The mistake most people make is assume all drivers generate the same kind of wave, if you know exactly what the specific driver is doing, and design accordingly, the you are okay. Most research assume direct radiated plane wave, simplifies the research process but cannot be reliably applied to real situations.
I have built an elliptical OS and posted early in this thread.
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"Optimisation Techniques for Horn Loaded Loudspeakers" by Richard C. Morgans.
Hello Kindhornman,
Yes it will cause what Hughes describes as an astigmatism in his QT paper, with a different apparent source location in the horizontal and vertical plan.
But is this really a problem?
I think it can be argued that having a narrowing in the vertical profile is probably better than having parallel walls like in the A290FL horn (or the smith horn, as an extreme example).
For one thing it helps with directivity.
The H9800 horn (used in the JBL S9800, with similar horns in the Everest2 and Array 1400 for example) has such a narrowing in the vertical plan (and a rather brutal transition, forming a diffraction slot). Its directivity indeed measure funky in the horizontal plan (as did the 2344 in the vertical plan, with the slot in the vertical plan) with a "wider than normal" horizontal directivity between ~3kHz and 8kHz (45° being above the on-axis signal at some points!). This is probably due to diffraction along the slot.
In the TH4003 there is no brutal transition tho: the profile is continuous so it might not have such problems.
Here are some measurements of its horizontal directivity behavior:
I'd say it is looks well behaved on paper, and listening reports are all very good.
Attachments
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At the Phase Plug Exit
The drivers normally used with MF/HF horn come equipped with a phase plug, the output from which is the principal determinate of what the horn 'sees' acoustically. Ideally, for an OS horn, the wave exiting the plug should have a coherent, flat front, positioned at the apex of the bounding hyperbola. Typically there is approximately a magnitude of difference between the area of this surface and that of the driver diaphragm producing it.
Regards,
WHG
The drivers normally used with MF/HF horn come equipped with a phase plug, the output from which is the principal determinate of what the horn 'sees' acoustically. Ideally, for an OS horn, the wave exiting the plug should have a coherent, flat front, positioned at the apex of the bounding hyperbola. Typically there is approximately a magnitude of difference between the area of this surface and that of the driver diaphragm producing it.
Regards,
WHG
URL Added
"WHAT'S SO SACRED ABOUT EXPONENTIAL HORNS?"
D.B. Keele, AESP1038
http://www.xlrtechs.com/dbkeele.com/PDF/Keele (1975-05 AES Preprint) - Whats So Sacred Exp Horns.pdf
"WHAT'S SO SACRED ABOUT EXPONENTIAL HORNS?"
D.B. Keele, AESP1038
http://www.xlrtechs.com/dbkeele.com/PDF/Keele (1975-05 AES Preprint) - Whats So Sacred Exp Horns.pdf
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How do they verify the plane wave is achieved?
How does the compression factor effect the expansion profile? The more compression, the air velocity vector pattern will change.
This is not exactly true, but it is to first order. In my book I show how a slight modification of the phase plug towards non-flat can suppress the first HOM. Whether this is worth the trouble or not is unknown.
While I was at B&C we did some measurements of this. It is true for the most part at the lower end of the bandwidth, but not at the upper end.
I later did some work on how to measure the exit wave front in general and found that using a plane wave tube with mics spaced along the tube at known distances could determine the axisymmetric part of the wave front at the aperture to a fairly high degree of accuracy. If you rotate the driver on the tube you can get a very good idea of the whole wave front. I have always wanted to build one of these tubes, but the space requirements are beyond my means. (And the information is of questionable value if you can't change the driver anyways.)
The compression factor has no effect as long as the SPLs stay down below about 140 dB - which is virtually always going to be the case. The air is not perfectly linear but it is linear enough so as not to have a strong effect on the wave front shape.
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Virtual [Sd] vs. REAL [Sd]
The higher the compression ratio the smaller the effective radiating area of the virtual piston the horn 'sees'. If you like, the driver piston characteristics (size particularly) are masked by the phase plug to a degree reflected by the compression ratio. The larger the piston, the lower the frequency that marks the onset of beaming. Due to the viscosity of air exhibited in small confined spaces, compression ratios over, say 10:1, are contraindicated.
Regards,
WHG
N.B., The horn acoustically begins at the phase plug exit, not at the arbitrarily point where the horn body is joined to the driver.
The higher the compression ratio the smaller the effective radiating area of the virtual piston the horn 'sees'. If you like, the driver piston characteristics (size particularly) are masked by the phase plug to a degree reflected by the compression ratio. The larger the piston, the lower the frequency that marks the onset of beaming. Due to the viscosity of air exhibited in small confined spaces, compression ratios over, say 10:1, are contraindicated.
Regards,
WHG
N.B., The horn acoustically begins at the phase plug exit, not at the arbitrarily point where the horn body is joined to the driver.
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WFM Addendum
For acoustic wave front measurement see:
WaveMaster - Fast wavefront measurement - TRIOPTICS GmbH
For acoustic wave front measurement see:
WaveMaster - Fast wavefront measurement - TRIOPTICS GmbH
Maybe
Possibly working within the realm of measurement error and the slight adjustment is the correction of a bias.
Regards,
Bill
Possibly working within the realm of measurement error and the slight adjustment is the correction of a bias.
Regards,
Bill
I am wondering whether it would be possible to glue little flaps at the exit of the phase plug along the radial direction and record with high speed camera. I think it would be tricky to get the wave front form and the velocity profile to both act like a plane wave would. Even if one cannot change the driver, you still want the relevant information to have better control how to match the horn.
SWAG
You may be able to get some sort of picture with a strobe light, signal generator, an elastic membrane lightly stretched over a ring and some fine sand sprinkled on top. Place the driver exit, sans horn near the underside of the diaphragm. Run at different frequencies and distances and observe the patterns that emerge. I have not tried this, so it remains a SWAG.
Note: The generator needs to supply a trigger for the strobe.
The measurement problem is three fold, given a DIY non-commercial setting:
1. The measurement protocol needs to be transparent to the process you are trying to quantify.
2. The instrument to do this cost more than the DUT.
3. The degrees of design freedom are limited to the horn parameters only, and picking a driver to meet the intended mission. Beyond this, changing driver parameters is not tenable here as more instruments and manufacturing facilities would be required to implement them.
For an OS Horn design and driver picking of this horn type, follow Earl's recommendations. He has more experience here than most of the rest of us whgen put together.
In any event the flare tangent angle at the horn throat should march that of the driver exit.
Regards,
WHG
You may be able to get some sort of picture with a strobe light, signal generator, an elastic membrane lightly stretched over a ring and some fine sand sprinkled on top. Place the driver exit, sans horn near the underside of the diaphragm. Run at different frequencies and distances and observe the patterns that emerge. I have not tried this, so it remains a SWAG.
Note: The generator needs to supply a trigger for the strobe.
The measurement problem is three fold, given a DIY non-commercial setting:
1. The measurement protocol needs to be transparent to the process you are trying to quantify.
2. The instrument to do this cost more than the DUT.
3. The degrees of design freedom are limited to the horn parameters only, and picking a driver to meet the intended mission. Beyond this, changing driver parameters is not tenable here as more instruments and manufacturing facilities would be required to implement them.
For an OS Horn design and driver picking of this horn type, follow Earl's recommendations. He has more experience here than most of the rest of us whgen put together.
In any event the flare tangent angle at the horn throat should march that of the driver exit.
Regards,
WHG
At the frequencies of interest I don't think that this would work. The flaps will be too heavy.
This is a system to measure optical wave fronts, as in a vision correction lens.
Best wishes
David
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Mistake
This article was in my reading list. I finally got around to reading it. Acoustic Holography is what I had in mind.
Regards,
WHG
This article was in my reading list. I finally got around to reading it. Acoustic Holography is what I had in mind.
Regards,
WHG
If by "it" you mean my technique, it measures inside of a plane wave tube. One could do the same calcs inside of an OS waveguide if you were willing to find the OS modes (for which there do exist numerical solutions), but inside of a tube the solutions are just Bessel functions which are very well known.