gedlee said:
Yes, I understand this.
I am turning the page a bit and getting out past the driver's boundary and into the planewave tube itself, and am interested in knowing more about how it functions in practical terms, and what sort of changes there might be that are significant in terms of distortion of the wavefront and or "distortions" while traveling vs. distance.
Also, in practical terms - not for measurement purposes - what effects if any are you aware of that are problematic or of concern for tube lengths that are made shorter and shorter. Let's assume the end is terminated in a manner that does not produce reflections for this discussion.
Ok, I don't know exactly what the set up would consist of. If you would like to describe it here, fine. If you would prefer to communicate it privately, that is ok too. In advance I do not know if it is something that I will have the time or resources to persue, but it is of considerable interest to know the test setup.
Thanks.
_-_-bear
bear said:
But you see this assumption is everything since it can never be achieved. You are looking for the "least" reflection and the longer the PWT the better. There was a paper in AES by Magalotti which was excellent on how to make a good PWT.
bear said:
First you need a good PWT - not trivial. Then you need microphone locations spaced along the length of the tub in the side wall. Its possible to use just one mic and move it as long as the empty holes are plugged.
Thats all that is required on the measurement side. The rest is analysis.
These things take up a lot of space that I just don't have. That and the time.
Ok, I do understand in general terms how a PWT is constructed. Read about it and seen pix.
My question is more about the effect of a simple tube placed in series with a compression driver and a termination - how is or is it any different than a direct connect between a given termination and the compression driver?
I can intuitively see that the time that it takes for any reflected energy (non ideal real world) to travel back to the diaphragm increases with the length of the tube - so would this increase the total negative effect the reflected energy creates or would it shift the frequency that it has an effect because the tube is N wavelengths long and the transition time is longer, or would it shift in time the point that the interference takes place (wrt a given bit of reflected energy)? I am thinking of lengths far less than a PWT and more in the 1M range...
On a bonafide PWT:
I do have the physical space. Up to 70ft indoors and almost unlimited outdoors. So it is possible for me to set it up.
Yes, I do understand that it is impossible to eliminate all reflections - I said "does not produce reflections for this discussion" alluding to the difference between ideal and real-world.
How many test locations are required along the tube?
Are we picking up on the perimeter of the tube, flush, or are we looking to probe to the center of the tube?
_-_-bear
My question is more about the effect of a simple tube placed in series with a compression driver and a termination - how is or is it any different than a direct connect between a given termination and the compression driver?
I can intuitively see that the time that it takes for any reflected energy (non ideal real world) to travel back to the diaphragm increases with the length of the tube - so would this increase the total negative effect the reflected energy creates or would it shift the frequency that it has an effect because the tube is N wavelengths long and the transition time is longer, or would it shift in time the point that the interference takes place (wrt a given bit of reflected energy)? I am thinking of lengths far less than a PWT and more in the 1M range...
On a bonafide PWT:
I do have the physical space. Up to 70ft indoors and almost unlimited outdoors. So it is possible for me to set it up.
Yes, I do understand that it is impossible to eliminate all reflections - I said "does not produce reflections for this discussion" alluding to the difference between ideal and real-world.
How many test locations are required along the tube?
Are we picking up on the perimeter of the tube, flush, or are we looking to probe to the center of the tube?
_-_-bear
Well, if you don't think I can do it, I'll leave you the glory to show that it can be done just once. I certainly cannot see the logic in proving such design cannot be done because if it does not already exist in reality up to now, it means that nobody feels it's worth the effort to even give it a go.gedlee said:
bear said:
If there are no reflections from the termination then there is no effect of a length of tube, but since there are always reflections it is a moot point.
bear said:
Its a matter of resolution. Think of it this way, each location along the PWT is the same as a location across the aperature. The more you have the higher the resolution. The measurements are all taken at the wall.
The shape of the wave at the compression driver exit depends on what it's driving. A little test that one can do is place the mic right at the end of the compression driver ends and the throat of the wave guide.
1. Run sine wave. and take note of the symmetry of the sine wave as well as the harmonics content. Change the frequency and look at how the symmetry changes and the harmonics.
2. Do the same test on different horns/guides and see how things change.
1. Run sine wave. and take note of the symmetry of the sine wave as well as the harmonics content. Change the frequency and look at how the symmetry changes and the harmonics.
2. Do the same test on different horns/guides and see how things change.
soongsc said:
That shows something the question is what?
It seems to me that unless the mic is exceedingly small (less than a 1/4wave at the face) it is going to create a reflection of some sort - so the result that you see is then a function of that reflection vs the incoming wave and the load behind it (the horn) which is now altered by the impediment placed there (the mic).
Also, this does not directly show the shape of the wavefront at all, it does show a sample at one point (or area summed), but the relationship to the rest of the wavefront and any interference is unknown. Yes?
_-_-bear
bear said:
To be precise, each microphone position allows for another mode of resolution. Then the driver can be rotated on its mount to get three rotational positions. With four positions there are twelve modes possible and this is pretty good resolution. The location along the tube is not critical, its just a variable, but it would probably be best to be about 1" or lets say 20 mm.
Assuming you can get hold of a really small mic, something like the 2~3mm ones attached to a thin post. The asymetric wave form that varies with frequency indicates that plane wave for all frequency is not possible. When you change the wave guide using the same driver, you will see how the assymetry changes which indicates dependency of wave front shape on type of horn/guide.bear said:
Soongsc
You seem to be lost again. What we are talking about is wavefront shape in space and what you are talking about is wavefront shape in time. They are not the same thing. Your "experiment" has nothing to do with "plane waves" since it is a measurement of the effect in time not in space.
You seem to be lost again. What we are talking about is wavefront shape in space and what you are talking about is wavefront shape in time. They are not the same thing. Your "experiment" has nothing to do with "plane waves" since it is a measurement of the effect in time not in space.
Odd, a post I thought I made here is MIA...
Anyhow, I asked if you could comment further on the idea of the effect of a tube of relatively short length inserted between a compression driver and a terminus (a real world one for example a Geddes OS waveguide properly designed to interface with said parallel wall tube)? IF this is done what are the obvious and "well-known" issues/effects that will occur that are not present when the "properly designed and applied" waveguide is connected to the compression driver without the length of tube?
Let's
Additionally, what role does the length of tube play? How long does the tube have to be before its effect (and I am only assuming there are some negatives) becomes a factor?
_-_-bear
Anyhow, I asked if you could comment further on the idea of the effect of a tube of relatively short length inserted between a compression driver and a terminus (a real world one for example a Geddes OS waveguide properly designed to interface with said parallel wall tube)? IF this is done what are the obvious and "well-known" issues/effects that will occur that are not present when the "properly designed and applied" waveguide is connected to the compression driver without the length of tube?
Let's
Additionally, what role does the length of tube play? How long does the tube have to be before its effect (and I am only assuming there are some negatives) becomes a factor?
_-_-bear
bear said:
Bear
I answered this already.
Since there are always reflections from the mouth, the tube adds a longer delay to the path. The longer the delays the more audible the problems. If you could somehow not have any reflections then this delay would only be a delay of the direct sound and not an audible factor at all. But, as I said, this is not possible.
Once the duration becomes longer, it's more appoaching to a "flow" instead of a "wave" in relation with the dimension of the throat. However, one side is still more pressurized than the other, plus the fact that boundary layer plays a more significant role, thus the physical model cannot be represented by simple math equations.gedlee said:
I actually like Improv better.
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