A while back I macgyvered this setup to measure the Bl product:
The minimal ribbon frame lies on the scale on top of three aluminium profiles (to create some distance to the scale). Mounted to the table above the piece of wood is positioned in the gap, a 10cm x 1.9cm copper foil is glued to the bottom. I figured that this would get me the average flux density. The scale is accurate to .01g. If you do the math on those two numbers you get a Bl product of 19mN, so the average flux density in the gap is 0.19T. This is a bit below the 0.24T predicted by FEMM but this is a view from above simulation and I don't know how accurate the simulation is.
regards,
Gerrit
The minimal ribbon frame lies on the scale on top of three aluminium profiles (to create some distance to the scale). Mounted to the table above the piece of wood is positioned in the gap, a 10cm x 1.9cm copper foil is glued to the bottom. I figured that this would get me the average flux density. The scale is accurate to .01g. If you do the math on those two numbers you get a Bl product of 19mN, so the average flux density in the gap is 0.19T. This is a bit below the 0.24T predicted by FEMM but this is a view from above simulation and I don't know how accurate the simulation is.
regards,
Gerrit
I'm quite psyched about this. One of my most memorable listening events was in Munich hifi show and a rig made up of; DCS, VTL 450 monos and Crystal Cable Arabesque glass speakers. The Arabesque uses a RAAL ribbon. They played orchestra music in a quite large room to realists levels with a fantastic force, resolution and stage. Really fantastic - especially brass and horn where really well recreated.
http://www.innerearmag.com/images/photos_reviews/CCAbaresque02_large.jpg
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http://www.innerearmag.com/images/photos_reviews/CCAbaresque02_large.jpg
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It's a single 10mm thick steel plate.
The steel frame gives you 6dB more efficiency but the price is a response that requires more equalisation to flatten which more or less rules out passive filtering. The off-axis behaviour is also not as good as the for minimal ribbon.
Another option is the minimal ribbon as a cardioid, this is something I still want to try just for the fun of it. It should require less eq than the steel frame ribbon while delivering more output than the minimal ribbon. A cardioid can also be placed on top of more traditional loudspeakers and of course closer to the back wall.
I never heard the Raal ribbons but I like to think I captured what they are about in my designs. The measurements prove that I'm not mistaking higher harmonics for brilliance or an elevated mid range for detail. This is why they are so important, it's just to easy to fool yourself.
You mention brass and horn. I was pleasantly surprised by the way my system renders Philip Glass' 'Songs from Liquid Days', very natural and spacious.
regards,
Gerrit
Time for a quiz!
Slap my face if You think I'm hijacking....
Speaking 'bout distortion, I have had this pair of B&G Neo8 laying in a drawer, not knowing what to do with them.
Almost useless with their hump and distortion peak at 13 kHz.
So, today, I decided to investigate what this peak is all about, and I have found a cause and a solution to the problem. (I think). See the differences in the graph...
Blue is before, red after remedy.. blue impulse is delayed on graph for clarity..electrical signal is not altered.....no new damping material...
The solution is quite amusing, lets see if You can nail it...
Slap my face if You think I'm hijacking....
Speaking 'bout distortion, I have had this pair of B&G Neo8 laying in a drawer, not knowing what to do with them.
Almost useless with their hump and distortion peak at 13 kHz.
So, today, I decided to investigate what this peak is all about, and I have found a cause and a solution to the problem. (I think). See the differences in the graph...
Blue is before, red after remedy.. blue impulse is delayed on graph for clarity..electrical signal is not altered.....no new damping material...
The solution is quite amusing, lets see if You can nail it...
Attachments
Real seem to succeed with a "flat" foil... using "symmafield"...
These where the ones in the Arabesque I belive.
http://www.raalribbon.com/download/raal_140-15d.pdf
Jonas - curious about your 13k fix...
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These where the ones in the Arabesque I belive.
http://www.raalribbon.com/download/raal_140-15d.pdf
Jonas - curious about your 13k fix...
//
The peak is a mechano-acoustic resonance between the mass of the (diaphragm + aluminum traces) and the compliance of the air in the cavity between the diaphragm and the openings in the outer steel plates.
From the measurement, my guess is you removed one of the outer steel plates. You would of course lose half the magnets so that would explain the loss in sensitivity. The resonance peak is reduced in level since there is now only one resonant system and the diaphragm is now better damped by the radiation impedance. Simulation would suggest you are measuring on the side from which you removed the steel plate. I’m not sure if you measured from the other side yet, but response will be slightly different in the top octave.
I think this topic is relevant to the thread as it explains why the top octaves of the Gerrit Boers ribbon configuration has such smooth response.
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bolserst, you are spot on! (as usual!)
And, yes, the measurement is taken from removed plate side.
Adding a graph of measurement from magnet side, green trace.
I think that adding two rows of holes so that you had six rows instead of four, would make the problematic distortion peak less prominent...
And, yes, the measurement is taken from removed plate side.
Adding a graph of measurement from magnet side, green trace.
I think that adding two rows of holes so that you had six rows instead of four, would make the problematic distortion peak less prominent...
Attachments
Hey there everybody I have one question for my thread gets no answers and suspenss kills me! I have a pair of Infinity RS-5001 but one tweeter not working, got it out, I have put the multi meter on it no resistance, I have switched left with right tweeter and no sound from the same tweeter while the other works fine! Is there room for even hope for salvage or no more hope? Thank you!
Inspiring solution from first principles.
I like the way you think!
Thank you
I already had the scale available and with d=0.01g it's pretty accurate. The max load is 1000g so the dynamic range is 100dB
What are the measurement conditions (distance, SPL, smoothing)? You also need to compensate for the loss in efficiency and keep the levels equal otherwise you can't really compare the results.
To me it looks like you exchanged the distortion associated with the main resonance for distortion peaks at lower frequency. For this kind of comparison it's also important to look at the different harmonics.
regards,
Gerrit
Bolserst: Can I ask for a bit more detail?
- Perhaps I'm misinterpreting your statement. Are you saying that removing one plate increases the radiation impedance? I would have thought each perforated plate would add to the radiation impedance seen by the diaphragm so removing one plate would decrease the radiation impedance.
- When you did the modeling did you find that the resonance frequency matches the diaphragm-plate distance and not the plate-plate distance? I was thinking of the resonator as consisting of two perforated plates separated by a gap (and driven by a central diaphragm). Should I instead think of each diaphragm-gap-plate system as a resonator and then couple the two resonators to form the two-plate system?
Also, Jonas, is the distortion in your "before and after" plot the total harmonic distortion or just one distortion product (second or third, for example)? With one plate and set of magnets removed the symmetry of the system is broken and it seems that change should be reflected in the even vs. odd distortion profile.
This has been an educational thread!
Few
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Few, its THD that is showed in graphs....
I will do some new measurements showing 2:nd and 3:rd harmonics in B&g Neo4 and Neo8. (Neo4 is my name on Neo8 without front magnet plate.)
This Neo8 that I opened had a loose connection on the positive input, but very easy to
fix with speaker opened. Oddly, I had no problem with soldering on the start of the alu track.
I will do some new measurements showing 2:nd and 3:rd harmonics in B&g Neo4 and Neo8. (Neo4 is my name on Neo8 without front magnet plate.)
This Neo8 that I opened had a loose connection on the positive input, but very easy to
fix with speaker opened. Oddly, I had no problem with soldering on the start of the alu track.
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Simplified modeling can be done using a lumped parameter analogous circuit as outlined in Chapter 3 of the Beranek Acoustics text book. Without going into great detail, the attached figure should give you an idea of what it is about. The radiation impedance can be simplified by assuming we are dealing only with high frequencies (ka>1) where the resistive component is dominant. This is an impedance analogy, so inductors are masses, capacitors are springs, the voltage source represents Force on the diaphragm, and the output pressure is voltage across either the front or rear radiation impedance. Of course the tricky part is determining the values of the elements. Also, the %open area of the steel plates will affect the “lossiness” of the compliance, making it not act like a perfect spring.
For the Neo-8 the diaphragm is so heavy, the mass of the air in the holes in the steel plates is not a bigger player in defining the frequency of the peak. However, the same model applies to ESLs and the mass of the air in the holes of the stators can be as large as the diaphragm mass…sometimes larger.
Does this brief explanation address all your questions?
Attachments
Ugh...just noticed in my haste to post last night I failed to paste in response to your first question.
Not sure what happened
Simplified modeling can be done using a lumped parameter analogous circuit…
(continued in previous post)
Not sure what happened
I guess it depends on your definition of radiation impedance. What I was trying to say was that the airload on each side of the tranducer is unchanged at high frequencies and is the characteristic impedance of air = (air density) x (speed of sound). With the (steel plate + magnets) removed from one side of the Neo8, the airload is now directly imparted to the diaphragm without the reactive acoustic components of cavity compliance or small slugs of mass in the holes in between. This would increase damping of the resonance.
That’s a good question. Although we like to think in terms of individual resonator systems, Spice simulation indicates there is significant interaction between the acoustic elements. With that in mind, I’ll try to describe what I did, and see if that answers your questions.
Simplified modeling can be done using a lumped parameter analogous circuit…
(continued in previous post)
Thanks, Bolserst, for the thorough replies. I had a different picture in mind so it's enlightening to see your approach.
First, regarding your circuit simulation of the system. Did you have realistic values for the various components in the circuit, and if so, did it accurately predict the resonance frequency?
Might there be a resistive contribution from the air moving through the perforated metal plate or are the holes' diameters typically too large for that to be significant? I was first thinking that the resistance from the holes would help damp the diaphragm so removing one plate would reduce the damping, but perhaps that's incorrect.
I tried looking at the problem by thinking of the partially reflecting perforated steel plates, separated by a gap, as an acoustic etalon, analogous to the ones used in optical systems. There, two partly reflecting mirrors are placed parallel to each other, separated by a small gap. The system exhibits optical resonances resulting from the "echo" of the light between the mirrors. The resonances are characterized by wavelengths of light at which the system exhibits very high transmission of light.
When I use your reported Neo 8 plate-to-plate distance of 6 mm I calculate a resonance frequency of 28.8 KHz so that doesn't encourage me to continue that line of thinking. I'd need a plate-to-plate distance of about 13 mm to get a resonance frequency of 13 KHz (unless I'm missing a two somewhere...).
Few
First, regarding your circuit simulation of the system. Did you have realistic values for the various components in the circuit, and if so, did it accurately predict the resonance frequency?
Might there be a resistive contribution from the air moving through the perforated metal plate or are the holes' diameters typically too large for that to be significant? I was first thinking that the resistance from the holes would help damp the diaphragm so removing one plate would reduce the damping, but perhaps that's incorrect.
I tried looking at the problem by thinking of the partially reflecting perforated steel plates, separated by a gap, as an acoustic etalon, analogous to the ones used in optical systems. There, two partly reflecting mirrors are placed parallel to each other, separated by a small gap. The system exhibits optical resonances resulting from the "echo" of the light between the mirrors. The resonances are characterized by wavelengths of light at which the system exhibits very high transmission of light.
When I use your reported Neo 8 plate-to-plate distance of 6 mm I calculate a resonance frequency of 28.8 KHz so that doesn't encourage me to continue that line of thinking. I'd need a plate-to-plate distance of about 13 mm to get a resonance frequency of 13 KHz (unless I'm missing a two somewhere...).
Few
Yes. Using equations in Beranek text to determine component values produced surprisingly accurate prediction of the resonance frequency and Q. It’s been a while since I’ve messed with the Neo-8, been mainly concentrating on ESL modeling as of late…will see if I can locate a comparison plot.
Indeed there is, but the resistance is exceedingly small until the holes get < 0.1mm diameter.
Similarly, to a first approximation the acoustic resistance of damping mesh is inversely proportional to the square of the hole size.
Mesh Acoustic Resistance vs pore size
@ Gerrit Boers:
Let us know if this is drifting too far off topic for your liking and we can rein it in or move to a new thread.
