can help in designing and optimizing sound-producing devices, studying sound propagation, and creating better sound quality in musical instruments and audio systems.

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