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# tuning fork sound/sine waves - 2D or 3D

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Hi,

Sorry if this seems a bit dumb...

The sine/sound waves produced when a tuning fork is struck....are they better understood as 2 dimensional waves of the kind youy might plot on a graph, or 3 dimensional waves like a stretched out spring?

If you can explain why as well that would help a lot.
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Commented:

Commented:
What i want to understand is what actually happens in the air space. It seems from the first link that what is happening is actually a 3 dimensional phenomenon. The article speaks of cylindrical spaces.

But waqtching the 3 dimensional animations and trying to understand the text, it seems that in the air space the disturbance is not actually of a sine wave shape...as far as i can tell. It is sort of the sum of a horizontal and a vertical disturbance.

....so...the 2 dimensional sine wave is the representation of the 3 dimensional activity which is not actually of a sine wave pattern itself.

Have I understood correctly?

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Commented:
the sine waves one finds associated with sounds often represent pressure at a particular point in space, plotted against time.
In 3, 4 and 5 dimensions, one see how pressure varies in time and space
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Commented:
"are they better understood as 2 dimensional waves of the kind youy might plot on a graph, or 3 dimensional waves"

2 D

On an elementary basis it is easiest to draw a line (in any direction) through the point halfway between the ends as the x axis and plot the pressure on the y axis.
(The physical reality is, of course,  three dimensional.)
The usual example has the straight line going through the ends of the fork.
Commented:
If the sound source is omnidirectional, you have a spherical air disturbance (in 3 dimensions...). If all movement could be "frozen", you'd have like an onion of air "layers" of different pressure. The sound is experienced as these layers travel through air (the imagined "freeze" unfrozen) and affects your eardrum. Possibly the pressure gradient plotted against the distance from the sound source is a sine wave, but the disturbance itself isn't; the air particles don't move along with the disturbance but rather just back and forth (so to speak) as the pressure waves pass; I'd expect the main direction of their movement to be perpendicular to the pressure gradients (the "layers" of the sound waves).
/RID
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Commented:
1.  my previous post responds to the specific question.
2.  The sound produced by a tuning fork is not omni-directional
3.  The important quantity is the pressure, not the pressure gradient. From an ideal tuning fork, the pressure (and the pressure gradient) definitely is a sine wave.
4.  The air particles do move (back and forth) in the direction of the "disturbance".
5.  Sound waves are longitudinal not transverse
6. The main direction of air particle movement will indeed be perpendicular to the pressure gradient.
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Commented:
Is it the tuning fork you want to understand, or the sine wave?
The full 3D pattern of sound surrounding a tuning fork involves more than just a single sine wave,
especially if the sound may also be bouncing off other objects in the room.
But the sine wave can be most easily understood as a graph of pressure vs time at a particular point.
It is also possible to understand it as a graph of air particle velocity at a particular point, but you may
want to filter out drifting air currents.
It might also describe pressure vs position at a given time, but then the amplitude would diminish with distance.

Commented:
I want to understand the tuning fork in particular because it gives the most regular sound in sine wave terms.

I guess what i really want is a real time shape in 3 dimensions that i can get my head around. I've got some ideas about this from the discussion. But not definitevly.

but there again, really the crux of the answer for me is, with a tuning fork, what is the shape of the waves it sends out into the air, [not that they can  be shown in useful way with sine waves on a graph.]

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Commented:
You can think of it as the sum of two dipole sourcces

Commented:
The tuning fork in itself is a bad sound transmitter, as the air displacement is minute. The sound source is often some object that the tuning fork is held against; the sound quality is affected by this object.

The wavelength of a 440 Hz (A) note is a little shorter than 1 metre and the sound source (the fork itself) is small in comparison. Assuming a point-shaped sound source may well be OK. However, a sound fork is probably not by any means omnidirectional, even though it's difficult to "direct" sound waves of such low frequencies.
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Commented:
for the far field region, treating it as a linear quadrupole should be a good approximation.
In the near field region, you'd want to take into account that ends of the tines will have the largest
displacement.  you may be able to just sum quadrupoles at different amplitudes along the fork.
If you also want to model any objects acting as resonators or reflectors or absorbers in the environment,
Commented:
Here is a video that might help you visualize what is happening:
Now instead of the colored bars, imagine air molecules. As the wave travels through the air, the molecules move back and forth. At some points they are more close together and the pressure is higher, at other places they are far apart and the pressure is lower. The wave moves from left to right, but the air molecules don't all move left to right.

Commented:
Hi,

Thanks everyone. I'm digesting.

Commented:
Hi,

Thanks for all the comments. I'm finding it harder to assign the points than understand the answers...almost :) . But I got the sense of what I was after. I think there are a lot of relevant points in many of the comments.

I am splitting the points as well as I can. Hope it's ok with the experts.