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In contrasting an OS throat to a large circular arc, it seems clear the OS has an advantage in HF constant directivity. (A large circular arc would "shadow" more of the driver exit at higher off-axis angles...OS opens quickly with minimal shadowing.)
However, it is not so clear (to me) why the OS throat offers lower diffraction/scattering than a large circular arc (both corrected for driver exit angle). The large circular arc would seem to offer lower rate-of-change over a longer path rather than focus it in a small area.
In simple words, what are the diffraction/scattering difference(s) between the two curves?
However, it is not so clear (to me) why the OS throat offers lower diffraction/scattering than a large circular arc (both corrected for driver exit angle). The large circular arc would seem to offer lower rate-of-change over a longer path rather than focus it in a small area.
In simple words, what are the diffraction/scattering difference(s) between the two curves?
The second derivative of the area expansion of a duct with circular arc walls is sensibly constant over a certain domain but it is one of those difficult functions that have zeros and infinities in their first derivative.
The o.s. device has an expansion that is a parabola symmetrical about the y axis and has a second derivative that is constant for all values of x.
From this it would seem that devices of both sorts of the same length, (where the circle is well behaved), would have the same modal production, but different directivity.
Rcw.
The o.s. device has an expansion that is a parabola symmetrical about the y axis and has a second derivative that is constant for all values of x.
From this it would seem that devices of both sorts of the same length, (where the circle is well behaved), would have the same modal production, but different directivity.
Rcw.
rcw said:
If its the "second derivative" *exclusively* you relate diffraction to, you are getting into severe trouble to judge the sonic impacts of a scattering junction.
Such a limited "math picture" leads to an exaggerated view related to diffraction issues – kind of diffraction-phobia 😀.
I don't think that very "math picture" – if this is what you or Earl rely on - mirrors "reality" reasonably well if you consider the decent behaviour of such device that deliberately use this effect:
Michael
I do not rely upon this “limited picture” as you care to call it, what I am doing is to outline in a simple way the physics behind the o.s. waveguide idea.
My own preference is for shallow devices driven by domes and have had many disagreements with Earl Geddes regarding the merits of otherwise of such devices, but the point is the physics keeps on being true however it might disagree with the pet notions that people might have about how things work.
The device you picture is known by some in the trade as a “blurter” horn, because to quote one of my acquaintances, “it looks like an **** and sounds like someone farting”, and was one that in the double blind tests conducted on a set of horns clearly identified as a horn.
This study was conducted to find out why people seemed to prefer direct radiators for high quality sound reproduction, and the reason seemed to be very much connected with the simple observation that the sound radiating inside a horn can move multimodally and is dispersive whereas in free air it only moves in the longitudinal mode and the velocity is constant.
Horns with the minimum of these effects, i.e. low reflection low diffraction devices were not identified as horns to a statistically significant degree, and horns that had a lot of them were identified as horns to a statistically significant extent, being a scientist I find such double blind data significant, and accept it regardless of what prejudices I might have.
Rcw.
My own preference is for shallow devices driven by domes and have had many disagreements with Earl Geddes regarding the merits of otherwise of such devices, but the point is the physics keeps on being true however it might disagree with the pet notions that people might have about how things work.
The device you picture is known by some in the trade as a “blurter” horn, because to quote one of my acquaintances, “it looks like an **** and sounds like someone farting”, and was one that in the double blind tests conducted on a set of horns clearly identified as a horn.
This study was conducted to find out why people seemed to prefer direct radiators for high quality sound reproduction, and the reason seemed to be very much connected with the simple observation that the sound radiating inside a horn can move multimodally and is dispersive whereas in free air it only moves in the longitudinal mode and the velocity is constant.
Horns with the minimum of these effects, i.e. low reflection low diffraction devices were not identified as horns to a statistically significant degree, and horns that had a lot of them were identified as horns to a statistically significant extent, being a scientist I find such double blind data significant, and accept it regardless of what prejudices I might have.
Rcw.
Oho, oho – another scientist with some fiery temper, how lovely!
🙂
But to be serious – no need to take it personal if I just state the obvious.
good to know, thanks for illuminating your background...
Good to know there is Australian english that more easily goes through censoring 😉
But to be serious – there definitely is more than one question about the conclusion you draw.
- how was the degree of diffraction measured when these groups of horns got categorised in being evil diffractive or holly - äh - what ?
- I'm a little light on own experience with the JBL BiRadial shown (having heard probably two or three times but haven't payd any attention) – but are more familiar with the EV ST350 which is different but pretty similar (same – decent but not stellar). As I haven't heard the "farting" sound with the EV it might be that this specific "farting" sound originates from the looooong cavity behind the scattering junction – generating a looooot of resonance. Meaning the conclusion that this "farting" sound is due to diffraction is a little bit ....
No – in free air you have exactly the same overlay of diffraction – were just showing this in the other thread.
http://www.diyaudio.com/forums/showthread.php?postid=1872295#post1872295
There is no qualitative difference (besides magnitude of course *if* you like to see this as kind of quality) for free air radiating device and for radiating into boundaries - where people like to call the "diffraction > reflection > delay > interference" complex as HOM
Michael
🙂
But to be serious – no need to take it personal if I just state the obvious.
rcw said:
good to know, thanks for illuminating your background...
rcw said:
Good to know there is Australian english that more easily goes through censoring 😉
But to be serious – there definitely is more than one question about the conclusion you draw.
rcw said:
- how was the degree of diffraction measured when these groups of horns got categorised in being evil diffractive or holly - äh - what ?
- I'm a little light on own experience with the JBL BiRadial shown (having heard probably two or three times but haven't payd any attention) – but are more familiar with the EV ST350 which is different but pretty similar (same – decent but not stellar). As I haven't heard the "farting" sound with the EV it might be that this specific "farting" sound originates from the looooong cavity behind the scattering junction – generating a looooot of resonance. Meaning the conclusion that this "farting" sound is due to diffraction is a little bit ....
rcw said:
No – in free air you have exactly the same overlay of diffraction – were just showing this in the other thread.
http://www.diyaudio.com/forums/showthread.php?postid=1872295#post1872295
There is no qualitative difference (besides magnitude of course *if* you like to see this as kind of quality) for free air radiating device and for radiating into boundaries - where people like to call the "diffraction > reflection > delay > interference" complex as HOM
Michael
"I suppose its a high frequency horn only"
There are 2 horns like that the 2344 and 2342. They were used in the JBL 4425/4430 and 4435 monitors and in some of the SR boxes. The larger horn was used from 1K up the smaller horn about 1.2K and up.
There are also a couple of imitations out there as well. The imitations don't have the same throat as the originals or hold directivity as well as the originals.
They are classic 80's difraction horns.
Rob🙂
There are 2 horns like that the 2344 and 2342. They were used in the JBL 4425/4430 and 4435 monitors and in some of the SR boxes. The larger horn was used from 1K up the smaller horn about 1.2K and up.
There are also a couple of imitations out there as well. The imitations don't have the same throat as the originals or hold directivity as well as the originals.
They are classic 80's difraction horns.
Rob🙂
doug20 said:
Doug, I read an article in the AES which came to a similar conclusion. Not certain that this is the one that RCW is referring to, but give it a look:
http://www.aes.org/e-lib/browse.cfm?elib=7915
A blind listening test is described in which 16 loudspeakers are compared with four reference loudspeakers under anechoic conditions. The test is concerned with the perceived sonic similarity between midrange horn loudspeakers and direct radiators and is intended to pinpoint the physical cause of a "characteristic sound" attributed to many studio monitor systems equipped with midfrequency-range horns. Comparisons are made between the listening test results and measurements of on-axis frequency response. The results indicate that short horns sound more similar to direct-radiating loudspeakers than long horns.It is concluded that the reflections from the mouth termination of long horns is responsible for the characteristic sound and that for studio monitor applications, a midrange horn should have a length not exceeding 340 mm and should be free of flare discontinuities.
Authors: Holland, Keith R.; Fahy, Frank J.; Newell, Philip R.
Affiliation: Institute of Sound and Vibration Research, University of Southampton, Southampton, UK
JAES Volume 44 Issue 1/2 pp. 23-36; February 1996
The study I referred to is an AES paper entitled, “The sound of midrange horns for studio monitors”,
AES Journal, Vol. 44, No. ½, (1996 Jan/Feb).
I have no strong personal feelings about the horn in question, however people I know in the rock n’ roll business , (most of whom are given to robust expression, that being “rock n‘ roll“), tend to make such comments about the device and the speaker system it is most associated with.
If you look at any acoustics text book you will see what I said about free air is correct. It is true that in real rooms with boundaries and obstacles that effects reflections and diffractions occur in the sound field. However on the way to our ears the first arrivals of sound do conform to the non dispersive constant velocity rule. Higher modes and diffractions cannot exist in free air, but the effects of them can. Inside ducts they can, and if these ducts are interposed between our ears and the primary transducer the effects can be readily heard, in the sound.
My opinion about computer simulations is mixed; on the one hand it is a useful conceptual tool, but it is a long way from being a sufficient one.
One tends to forget the underlying science and make pretty pictures. In the end if all I want to do is look at pretty pictures I go to an art gallery.
What I want is facts and figures, about the physical processes happening, and I am afraid all I get from boundary element number crunching solutions to the Helmholtz equation is pretty pictures, and with ought considering the underlying science all you can do is to fiddle about until you get the prettiest one.
The point about this is that you only get the prettiest one from the set of parameters you use and it seems that a lot of people, (present company excepted I am sure), seem to get the starting point from rigorous scientific methods such as consulting a ouija board.
Rcw.
AES Journal, Vol. 44, No. ½, (1996 Jan/Feb).
I have no strong personal feelings about the horn in question, however people I know in the rock n’ roll business , (most of whom are given to robust expression, that being “rock n‘ roll“), tend to make such comments about the device and the speaker system it is most associated with.
If you look at any acoustics text book you will see what I said about free air is correct. It is true that in real rooms with boundaries and obstacles that effects reflections and diffractions occur in the sound field. However on the way to our ears the first arrivals of sound do conform to the non dispersive constant velocity rule. Higher modes and diffractions cannot exist in free air, but the effects of them can. Inside ducts they can, and if these ducts are interposed between our ears and the primary transducer the effects can be readily heard, in the sound.
My opinion about computer simulations is mixed; on the one hand it is a useful conceptual tool, but it is a long way from being a sufficient one.
One tends to forget the underlying science and make pretty pictures. In the end if all I want to do is look at pretty pictures I go to an art gallery.
What I want is facts and figures, about the physical processes happening, and I am afraid all I get from boundary element number crunching solutions to the Helmholtz equation is pretty pictures, and with ought considering the underlying science all you can do is to fiddle about until you get the prettiest one.
The point about this is that you only get the prettiest one from the set of parameters you use and it seems that a lot of people, (present company excepted I am sure), seem to get the starting point from rigorous scientific methods such as consulting a ouija board.
Rcw.
rcw said:
Allow me to add another argument why I don't follow the conclusions of that paper with respect to "diffraction" (If I would put it more pronounced I'd say "its based on flimsy data" 😀 ).
If you have a look at a modern AMT you immediately see that there is a lot of diffraction going on due to its "scattering junctions" (180 deg included angle)
Same for any electrostat.
Have you ever heard someone complaining about any "honk" or "farting" sound of these device?
See what I mean?
My take on this is that it isn't diffraction "per se" that we have to spot for unpleasant sound but its impact on the sound field which highly depends on circumstances.
The BiRadial's and MantaRay's below as the (currently 😉 ) "most evil" diffraction horns thus should possiby be considered in a different context?
BEM in contrast seems to calculate rather for the radiation pattern only (the wave front radiating into virgin space) which *is* part of an established sound field but the interference part is missing.
With respect to constant directivity this tells us that "CD" is basically related to the radiation pattern only – but the "how to" – better revealed in a sound field analysis - makes all the difference.
Maybe someone more familiar with BEM can confirm?
rcw said:
🙂
Michael
I agree Michael that we are not talking about diffraction per se, we are talking about diffraction in ducts that have dimensions that are not too dissimilar to the wavelengths we are propagating.
In the two devices you mention the first example can be seen as a diffraction slot that has very small dimensions with respect to the wavelengths it is propagating and the impedance anomaly that this presents does not have any significant effect because of this.
In the second case however we again have a impedance anomaly and it is virtually the biggest one we can get in a horn.
The point now is that the dimensions encountered by the back and forward scattering waves are such that they have significant effect. The back scattered wave reflects to the driver diaphragm and the impedance anomaly that this presents reflects the wave back again.
The forward scattering wave reflects from the impedance anomaly at the junction to the flared mouth section which all in turns reflects from the mouth, all refecting back to the main scattering junction, and so on.
If you want to know the exact modal breakdown inside the horn it is necessary to solve the Helmholtz equation for eigenvalues.
It is possible to get some idea from boundary elements, but it is far better to use a combination boundary and finite element model because the eigenvalues are solutions to a three dimensional matrix.
To do this still, as far as I know, requires professional finite element software that involves big bucks.
Rcw.
In the two devices you mention the first example can be seen as a diffraction slot that has very small dimensions with respect to the wavelengths it is propagating and the impedance anomaly that this presents does not have any significant effect because of this.
In the second case however we again have a impedance anomaly and it is virtually the biggest one we can get in a horn.
The point now is that the dimensions encountered by the back and forward scattering waves are such that they have significant effect. The back scattered wave reflects to the driver diaphragm and the impedance anomaly that this presents reflects the wave back again.
The forward scattering wave reflects from the impedance anomaly at the junction to the flared mouth section which all in turns reflects from the mouth, all refecting back to the main scattering junction, and so on.
If you want to know the exact modal breakdown inside the horn it is necessary to solve the Helmholtz equation for eigenvalues.
It is possible to get some idea from boundary elements, but it is far better to use a combination boundary and finite element model because the eigenvalues are solutions to a three dimensional matrix.
To do this still, as far as I know, requires professional finite element software that involves big bucks.
Rcw.
What about the smith horn, aka "diffraction horn"? 😀mige0 said:
JBL 2397 :
An externally hosted image should be here but it was not working when we last tested it.
rcw said:
RCW
Whatever disagreements we may have had in the past, I certainly respect your statements here. You are about the only one who makes any sense.
I have been out of town for a few days and not in contact with the ether. "I'll be back."
One comment - its not the "area" change that is important to diffraction, but the "contour" change. This is discussed in my various wrttings. The OS does have the minimum 2nd derivative of contour over an extended length.