Some of the diffraction/wave theory results are counter intuitive.
Thank you for the link to the relevant post, I will read it before I comment more.
Best wishes
David
Think of sound like ripples in a fluid. Toss a stone into a still pond you will see the ripples. Toss a stone into a smoothly flowing stream; you will see the ripples going with the flow.
All those slots and sharp changes of direction in the throat and walls of a horn/waveguide are placed there to direct the flow of air (fluid) and the sound that goes along with the flow.
Horn/waveguide theory would have us believe that smooth spherical sound waves travel the length of the horn. I would like to believe that. This might be true if the fluid flow was always laminar. Due to friction at the horn/waveguide walls the fluid flow may become turbulent. Diffraction edges also cause turbulence.
It is my impression that turbulence caused eddy flow is the stuff of HOM’s.
File:Vortex-street-animation.gif - Wikipedia
DT
All those slots and sharp changes of direction in the throat and walls of a horn/waveguide are placed there to direct the flow of air (fluid) and the sound that goes along with the flow.
Horn/waveguide theory would have us believe that smooth spherical sound waves travel the length of the horn. I would like to believe that. This might be true if the fluid flow was always laminar. Due to friction at the horn/waveguide walls the fluid flow may become turbulent. Diffraction edges also cause turbulence.
It is my impression that turbulence caused eddy flow is the stuff of HOM’s.
File:Vortex-street-animation.gif - Wikipedia
DT
I think you misconceive it, that fluid flow is like sound flow is a faulty way to conceptualize the physics.
Like many popular misconceptions there is kernel of... not actually "truth" but facts that are misinterpreted.
In this case perhaps the fact that bass reflex ports can chuff from turbulent flow.
This is quite distinct from transmission of sound in a horn.
Sound transmits perfectly well in solids, with exactly the same wave equation.
In fact there are nice solid "horns" used for ultrasonic welders and such.
Obviously these have no flow, and there is practically none in an audio horn either.
Best wishes
David
David,
I did not say that sound traveled like fluid. I said that sound travels through moving fluid. As you say sound also travels through solids. An example of sound traveling through moving air: When there is an open air concert at the county fair and the breeze is blowing, the neighbors downwind are complaining about the noise. If the wind is not blowing the phone is not ringing.
Take a look at Earl R. Geddels’ paper “Audibility of Linear Distortion with Variations in Sound Pressure Level and Group Delay”. Geddels tests the concept that the distortion specific to compression drivers and horns is due to delay or group delay. The delay increases and the audibility of this horn specific new type of distortion become increasingly more audible with an increase in sound pressure level. In this paper Geddels does not name this new distortion metric, he says that it is linier unlike the nonlinear THD and IMD. Geddles does not identify what causes the increasing delay other than Variations in Sound Pressure Level and that and horn geometry can make it worse.
In an duct if the air flow is increased by 10% the power required to push it increases by the third power (1.1)^3 or by 33%. The velocity change is linear; the increase in power used is not. The increased power is due to friction and turbulence.
I am saying that fluid (air) friction and turbulence is the likely suspect for "horn delay distortion". (Okay I gave it a name.)
http://www.gedlee.com/Papers/AES06Gedlee_ll.pdf
Thanks DT
For fun
YouTube
I did not say that sound traveled like fluid. I said that sound travels through moving fluid. As you say sound also travels through solids. An example of sound traveling through moving air: When there is an open air concert at the county fair and the breeze is blowing, the neighbors downwind are complaining about the noise. If the wind is not blowing the phone is not ringing.
Take a look at Earl R. Geddels’ paper “Audibility of Linear Distortion with Variations in Sound Pressure Level and Group Delay”. Geddels tests the concept that the distortion specific to compression drivers and horns is due to delay or group delay. The delay increases and the audibility of this horn specific new type of distortion become increasingly more audible with an increase in sound pressure level. In this paper Geddels does not name this new distortion metric, he says that it is linier unlike the nonlinear THD and IMD. Geddles does not identify what causes the increasing delay other than Variations in Sound Pressure Level and that and horn geometry can make it worse.
In an duct if the air flow is increased by 10% the power required to push it increases by the third power (1.1)^3 or by 33%. The velocity change is linear; the increase in power used is not. The increased power is due to friction and turbulence.
I am saying that fluid (air) friction and turbulence is the likely suspect for "horn delay distortion". (Okay I gave it a name.)
http://www.gedlee.com/Papers/AES06Gedlee_ll.pdf
Thanks DT
For fun
YouTube
This paper identifies an apparent difference between a longer horn and a waveguide that has been optimised to have reduced reflections and diffraction. Since energy that doesn't follow the intended primary mode can manifest itself as a secondary source, it then goes on to consider the audible effects of delayed sources.
Some of these sources have been referred to as higher order modes. There is an implication as to how to reduce these by trying to maintain the wavefront.
Some of these sources have been referred to as higher order modes. There is an implication as to how to reduce these by trying to maintain the wavefront.
I always read Earl's stuff with interest, thanks for the prompt to reread that one, but I don't see that it supports your idea.
Not just in a duct, as a fairly serious cyclist, I'm painfully aware of the increase in power required to improve a race speed just a little!
But I think you misconceive this, there is very little actual air flow in an audio horn driven by a compression driver.
It's not like a bass reflex duct.
I think this very unlikely, the HOM that Earl discusses are intrinsic to the sound wave and would occur equally in a solid medium.
I could, of course, be mistaken, so why not PM Earl and have him check your claim?
If he does support it then I will be both amazed and sincerely appreciative.
Best wishes
David
I read the discussion, especially Earl's comments.
I think he's correct in theory but that it's practically irrelevant unless you plan to build custom compression drivers.
Any commercially available driver has a more or less planar wave-front at the throat and will thus narrow more or less as you have stated.
Best wishes
David
Well spotted
I almost mentioned this but decided to wait until I had studied it a little more.I see how the directivity can be controlled by the wavefront at the throat.
I find it hard to understand how the directivity can be "opened up" by a waveguide, at least usefully in practice.
Some of wave optics is counterintuitive and Earl is an expert but I have some concerns and questions.
What are the limits? (presumably there are limits).
Where is it demonstrated that the measured DI is not due to non-planar throat wavefront?
I haven't found theory or specific experimental data from Earl on this, or references, have you?
Best wishes
David
Last edited:
If the waveguide were not significantly within the bounds of the projected wavefront, as is the case with a piston on a flat baffle at higher frequencies, then its directivity seems dependent on the source size.
I have a reference regarding laminar flow.
There is then the interaction of diffracted energy, reflected by the walls of the waveguide and combining in a constructive way to produce the right wavefront. What do you think?
Edit: another interesting post
I have a reference regarding laminar flow.
There is then the interaction of diffracted energy, reflected by the walls of the waveguide and combining in a constructive way to produce the right wavefront. What do you think?
Edit: another interesting post
Last edited:
Compression drivers and horn/waveguides are thermodynamic machines. We are speaking about the air when we call them compression drivers. Air is what provides the impedance, the load, as in, no air, no sound. Air is an integral part of these machines. Trouble is we cannot watch this moving part move or do its’ work. This is different than a clock, we can watch it move and then gain the insight of how it works.
Thake a look at https://www.jblpro.com/pub/technote/tn_v1n08.pdf JBL Technical Notes Volume 1, Number 8. At long length this JBL Tech Note documents (and calculates) how the nonlinear compression of air in Compression Drivers’s and Horns is the mechanism for Harmonic Distortion and IMD.
Geddles tells us that perceived HOM horn distortions are THD’s and IMD’s delayed. It is difficult to take the air out of this argument.
Thanks DT
Thake a look at https://www.jblpro.com/pub/technote/tn_v1n08.pdf JBL Technical Notes Volume 1, Number 8. At long length this JBL Tech Note documents (and calculates) how the nonlinear compression of air in Compression Drivers’s and Horns is the mechanism for Harmonic Distortion and IMD.
Geddles tells us that perceived HOM horn distortions are THD’s and IMD’s delayed. It is difficult to take the air out of this argument.
Thanks DT
In terms of Horns/waveguides, directivity is largely affected by throat size. Look at Geddles’ OS formulas.
The driver throat radius is in the denominator, the smaller the throat the greater the throat impedance. A 2 inch throat has 4 times the area of a 1 inch throat. For a given sound pressure level a 1 inch throat has 4 times the air velocity impacting with higher throat impedance the air flow sharply slows down and spreads out to the limits of the horn/waveguide walls. Similar story with the 2 inch throat with less velocity, less throat impedance and less spreading. This is this reason why Geddles uses 1 inch throats.
Interesting thing about a 1 inch driver in a baffle, there is less velocity, less impedance (because there is no throat) and much tighter directivity.
Thanks DT
I think is different from this..
Which leads to the listener having a non-linear perception with regard to group delay, where group delay is a linear phenomenon.gedlee said:
gedlee said:
That was pretty much my point, if the words were any more similar I would worry that people may think I stole the quote.
Actually it's from before I joined DIYaudio, I should read that thread.
I did consider that this could happen at certain points and frequency bands.
But I suspect they would be too limited to contribute "usefully in practice".
That's why I added the phrase, but, like James, I'm still unsure.
That's another classic paper but, once more, I don't see that it supports your claim.
Like Allen, I don't think this is his claim at all, almost the opposite.
Best wishes
David
I did once, and was convinced to try it. I built a round OS waveguide. Then I built a large elliptical waveguide, moving to corner positioned. Then I went back to round with a substantial rebuild, full corner mounting but retaining the space around the treble, so keeping the waveguide. I can't see a time in the future when I will change it (modify the mouth maybe but not change from round OS).
https://www.jblpro.com/pub/technote/tn_v1n08.pdf
This tech note documents that CD’s and horns are thermodynamic machines. The thermodynamic properties of air including formulas and calculations are integral to horn theory. Pick up your acoustics text, you need not flip too many pages. To say that air is unimportant to understanding horn theory is not correct. This is why I referred you to this JBL tech note. Just saying.
This is exactly what Geddes & Lee proposed and did subjective tests for their 2006 AES Convention Paper.
They took a 10 second clip of a Tracy Chapman tune. Using MathCad they digitally created multiple versions of playback level, distortions and delay. They played these stimuli with ear buds to 30 test subjects. Geddes and Lee formed their conclusions using this test protocol.
“…The combined effects of playback level and delay have strong implications to the perceived perception of nonlinear distortion.”
They did this testing without a Compression Driver Horn/Waveguide in the room.
This all gets a little squishy from here. Geddes and Lee make the logical leap to HOM’s perceived linear distortions and horns/waveguides with no objective test results.
Now twelve years later there are still no objective graphs, plots or other data.
The only test results were generated with computer generated sample tracks played back over ear buds.
In the introduction to the Convention Paper Geddes and Lee tell us that “the perception of nonlinear distortion does not correlate with the commonly used metrics of THD and IMD but that it does correlate with a new metric”.
This new metric remains undocumented twelve years later.
Questions:
What complicated objective test did Geddes and Lee have in mind? Did they or others others do this testing?
Santa is bringing a Audio Precision APx1701 and microphone this season.
Thanks DT
I'd like to hear that, if you want to show it off of course. Where are you in Aus?
My acoustics texts tell me that the classical Wave Equation is linear, and that sound waves obey essentially the same physics in air or any fluid.
The JBL note is about second order non linearity that causes distortion.
This is not the essential element of HOM, rather a separate issue.
One that Earl considers rather unimportant, in fact.
This is not true, Earl published the new metric years back.
Your other questions you should take that up with him, I expect James would prefer his tread not to be hijacked.
Enjoy Xmas and the AP.
Best wishes
David
Last edited:
NSW. The timing would have to be right. I'm in the process of a lower midrange build. This will take several months at least. I'll do a build thread when I'm finished.