You know, on axis...like a coaxial driver has 2 sources lined up on one axis...woofer and a tweeter on something like an altec coaxial...loading adds extension to how low the driver on that particular axis, can play. I don't know how else to get a compression driver to play lower than what the products are offered today, other than using loading...the only other option would be to raise the xover...which may not be what I want to do... pros n cons
These are the duct modes, I assume: Acoustic modes on a circular duct - GIF on Imgur
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Of course it is possible. I do it in my software now. The problem is that it can be very hard to see the variations in that the HOMs are certainly going to be much smaller than the lowest mode. Hence, unless you amplify the HOMs in the video playback, you won't be able to actually "see" much of anything with the eye.
In a plane wave tubes, pressure and velocity track each other very precisely, no phase or amplitude changes at all with frequency or space, unlike in free space. (Lowest mode only.) i.e. the relationship between velocity and pressure is a single complex constant.
In a plane wave tubes, pressure and velocity track each other very precisely, no phase or amplitude changes at all with frequency or space, unlike in free space. (Lowest mode only.) i.e. the relationship between velocity and pressure is a single complex constant.
That wasn't clear to me, thanks. The more I actually think about it the more questions arise -
So, do we assume here that the exit of a driver is a tube? I mean, it seems to me that the duct modes can't describe a spherical wave (if there was one) - is that correct? Exits of CDs are typically conical (tapered), not tubular, so I guess a spherical wave can actually be formed at the exit. So this is the simplification? Would I have to use spherical harmonics as a general solution?
So, do we assume here that the exit of a driver is a tube? I mean, it seems to me that the duct modes can't describe a spherical wave (if there was one) - is that correct? Exits of CDs are typically conical (tapered), not tubular, so I guess a spherical wave can actually be formed at the exit. So this is the simplification? Would I have to use spherical harmonics as a general solution?
Tough questions 
First, duct modes can describe a spherical wave, but it converges very slowly depending on the curvature of the wave. The duct modes all have zero slope at the boundary while a spherical wave does not. In order for the modal sum to converge to the spherical wave it will take an infinite number of modes to yield the exact shape right at the boundary. The same thing is true of a small source in a room. It takes an infinite summation of the modes to simulate the wave right up to the source. The fewer modes in the sum the farther aware the sum converges. If one calculates the intensity of the waves from the source one will see that right at the source there is no intensity, it falls to zero, an obviously incorrect result. With less modes the intensity won't appear until some distance from the source, about 1/4 wavelength of the highest mode in the sum. More modes and the intensity starts closer to the source. I wrote about this way back in the 90's.
Now if the throat is conical then the duct modes that you are using are not correct, but then neither are the spherical modes because they assume a free field not a bounded one. As the angle of the cone gets smaller the modes converge to the cylindrical modes and as it gets wider they converge to the spherical ones. In between the eigenvalues for the angular modes must be calculated from the angle of the cone. This situation and example are discussed in my book.
For our purposes, where the cone angle is generally small the cylindrical duct modes are the closest and hence this approximation works best. For a very wide conical angle this simplification would not work very well.
First, duct modes can describe a spherical wave, but it converges very slowly depending on the curvature of the wave. The duct modes all have zero slope at the boundary while a spherical wave does not. In order for the modal sum to converge to the spherical wave it will take an infinite number of modes to yield the exact shape right at the boundary. The same thing is true of a small source in a room. It takes an infinite summation of the modes to simulate the wave right up to the source. The fewer modes in the sum the farther aware the sum converges. If one calculates the intensity of the waves from the source one will see that right at the source there is no intensity, it falls to zero, an obviously incorrect result. With less modes the intensity won't appear until some distance from the source, about 1/4 wavelength of the highest mode in the sum. More modes and the intensity starts closer to the source. I wrote about this way back in the 90's.
Now if the throat is conical then the duct modes that you are using are not correct, but then neither are the spherical modes because they assume a free field not a bounded one. As the angle of the cone gets smaller the modes converge to the cylindrical modes and as it gets wider they converge to the spherical ones. In between the eigenvalues for the angular modes must be calculated from the angle of the cone. This situation and example are discussed in my book.
For our purposes, where the cone angle is generally small the cylindrical duct modes are the closest and hence this approximation works best. For a very wide conical angle this simplification would not work very well.
Thank you, it makes much more sense to me now.
So here we go, easy first - this is the very first attempt to define a desired wavefront in ABEC - duct mode 0,1. Waveguide is the ST260, simulated in constant acceleration mode.
So what are we looking at? Does it make any sense? Should I continue?
So here we go, easy first - this is the very first attempt to define a desired wavefront in ABEC - duct mode 0,1. Waveguide is the ST260, simulated in constant acceleration mode.
So what are we looking at? Does it make any sense? Should I continue?
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It makes perfect sense to me. The cut-in is obvious and clearly too high to really worry about. The peak cut-in shown here is about 17 kHz, which does seem extremely high to me. But the pictures don't lie, that's exactly what I would expect.
What you are seeing is the evanescent waves that reach to the measurement location up to 7 kHz where you can see the evanescent transition to real as a null. This is followed by a huge peak in efficiency (which is sometimes called coincidence,) as the system uniquely couples this mode to the environment most efficiently. After that the efficiency falls.
PS. This device could beam terribly above 10 kHz if the diaphragm where bending up there, which is likely. Or, lets remember that at those frequencies we know that the phase plug is also critical.
PPS. Those plots are pieces of beauty to me. Thanks. Bravo.
What you are seeing is the evanescent waves that reach to the measurement location up to 7 kHz where you can see the evanescent transition to real as a null. This is followed by a huge peak in efficiency (which is sometimes called coincidence,) as the system uniquely couples this mode to the environment most efficiently. After that the efficiency falls.
PS. This device could beam terribly above 10 kHz if the diaphragm where bending up there, which is likely. Or, lets remember that at those frequencies we know that the phase plug is also critical.
PPS. Those plots are pieces of beauty to me. Thanks. Bravo.
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Marcel,
Thank you for this software, even a non technical person like me can grasp the idea of designing a waveguide.
With that say, can someone point me in the right direction... I am looking to design a waveguide 6.5" in diameter for a 1"threaded compression driver.
I have modeled some using your the demo files than come with ATH and looked the plots in ABEC, but I would really like to try something like the JBL M2, sadly most of the parameters shown in previous posts don't work in ATH 4.7.-
I am using the CD in a 2 way design crossed around 2.4kHz
Cheers and thanks again
Thank you for this software, even a non technical person like me can grasp the idea of designing a waveguide.
With that say, can someone point me in the right direction... I am looking to design a waveguide 6.5" in diameter for a 1"threaded compression driver.
I have modeled some using your the demo files than come with ATH and looked the plots in ABEC, but I would really like to try something like the JBL M2, sadly most of the parameters shown in previous posts don't work in ATH 4.7.-
I am using the CD in a 2 way design crossed around 2.4kHz
Cheers and thanks again
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We have already seen a few measurements of this very device with several different drivers:
Acoustic Horn Design – The Easy Way (Ath4)
Acoustic Horn Design – The Easy Way (Ath4)
Acoustic Horn Design - The Practical Way
Acoustic Horn Design – The Easy Way (Ath4)
You can try these, that's what I found. Use the latest Ath release: ATH - Advanced Transition Horns
- With the "Slot" feature you can get really close (if that's what you want): #6623 etc
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huh, so the 2nd mode is basically already out of band? and the 1st mode just barely makes it in?
so is it maybe not too much to worry about in these waveguides?
I wonder how much worse the results will be with a worse design, something like the PT waveguides, spherical, or something with a more pronounced diffraction slot?
so is it maybe not too much to worry about in these waveguides?
I wonder how much worse the results will be with a worse design, something like the PT waveguides, spherical, or something with a more pronounced diffraction slot?
Anyone can easily test the same on their designs (1" throat in this case): Set the attached file as the source definiton script, put e.g. Source.Contours = D:\projects\ath\cfg\duct_mode_01.txt into your script.
I'm also going to prepare some batch run - it's not a problem to simulate automatically hundreds of different geometries by varying the shape parameters randomly and skim through the reports. We should see then where we can expect the cut-ins in some suboptimal designs.
I'm also going to prepare some batch run - it's not a problem to simulate automatically hundreds of different geometries by varying the shape parameters randomly and skim through the reports. We should see then where we can expect the cut-ins in some suboptimal designs.