No, the dampening in a horn provided by foam or any other dampening material has nothing to with that HOM bogus.
Its simply a measure to reduce lopped reflections from the "mouth" of a horn / waveguide / diffraction alignment device.
Actually its there (if you feel its needed) to make a horn / waveguide / diffraction alignment device to act less as a transmission line / pipe (=lowering horn honk).
Michael
Its simply a measure to reduce lopped reflections from the "mouth" of a horn / waveguide / diffraction alignment device.
Actually its there (if you feel its needed) to make a horn / waveguide / diffraction alignment device to act less as a transmission line / pipe (=lowering horn honk).
Michael
Its feasible to use other materials, I have, but they are not so easy to use and not as consistant as the foam.
With respect to effects related to all sort of horns that are caused by simple diffraction "HOM bogus" is more often than not used here as a "smoke discharger / mind blocker term".
Just keep it simple: this horn stuffing is mainly the same thing like transmission line stuffing = dampening of looped echoes
Michael
Just keep it simple: this horn stuffing is mainly the same thing like transmission line stuffing = dampening of looped echoes
Michael
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Now you have done it!
Hmm... I didn't mean to open a can of worms.
Continuing this line of thought: don't you think materials with a higher damping coefficient are better?
Never mind - *this* can that has been opened and discussed in each and every respect quite a while ago - nothing substantial thats left over...
Michael
Getting it right is the key, not too high and not too low. That's the principle reason that I settled in on the foam - the high level of control possible with the pore density. This isn't possible with other materials - the damping is what it is and you have no control over it. I tried densities of 10 PPI to about 90 and it was a fairly small range of 30-40 that did precisely what I was looking for.
Hello Earl,
-0.07dB was the mean calculated all over the differential spectrogram.
On the week end I refined the analysis and could separate the frequency/time couples for which the absolute variation is more than 0.01dB.
Doing this the attenuation of the diffraction + Homs + submilliseconds reflections arise to -8dB. This means that only 40% of the level of those is absorbed by the foam plug.
I don't know if this value can be judged as an efficient absorption.
The method is IMHO very powerful and we have to learn how to improve the use of it, but yet is give very interesting informations.
Best regards from Paris, France
Jean-Michel Le Cléac'h
-0.07dB was the mean calculated all over the differential spectrogram.
On the week end I refined the analysis and could separate the frequency/time couples for which the absolute variation is more than 0.01dB.
Doing this the attenuation of the diffraction + Homs + submilliseconds reflections arise to -8dB. This means that only 40% of the level of those is absorbed by the foam plug.
I don't know if this value can be judged as an efficient absorption.
The method is IMHO very powerful and we have to learn how to improve the use of it, but yet is give very interesting informations.
Best regards from Paris, France
Jean-Michel Le Cléac'h
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As far as I am concerned no single spatial point measurement could sort out HOMs, reflections and diffractions. These are spatial problems and only a spatial analysis of the sound field could have any chance of accurately getting at this data.
I am researching this kind of analysis, but alas I do not get much time for this kind of thing anymore so I don't know when I would have any results. But the bottom line here is that I do not see what you are doing as all that useful. You show some data and jump to some conclusions - not very scientific (but better than "well, it sounds good to me!")
I am researching this kind of analysis, but alas I do not get much time for this kind of thing anymore so I don't know when I would have any results. But the bottom line here is that I do not see what you are doing as all that useful. You show some data and jump to some conclusions - not very scientific (but better than "well, it sounds good to me!"
)
yeah right, HOM is pretty irrelevant stuff, actually a "fogware" synonym for diffraction effects
Better to investigate / understand the root cause than to stay mind blocked by fancy "scientifically sounding" terms...
LOL
Besides that - HOM being a subcategory of diffraction effects *at best* - one first has to understand implications of diffraction effects.
This directly leads to the necessity of understanding CMP effects, which (as spatial effects) - by no means require sound field investigation at first hand.
Michael
Better to investigate / understand the root cause than to stay mind blocked by fancy "scientifically sounding" terms...
LOL
Besides that - HOM being a subcategory of diffraction effects *at best* - one first has to understand implications of diffraction effects.
This directly leads to the necessity of understanding CMP effects, which (as spatial effects) - by no means require sound field investigation at first hand.
Michael
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Hello Earl,
As I don't possess any OS waveguide myself I have to rely on others for the mesurements (tanks to them).
What I would like to have are measurement every 10 degrees (from 0 to 90° min) in the 2 cases: without the foam plug and with the foam plug.
I begun to write a Matlab routine that will be a kind of 3D wavelets differential spectrogram.
What interests me a lot is to see if the delay that we could see on the on axis differential spectrogram are related to a non axial propagation.
Best regards from Paris, France
Jean-Michel Le Cléac'h
As I don't possess any OS waveguide myself I have to rely on others for the mesurements (tanks to them).
What I would like to have are measurement every 10 degrees (from 0 to 90° min) in the 2 cases: without the foam plug and with the foam plug.
I begun to write a Matlab routine that will be a kind of 3D wavelets differential spectrogram.
What interests me a lot is to see if the delay that we could see on the on axis differential spectrogram are related to a non axial propagation.
Best regards from Paris, France
Jean-Michel Le Cléac'h
As far as I am concerned no single spatial point measurement could sort out HOMs, reflections and diffractions. These are spatial problems and only a spatial analysis of the sound field could have any chance of accurately getting at this data.
I am researching this kind of analysis, but alas I do not get much time for this kind of thing anymore so I don't know when I would have any results. But the bottom line here is that I do not see what you are doing as all that useful. You show some data and jump to some conclusions - not very scientific (but better than "well, it sounds good to me!"
Interesting reading: "Fourier acoustics : sound radiation and nearfield acoustical holography" by Earl G. Williams, Academic Press 1999.
Relevant regarding higher order modes and modal decomposition/analysis. Also includes some practical details on measurements and processing of the signals.
Bjørn
Relevant regarding higher order modes and modal decomposition/analysis. Also includes some practical details on measurements and processing of the signals.
Bjørn
Think this would be even more complex.
As the travel distance of all "rays" are different - thus is the delay elongation different for any "ray" of different origin and any point of measurement - so - the whole sound field pattern is slightly shifting
But, "HOM" as a subset of diffraction (at best) - is only affected via diffraction in the presence of dampening materials, no matter how.
Michael
As the travel distance of all "rays" are different - thus is the delay elongation different for any "ray" of different origin and any point of measurement - so - the whole sound field pattern is slightly shifting
But, "HOM" as a subset of diffraction (at best) - is only affected via diffraction in the presence of dampening materials, no matter how.
Michael
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Bjorn, this is 321 pages of dense math
couldn't you give us a brief summary ?
Michael
The math should not be a problem I think, because we are talking about very simple things...
LOL - a good one !
BUT
understanding of underlying physical effects and the translation from and into math terms are pretty different things - as yo might agree with.
You may lack the one or the other - I myself prefer to *not* lack "in understanding" - *if* I have the choice
Even better to have both talents - as Bjorn has proven to be capable more than once...
Michael
BUT
understanding of underlying physical effects and the translation from and into math terms are pretty different things - as yo might agree with.
You may lack the one or the other - I myself prefer to *not* lack "in understanding" - *if* I have the choice
Even better to have both talents - as Bjorn has proven to be capable more than once...
Michael
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Bjorn
Yes, a good reference. Earl Williams was a classmate of mine at Penn State.
And you are correct to imply that model decomposition is the correct way to analyze what the current discussion is trying to azzez (won't use this word with "s") (HOMs). Radiation mode decomposition is precisely what my current software investigation is about. I use a technique that Dr. Williams discusses starting in Chapter 6 - decomposition using spherical harmonics. (Basically see section 6.7, with some discussion in section 7.2) For far field polar plots this technique is ideal, but it does not have high enough resolution for a detailed analysis of the HOMs in a waveguide device.
To do that one needs an "infinite" baffle to measure in and then to use a Bessel function expansion (which Williams does not discuss, but this technique can be found in "An Introduction to Fourier Optics" by Goodman which use Fourier-Bessel Transforms aka Hankel Transform). Then use this data to find the spherical expansion in the mouth. This will show what portion of the wavefront in the mouth is due to the primary mode and what is due to the HOM. I think that you can see that this is no small programming task.
One could also propagate these modes backwards to the throat with a "bit" more math, as shown in my book or my papers on waveguides.
Since an infinite baffle is impossible for me, I am trying to find a way to use a finit baffle and to account for this in the modal expansion. That is not coming out so well!! The diffraction at this baffle edge is problematic and hard to account for unless it is outside of the window used for the far field data. - Measurements can be tricky sometimes.
Jean-Michel
I have data at 7.5 degrees with and without the foam, but I'm not interested in supplying you with data so that you can misuse it to debunct my concepts. If you show me how you would calculate the radiation modes from this data (the correct method) then I would supply it. But to just use "wavelets" as you have been doing, which is, as I said, incorrect, I have no wish to further an incorrect investigation into this matter.
Yes, a good reference. Earl Williams was a classmate of mine at Penn State.
And you are correct to imply that model decomposition is the correct way to analyze what the current discussion is trying to azzez (won't use this word with "s") (HOMs). Radiation mode decomposition is precisely what my current software investigation is about. I use a technique that Dr. Williams discusses starting in Chapter 6 - decomposition using spherical harmonics. (Basically see section 6.7, with some discussion in section 7.2) For far field polar plots this technique is ideal, but it does not have high enough resolution for a detailed analysis of the HOMs in a waveguide device.
To do that one needs an "infinite" baffle to measure in and then to use a Bessel function expansion (which Williams does not discuss, but this technique can be found in "An Introduction to Fourier Optics" by Goodman which use Fourier-Bessel Transforms aka Hankel Transform). Then use this data to find the spherical expansion in the mouth. This will show what portion of the wavefront in the mouth is due to the primary mode and what is due to the HOM. I think that you can see that this is no small programming task.
One could also propagate these modes backwards to the throat with a "bit" more math, as shown in my book or my papers on waveguides.
Since an infinite baffle is impossible for me, I am trying to find a way to use a finit baffle and to account for this in the modal expansion. That is not coming out so well!! The diffraction at this baffle edge is problematic and hard to account for unless it is outside of the window used for the far field data. - Measurements can be tricky sometimes.
Jean-Michel
I have data at 7.5 degrees with and without the foam, but I'm not interested in supplying you with data so that you can misuse it to debunct my concepts. If you show me how you would calculate the radiation modes from this data (the correct method) then I would supply it. But to just use "wavelets" as you have been doing, which is, as I said, incorrect, I have no wish to further an incorrect investigation into this matter.
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What would be too high? The way I see it, the worst that could happen is you sacrifice a bit more efficiency than necessary, right?