Ok, any horn or waveguide produces HOMs. But are we saying that they are generated by diffraction?
Diffraction due to friction on the surface that the wave travels across??
Or??
Thought experiment: take a square conic horn, remove two parallel sides, make the remaining sides "nearly infinite" in width will there still be HOMs present? (the idea being the wave expands along the surface in one axis... and no surface on the other)
_-_-bear
Diffraction due to friction on the surface that the wave travels across??
Or??
Thought experiment: take a square conic horn, remove two parallel sides, make the remaining sides "nearly infinite" in width will there still be HOMs present? (the idea being the wave expands along the surface in one axis... and no surface on the other)
_-_-bear
bear said:
Perhaps I am not understanding what you intend to say.
As far as I know the mere bending of the horn contour does not in and of itself create "diffraction", if the bend is for example the exponential bend of an exponential horn, or even
I'm all "ears" wrt this...
_-_-bear
Yes, you got the picture – though you still doubt
Best you can do is to reread Earls and Bjorns papers from that point of view.
Its a littlle bit tough to cut through Earls "woodoo dancing" he likes to wrap around the issue – but basically you get the best idea of whats happening is to look at my "bunch of pretty pictures" at Earls thread first and read the papers afterwards.
Diffraction has no other meaning as that the wave front reaches a point where more space is available further on as it can fill when travelling in its original shape.
To make it more clear
– in an (infinite) duct for plane wave fronts this never happens – in a duct of finite length this happens when the wave front reaches its mouth.
- now what happens at the mouth? Clearly part of the wave front will be bent around the rim whereas part of the wave front will continue to go ahead.
- the part of bending around the rim is the diffraction part
- now this mechanism of diffraction also happens if you – lets say attach a conical horn at the end of our finite duct (slightly less, as the change in available area further on is less)
- now – if you assume to replace our finite duct by a standard compression driver (which is said to provide fairly plane wave fronts) and you attach a conical horn like the OS (at its mouth) you get some severe diffraction at the point of the driver / throat joint.
- this raises the need of a "as smooth as possible" transition section from the joint to the conical section – which has been found by Earl to be the OS contour.
- keep in mind that "as smooth as possible" is the best you can do but still is pretty bad with respect to diffraction
- once the wave front is bent from a plane wave front to meet the requirements of a conical horn – which is a spherical wave front with its centre in the point of the contour origin – the conical horn is the *only* shape that does not require further bending of the wave front (you can look at a conical horn as a cut out section of a omni directional source radiating in free space)
To sum up this is the basic meaning of what Earl tells us with ""there are only two co-ordinate systems of 1P"
Now we have clarified (to some extent) the meaning of diffraction.
Now - *if* at the point of diffraction there follows some more contour - of a duct of larger diameter or a horn contour with increased flare – part of the diffracted wave front gets reflected from that following contour – creating the axisxmmetric overlay of interference called "HOM".
Pretty simple in its basic mechanism – no need for all that fuss around "HOM"
If you follow Jean Michel's approach you will see that he treats the issue different – concentrating on avoiding heavy diffraction / reflection along the whole contour and at the mouth too...
Michael
ZilchLab said:
Yes.
ZilchLab said:
You invite me for a ride on a mary go round on vanitiy fair?
Basics:
http://www.exdreamnet.de/download/WgRevisited.zip
(chapter 3.5)
Practice:
http://www.exdreamnet.de/download/WgPractice.zip
The OS waveguide is nearly double as long as the CD horn, it has the double mouth area, and the designed coverage - difficult to achieve - is much wider with the CD device.
Besides a bit - to say it deridingly - unfair conditions the OS shows severe "HOM" at around 8..9kHz, the CD is free from that. CD avoids "HOM" by diffraction - I mean it. The impulse response of the CD is much cleaner.
Horizontal coverage is all in all more consitent - with CD - as it shows less attentuation at small angles. Vertical coverage of the CD is much less impressive as can be seen from the "Q" over frequency plots. That CD must be a JBL biradial or a knockoff ... we all know that these devices intentionally blend off the high frequency content in vertical direction. True, people designing CD devices design them for their needs in spite of being stuck to idealistic assumptions**.
So, to measure "HOM" we should know the basic mode by which "HOM" is defined as being opposed to.
** You may lookup the RCF H100 as an example for a vertically more "constant" CD device
wxa666 said:
Mileage is mileage, apparently.
wxa666 said:
Despite them both being axi-asymmetric, the smaller CD has ~50% wider beamwidth, and with a shorter vertical height, it would be expected to lose pattern control at a somewhat higher frequency, as shown.
The comparison was not apples to apples, but did show how the behavior differed in general terms.
I'm not sure about the HOM part, since we're supposed to ignore the OS's on-axis anomaly....
ZilchLab said:
I'm not sure about the HOM part, since we're supposed to ignore the OS's on-axis anomaly....
Do we? Besides the on axis "something" that the OS shows dramatically what about the rest of the "inconsistency" plots? I don't know exactly what this measurment is addressed to. The authors state it shows IM distortion for the most. But that could nothing have to do with reflections and the like what the authors take primally into account. Still wondering to what this kind of measurement will evolve to in the next decades. The OS is 1% better within a certain frequency interval, though.
I really would like to examinate my JBL 2352 horns in respect to HIGHER ORDER MODES. They sound very pleasing all in all. But getting annoying when level is higher than 100dB for longer periods. Strange things around. I have to find the BASIC MODE! Any help appreciated.
wxa666 said:
Do you use a low midrange horn and bass horn with the 2352? At higher levels I find horns to sound best with horns. If you use a direct radiator woofer it may start to compress at the higher levels where the compression driver is still delivery near full output. This can skew the whole perception of the response and can be annoying.
Maybe the use of foam and it's attenuation and higher dynamic/sooner compression make a direct radiator woofer / horn top end less "skewed" at elevated levels? Something to chew on..
Also the use of metal diaphragms at elevated levels can be annoying in my experience. Have you tried coating your diaphragms?
TrueSound said:
Hi,
You didn't get the twinkling? 100 dB for longer periods as mentioned will wreck Your hearing forever.
Yes I tried coating with JBLs ... but it didn't work at all. 3dB attenuation but same ragged response above 13kHz. An aftermarket dia was best since. Strange enough the 2352 with 2447 suffers from irregularities at 10kHz as the OS does - but reverse somehow. Coincidence or what is it?
so long
Ok... having done more reading it seems that HOMs were first identified, measured and reduced to practice in 1925 by Hoersch in conical horns.
According to what I now understand any expansion of a wavefront can cause HOMs to occur.
The slower the expansion, the less HOMs are apparently created...
Earlier we discovered that HOMs occur at a frequency that is related to the dimensions of the walls... and also that HOMs that are predicted by a computer model do not all occur, and that they seem to occur mainly at higher frequencies. This apparently corresponds to the throat area, and not to the larger horn expansion - since that corresponds to a lower frequency.
To some extent I guess we can all call "horns" and "waveguides" now "soft diffraction devices" as well...
_-_-bear
According to what I now understand any expansion of a wavefront can cause HOMs to occur.
The slower the expansion, the less HOMs are apparently created...
Earlier we discovered that HOMs occur at a frequency that is related to the dimensions of the walls... and also that HOMs that are predicted by a computer model do not all occur, and that they seem to occur mainly at higher frequencies. This apparently corresponds to the throat area, and not to the larger horn expansion - since that corresponds to a lower frequency.
To some extent I guess we can all call "horns" and "waveguides" now "soft diffraction devices" as well...
_-_-bear
bear said:
Nop the (infinite) conical is the exception when driven by the right wave front – unfortunately infinite does not make that much sense for home application
and the "right wave form" has to be created first .Its not that the wave front *expands* (this is what wave front from a omni source radiating in 4Pi does anyway) what creates "diffraction" (its "diffusion" if I translate correctly) - its all about *bending* the wave front.
bear said:
yes, in the sense of *less severe* not necessarily in the sense of *less discrete modes*
bear said:
To some extent I guess we can all call "horns" and "waveguides" now "soft diffraction devices" as well...
_-_-bear
So you think everything about HOM now falls into place?
Michael
wxa666 said:
How about this: Do the X/Y mesh thing at some nominal distance in front of the waveguide, say 1M. If Michael's plots are correct, we don't care about the SPL at this point, it's the arrival times that define the shape - plane, warped plane, sphere, whatever, on the assumption that a wavefront actually has a 3D "edge," and that no HOMs are able to get there first. Good so far?
Add a Z axis, forward distance, and map the SPL spectrum of the basic modal plane. Windowing will then reveal the anomalies, perhaps.
wxa666 said:
Move up to 2452H-SL. It's a different animal, aquaplas'd titanium diaphragm, but also something else, phase plug changes, maybe. Look at the PWT measurements; something's up. I was able to make 2435HPL and 2431H work nicely on 2352, but never so well as 2452H-SL.
435Be and 435Al are the Consumer versions of 2435HPL and 2431H, respectively, with aquaplas'd diaphragms. Drivers and diaphragms are available. Ferrofluid's also a factor to sort out. We couldn't get 476Be, and I suspect the same will be the case with 476Mg. JBL Consumer knows better than to let those into the hands of DIYers; the back door is locked up tight....
Double blind testing
I propose a DBT of the audibility of HOM.
I do not doubt the existance of HOM, so that shouldn't be inferred by my post. I have always thought horns with sharp edges sounded harsh, long before this HOM subject was ever discussed. To me, there's definitely something to it.
However, what I am not at all sure about is the amount of psychoacoustics involved. These guys putting foam in their horns all think they hear a difference. I can see how the internal reflections would be absorbed by the foam more than the axial wave. But I can also see how the guy putting the foam in the horn would expect it so sound better, especially after getting excited by the prospect here.
My question is, which is the most responsible for a perception of improved sound quality here: Actual detection or preconceived notion?
So I propose a DBT. Identical speakers with and without foam. Might also be useful to include other CD horn shapes with varying degrees of diffraction.
I think that's really the only way to determine the audibility of varying amounts of HOM. Without that, any comments on the effectiveness of specific flare profiles or foam treatments are subjective opinions, likely biased by preconceived notions.
I propose a DBT of the audibility of HOM.
I do not doubt the existance of HOM, so that shouldn't be inferred by my post. I have always thought horns with sharp edges sounded harsh, long before this HOM subject was ever discussed. To me, there's definitely something to it.
However, what I am not at all sure about is the amount of psychoacoustics involved. These guys putting foam in their horns all think they hear a difference. I can see how the internal reflections would be absorbed by the foam more than the axial wave. But I can also see how the guy putting the foam in the horn would expect it so sound better, especially after getting excited by the prospect here.
My question is, which is the most responsible for a perception of improved sound quality here: Actual detection or preconceived notion?
So I propose a DBT. Identical speakers with and without foam. Might also be useful to include other CD horn shapes with varying degrees of diffraction.
I think that's really the only way to determine the audibility of varying amounts of HOM. Without that, any comments on the effectiveness of specific flare profiles or foam treatments are subjective opinions, likely biased by preconceived notions.
Re: Double blind testing
Agree that DBT's would confirm audibility/non-audibility of the foam effects. I have a pretty ideal system for that. My crossover/volume control is a DEQX, so I can easily eq to the same level for different combinations. I'd do it myself, but that's pretty hard.
Obviously, I can't rule out psychoacoustic effects from my own experience. But I can say that I've tried lots of the usual minor tweaks, and can't reliably hear differences, even for things where I expected to hear a difference. But I am sensitive to the megaphone effect. The first pair of speakers I built were a well regarded back loaded horn. The enclosures are fairly tedious to build. So after all that work, I sure wanted to like them. But immediately upon hearing them, I could hear a horn like character, though lower in the frequency range than with mid-high horns. I tried lots of fixes and got them better, but still couldn't get them to the point where I could ignore it.
When I built my Unity's I could hear some of that effect. Not much, and not on all music, but still distracting on some, particularly female voices. I put the foam in and have not detected the same issue. Conditioning? Possibly, but I don't think so. Not worth it to me to determine absolutely, as I'm satisfied, the cost isn't high, and there is no real downside.
Sheldon
Wayne Parham said:
Agree that DBT's would confirm audibility/non-audibility of the foam effects. I have a pretty ideal system for that. My crossover/volume control is a DEQX, so I can easily eq to the same level for different combinations. I'd do it myself, but that's pretty hard.
Obviously, I can't rule out psychoacoustic effects from my own experience. But I can say that I've tried lots of the usual minor tweaks, and can't reliably hear differences, even for things where I expected to hear a difference. But I am sensitive to the megaphone effect. The first pair of speakers I built were a well regarded back loaded horn. The enclosures are fairly tedious to build. So after all that work, I sure wanted to like them. But immediately upon hearing them, I could hear a horn like character, though lower in the frequency range than with mid-high horns. I tried lots of fixes and got them better, but still couldn't get them to the point where I could ignore it.
When I built my Unity's I could hear some of that effect. Not much, and not on all music, but still distracting on some, particularly female voices. I put the foam in and have not detected the same issue. Conditioning? Possibly, but I don't think so. Not worth it to me to determine absolutely, as I'm satisfied, the cost isn't high, and there is no real downside.
Sheldon
There are some essential problems in terms of using a listening test to make a determination about these matters.
I'd say stick to the objective empirical measurements, the engineering simulations and the mathematical descriptions as far as determining their existence.
In terms of "better or worse" that is a can of big fat juicy worms. Any one of us can make that determination WRT their own system, or their own preferences, but that proves not a thing. You can use DBT, ABX or whatever.
Another thought, merely on the foam vs no foam comparison, there are several variables in just setting up the two tests to be "equivalent" that makes them automatically not equivalent. Although I think it is self-evident, here are a few:
- the EQ means a phase shift
- the EQ means a different diaphragm displacement vs frequency
- the EQ means another signal path with active and passive devices
- and if the EQ is DSP and always in, how clean is the DSP path?
- is the foam removing something that is being generated NOT by the driver/horn? Rather, it is annoying yet faithfully reproduced??
- and more...
Of course a listener can still have a preference... but I'd like to be able to measure "distortion" due to HOMs... since if you can hear them not be present then you ought to be able to measure them and then measure not them??
Or are we talking the sort of difference between copper wire and silver wire, or film/foil vs. metalized?? From the report that Sheldon gives, it seems like a larger difference, which would lead me (at least) to think that it ought to be rather measureable via somewhat normal means??
_-_-bear
I'd say stick to the objective empirical measurements, the engineering simulations and the mathematical descriptions as far as determining their existence.
In terms of "better or worse" that is a can of big fat juicy worms. Any one of us can make that determination WRT their own system, or their own preferences, but that proves not a thing. You can use DBT, ABX or whatever.
Another thought, merely on the foam vs no foam comparison, there are several variables in just setting up the two tests to be "equivalent" that makes them automatically not equivalent. Although I think it is self-evident, here are a few:
- the EQ means a phase shift
- the EQ means a different diaphragm displacement vs frequency
- the EQ means another signal path with active and passive devices
- and if the EQ is DSP and always in, how clean is the DSP path?
- is the foam removing something that is being generated NOT by the driver/horn? Rather, it is annoying yet faithfully reproduced??
- and more...
Of course a listener can still have a preference... but I'd like to be able to measure "distortion" due to HOMs... since if you can hear them not be present then you ought to be able to measure them and then measure not them??
Or are we talking the sort of difference between copper wire and silver wire, or film/foil vs. metalized?? From the report that Sheldon gives, it seems like a larger difference, which would lead me (at least) to think that it ought to be rather measureable via somewhat normal means??
_-_-bear
"I'd say stick to the objective empirical measurements, the engineering simulations and the mathematical descriptions as far as determining their existence."
Hello Bear
I have no doubt about the existence of HOM. If they are in the proofs they are real. The question seems to the how audible they are. That said there seems to be no easy way to measure them using the readily available measurement suites that many of us have and are familiar with.
More importantly no matter what we measure it all comes down to what we hear. Why would a listening test be a bad idea??
The inventor has already done this. If we can confirm his findings that seems to be a real step forward.
"Seems a little odd that many advise taking the bug screens out of compression drivers for a "smoother" sound"
Hello Panomaniac
I won't be taking mine out any time soon. The foam absorbs reflections the screen has the potential make more. All I know is I have about .020 between my diaphrams and my phase plugs. I am not taking any chances of some damn bug messing up the works.
Rob
Hello Bear
I have no doubt about the existence of HOM. If they are in the proofs they are real. The question seems to the how audible they are. That said there seems to be no easy way to measure them using the readily available measurement suites that many of us have and are familiar with.
More importantly no matter what we measure it all comes down to what we hear. Why would a listening test be a bad idea??
The inventor has already done this. If we can confirm his findings that seems to be a real step forward.
"Seems a little odd that many advise taking the bug screens out of compression drivers for a "smoother" sound"
Hello Panomaniac
I won't be taking mine out any time soon. The foam absorbs reflections the screen has the potential make more. All I know is I have about .020 between my diaphrams and my phase plugs. I am not taking any chances of some damn bug messing up the works.
Rob
panomaniac said:Seems a little odd that many advise taking the bug screens out of compression drivers for a "smoother" sound - and yet others suggest a big hunk of foam makes things better.
Hmmmm.....
Not necessarily.
If you look at a bug screen as kind of hurdle the wave front has to pass.
In terms of acoustics its nothing less than a halfway mirror – uhhh – reflections, reflections – no good
Looking at acoustic foam instead its like shouting into space of infinity.
Meaning if you make it dense and large enough quite anything will get absorbed.
Pretty good for not getting any echos and especially good for the driver who gets decoupled from the otherwise rugged horn impedance (not fully - only very slightly of course but obviously enough to be noticed)
bear said:
I too think it should be measurable.
As said - quite contrary to Earls opinion – I think its useful to get the lumped HOM measured and I also think it shouldn't be so complicated except for bringing up the reflections that are several decades below in an impulse response graph.
With linear scale, signals down at around 30-40 dB got lost (not possible to identify them) – and i'm pretty sure we will have to look another 20-40dB below that at least.
Michael
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