Sheldon this interesting. Has obvious limitation because its an appendix to a chapter and only deals with one segment of an untypical simplified setup but the discussion and graphs are descriptive of the general HOM situation.
http://www.gedlee.com/downloads/Appendix_A.pdf
"A pdf file with Appendix A (not in printed text). This appendix has an example of a waveguide calculation with higher order modes considered."
I have to run, but:
I’m wondering about the utility of an empirical approach: if we take a OS waveguide which theoretically produces the least HOMs, measure its output parameters exhaustively, and after that install a temporary kink, bump or pimple designed to create HOMs, re-measure and subtract the first set of measurements from the second, then what remains would be the sample HOMs.
I wouldn't bother, I know what they are and it seems pretty obvious how they can be minimized but there are folk who want to look at the little beasts, so this might be a way
http://www.gedlee.com/downloads/Appendix_A.pdf
"A pdf file with Appendix A (not in printed text). This appendix has an example of a waveguide calculation with higher order modes considered."
I have to run, but:
I’m wondering about the utility of an empirical approach: if we take a OS waveguide which theoretically produces the least HOMs, measure its output parameters exhaustively, and after that install a temporary kink, bump or pimple designed to create HOMs, re-measure and subtract the first set of measurements from the second, then what remains would be the sample HOMs.
I wouldn't bother, I know what they are and it seems pretty obvious how they can be minimized but there are folk who want to look at the little beasts, so this might be a way
I guess it wouldn't work because any change in the profile (say, a circumferical thin rim, protruding 0.5" and located near the throat) also changes the basic overall response. So it is not so easy to interpret the residual beacuse it contains any differences, not the HOMs alone. But looking at the individual impulse responses would still give some insight, preferably with a waveguide deep enough or with a very long and smooth mouth-to-baffle transition that the interference from the mouth can be factored out.FrankWW said:
- Klaus
Frank
Yoyr comments are kind of my take on this.
From what I know HOM are not good and everything that I have done to reduce them has been positive. Do I know for absolute sure that it is the HOM that are the culprits? Have I measured the effects and quantified the improvment? No, I haven't done any of that. I have not had the time. I stumbled into the foam (it was almost a mistake!), but the OS contour was solidly done and proven. Since then I have not had the time to do much of anything academic (I'm too busy build great sounding speakers 🙂 ). I did all my analytical work when I had a full time job and did not work for myself. Working for yourself gets you very focused on doing what's necessary and cutting out anything that isn't.
Hence, I have not taken many steps towards measuring and quantifying HOM. It is know that they exist, for instance Markaski measured them in an AES paper, but he concludde that they did not have much effect on the polar response. But then THD doesn't have much effect on the polar response either and many people claim that they hear that. I KNOW that THD is not what we hear, so it's fair to conclude that the HOM might be quite audible despite the fact they they are fairly low level. My data indicates that this is certainly plausible.
As to impulse response measurements, the Farina technique is indeed the ideal AFAIK. I am looking at the HOLMimpulse software that is free (see other thread). If he can provide me with the actual impulse response, with the harmionics impulses, then there is more that I could do. I am quite interested in this.
Yoyr comments are kind of my take on this.
From what I know HOM are not good and everything that I have done to reduce them has been positive. Do I know for absolute sure that it is the HOM that are the culprits? Have I measured the effects and quantified the improvment? No, I haven't done any of that. I have not had the time. I stumbled into the foam (it was almost a mistake!), but the OS contour was solidly done and proven. Since then I have not had the time to do much of anything academic (I'm too busy build great sounding speakers 🙂 ). I did all my analytical work when I had a full time job and did not work for myself. Working for yourself gets you very focused on doing what's necessary and cutting out anything that isn't.
Hence, I have not taken many steps towards measuring and quantifying HOM. It is know that they exist, for instance Markaski measured them in an AES paper, but he concludde that they did not have much effect on the polar response. But then THD doesn't have much effect on the polar response either and many people claim that they hear that. I KNOW that THD is not what we hear, so it's fair to conclude that the HOM might be quite audible despite the fact they they are fairly low level. My data indicates that this is certainly plausible.
As to impulse response measurements, the Farina technique is indeed the ideal AFAIK. I am looking at the HOLMimpulse software that is free (see other thread). If he can provide me with the actual impulse response, with the harmionics impulses, then there is more that I could do. I am quite interested in this.
gedlee said:
Thanks a lot for clarification, Earl.
To sum up, there is practically no evidence on HOM in loudspeaker so far. (despite Markaski's work)
All we have is a nice little hypothesis - as HOM *does* exist (also proven in many other applications)
It further might be that Bjorn Kolbreks take on the sonic impacts given is quite correct.
If so, we have here a good chance to scratch our head to conduct a set up that possibly could reveal the impacts of HOM by quantifying it one way or the other.
From you experience Earl, would you agree that it might make sense to split the implications of HOM
1. into defects in the sound field and
2. into impedance interaction with the driver
?
ad 1) acoustic measurements on a fine grid would be necessary.
ad 2) we possibly can take advantage of thend's set up.
http://www.diyaudio.com/forums/showthread.php?postid=1843034#post1843034
It allows to separate the impedance of the horn with all its reflections back to the throat and also does not generate HOM by itself (below cut off of the pipe) *if* we can choose the right dimensions.
Possibly to attach a fairly large low HOM horn and an excessive HOM horn of comparable size would be a good starting point – no?
Michael
mige0 said:
Michael
You always sem to see things the way you want them to be.
There is STRONG evidence for HOM in horns and waveguides. That HOM exist is not even in contention, they do - proven. How audible they are and that they are a major factor in the character known as "harshness", is an unpreven hypothesis, but is supported by all the existing data. It is NOT nonlinear distortion - proven, it is NOT frequency response - proven, there is strong evidence that HOM types of aberation are highly audible with distortion like characteristics. Betting on anything would be rather foolish.
To prove that certain audible effects are related with HOM, it is necessary to be able to distinguatish what is HOM and what is not during measurements. Currently nobody gas done this. The data already presented shows what could also be interpreted as a low pass filter effect.
"That HOM exist is not even in contention, they do - proven."
Hello Earl
I am no scientist but I have an idea. I am assuming that thier existence is based on your waveguide theory and that they can be seen in your mathematical proofs. If this is so can a noise signal be derived that has the same characteristics??
If that is possible you could take a device that does not generate HOM or has a low incidence of them and use the noise signal to determine the threshold of audibility.
Is that possible as audibility seems to be the key to this discussion??
Rob 🙂
Hello Earl
I am no scientist but I have an idea. I am assuming that thier existence is based on your waveguide theory and that they can be seen in your mathematical proofs. If this is so can a noise signal be derived that has the same characteristics??
If that is possible you could take a device that does not generate HOM or has a low incidence of them and use the noise signal to determine the threshold of audibility.
Is that possible as audibility seems to be the key to this discussion??
Rob 🙂
Shouldn't we care more about the evidence of HOM audibility and not really if HOM are proven through mathematics??
If they are not audible then who cares?? Also, If the "HONK" of other waveguides isnt a direct correlation to HOMs then we still need to figure out what the "HONK" is from.
If they are not audible then who cares?? Also, If the "HONK" of other waveguides isnt a direct correlation to HOMs then we still need to figure out what the "HONK" is from.
gedlee said:
Yes - sure – you already know – I stand to my "truth is subjective" - depending on knowledge and experience
😀
gedlee said:
Possibly I'll be back with some "rather foolish" measurements soon.
🙂
I have not repeated my measurements precisely enough to post 'em right away - but as far as I'm now, under certain circumstances there is "sound field pattern *modulation*" with horns – something well beyond HOM IMO – and also something I wasn't aware of that it exists at all.
Would that be interesting for you - or did I just miss something everybody knows who is "in horns"?
Robh3606 said:
Possibly better put – as far as I know HOM isn't Earls "invention" 😉 (please correct me if I am wrong) – but he certainly has repeatedly introduced HOM to the audio community (though he always likes to wrap a little "woodoo dancing" around it)....
Michael
mige0 said:
Michael
I have no idea what you are talking about.
mige0 said:
That is incorrect. I was the first person to define their existance.
doug20 said:
You seem to be missing some critical points.
1) "HONK" has been completely removed in my waveguides, on this point there can be no argument because everyone who has heard them would agree.
2) What removed is "HONK" isn't perefectly clear, but several things can be elliminated, like distortion, or frequency response
3) in an attempt to quantify why my new waveguides have no "HONK" I did a psychoacoustic study on the audibility of HOM types of aberations (its not possible to control HOM in an arbitrary way in real devices so "simulated" HOM had to do). The audiblity was clearly shown, it was shown to depend on SPL, which made it sound like nonlinear distortion (whcih, as I said, was shown to NOT be the cause).
Hence to repeat, there is no data to say that its NOT HOM and there is data to say it is. People just don;t seem to want to accept it - nothing new there either, thats just human nature.
But getting rid of "Horn HONK", is done. Figuring out all the details of why, well thats probably quite a ways off yet.
yes, that is why Im reading, learning and asking all the dumb questions.....I hope that doesnt bother to many people 😀
No polite question or comment is ever a problem. It's the pointless attacks and sarcastic critiques that get us nowhere.
gedlee said:
forget about - its obsolet - somewhere within my measurement tolerance if I more precisely control the variables...
gedlee said:
Wow - thanks than for correcting me !
To come back to my proposed measurement set up for HOM quantification - what you think about?
Michael
mige0 said:
I'm referring to the above.
The idea behind is that HOM is – correct me if I'm wrong – a pure pattern of interference's in the first place.
(Though it needs some time to build up, as time of flight for the axial wave is shorter than for the waves bouncing between horn walls).
So - for a start - lets say we don't care for the defects it creates in the sound field (which possibly could best be measured by highly directive mics in a fine mesh) but look at the lumped impact on impedance changes due to reflection back into the horn instead.
If we have a fairly deep horn it should be possible to distinguish between the reflections of the axisymmetric wave front at the horn mouth (coming back - to be measured - in the impedance plot at the shortest delay) and the reflections from the "bouncing around wave fronts" (=HOM) at the mouth (coming back to the throat at a slightly increased delay).
Michael
I thought Dr. Geddes mentioned earlier that reflections back into the throat and hitting the diagram was not a primary issue here and would be too small to cause an appreciable effect on the impedance plot.
First I would think that you would need extremely clean impedance measurements with as low a noise floor as possible.
Second, I would think you would need match the impedance ripples to the mathematically derived frequencies for the modes.
It seems to me like an impulse response is a better way of measuring this given that it's delayed energy. All the same issues I mentioned above apply, but I think they are more likely to show up than the impact on impedance of the wave's reflected back through the throat.
First I would think that you would need extremely clean impedance measurements with as low a noise floor as possible.
Second, I would think you would need match the impedance ripples to the mathematically derived frequencies for the modes.
It seems to me like an impulse response is a better way of measuring this given that it's delayed energy. All the same issues I mentioned above apply, but I think they are more likely to show up than the impact on impedance of the wave's reflected back through the throat.
pjpoes said:
If you go with my (and Bjorns) argument that HOM is a *pure* pattern of interference's in the first place - it does not make any sense IMO to look at acoustic measurements unless you are ready for *very* complicated set up (to find out the direction of wavefronts and its respective origin).
Also - pure electric measurements can be done at way lower noise (correction: this may not apply here as we use the driver as kind of mic right at the throat).
I would do some specific acoustic measurements probably afterwards.
Michael
Due to all sorts of reasons, I find even course impedance measurements somewhat difficult. I can measure the same driver 50 times on the same setup and tested in the same way and get 50 slightly different results. Given how small these impedance fluctuations are going to be, I just think it's going to be very difficult.
As you realized, the driver is being asked to be a mic, worse yet, it's loaded in a horn or waveguide, which will only amplify the cause of the S/N problem. I won't say it's an impossibility, but I think you would have to construct a completely dead and quite, resonant free, isolation box with full anechoic properties down to 1000hz or so.
This may be a dumb question, but how would the impedance show the delayed energy? I usually use sweep tones when I measure impedance. Going with your idea for moment, and assuming your initial claim of pure interference pattern, would you need to look at the impedance in a fashion more akin to an impulse or waterfall. How it changes in say a 5ms window?
As you realized, the driver is being asked to be a mic, worse yet, it's loaded in a horn or waveguide, which will only amplify the cause of the S/N problem. I won't say it's an impossibility, but I think you would have to construct a completely dead and quite, resonant free, isolation box with full anechoic properties down to 1000hz or so.
This may be a dumb question, but how would the impedance show the delayed energy? I usually use sweep tones when I measure impedance. Going with your idea for moment, and assuming your initial claim of pure interference pattern, would you need to look at the impedance in a fashion more akin to an impulse or waterfall. How it changes in say a 5ms window?
mige0 said:
Michael
I'd say that in theory this might be possible within some given limitations. But in practice what you would be looking for is a needle in a haystack. The acoustic effect is hard to measure and the impedance reflects the acoustic effects only to the extent of its efficicency - maybe 5%. Hence this effect is twenty times lower in the electrical domain than it is in the acoustical domain - very small indeed. Good luck with that.
And even if you do achieve this task, you could, at best, only get the "average" HOM effect on the diaphragm since the electrical impedance is a bulk or average quantity. How do you know that the HOM don't "average" to zero at the diaphragm? I would think that they would - hence, no effect on the diaphragm, on average, at all.