The thing that is missing in this discussion is the fact that the wave sent back down the device, from mouth diffraction, is dominantly HOMs. Because the higher order modes propagate differently from the dominantly first order one (both in the device as well as into free space,) the effect is different along every axis. Hence, correcting the resonance at any single point (axis) is not going to be globally correct.
This is why I believe that it is much better not to have these resonances in the first place, than try to correct them after the fact.
This is why I believe that it is much better not to have these resonances in the first place, than try to correct them after the fact.
I understand this to be an angularly non-symmetric aberration in a lens, usually the eye. Horn does this apply to a waveguide?
Obviously you don't.
If you could look outside your own bubble, you would have known that astigmatism is the root of all the problems associated with diffraction slots, or
pronounced discontinuities.
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What I referred to are resonances at the lower end of the passband, as we see in camplo's horn.
"Low frequencies tend to reflect more strongly than high frequencies, so the reflections are most prevalent in the lowest octaves of the horn’s usable range. It is this precise phenomenon that produces an audible artifact commonly described as a 'honk'" (Gunness)
I can't imagine that these resonances (at wavelengths comparable or larger than the whole horn) are "dominantly HOMs". I don't doubt that at high enough frequencies the situation will be different and more complicated. And of course I agree that it is far better to not have those deffects in the first place.
I think the goal as i read the Gunness paper, is to catch the HOMs at the throat, like in a net, so that they don't radiate back out.
Catching the HOMs in the time domain, as they return to their place of origin.
I also don't think anyone assumes such time domain EQ will make all things right at every point on the globe. But I think there's strong evidence it can do a lot of average good globally.
Have you ever looked into FIR based impulse inversion to correct CD/horns, ala the Gunness paper?
Very curious if so....
Surely so.
I guess for me though, I look to how to integrate the CD/horn into an entire speaker, and the price of "horn perfect" seems too high my speaker obectives that are somewhat at odds with a standalone CD/horn. To each our own, eh?
No, I never looked at it, but I don't see it as ground breaking using FIR filters to EQ horns. My focus was on not having any sharp resonance or diffraction to EQ.Have you ever looked into FIR based impulse inversion to correct CD/horns, ala the Gunness paper?
Very curious if so....
I guess for me though, I look to how to integrate the CD/horn into an entire speaker, and the price of "horn perfect" seems too high my speaker obectives that are somewhat at odds with a standalone CD/horn. To each our own, eh?
My goal too has always been the optimized "system" for its intended space, not the individual components. I do always look from the perspective of small spaces not PA. I suspect your interests are in PA and that is a whole other ball game. In that world Danley's approach seems to me to be the optimum.
Gotcha. Thx.
My DIY goals started as PA, but i found the live-gig scene far too taxing, with time demands beyond what this old man was willing to cope with.
So exit PA world.
The PA stuff i initially built ( designed by an accomplished PA guy) sounded good enough in comparison to home gear, fidelity wise,
and so far above usual home gear SPL/dynamics/bass wise....
that I decided in order to keep DIY speaker building going, I'd try a "hi-fi PA" route.
After some conventional CD/waveguide builds over 12"s or 15"s, I got enamored by the idea of true 1/4 wave spacing between all sections.
Been synergy ever since ... on version 10 now Lol.
They have all been designed for indoor & outdoor use at home ...... as a direct replacement for conventional home gear.
But doesn't this dictate a fairly low DI? I seek as high a DI as I can get within reason.
Charlie Hughes uses the term when describing the Manta Ray horn.
https://peaveycommercialaudio.com/w...atic-Throat-Waveguide-by-Charles-E-Hughes.pdf
"However, this approach does introduce a new problem for horns used in arrays. The term “apparent apex” was coined to describe the angular focal point of dispersion, where the coverage angles converge in each plane. The apparent apices do not occur at the same point on the axis of this particular horn design. This causes the curvature of the wavefront as it emanates from the horn to be ellipsoidal, rather than spherical and is, therefore, astigmatic".
In that context and using his definition, it makes sense. But I have never heard the term used outside of optics, but I guess that it can be applied.
The dictionary says:
I have seen it in optics for discussing a lens aberration, such as is applied to the eye.
The dictionary says:
None of that seems to apply.
I have seen it in optics for discussing a lens aberration, such as is applied to the eye.
Yea, for sure. As one Czech (non-existent) classic wrote - "...and the worst of all are the gnomes. Those little bastards get everywhere."
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I can totally feel 10hz from my nearfield 1x4" monitors. I have to EQ the everloving **** out of it, and I'll maybe hit 70dB flat before I start getting distortion you can hear, but it's a two-way and it covers the full range.
It's just not a real set of parameters to judge something by.
It's just not a real set of parameters to judge something by.
Astigmatism is discussed several times in what is considered by many to be the only book an audio engineer needs on their shelf:
"The wave fronts in the interior of the throat
section have very little curvature. When these waves
encounter the slot two different events occur. A
portion of the wave is reflected back towards the
driver as the exit from the throat section represents a
discontinuity in the wave medium. The portion of
the wave which emerges from the slot undergoes
diffraction at the edges of the slot and emerges as a
diverging wave such that the wavefront has a small
radius of curvature in a plane perpendicular to the
length of the slot and a larger radius of curvature in
a plane parallel to the length of the slot.
The diverging wave is allowed to expand into a
bounded region whose cross-sectional area expands
conically except that the horizontal angle is greater
than the vertical angle so as to match the differing
radii of curvature formed at the slot. Wave propagation
in the expanding conical section is well
behaved as the horn walls provide a reasonable
match for the diverging wavefronts. The wavefronts,
of course, are astigmatic by virtue of the differing
radii of curvature in the horizontal and vertical
planes. With a sufficiently large mouth, the horn
coverage angles will be independent of frequency up
to a limit where the wavelength becomes comparable
to the width of the slot. Beyond this limit the
horn will begin to beam. In spite of the constant
directivity property, horns of this type have two
distinct and serious drawbacks. The internal reflection
that occurs at the diffraction slot gives rise to
standing wave production with associated resonances
occurring in the throat section. The astigmatism
associated with the emerging wavefronts from
the horn implies separated sources of sound rather
than a single point source, as would be the case for a
purely spherical wavefront."
David Gunness (Electro-Voice · Eastern Acoustic Works · Fulcrum Acoustic), "is known for his work on loudspeaker design, especially high-output professional horn loudspeakers for public address, studio, theater, nightclub, concert and touring uses." He obviously doesn't refer to ancient exponential, hypex, nor to axisymmetrical Tractrix or JMLC horns in this case:
"Horn Resonance, a second loudspeaker behavior, which yields well to digital preconditioning, is horn resonance. A wavefront progressing down any horn will encounter one or more discontinuities in the area expansion. All horns present a discontinuity at their mouths. Constant directivity horns often employ a diffraction slot to achieve a wide coverage pattern at high frequencies. The exit of this slot represents a severe discontinuity. A discontinuity in a horn’s expansion produces a reflection. A fraction of the sound power reverses course and returns to the compression driver where it is partially absorbed and partially re-emitted, often several milliseconds late. This process is, of course, regenerative, once again producing a decaying series of arrivals. Low frequencies tend to reflect more strongly than high frequencies, so the reflections are most prevalent in the lowest octaves of the horn’s usable range. It is this precise phenomenon that produces an audible artifact commonly described as a “honk”. The wavefront does not return to the compression driver in a perfectly coherent fashion, but it appears that the bulk of the reflected energy does converge back at the driver. To the extent that it does, the phenomenon acts as a two-port system, and is correctable via signal preconditioning."
"The wave fronts in the interior of the throat
section have very little curvature. When these waves
encounter the slot two different events occur. A
portion of the wave is reflected back towards the
driver as the exit from the throat section represents a
discontinuity in the wave medium. The portion of
the wave which emerges from the slot undergoes
diffraction at the edges of the slot and emerges as a
diverging wave such that the wavefront has a small
radius of curvature in a plane perpendicular to the
length of the slot and a larger radius of curvature in
a plane parallel to the length of the slot.
The diverging wave is allowed to expand into a
bounded region whose cross-sectional area expands
conically except that the horizontal angle is greater
than the vertical angle so as to match the differing
radii of curvature formed at the slot. Wave propagation
in the expanding conical section is well
behaved as the horn walls provide a reasonable
match for the diverging wavefronts. The wavefronts,
of course, are astigmatic by virtue of the differing
radii of curvature in the horizontal and vertical
planes. With a sufficiently large mouth, the horn
coverage angles will be independent of frequency up
to a limit where the wavelength becomes comparable
to the width of the slot. Beyond this limit the
horn will begin to beam. In spite of the constant
directivity property, horns of this type have two
distinct and serious drawbacks. The internal reflection
that occurs at the diffraction slot gives rise to
standing wave production with associated resonances
occurring in the throat section. The astigmatism
associated with the emerging wavefronts from
the horn implies separated sources of sound rather
than a single point source, as would be the case for a
purely spherical wavefront."
David Gunness (Electro-Voice · Eastern Acoustic Works · Fulcrum Acoustic), "is known for his work on loudspeaker design, especially high-output professional horn loudspeakers for public address, studio, theater, nightclub, concert and touring uses." He obviously doesn't refer to ancient exponential, hypex, nor to axisymmetrical Tractrix or JMLC horns in this case:
"Horn Resonance, a second loudspeaker behavior, which yields well to digital preconditioning, is horn resonance. A wavefront progressing down any horn will encounter one or more discontinuities in the area expansion. All horns present a discontinuity at their mouths. Constant directivity horns often employ a diffraction slot to achieve a wide coverage pattern at high frequencies. The exit of this slot represents a severe discontinuity. A discontinuity in a horn’s expansion produces a reflection. A fraction of the sound power reverses course and returns to the compression driver where it is partially absorbed and partially re-emitted, often several milliseconds late. This process is, of course, regenerative, once again producing a decaying series of arrivals. Low frequencies tend to reflect more strongly than high frequencies, so the reflections are most prevalent in the lowest octaves of the horn’s usable range. It is this precise phenomenon that produces an audible artifact commonly described as a “honk”. The wavefront does not return to the compression driver in a perfectly coherent fashion, but it appears that the bulk of the reflected energy does converge back at the driver. To the extent that it does, the phenomenon acts as a two-port system, and is correctable via signal preconditioning."
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I like high DI too, or maybe i should say i like a high constant directivity, that extends as low in frequency as possible.
With the straight sided conical synergies, it seems to me it's a matter of deciding what H-V pattern i want, and then how low in frequency do i want horizontal pattern control to hold (which of course just means how big am i willing to build.)
I've tried H-V 60x60, 60x40, 75x50, 75x60, & 90x60. Mouth widths from 31" to 48", with height's proportional to H-V pattern.
Horizontal pattern typically falls between 190-280Hz (using Keele's formula)
Depth is of course, a function of H pattern and mouth size. Trying to hold a narrow pattern low makes for a deep box indeed!!
I don't follow the reasons why 1/4 WL spacing would dictate a fairly low DI ??
Oh, let me stop and define what I meant by 1/4 spacing.....
My definition is all frequencies in the critical xover summation range, are within 1/4 WL of each other.
Which of course equates to the side of frequency range above xover freq. (Minimizing that critical range is one reason why i like steep linear phase xovers)
A synergy is the only design I've seen so far, that allows multiple driver sections to accomplish that....(other than maybe planars of various sorts.)
It seems the only way to overcome the simple geometry of piston drivers' centers-to-centers exceeding 1/4 WL in the horizontal plane, is to collapse the horizontal plane.... the horizontal spacing distance between drivers' centers......onto a cone or other suitable horn.
That's pretty much the entire reason i've been devoted to synergies.....to get 1/4 WL between sections all the way down in freq.
Please do explain what you mean by dictating a fairly low DI.... i'm missing something, thx
Thx for that and the Don Davis info.
Yes, I firmly believe (and measure) that correcting the bulk of the reflected energy that returns to the compression driver, is not too hard to do and of audible benefit.
That said, i think it's obvious the best of all worlds would be employing FIR "preconditioning", on a CD/horn needing very little of it to begin with
What is "considered by many" doesn't mean a thing. There was a time when the Earth was considered flat by many. You should stop using this phrase as if it meant something.
- No one questioned the possibility to define the term "acoustic astigmatism" in a meaningful way. But it's obviously something different than what was discussed and to what you reacted using that term.
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To refresh your - rambling short and long term - memory and assist your capacity to understand:
It is all about astigmatism.
The fact that he doesn't mention the name doesn't change the fact that he describes the same consequential issues as Don Davis.
It is all about astigmatism.
The fact that he doesn't mention the name doesn't change the fact that he describes the same consequential issues as Don Davis.
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